Analysis of global research hotspots and trends in immune cells in intervertebral disc degeneration: A bibliometric study

ABSTRACT

Intervertebral disc degeneration is an important pathological basis for spinal degenerative diseases. The imbalance of the immune microenvironment and the involvement of immune cells has been shown to lead to nucleus pulposus cells death. This article presents a bibliometric analysis of studies on immune cells in IDD in order to clarify the current status and hotspots. We searched the WOSCC, Scopus and PubMed databases from 01/01/2001 to 08/03/2023. We analyzed and visualized the content using software such as Citespace, Vosviewer and the bibliometrix. This study found that the number of annual publications is increasing year on year. The journal study found that Spine had the highest number of articles and citations. The country/regions analysis showed that China had the highest number of publications, the USA had the highest number of citations and total link strength. The institutional analysis found that Shanghai Jiao Tong University and Huazhong University of Science Technology had the highest number of publications, Tokai University had the highest citations, and the University of Bern had the highest total link strength. Sakai D and Risbud MV had the highest number of publications. Sakai D had the highest total link strength, and Risbud MV had the highest number of citations. The results of the keyword analysis suggested that the current research hotspots and future directions continue to be the study of the mechanisms of immune cells in IDD, the therapeutic role of immune cells in IDD and the role of immune cells in tissue engineering for IDD.

KEYWORDS:

• Immune cells
• intervertebral disc degeneration
• immune response
• bibliometric analysis
• web of science
• Scopus

Introduction

Intervertebral disc degeneration (IDD) is an important pathological basis for spinal degenerative disease (SDD) and is a major cause of low back pain (LBP) and disability. Epidemiological findings suggest that approximately 4/5 of the adult population will suffer from LBP at some point in their lives, and approximately 637 million people worldwide are affected by it.¹ It is a common degenerative disease that severely affects the quality of life of patients in their senior years. Studies reveal that about 10% of people in their 50s suffer from IDD, while the proportion of people with IDD increases to about 60% in their 70s.² As lifestyles keep changing, the incidence of the disease is becoming younger. It is estimated that about 20% of adolescents have mild disc degeneration.³ The high incidence and widespread impact of degenerative disc disease (DDD) has made it a serious health problem for patients. To date, there is no uniformly effective treatment for it.⁴

IDD is a complex cell-mediated process that ultimately leads to functional and structural alterations; however, the specific molecular mechanisms underlying the pathogenesis of IDD are still not fully understood.⁵ Immune response, apoptosis, autophagy, oxidative stress, inflammatory response, biomechanical deterioration, genetics, and infection are important factors that contribute to the development of IDD. Adequate study of the specific role of these factors in the development of IDD is a necessary way to find effective treatments based on the pathogenesis. Of the numerous mechanisms, imbalances in the immune microenvironment and the involvement of immune cells have been shown to lead to nucleus pulposus cells (NPCs) death and have received a great deal of attention in recent years.⁶ It has been found that when a disc degenerates, the body develops an immune response and a large number of inflammatory cytokines are released, causing symptoms such as LBP.⁷ Nevertheless, the immunological pathogenesis of intervertebral disc (IVD) is relatively complex and only an adequate understanding of the role of immune cells and immune responses in IDD can provide certain building blocks for subsequent therapeutic studies.

Bibliometrics is an important division of the information subject system and is a branch that reveals the laws of documentary information and the science of documentary information management.^8,9 It can use a combination of mathematical and statistical methods to analyze a number of characteristics of the literature.¹⁰ Through quantitative analysis of bibliometrics and understanding the development of the field, we can identify patterns in scientific activity, quickly understand the state of development of a field and recent research hotspots, and use various bibliometric indicators to provide guidance for effective scientific research activities in the future.¹¹

Hence, this study is the first to analyze relevant studies in the field of the role for immune cells in IDD by means of a bibliometric approach and using various software to reveal current research hotspots and trends with a new perspective, providing a reference for future researchers to further investigate the immune mechanisms of IDD.

Materials and methods

Data sources and search strategies

We searched the Web of Science Core Collection (WOSCC), Scopus and PubMed databases for literature related to the study of immune cells in IDD. We searched the databases for articles published in the period 2001.01.01–2023.03.08, and all content was searched on the same day to avoid errors caused by database updates. The type of publication is restricted to “article” and “review” and the language type of the article is “English.” Taking Scopus as an example, the search formula is: ((TITLE-ABS-KEY (immune AND cell*) OR TITLE-ABS-KEY (b AND cells*) OR TITLE-ABS-KEY (t AND cells*) OR TITLE-ABS-KEY (plasma AND cell*) OR TITLE-ABS-KEY (macrophage*) OR TITLE-ABS-KEY (nk AND cell*) OR TITLE-ABS-KEY (monocyte*) OR TITLE-ABS-KEY (dendritic AND cell*) OR TITLE-ABS-KEY (mast AND cell*) OR TITLE-ABS-KEY (neutrophil*) OR TITLE-ABS-KEY (natural AND killer AND cell*) OR TITLE-ABS-KEY (lymphocyte*)) AND PUBYEAR > 2000 AND PUBYEAR > 2000) AND ((TITLE-ABS-KEY (intervertebral AND disc*) OR TITLE-ABS-KEY (intervertebral AND disk*) OR TITLE-ABS-KEY (disc*, AND intervertebral) OR TITLE-ABS-KEY (disk*, AND intervertebral)) AND PUBYEAR > 2000 AND PUBYEAR > 2000). Two authors independently read the titles and abstracts of downloaded literature to exclude irrelevant and duplicate articles. The literature search strategy and the main framework elements are shown in Figure 1.

Figure 1. Flow chart about the search strategy.

Data collection

In the WOSCC and Scopus databases, we exported the content of the search in plain text file format and tab-delimited files, and after manual filtering and CiteSpace removal of duplicates, we finally saved the valid results to be used for subsequent analysis. Carrot2 was used to analyze the PubMed database for subject terms in the field and for testing the hot content derived from the two databases mentioned above.

Bibliometric analysis

Literature downloaded from the WOSCC and Scopus databases was imported into Citespace (6.2.R2), Vosviewer (1.6.17), the bibliometrix package in R4.2.2 and Microsoft Excel 2016 for data processing and visual analysis. The online analysis platform for bibliometrics (https://bibliometric.com/app) presented the inter-country collaboration chord diagram. Line graphs of annual publication volume in different databases and different immune cell research in WOSCC are plotted by Excel. Country, institution, author frequency and collaboration networks are analyzed and visualized through Vosviewer; keyword networks, clustering, literature citation, and co-citation networks are presented through Vosviewer and R; keyword clustering, emergence and double mapping overlay plots of journals are presented through Citespace; the Bibliometrix package is used to analyze the development in Scopus.

Results

The trend of publications

From 2001.01.01 to 2023.03.08, we retrieved a total of 2661 publications through the WOSCC database, including a total of 2303 publications of “article” type and 301 “reviews,” and after excluding non-English literature, a total of 2592 valid publications were obtained. In the Scopus database, a total of 1950 documents were retrieved, including 1593 publications of “article” type and 258 “reviews,” excluding non-English literature, resulting in 1738 valid documents. In the PubMed database, we obtained a total of 1198 valid articles. The annual trend in the number of articles is shown in Figure 2a, from which it can be seen that the year with the highest number of articles in both databases is 2022. Since 2001, the number of articles has shown a year-on-year increase, which shows that the study of immune cells is becoming more and more important to researchers in the field of IDD.

Figure 2. Number of publications on immune cells in disc degeneration according to the year in different databases. (a) Annual number of publications in different databases. (b) Annual publications of different immune cells in the WOSCC database.

The WOSCC database was searched separately for types of immune cells, mainly Leukomonocyte, Macrophage, Monocyte and Neutrophil. As seen in Figure 2b, lymphocytes are a major component of immune cells, and the number of publications continues to increase each year, with an upward spiral, as with other immune cells. This is followed by macrophages.

Journal analysis

In the WOSCC database, 569 journals have appeared since the turn of the 21st century, with the highest number of articles published being Spine, and it has the highest number of citations and average citations. The next most popular journals are the Journal of Orthopaedic Research and the European Spine Journal, both of which are prominent journals in this area. Among the top 10 journals in terms of publication volume (Table 1), the journal with the highest impact factor is the International Journal of Molecular Sciences with an IF of 6.208 in 2021, followed by Arthritis Research & Therapy with an IF of 5.606. The top ten journals are in Q1 and Q2, with Q1 accounting for 60% and Q2 accounting for 40%. In addition to specialist journals such as Spine, European Spine Journal and Spine Journal, general journals such as PLoS one and Scientific Reports have also made an important contribution to the development of the field. In the Scopus database, eight of the top ten journals in terms of publications are duplicates of the WOSCC database, and in particular, the top three journals in terms of publications searching the two databases yielded the same results. This result reaffirms that Spine, the Journal of Orthopaedic Research, and the European Spine Journal have made important contributions to the field.

Table 1. Top 10 journals in terms of the documents on immune cells in disc degeneration.

Rank	WOSCC						Scopus
	Journals	Documents	Citations	Norm. citations	IF (2021)	JCR (2021)	Journals	Documents
1	Spine	179	9206	170.1832	3.269	Q2	Spine	87
2	Journal of Orthopaedic Research	106	3008	91.8649	3.103	Q2	Journal Of Orthopaedic Research	66
3	European Spine Journal	78	3836	86.4868	2.721	Q2	European Spine Journal	46
4	European Cells & Materials	67	2272	69.1828	4.325	Q1	Spine Journal	45
5	Spine Journal	66	1756	57.2455	4.297	Q1	Oxidative Medicine And Cellular Longevity	30
6	JOR Spine	51	554	48.8313	3.747	Q1	Arthritis Research And Therapy	28
7	Arthritis Research & Therapy	47	2394	59.538	5.606	Q1	International Journal Of Molecular Sciences	26
8	International Journal of Molecular Sciences	46	830	61.2817	6.208	Q1	Molecular Medicine Reports	23
9	PLoS One	39	1183	30.2029	3.752	Q2	PLoS One	22
10	Scientific Reports	37	861	35.8734	4.997	Q2	JOR Spine	18

In the WOSCC database, a double map overlay of journals shows the main citation path from the citing journal to the cited journal, indicating the thematic distribution of scholarly journals. Citing journals are on the left, cited journals are on the right, and the thicker distinct colored paths indicate citation links.¹² One pink primary citation path and one orange primary citation path are defined (Figure 3a). The orange path suggests that publications in Molecular/Biology/Immunology journals typically cite publications in Molecular/Biology/Genetics journals. In addition, the pink paths represent that publications in Neurology/Sports/Ophthalmology journals usually cite publications in Molecular/Biology/Genetics journals. Vosviewer is used to construct a visual network between journals (Figure 3b) and journals with ≥ 5 publications are used for the collaborative network analysis, obtaining a total of 108 journals clustered into 7 categories. The results show that journals such as Spine and Journal of Orthopaedic Research were classified as category 3, journals such as European Cells & Materials, and European Spine Journal were classified as category 2, and journals such as Arthritis Research & Therapy and Scientific Reports were classified as category 5, which had a high intensity of association and were important in the collaborative network.

Figure 3. Journal analysis on immune cells in disc degeneration. (a) the dual-map overlay of journals in WOSCC. Citing journals are on the left, while cited journals are on the right. Colored paths indicate citation relationships. In this area, one pink primary citation path and one orange primary citation path are defined. The orange path suggests that publications in Molecular/Biology/immunology journals typically cite publications in Molecular/Biology/Genetics journals. In addition, the pink path represents that publications in Neurology/Sports/Ophthalmology journals usually cite publications in Molecular/Biology/Genetics journals. (b) the journal network graph in WOSCC. The size of the nodes indicates the number of journal publications, the thickness of the connections between the nodes reflects the strength of cooperation between the journals, and the color of the nodes corresponds to the clustering of the different journals, with nodes of the same color belonging to the same cluster.

Country/Regional analysis

In WOSCC, 68 countries are involved in the publication of literature, and the distribution of the number of publications by country/region is shown in Figure 4a. As can be seen, China has the highest number of publications (n = 1060), followed by the USA (n = 670), Japan (n = 309) and Switzerland (n = 155), as detailed in Table 2. In Scopus, nine of the top ten countries in terms of number of publications coincide with those in WOSCC, of which China (n = 655), USA (n = 444) and Japan (n = 155) rank in the top three, in line with the WOSCC database. In addition, the only difference in Scopus, Italy, with 50 publications, is ranked ninth and should also receive some attention. In terms of citation frequency, although China has the highest number of publications, the USA has the highest citation frequency (n = 27,472), followed by China (n = 20,119) and Japan (n = 12,476). Total link strength is the main observer of the degree of cooperation between countries in a cooperative network, with higher link strength indicating a higher position in relation to this network. From this perspective, USA is found to have the highest total link strength (n = 414), followed by Switzerland (n = 217), China (n = 200) and Japan (n = 181).

Figure 4. Country/Regions analysis on immune cells in disc degeneration. (a) world map of the number of publications issued by country/regions. Darker colors indicate a higher volume of publications by country/regions. (b) the country/regions cooperation network graph in WOSCC. The size of the nodes indicates the number of country publications, the thickness of the connections between the nodes reflects the strength of cooperation between the countries, and the color of the nodes corresponds to the clustering of the different countries, with nodes of the same color belonging to the same cluster. (c) chord diagram of inter country/regions cooperation network in WOSCC. The size of the color block in front of each country region indicates the number of publications in that country. Links between countries indicate a partnership, with thicker lines indicating closer cooperation between countries. (d) world map of inter country/regions cooperation network in Scopus. The stronger the line, the closer the cooperation between countries. (e) histogram of corresponding author’s country. SCP: single country publication, MCP: multiple country publication.

Table 2. Top 10 countries in terms of the documents, citations and total link strength on immune cells in disc degeneration.

Rank	WOSCC						Scopus
	Country	Documents	Country	Citations	Country	Total link strength	Country	Documents
1	China	1060	USA	27472	USA	414	China	655
2	USA	670	China	20119	Switzerland	217	USA	444
3	Japan	309	Japan	12476	China	200	Japan	155
4	Switzerland	155	England	5934	Japan	181	England	98
5	Germany	153	Switzerland	5408	Germany	180	Germany	95
6	England	149	Germany	4681	England	146	Switzerland	71
7	Netherlands	98	Canada	4093	Netherlands	121	Canada	63
9	Canada	95	Netherlands	3196	Canada	84	South Korea	51
8	South Korea	71	South Korea	1988	South Korea	54	Italy	50
10	Australia	69	Sweden	1923	Australia	48	Australia	45

We used Vosviewer software to visualize and analyze the clustering and cooperation between countries, setting the minimum number of occurrences to 2. A total of 47 countries were filtered and eventually clustered into 9 categories (Figure 4b). Among them, the USA was classified as category 7, Switzerland, Japan and Germany as category 5, China as category 8 and England as category 2, all of which have an important position in the cooperation network and have close cooperation with other countries. We constructed a chord diagram of inter-country cooperation through an online analysis platform (Figure 4c). It is evident from this that China, USA and Japan are the three most important countries in cooperation network. Most of the articles from China are based on domestic cooperation, while a small number of articles on international cooperation are based on developed countries such as the USA, Japan, Germany, Switzerland and the UK. The USA, on the other hand, is actively involved in international collaborations in addition to domestic collaboration, working closely with countries such as China, Japan, Netherlands, Germany, UK and Italy. We constructed a world map of inter-country collaborations in the Scopus database through the R package, see Figure 4d. Additionally, we constructed single country publication (SCP) and multiple country publication (MCP) for the top ten countries, drawing similar conclusions to the WOSCC database (Figure 4e).

Institution analysis

In WOSCC, a total of 2252 institutions are involved in the publication of literature, with the specific ranking and influence shown in Table 3. The largest number of articles is published by Shanghai Jiao Tong University from China with 77 articles, followed by the University of Bern from Switzerland and the University of California System from the USA with 70 and 68 articles respectively. In terms of citations, Tokai University from Japan is ranked first with 4,263 citations despite having published only 67 papers. Thomas Jefferson University from the USA is second with 3270 citations, while Duke University is third with 2842 citations. In Scopus, seven of the top ten institutions duplicated the results obtained by WOSCC. The institution with the highest number of publications is Huazhong University of Science Technology from China with 51 publications, followed by Thomas Jefferson University from the USA (n = 43) and Sun Yat-Sen University from China (n = 39).

Table 3. Top 10 institutions in terms of the documents, citations and total link strength on immune cells in disc degeneration.

Rank	WOSCC						Scopus
	Institution	Documents	Institution	Citations	Institution	Total link strength	Institution	Documents
1	Shanghai Jiao Tong University	77	Tokai University	4263	University of Bern	118	Huazhong University of Science Technology	51
2	University of Bern	70	Thomas Jefferson University	3270	Tokai University	100	Thomas Jefferson University	43
3	University of California System	68	Duke University	2842	University of Hong Kong	98	Sun Yat-Sen University	39
4	Tokai University	67	University of Hong Kong	2228	AO Research Institute Davos	95	Rush University Medical Center	28
5	Sun Yat sen University	65	Rush University	2196	Rush University	85	Shanghai Jiao Tong University	28
6	Jefferson University	64	University of Manchester	2108	Sun Yat sen University	84	Université McGill	27
7	Naval Medical University	60	University of Pittsburgh	2034	Thomas Jefferson University	81	University of Bern	27
8	Huazhong University of Science Technology	58	University of Bern	1789	University Medical Center Utrecht	74	Zhejiang University	22
9	Soochow University	57	Sun Yat sen University	1786	University of Pennsylvania	71	AO Research Institute Davos	22
10	Zhejiang University	57	McGill University	1739	University of Utrecht	69	University of California System	21

In terms of citations, Tokai University from Japan is ranked first with 4,263 citations despite having published only 67 papers. Thomas Jefferson University from the USA is second with 3270 citations, while Duke University is third with 2842 citations. The University of Bern is clustered in category 5, Tokai University is clustered in category 12 and Thomas Jefferson University is clustered in category 7 (Figure 5a). They are all significant and highly representative of the institutions in the different clusters. Moreover, the total link strength is a useful indicator for the importance of an institution in the network (Table 3). Of these, the University of Bern, although not the institution with the highest number of posts, has the highest total link strength in the network at 118, followed by Tokai University and the University of Hong Kong at 100 and 98, respectively. The chronological chart can reflect the chronological order of the emergence of institutions. From Figure 5b, we can find that institutions such as Tokai University and Rush University emerged earlier in the field, while some Chinese institutions, such as Shanghai Jiao Tong University, emerged relatively later despite their high number of publications.

Figure 5. Institution analysis on immune cells in disc degeneration. (a) the institution cooperation network graph in WOSCC. The size of the nodes indicates the number of institution publications, the thickness of the connections between the nodes reflects the strength of cooperation, and the color of the nodes corresponds to the clustering of the different institutions, with nodes of the same color belonging to the same cluster. (b) the institutions’ collaboration network visualization map in WOSCC. Each node represents an institution and the color of the node corresponds to the main publication time of the institution. The size of the node indicates the number of publications in the literature, and the thickness of the connections between the nodes reflects the strength of the partnership. Different colors indicate different publication times.

Author analysis

Taking WOSCC as an example, a total of 10,952 authors have been involved in the literature, and the top ten authors in terms of number of publications are shown in Figure 6a. Sakai D has the highest number of publications (n = 57), followed by Zhang Y (n = 55) and Risbud MV (n = 53), as shown in Table 4. In terms of citation volume, it can be found that Risbud MV published the largest number of citations at 3316, followed by Risbud MV and Sakai D at 2883 and 2341, respectively. Starting from the total link strength it can be found that Sakai D has the highest strength with 191, followed by Yurube T with 165 and Risbud MV with 159. In Scopus, five of the top ten authors in terms of publications overlap with the top ten authors in terms of publications in WOSCC, reinforcing the fact that these scholars have an important position. In this, Risbud MV has published 39 publications and is ranked number one, followed by Shapiro IM who has published a total of 30 and Sakai D who has published 20.

Figure 6. Author analysis on immune cells in disc degeneration. (a) dot line chart of top 10 authors in terms of publications in WOSCC. (b) the author cooperation network graph in WOSCC. The size of the nodes indicates the number of author publications, the thickness of the connections between the nodes reflects the strength of cooperation, and the color of the nodes corresponds to the clustering of the different authors, with nodes of the same color belonging to the same cluster. (c) Sankey diagram between important authors and countries in WOSCC. The left side is the node for the author, the right side is the node for the country, the line indicates that the author belongs to this country, and the thicker the line, the higher the number of publications of the author. (d) authors’ production over time in Scopus. The size of the node indicates the author’s annual publication volume.

Table 4. Top 10 authors in terms of the documents, citations and total link strength on immune cells in disc degeneration.

Rank	WOSCC						Scopus
	Author	Documents	Author	Citations	Author	Total link strength	Author	Documents
1	Sakai D	57	Risbud MV	3316	Sakai D	191	Risbud MV	39
2	Zhang Y	55	Shapiro IM	2883	Yurube T	165	Shapiro IM	30
3	Risbud MV	53	Sakai D	2341	Risbud MV	159	Sakai D	20
4	Wang H	50	Grad S	1690	Grad S	158	Shao Z	19
5	Li B	44	Masuda K	1648	Kakutani K	148	Grad S	18
6	Shapiro IM	44	Alini M	1460	Yang C	141	An HS	17
7	Grad S	43	Hoyland JA	1353	Nishida K	140	Wang K	16
8	Liu Y	38	Mochida J	1305	Shapiro IM	139	Zheng Z	16
9	Alini M	36	Setton LA	1236	Takada T	139	Hoyland JA	15
10	Hoyland JA	36	Iatridis JC	1189	Song Y	136	Iatridis JC	15

Vosviewer and R software are used for visual analysis of the author network. WOSCC data showed that there were 567 authors with postings greater than or equal to 5, and the results of the cluster analysis clustered the authors into a total of 27 categories (Figure 6b). Sakai D, the author with the highest number of publications and the highest total link strength, was clustered into category 17, shown in gray; Risbud MV and Shapiro IM, the authors with the highest number of citations, were clustered into category 13, shown in light yellow; and Yurube T, the author with the second highest total link strength, was clustered into category 18. We plotted the Sankey diagram between authors and countries through R package (Figure 6c) and found that the top 20 authors were mainly from six countries, with the top three countries remaining China, the USA and Japan. We used the data in Scopus as a validation set and visualized it through R software. A bubble plot of authors’ output over time was shown in Figure 6d, which visualized the authors who have published more articles and on average more articles at different points in time, and found that the top ten authors are mainly from China, and their output has gradually increased in recent years, represented by Zhang Y, LI X and Wang X.

Citation and co-citation analysis

In WOSCC, we have summarized the specific information of the top 10 cited publications in Table 5. In total, five of these ten documents are “reviews” and five are “articles.” The most cited article was “Scaffolding in tissue engineering: general approaches and tissue-specific considerations” in the European Spine Journal in 2008, with 876 citations. The second was a review by Risbud MV in Nature Reviews Rheumatology in 2014 called “Role of cytokines in intervertebral disc degeneration: pain and disc content,” with 854 citations. In addition, articles by Le Maitre CL, Sakai D and Shamji MF were all highly regarded. Sakai D’s team published half of 10 highly cited literature, which shows his outstanding contribution to the field. Norm. citations is an important indicator of the extent to which an article has been cited, highlighting the average number of citations per year since the publication of the literature. The highest Norm. citations was for the article published by Risbud MV in 2014, with a value of 19.2708. This was followed by the literature published by Chan, BP in 2008 and Richardson SM in 2016 with values of 14.5792 and 9.5823, respectively. We visualized the cited literature through Vosviewer and found that they were clustered into a total of eight categories, as shown in Figure 7a.

Figure 7. Cited and co-cited articles analysis on immune cells in disc degeneration. (a) the cited articles’ cooperation network graph in WOSCC. (b) the co-cited articles’ cooperation network graph in WOSCC. The size of the nodes indicates the number of articles, the thickness of the connections between the nodes reflects the strength of cooperation, and the color of the nodes corresponds to the clustering of the different articles, with nodes of the same color belonging to the same cluster.

Table 5. Top 10 highly cited articles on immune cells in disc degeneration.

Rank	Authors	Title (WOSCC)	Journal	Year	Citations	Norm. citations	Article Type
1	Chan, BP	Scaffolding in tissue engineering: general approaches and tissue-specific considerations	European Spine Journal	2008	876	14.5792	Review
2	Risbud MV	Role of cytokines in intervertebral disc degeneration: pain and disc content	Nature Reviews Rheumatology	2014	864	19.2708	Review
3	Le Maitre CL	Matrix synthesis and degradation in human intervertebral disc degeneration	Biochemical Society Transactions	2007	359	5.3829	Review
4	Sakai D	Transplantation of mesenchymal stem cells embedded in Atelocollagen gel to the intervertebral disc: a potential therapeutic model for disc degeneration	Biomaterials	2003	319	3.7021	Article
5	Shamji MF	Proinflammatory cytokine expression profile in degenerated and herniated human intervertebral disc tissues	Arthritis and Rheumatism	2010	291	5.3962	Article
6	Sakai D	Exhaustion of nucleus pulposus progenitor cells with aging and degeneration of the intervertebral disc	Nature Communications	2012	287	5.6537	Article
7	Sakai D	Regenerative effects of transplanting mesenchymal stem cells embedded in atelocollagen to the degenerated intervertebral disc	Biomaterials	2006	285	3.9993	Article
8	Richardson SM	Mesenchymal stem cells in regenerative medicine: Focus on articular cartilage and intervertebral disc regeneration	Methods	2016	284	9.5823	Review
9	Sakai D	Stem cell therapy for intervertebral disc regeneration: obstacles and solutions	Nature Reviews Rheumatology	2015	271	6.0984	Review
10	Sakai D	Differentiation of mesenchymal stem cells transplanted to a rabbit degenerative disc model: potential and limitations for stem cell therapy in disc regeneration	Spine	2005	271	4.7711	Article

We have analyzed the co-citations of the literature to find another important category of literature affecting the field, and the specific information on the top ten co-citations is summarized in Table 6. From Table 6 we can notice that the top ten co-citations are mainly in the form of “articles,” seven in total, and “reviews,” three in total. The most frequently co-cited article was “Role of cytokines in intervertebral disc degeneration: pain and disc content” by Risbud MV, published in Nature Reviews Rheumatology in 2014, with 374 citations. This was followed by Pfirrmann CWA’s article in Spine in 2001 and Le Maitre, CL’s article in Arthritis Research & Therapy in 2005 with 340 and 264 citations respectively.

Table 6. Top 10 highly co-cited articles on immune cells in disc degeneration.

Rank	Authors	Title (WOSCC)	Journal	Year	Citations	Total link strength	Article Type
1	Risbud MV	Role of cytokines in intervertebral disc degeneration: pain and disc content	Nature Reviews Rheumatology	2014	374	5882	Review
2	Pfirrmann CWA	Magnetic resonance classification of lumbar intervertebral disc degeneration	Spine	2001	340	4377	Article
3	Le Maitre, CL	The role of interleukin-1 in the pathogenesis of human intervertebral disc degeneration	Arthritis Research & Therapy	2005	264	5416	Article
4	Adams, MA	What is intervertebral disc degeneration, and what causes it?	Spine	2006	243	4728	Review
5	Urban, JPG	Degeneration of the intervertebral disc	Arthritis Research & Therapy	2003	234	4019	Review
6	Luoma, K	Low back pain in relation to lumbar disc degeneration	Spine	2000	222	3314	Article
7	Le Maitre, CL	Catabolic cytokine expression in degenerate and herniated human intervertebral discs: IL-1 beta and TNF alpha expression profile	Arthritis Research & Therapy	2007	215	4112	Article
8	Antoniou, J	The human lumbar intervertebral disc – Evidence for changes in the biosynthesis and denaturation of the extracellular matrix with growth, maturation, aging, and degeneration	Journal of Clinical Investigation	1996	191	3471	Article
9	Roberts, S	Histology and pathology of the human intervertebral disc	The Journal of Bone and Joint Surgery-American Volume	2006	181	3204	Article
10	Urban, JPG	Nutrition of the intervertebral disc	Spine	2004	179	4069	Article

We visualized the important co-cited literature using Vosviewer software, as shown in Figure 7b. These were clustered into a total of four categories, with the size of the nodes representing the number of citations in the literature and the position in the graph representing the total link strength of the literature. Literature with higher total link strength has an important role in the network. Of these, Risbud MV published in 2014 had the highest total link strength at 5882, followed by Le Maitre, CL published in 2005 and Adams, MA published in 2006 with total link strengths of 5416 and 4728, respectively.

Keywords analysis

Through data manipulation and software analysis, we have summarized in Table 7 the top 15 keywords appearing in the WOSCC and Scopus databases with the highest frequency of occurrence and total link strength. As we can see, the top 15 important keywords in both databases are almost identical. The keywords with the highest frequency rankings tend to have a higher total link strength as well, highlighting their importance in the field. The most frequent keywords in the WOSCC are “intervertebral disc,” “intervertebral disc degeneration,” “nucleus pulposus,” “inflammation,” “nucleus pulposus cells,” and others. The top five keywords in terms of frequency ranking derived from the Scopus database are generally consistent with the above, and it is evident that these keywords are hinting at research hotspots in the field. Additionally, “apoptosis,” “tissue engineering,” “mesenchymal stem cells,” “nf-kappa b,” “macrophages” and “regeneration” are also important keywords that need to be focused on and studied.

Table 7. Top 15 keywords in terms of the occurrences and total link strength on immune cells in disc degeneration.

Rank	WOSCC				Scopus
	Keywords	Occurrences	Keywords	Total link strength	Keywords	Occurrences	Keywords	Total link strength
1	intervertebral disc	645	intervertebral disc	1733	intervertebral disc degeneration	356	intervertebral disc degeneration	691
2	intervertebral disc degeneration	563	intervertebral disc degeneration	1220	intervertebral disc	272	intervertebral disc	637
3	nucleus pulposus	317	nucleus pulposus	958	inflammation	153	inflammation	390
4	inflammation	202	inflammation	560	nucleus pulposus	140	nucleus pulposus	372
5	nucleus pulposus cells	154	nucleus pulposus cells	377	nucleus pulposus cells	91	apoptosis	201
6	apoptosis	147	annulus fibrosus	375	apoptosis	86	nucleus pulposus cells	198
7	disc degeneration	119	apoptosis	372	disc degeneration	63	macrophages	192
8	tissue engineering	118	tissue engineering	365	low back pain	63	disc degeneration	164
9	annulus fibrosus	116	mesenchymal stem cells	352	macrophages	63	low back pain	158
10	mesenchymal stem cells	116	disc degeneration	328	mesenchymal stem cells	62	extracellular matrix	155
11	extracellular matrix	100	extracellular matrix	298	extracellular matrix	56	mesenchymal stem cells	149
12	low back pain	100	low back pain	280	annulus fibrosus	48	annulus fibrosus	148
13	degeneration	90	degeneration	254	platelet-rich plasma	41	platelet-rich plasma	117
14	regeneration	73	regeneration	229	nf-kappa b	40	nf-kappa b	97
15	nf-kappa b	73	nf-kappa b	205	tissue engineering	39	tissue engineering	95

We conducted a cluster analysis of the keywords through Vosviewer, which is shown in Figure 8a. In the WOSCC, important keywords are clustered into 12 categories, with “intervertebral disc,” “nucleus pulposus,” “tissue engineering” were clustered into category 4, “intervertebral disc degeneration,” “nucleus pulposus cells,” “apoptosis,” and others were clustered into category 2, and “inflammation,” “disc degeneration” and “low back pain” were clustered into category 1, which are all critical clusters and demand our attention. In Scopus, the keywords were clustered into 11 categories, with “intervertebral disc degeneration,” “nucleus pulposus cells,” and “apoptosis” were clustered into category 5; “intervertebral disc,” “extracellular matrix,” “nf-kb” and others were clustered into category 2; “inflammation,” “nucleus pulposus,” “annulus fibrosus” were clustered into category 8 (Figure 8b). We used Citespace to analyze the keywords in WOSCC to find the hot spots and directions of research (Figure 8c). We identified that previous research has focused on basic research on “nitric oxide,” “metabolism,” “matrix metalloproteinases” and “proteoglycans.” In the middle of the study, the keywords emerged mainly focusing on “intervertebral disc cells,” “progenitor cells,” “articular chondrocytes,” “hydrostatic pressure” and other in vivo and in vitro studies. In recent years, research in this field has focused on “nf kappa b,” “senescence,” “inflammation,” “bone marrow cells,” “oxidative stress,” “autophagy” and “apoptosis.” These are all significant research hotspots in recent years, and are also potential future trends and research directions. We performed a further bibliometric analysis of the macrophages that appeared in the keywords, using Citespace to highlight keywords related to macrophages in IDD, and clustered the keywords to investigate major research directions (Figure S1). The results revealed that prior to 2015, research focused mainly on “nitric oxide,” “matrix metalloproteinases,” “cytokine expression,” while after 2015 to date, researchers have focused on “tumor necrosis factor,” “tnf alpha,” “inflammation,” and so forth. The keyword cluster analysis clustered the keywords into 14 categories, with the main clusters also being “tumor necrosis,” “disc inflammation,” “collagen expression,” “anti-inflammatory cytokine,” “signaling pathway,” and others.

Figure 8. Keywords analysis on immune cells in disc degeneration. (a) the keywords cooperation network graph in WOSCC. (b) the keywords cooperation network graph in Scopus. The size of the nodes indicates the number of keywords, the thickness of the connections between the nodes reflects the strength of cooperation, and the color of the nodes corresponds to the clustering of the different keywords, with nodes of the same color belonging to the same cluster. (c) the top 25 keywords with the strongest citation bursts in WOSCC. The strongest citation bursts are those that occur over a short period of time with dramatic frequency changes. The red bar indicates the time period of the keyword burst. (d) the trend topics analysis used bibliometrix in Scopus. Larger nodes indicate higher frequency, the lines connecting the nodes represent the timeline of keyword appearances.

Moreover, in order to verify the accuracy of the content derived above, a temporal analysis of the keywords in Scopus was carried out in R, see Figure 8d. The trend topic map illustrated that in recent years the field has continued to emphasize topics such as “nf kb signaling,” “pyroptosis,” “oxidative stress,” “apoptosis,” “tumor necrosis factor,” “reactive oxygen metabolite,” and “tissue engineering,” which are comparable to the findings of our study above. In addition, we detected the topics of immune cells in IDD research in PubMed by Carrot2 and plotted the pie-chart and treemap, and the results are shown in Figure S2. we summarized the top 20 topic terms as shown in Table S1. It was found that “IDD, Genes, Hub,” “FasL, Privilege, Fas,” “IVD, Macrophages, Cytokines,” “IVDD, Group, Stem,” and “NP, Herniation, AF” ranked relatively high. Moreover, Chemokine, Senescence, Injection, Stem Transplantation, MSCs, IL-1β, and so forth are also significant components to focus on.

Discussion

The study of immune cells in IDD has gained a long history of development and attention. The hypothesis of the autoimmune theory of IDD was formulated in 1977 by Gertzbein, SD through clinical and experimental studies, who found that the products of the Extracellular Matrix (ECM) augmented inflammatory and immune responses.¹³ Since then, immunological studies of IDD have kicked off. At this stage, researchers believe that the IVD is an immune-privileged organ, whereas the healthy NP is immunogenic, and its unique structure is protected by the annulus fibrosus (AF) and cartilage endplates (CEP), isolating it from the immune system.^14–16 When IDD occurs, the body’s self-repair function allows neurovascular growth into it, exposing the NP to the host immune system and severely disrupting the immune-privileged microenvironment.^17,18 On the one hand, immune cells infiltration in degenerated discs can further amplify the inflammatory cascade response^18,19，on the other hand, the intervention of immune cells can lead to the release of vascular invasions, neurofactors, and pro-inflammatory factors, further aggravating the condition.²⁰ It has been reported that IDD occurs with a massive release of cytokines, particularly IL-1β, which induces a significant increase in nerve growth factor (NGF) and vascular endothelial growth factor (VEGF), thereby promoting nerve and blood vessel growth into the disc, and that Substance P released into the matrix sensitizes the nerves, leading to injury propagation and LBP.²¹ At the same time, in the disc microenvironment, matrix metalloproteinases (MMPs) and inflammatory mediators, such as interleukin-1β (IL-1β) and TNF-a, are significantly elevated, and these cytokines, which can be produced by IVD cells and immune cells, can stimulate the body to produce significant pain symptoms.^22–24 Evidently, the pain may be due to the infiltration of immune cells into the structurally defective area of the IVD.²⁵ Pain is the predominant symptom for which patients seek medical attention, and it is closely related to the involvement of immune cells.

The body’s initial line of defense against infection is intrinsic immunity, which consists mainly of lymphocytes, natural killer cells, mast cells, neutrophils, macrophages, and cytokines (such as chemokines, tumor necrosis factors, and interleukins).²⁶ A variety of immune cells such as monocytes/macrophages, lymphocytes, mast cells, natural killer cells, and dendritic cells are involved in IDD. Studies have found that NPCs secrete ECM such as collagen and proteoglycans, which can be perceived by the organism as autoantigens and can trigger T cell-mediated secondary immune responses.^17,27 Wang, L et al. revealed through a bioinformatic analysis that the LDH group contained a higher proportion of T cells as well as macrophages compared to the control group.²⁸ An immune cell infiltration analysis showed an increased proportion of CD8 T cells and M0 macrophages in IDD, while CD4+ T cells, neutrophils, resting dendritic cells, follicular helper T cells and monocytes were less abundant.²⁹ Another study also demonstrated that T cells, neutrophils, plasma cells and monocytes/macrophages are abnormally activated during the development of IDD.³⁰ In the course of the immune response, NPCs, AF cells and immune cells such as macrophages, T-cells and neutrophils release cytokines like TNF, ILG-6, IL-17, ILL-8, il-2, ILL-4, ILL-10, and IFN-T, as well as neurotoxins, which contribute to the pain and intensify the pathogenic process of IVD cells.²⁰ Thus, it can be seen that the mechanism of immune cells involved in IDD is precise, of which macrophages, mast cells, and lymphocytes are the most important components and play an important role in the whole process. Among these immune cells, macrophages are the pivotal indicator of IDD initiation, progression, and remission.^28,31 Macrophages, a major subpopulation of immune cells, can significantly contribute to the progression of IDD by secreting cytokines.³² M1 macrophages are also known as “classical” macrophages, particularly because they are involved in the inflammatory response and the release of pro-inflammatory cytokines, both of which can significantly exacerbate the progression of IDD and LBP.^31,33 By co-culturing macrophages and intervertebral discs, Ana J. Silva et al. found that degenerating IVD tissue preferred to polarize macrophages, making them more inclined to promote inflammation, which seemed to exacerbate IDD.²³ A study also found that degenerating nucleus pulposus-derived exosomes (dNPc-exo) promote macrophage polarization toward the M1 phenotype, which can stimulate inflammatory and immune responses.³⁴ Mast cells (MCs) are also involved in the immune process of IDD. MCs aggregates have been reported to be found in both NP cells and ECP of disc tissue in discogenic pain, whereas they were not found in the AF.³⁵ MCs infiltrate and trigger degeneration of IVD cells, and MCs-IVD cells interactions promote inflammation, catabolism and increased catabolism and angiogenesis in discogenic pain.³⁶ In addition, smoking promotes the exacerbation of IDD by a mechanism that may be related to the promotion of MCs hyperactivation and the release of proteases through the induction of mettl14-mediated modification of DIXDC1 m6.³⁷ The cellular and humoral immune responses are the two main subtypes of immune responses that occur in IDD. The development of vascular endothelial cells around the damaged disc can lead to lymphocyte aggregation to the injured area, recognition of antigens and activation of T lymphocytes.²⁸ T cell-mediated immune-inflammatory responses can significantly promote IDD and exacerbate painful symptoms. Studies have found that IVD tissues with full-layer AF tears and significant NP herniation show high expression of CD4+ and CD8+ T cells.³⁸ In recent years, studies on the role of T cells in IDD have gradually favored Helper T cells (Th), and the balance between Th1 and Th2 is extremely important for maintaining the homeostasis of the local internal environment in IVD. When the relationship between Th1 and Th2 is imbalanced, the secretion of inflammatory factors in the body will be disturbed, which in turn causes the occurrence and development of IDD. It has been found that high expression of Th1 promotes the production of inflammatory factors, while Th2 reduces the inflammatory response and protects IVD.^39,40 As can be seen, by intervening in the balance of Th may be an effective way to treat IDD. Besides the important immune cells mentioned above, dendritic cell (DCS) migration as well as antigen expression are also thought to be able to have an important influence on the development of inflammation and tolerogenic immune response in IVD tissue.⁴¹ From the above, it is clear that the mechanism of immune cells in IDD is complex, among which macrophage is the most studied and the most important type of cell, which, as a major subpopulation of immune cells, can significantly affect the development of IDD. In addition, lymphocytes and MCs act synergistically to influence the course of IDD.

General information

This study is the first to analyze the hot spots and trends in immune cell research in IDD from a bibliometric perspective. Our study demonstrated that the number of publications, both in the WOSCC database, the Scopus and PubMed databases, has shown a year-on-year increase since the beginning of the twenty-first century, revealing that the study of immune cells in IDD is gradually gaining importance. The results of the journal analysis showed that Spine has the highest number of publications and total citations. It is a particularly important journal and we can pay more attention to the literature from this journal when we study this field. In addition, the Journal of Orthopaedic Research, European Spine Journal and Spine Journal were also important specialist journals and worthy of study and research. The results of the country analysis indicated that China has the highest number of publications, while the USA has the highest total number of citations and total link strength. It is obvious that although China has the highest number of publications, the quality of the articles is less than that of the USA, and there is a need to further improve the quality and depth of the research. Inter-country cooperation and cluster analysis similarly confirm the same findings, discovering that China, despite its high volume of publications, has less international cooperation and that the USA occupies an important place in the cooperation network, maintaining intimate exchanges between it and a wide range of developed countries. It is evident that the development of a field can only be better promoted by active participation in international cooperation. Besides, countries such as Japan, Switzerland, Germany and England have also made important contributions to the development of the field and researchers need to pay further attention to the publications written by scholars from these countries. The institution analysis also indicated that Shanghai Jiao Tong University and Huazhong University of Science Technology had the highest number of publications in different databases, Tokai University had the highest number of citations and the University of Bern had the highest total link strength. These institutions are actively involved in research and have made important contributions to its development. Sakai D and Risbud MV have the highest number of publications separately in two databases and are the most prominent in the collaborative network of research, and their articles need to be read thoroughly. In terms of the number of publications alone, the top ranked authors are mainly from China and the USA, which shows that these two countries are important research countries and their annual output has actively contributed to the development of the field. Zhang Y, LI X, Wang X and other Chinese scholars have increased the number of publications year by year in recent years and have gained further insight into the field. Furthermore, Risbud MV, Shapiro IM, Grad S and others have a high number of citations for their articles, reflecting the recognition of their research content by the field. The authors mentioned above are all candidates for focus by later researchers. It is worth pointing out that the cluster analysis shows that although the number of publications in China is high, they are mostly domestic collaborations and need to actively learn from Western countries and strengthen international cooperation and exchange in order to gain further breakthroughs.

Analysis of key literature

Analyzing the content of highly cited and highly co-cited literature is useful for quickly capturing a field’s research hotspots and directions of high interest to researchers. The most cited study of immune cells in IDD is a 2008 article in the European Spine Journal entitled “Scaffolding in tissue engineering: general approaches and tissue- specific considerations” in the European Spine Journal in 2008. Although it is a review, the scaffolding aspect of tissue engineering is often cited in later studies on IDD.⁴² The second most cited paper is a review by Risbud, MV et al. entitled “Role of cytokines in intervertebral disc degeneration: pain and disc content,” published in Nature Reviews Rheumatology in 2014.²⁰ It demonstrates that the secretion of multiple pro-inflammatory cytokines in IDD further promotes the activation and infiltration of immune cells, which amplify the inflammatory cascade response and then induce pain and degeneration. The most cited research article is a 2003 publication by Sakai, D et al. in Biomaterials called “Transplantation of mesenchymal stem cells embedded in Atelocollagen gel to the intervertebral disc: a potential therapeutic model for disc degeneration.” They embedded mesenchymal stem cells (MSCs) in gels and transplanted them into a rabbit model of IDD, and found that depsipeptide collagen gels acted as an important vehicle for MSCs in transplants, allowing MSCs to proliferate, matrix synthesize and differentiate, a study that strengthens the feasibility of practical applications for MSCs in the treatment of IVD disease and has gained widespread recognition in the field.⁴³ Co-cited literature is an essential metric in bibliometric studies that provides a side-by-side response to some of the important literature. The results of the co-citation analysis revealed that the most cited literature remains the article published by Risbud MV et al. in 2014, which provides a solid foundation for the study of the immunology of IDD and deserves to be carefully studied and learned by contemporary researchers. The second most cited publication in the co-citation analysis is Pfirrmann, CW’s article on magnetic resonance grading of IDD, another important article, which is still referred to today in the classification of IDD.⁴⁴ In 2003, Le Maitre, CL et al. published a research study in Arthritis Research & Therapy entitled “The role of interleukin-1 in the pathogenesis of human intervertebral disc degeneration,” which found that as IDD increased, inflammatory factors such as IL1 and immune responses increased, which provided the basis for later treatment of IDD by IL-1 inhibition.⁴⁵ The literature mentioned above, as well as the top-ranked highly cited and highly co-cited literature, are all important and provide an essential foundation for later research. From these, it is not surprising to find that the study of immune cells in IDD is accompanied by an inflammatory factor response, an important manifestation of immune cell infiltration, and a hot topic of current research.

Research hotspots and trends

The bibliometric analysis of keywords is an important element in locating current hotspots, trends, and future directions. We searched for current hotspots and future trends in the study of immune cells in IDD through keyword frequency, total link strength, cluster analysis, burst analysis, and thematic trend analysis. Based on the keywords burst analysis, we can divide the research changes and ideological evolution of immune cells in IDD species into three phases. The beginning of the 21st century was the initial stage of this field, when the research hotspots mainly focused on the metabolism of ECM, such as “metabolism,” “matrix metalloproteinases” and “proteoglycan synthesis,” and the second is the developmental stage of the field, mainly through in vivo and in vitro experiments, to further study the physiological function, development and pathological changes of IVD cells, such as “in vivo,” “in vitro,” “basic science,” “intervertebral disc cells,” “anulus fibrosus,” “gene expression,” “progenitor cells,” and others. Beyond 2019, the mature phase of the field, the focus of research will be mainly on exploring the specific mechanisms of immune cells in IDD and possible therapeutic approaches and tissue engineering studies, such as “senescence,” “inflammation,” “bone marrow cells,” “oxidative stress,” “autophagy” and “apoptosis.” According to trend topics analysis, we can also identify the development process, previous research, current hotspots and cutting-edge issues of immune cells in IDD. After entering the 21st century, the research in this field mainly focuses on finding the possible immunological mechanisms of pain occurrence from the clinical point of view, such as “leukotriene b4,” “immunization,” “myelomonocytic,” “controlled clinical trial.” In the middle stage of development, the research hotspots mainly focus on the physiopathological study of diseases, such as “nucleus pulposus,” “intervertebral disk,” “lumbar vertebrae,” “proteoglycan,” and “tissue engineering” is also gradually appearing in this field of research. In recent years, research has focused on specific immunological mechanisms, such as “nf kb signaling,” “pyroptosis,” “oxidative stress,” “microrna,” “apoptosis,” and so forth. Based on the mutual validation of the two methods, we can easily visualize the development of this field. The study of immune cells in IDD has gained more and more attention since entering the 21st century, with initial studies focusing on the mechanism of immune response. With the continuous development of molecular biology tools, research has gradually crystallized, and the physiopathological role of immune cells in IVD tissues has been further explored through in vivo and in vitro studies. In recent years, the immunological mechanisms of IDD have been studied in depth, and the involvement of immune cells in inflammatory response, cellular pyroptosis, autophagy, and oxidative stress has been found to be indispensable. Based on the well-established pathomechanisms, scholars have gradually adopted drugs, stem cells (SCs), platelet-rich plasma (PRP), and bio tissue-engineered scaffolds for the treatment of IDD, and at the same time, further explored their modulation of the immune response of the IVD. Immune cells play a pivotal role in the pathogenesis, diagnosis and treatment of IDD, and only a clear grasp of the immunological mechanisms of IDD can provide guidance for later treatment. Even though the current emergence of multiple therapeutic approaches offers the possibility of IDD treatment, most of the research is mainly based on basic research and still lacks a large number of preclinical and clinical studies. How to apply methods through interfering with the immune response, such as SCs and tissue-engineered scaffolds, into the clinic and to benefit patients is the current difficulty, which requires further efforts and deepening of the research. Based on the keyword analysis, we have summarized the current hotspots and possible future trends. As a result, the current hot topics concentrate on the “Mechanisms of immune cells in IDD,” “Therapeutic role of immune cells in IDD” and “Immune response in tissue engineering of degenerated discs.” Future research directions include, but are not limited to, “inflammatory responses to IDD,” “autophagy,” “apoptosis,” “pyroptosis,” “stem cells,” “tissue engineering,” and others.

Mechanisms of immune cells in IDD

The IVD is a special organ that has no vascular nerve structure and when degeneration occurs there is an infiltration of immune cells which stimulates an inflammatory response and further accelerates degeneration. During the process of IDD, immune cells with different characteristics appear in the disc, some of which release cytokines that promote inflammatory responses and angiogenesis in the disc, making it an important driver of IDD.⁴⁶ As can be seen, inflammation is an important way in which immune cells mediate IDD. They can also interact with the NP and the extracellular matrix (ECM) to promote the progression of IDD.⁴⁷ With the advent of modern biological techniques, bioinformatics is being used to study the immunology of IDD. Xu, SX et al. found by bioinformatic analysis that many cell types in IDD tissue were altered, but mainly in activated dendritic cells, type 17 T helper cells, and mast cells.⁴⁸ Guo, Wei et al. also hypothesized that increased MFN2 expression and infiltration of CD8+ T cells, NK cells and neutrophils in NP tissues play an important role in the pathogenesis of IDD from a bioinformatic approach based on mitochondrial studies.⁴⁹ In addition, Li, WH et al. performed a combined analysis of bulk-RNA sequencing and single-cell sequencing, combined with a range of techniques including weighted gene co-expression network analysis (WGCNA), immuno-infiltration analysis and differential analysis to investigate cellular changes in IDD. It was found that the proportion and characteristics of late degenerating cells changed significantly compared to the early stages of IDD, where monocytes/macrophages were divided into five distinct subpopulations with different roles.⁵⁰ Macrophages play an important role in IDD. Macrophages are the only inflammatory cells that can penetrate the closed NP and are expressed in various structures of the IVD, with their number positively correlating with the degree of IVDD; macrophages exert phagocytosis or regulate the metabolism of dorsal root ganglia and muscle tissue through neuroimmune mechanisms, while an imbalance in macrophage polarization leads to the convergent aggregation of more inflammatory factors, creating a “positive,” “inflammatory waterfall” and “effect feedback” greatly exacerbates LBP.⁵¹ Another study found that inhibition of HSP90 had an effect on macrophage-induced NP fibrosis and pathological angiogenesis.⁵² However, it is worth mentioning that immune cells are not harmless in IDD. Macrophages have been found not only to act as phagocytes but also to promote disc metabolic homeostasis through trophic, regulatory and repair functions, and by down-regulating the ECM gene in the IVD, it promotes tissue remodeling.²³ A preclinical study found that injecting bone marrow-derived macrophages into herniated discs reduced the size of the herniation by 44%, suggesting that macrophages promote resorption of herniated discs, which could be an effective clinical therapy.⁵³ In addition, chemokines,⁵⁴ miRNAs,⁵⁵ cell pyroptosis,⁵⁶ endoplasmic reticulum stress,⁵⁷ oxidative responses,⁵⁸ and lipid metabolism⁵⁹ are all important pathways through which immune cells mediate IDD. Only a refined understanding of the role of immune cells in IDD will allow further strategies and protocols to be developed for later treatment.

In studies of the mechanisms of immune cells in IDD, “senescence,” “inflammation,” “nf kb signaling,” “microrna,” “oxidative stress,” “autophagy,” “pyroptosis” and “apoptosis” are key research areas of ongoing interest. The mechanisms of IDD are complicated, with multiple factors contributing to the pathogenesis during the immune cell-mediated inflammatory response. IDD has been reported to be closely associated with human aging, with senescence of NPCs playing an important role.^60,61 A study found that stem cell-derived extracellular vesicles + RGD composite medullary matrix hydrogel (sEV-RGD-DNP) attenuates IDD by targeting the HIPK2/p53 pathway to increase sEV bioavailability and attenuate NPSC senescence.⁶² ECM degradation and apoptosis are the most direct manifestations of IDD development. For example, one study found that down-regulation of PP2A reduced apoptosis and degeneration of AF cells via the p38/MAPK pathway.⁶³ With the continuous development of molecular biology techniques, a variety of cell deaths have been found to occur in IDD. Several studies have found that necroptosis-associated pathways are activated and play an important role in the pathogenesis of IDD.⁶⁴ Pyroptosis as pro-inflammatory programmed necrosis mediated by inflammatory vesicles plays an essential part in the onset and progression of IDD.⁶⁵ It functions as a newly identified pro-inflammatory cellular programmed necrosis that is the initiating and promoting event leading to IDD.⁶⁶ Inflammatory mechanisms are the most important way in which immune cells mediate the development of IDD. Immune dysregulation increases the level of inflammatory cytokine expression, a result that disrupts IVD homeostasis, leading to NPCs shrinkage, fibrosis, and epidermal changes, and restoration of cytokine homeostasis through modulation of the immune response and the inflammatory microenvironment is a possible way of achieving IVD repair and regeneration.⁶⁷ NF-κB, as a classical pathway of inflammation genesis, still continues to receive extensive attention in the mechanism study of IDD in recent years. That it is closely related to the occurrence of apoptosis, pyroptosis, oxidative stress response, ferroptosis, autophagy, and so forth in IVD cells.^68–71 Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, is cytoprotective, and is essential for the maintenance of neuroplasticity and normal function, which is associated with pain due to IDD.⁷² For example, Guo, D et al. demonstrated in vivo and in vitro that Panax notoginseng saponins (PNS) could protect HNPCs from apoptosis by inhibiting autophagy and improve IDD in vivo.⁷³ Not only that, oxidative stress is considered to be an important pathogenic factor in the complex pathophysiological process of IDD. It has been reported that high levels of reactive oxygen species (ROS) can cause damage to cellular nucleic acids, lipids, proteins, and other macromolecules, severely affecting normal cellular activity and function, and ultimately leading to the onset of cellular senescence or death.⁷⁴ Oxidative stress is also considered to be one of the key links in the development of IDD, and it often happens in conjunction with other mechanisms. For instance, it has been found that oxidative stress induces ferroptosis, which has an important role in the pathology of IDD.⁷⁵ In addition, recent studies have highlighted the critical role of microRNAs in regulating the onset and progression of IDD. Li, Y et al. screened miR-15a-5p as a potentially important biomarker by RNA sequencing, and targeting the miR-15a-5p-mRNA signaling pathway may be a promising strategy for the treatment of IDD disease.⁷⁶ Another study also revealed that melatonin impedes NPCs apoptosis and IL-1β-induced inflammation by regulating the MEG3-miR-15a-5p-PGC-1α/SIRT1 pathway.⁷⁷ The above results suggest that immune cells, cell death, senescence, changes in mRNA, inflammatory responses, and ECM degradation all contribute to the development of IDD, and that they often do not exist independently, but rather interact with each other and work together. Due to the specificity of the disc structure, more work needs to be done in later studies to improve precise immune control. Immunomodulation of the IVD will also be a hot direction for continued research in the future.

Therapeutic role of immune cells in IDD

Research into the immune mechanisms of IDD has provided a solid basis for the development of therapeutics, and a number of drugs, cellular therapies and platelet-rich plasma (PRP) have been developed in response to this research. In therapeutic strategies for IDD, such as SCs transplantation and the use of biomaterials, it is important not to lose sight of the immune-absorbent properties of the IVD. SCs may play an important role in immune amnesty reconstruction of degenerated discs, whereas biomaterials used for disc regeneration usually mimic the disc composition, and it is used together with ex vivo foreign bodies, which is more likely to elicit an immune response from the body. Therefore, we should be careful to avoid autoimmune reactions when treating IDD. For example, Bai, X et al. showed that embelin protects human NPCs from apoptosis and inflammation induced by IL-1β stimulation by regulating the PI3K/Akt signaling pathway in cell-based experiments.⁷⁸ Chen, W et al. also found that Rosuvastatin inhibited TNF-α-induced matrix catabolism, cell pyroptosis and senescence in NPCs through the HMGB1/NF-κB signaling pathway in vitro and in vivo study.⁷⁹ In addition, some traditional Chinese herbs have been found to mediate IDD by modulating the immune response. For example, one study showed that Bushen Huoxue Recipe can promote the conversion of adipose stem cells (ADSCs) to NPCs by regulating the TGF-β1/Smad pathway, which can promote the growth and proliferation of NPCs, delay aging, improve the microenvironment, and repair oxidatively damaged NPCs.⁸⁰ Duhuo Jisheng Decoction (DHJSD) effectively attenuates LPS-induced disc inflammatory response and further inhibits NLRP3 expression, which may inhibit NPC pyroptosis via the SDF-1/CXCR4-NF-kB-NLRP3 axis.⁸¹ Research into cell therapy is an important modality in the treatment of IDD. A systematic review has shown that basic experiments confirm that SCs are effective in the treatment of IDD, but reliable clinical studies are lacking.⁸² Mesenchymal stem cell (MSC) have been shown to have distinct immunomodulatory effects on several immune cell subpopulations including T lymphocytes, B lymphocytes, natural killer cells and dendritic cells.⁸³ In vitro, it was found to induce apoptosis of T-lymphocytes and significantly inhibit the proliferation of immune cells and the production of pro-inflammatory cytokines.⁸⁴ SCs can significantly promote tissue regeneration by modulating the subtype distribution and phenotype of immune cells, including macrophage polarization.⁸⁵ Our research group has also confirmed the extensive use of SCs in IDD by bibliometric methods and has already conducted some clinical studies.⁸⁶ Because of its proven efficacy, PRP has been used in clinical studies of IDD with favorable results. A long-term study found that intra-discal PRP injections were safe and effective in improving LBP with a follow-up period of over 5 years.⁸⁷ A systematic review has demonstrated the regenerative potential of PRP in previous basic studies and recent clinical studies have reported that PRP therapy is safe and effective in the treatment of many spinal conditions.⁸⁸ Its role is also related to the regulation of immune cells such as macrophages.⁸⁹ Researchers have increased regenerative capacity and anti-inflammatory effects through pluronics nanomicelles containing a mixture of PRP and simvastatin (SIM) in a hydrogel, which was found to promote disc healing and significantly reduce inflammatory responses.⁹⁰ Evidently, the current research is almost in the preliminary basic research, and only by continuously advancing the research on the mechanism of IDD, drugs, cells, PRP and other therapies can we provide reference for clinical research and further use in the clinic to alleviate patients’ suffering.

Immune response in tissue engineering of degenerated discs

Because conventional treatments cannot repair or reverse IVD damage, researchers are trying to find more effective treatments to inhibit the progression of IDD. In recent years, as tissue engineering techniques continue to develop and mature, their use in IDD is becoming a hot topic of research. So far, achieving sustainable, dynamic and effective ECM anti-inflammatory and cellular microenvironmental restoration in IDD has proved to be difficult.⁹¹ An animal study has shown that injectable hydrogels can promote ECM secretion and reduce the inflammatory response, which could be a powerful potential method for IDD.⁹² Another dynamic multifunctional nano-hybrid peptide hydrogel (NHPH) is not only injectable, biocompatible and biodegradable, but also scavenges reactive oxygen species and remodels the ECM, in addition to continuously delivering pro-regenerative cytokines, effectively suppressing the immune response and ultimately restoring the regenerative microenvironment of the ECM, facilitating structural and functional recovery of the IVD after severe injury.⁹³ Jiang, Y et al. developed a “stand-alone” injectable hydrogel capable of storing Mg²⁺ while carrying NPCs, a material that promotes local ECM synthesis and facilitates immunomodulation (up-regulation of M2/down-regulation of M1 macrophage polarization) to inhibit IDD progression.⁹⁴ In addition to hydrogels, scaffolds are an important component of tissue engineering. It can inhibit IDD by inhibiting macrophage polarization.⁹⁵ A study has developed a unique nano-scaffold that scavenges reactive oxygen species, inhibits immune cell responses and inflammatory stimuli, relieves pain and promotes IVD regeneration.⁹⁶ Multifunctional and multidimensional biomaterials, especially those that can self-scavenge inflammatory mediators, pro-ECM regenerative properties, or protect tissues from apoptotic signals, are considered as a means for long-term immunomodulation and effective tissue regeneration.⁹⁷ Liu, C et al, designed GLRX3-associated MSC-derived extracellular vector (EVs-GLRX3) hydrogels, which enhanced cellular antioxidant defenses, prevented cellular senescence, mitigated mitigated mitochondrial damage, and restored ECM deposition by regulating redox homeostasis.⁹⁸ Furthermore, it has also been reported that siRNA@G5-PBA@Gel can alleviate inflammatory storms by inhibiting the P65/NLRP3 signaling pathway, which in combination with cellular therapy can significantly increase the regeneration of IVD.⁹⁹ Thus, it can be seen that there is a wealth of ways to design tissue engineering, and they can be combined with cellular therapy, drug therapy and other therapies to effectively reduce the immune response, inhibit the M1 polarization of macrophages, among other things, to reduce the inflammatory response and promote tissue regeneration to achieve a good therapeutic effect. Clearly, when tissue engineering is used in the treatment of IDD, attention needs to be paid to biocompatibility and the immune response to the organism, and mechanistic research is necessary to improve the use of tissue engineering. Despite extensive research, the success of tissue engineering strategies in translating preclinical models into therapies that are beneficial to patients has been limited, and current research remains at the basic research level. In terms of clinical translation, current tissue engineering research still needs to consider reducing the immune response to the body, targeting immune cells to effect action, and improving the stability and biocompatibility of the material. In addition, combining other cellular therapies, and so forth, is also an important way to improve effectiveness. Once this is done, the translational work of bioengineering into clinical research can be continuously promoted. Others have argued that precision medicine can use multi-omics analysis to elucidate the pathology of IDD in each patient, prescribe individualized treatments and develop the most effective approaches.¹⁰⁰ However, a more complete understanding of the disc microenvironment, immunology, biomechanics, genetics, and others is urgent and necessary to assess which treatments or combinations of treatments will be effective for the patient.

Limitations

Although this study is the first to examine the hotspots and future trends in immune cell research in IDD through a bibliometric approach, it still has some common limitations. First, although three databases were searched in this article, the results were analyzed without fully aggregating the contents of multiple databases due to space, which caused some bias in the article. Secondly, although the search formulae have been planned and used more systematically, there is no guarantee of the accuracy and completeness of the results and risk that some important literature will be overlooked. Thirdly, this study searched the literature from 01/01/2001 to 08/03/2023, which made it difficult to replicate the results at a future time due to the real-time update nature of the database. Fourth, although we have performed a bibliometric analysis using a variety of common software, there are differences and biases in the way different software operates and the results of the analysis, so we have improved the reliability of this article by aggregating the results derived from different software. In summary, although the limitations are described above, these are inevitable drawbacks of bibliometric analysis and our findings can still provide some reference to the field of immune cell research in IDD.

Conclusion

This article provides a bibliometric analysis of the current state of research on immune cells in IDD research. This study found that the number of annual publications is increasing year on year, indicating that immune cells are gaining importance in the study of IDD. The journal analysis found that Spine has the highest number of publications and citations, indicating its importance in the field. The results of the country analysis showed that China has the highest number of publications, the USA has the highest number of citations and total link strength, and the USA has an important position in international collaboration, but China’s collaborations are mostly domestic and need to strengthen academic collaborations with other developed countries. The institution analysis revealed that Shanghai Jiao Tong University and Huazhong University of Science Technology had the highest number of publications, Tokai University had the highest number of citations, and University of Bern had the highest total link strength. The author analysis identified Sakai D and Risbud MV had the highest number of publications. Sakai D had the highest total link strength and Risbud MV had the highest number of citations. We analyzed the highly cited and highly co-cited literature and discovered that the top-ranked studies were mainly reviews, followed by basic studies with a bias toward the mechanisms by which immune cell-mediated inflammatory responses promote the IDD. The results of the keyword analysis suggested that the current research hotspots and future directions remain the study of the mechanisms of immune cells in IDD, the therapeutic role of immune cells in IDD and the role of immune cells in tissue engineering for IDD. We have also illustrated through our software the current focus of research and how immune cells are evolving over time in the study of IDD.

Authors’ contributions

All authors agree to be accountable for all aspects of the work, read and approved the final manuscript. NW WQX and LX: designed and conceptualized this study. NW, ZHM and SC wrote the manuscript and revised the draft. NW, ZPX, ZHM and XYF collected and analyzed the data. ZHM, WQX and SC prepared the figures and tables. NW, LX and ZPX: supervision and conceptualization.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The original results presented in the study are included in the article. Further inquiries regarding the data can be directed to the corresponding author.

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website at https://doi.org/10.1080/21645515.2023.2274220

Additional information

Funding

This work was funded by Natural Science Foundation of Jiangsu Province [No. BK20221420], Jiangsu Provincial Traditional Chinese Medicine Science and Technology Development Plan Project [No. ZD202008], Science and technology projects in Jiangsu Province [No. BE2019765], and Jiangsu Province Graduate Practice Innovation Program Project [No. SJCX23_0957].