Investigation of HEG1, VISTA (VSIR) and PD-L1 expression levels in malignant mesothelioma by immunohistochemical methods and their relationship with histological subtypes

Abstract

Malignant Mesothelioma is a primary malignant tumor of the mesothelium lining the pleura, pericardium, peritoneum and tunica vaginalis of the testis with a poor prognosis. The epithelioid subtype is graded as high and low grade in the 2021 WHO classification of thoracic tumors. Recently, promising results from clinical trials of treatment with immune checkpoint inhibitors are reported. The aim of our study was to shed light on clinical studies on potential immune checkpoint inhibitors by examining VISTA and PD-L1 expression levels as well as HEG1 in malignant mesothelioma subtypes. Our study included 69 cases diagnosed with ‘malignant mesothelioma, well-differentiated papillary mesothelial tumor, atypical mesothelial hyperplasia’ at Dicle University Faculty of Medicine Department of Pathology between 2015 and 2021. Primary antibodies against HEG1, VISTA, and PD-L1 were used in the immunohistochemical study. The results showed a significant relationship between PD-L1 expression and sarcomatoid and high-grade epithelioid malignant mesotheliomas. VISTA was detected at a high rate in epithelioid malignant mesotheliomas and was negative in non-mesothelial tumors. HEG1 was positively monitored in mesothelial-derived tumors and negatively monitored in non-mesothelial tumors. The obtained results suggest that HEG1 could be further explored as a useful marker in determining mesothelial origin and that the expression levels of VISTA and PD-L1 markers may vary in histological subtypes in the selection of drugs for the treatment of malignant mesothelioma.

Keywords:

• Mesothelioma
• HEG1
• VISTA
• pd-l1

Introduction

Malignant mesothelioma (MM) is the primary malignant tumor of the mesothelium lining the pleura, pericardium, peritoneum, and tunica vaginalis of the testis, and 80% of it is located in the pleura [1]. Although MM had not been graded until 2021, a two-stage system (low and high grade), including nuclear grade (mitotic count and nuclear atypia) and the presence of necrosis was proposed in the 2021 WHO classification of thoracic tumors [2]. MM, which is usually diagnosed at a late stage, has a low treatment response and a poor prognosis [3]. Cisplatin-pemetrexed is approved front-line regimen for unresectable MM. In recent years, immune checkpoint inhibitors have emerged as a new treatment option [4].

PD-L1 is an immunomodulatory surface glycoprotein on antigen-presenting cells, activated T cells and some non-haematopoietic tissues. The interaction between PD-L1 and the PD-L1 receptor controls tumour escape from immunity and limits T lymphocyte activation during an inflammatory response. Inhibition of the PD-1/PD-L1 interaction results in prolonged T-cell activation and enhanced anti-tumour activity [5]. The V-set immunoregulatory receptor (VSIR) is a member of the B7-CD28 ligand and receptor family and is also known as V-domain Ig suppressor T-cell activation (VISTA). VISTA is a novel immune checkpoint gene that is structurally similar to PDL-1. Expressed on myeloid cells and T lymphocytes, VISTA suppresses early T cell activation and proliferation and reduces cytokine production when overexpressed [6,7]. A novel and sensitive marker for the diagnosis of MM is the sialylated protein HEG homolog 1 (HEG1). HEG1 in MM cells is a 400 kDa mucin-like membrane protein. About 70% of its molecule is a densely O-glycosylated Ser/Thr-rich region, but there are no tandem repeat sequences. HEG1 is expressed from the apical cell surface. It is associated with MM cell proliferation [8,9].

The aim of our study was to examine the expression levels of HEG1 as well as VISTA and PD-L1 in MM subtypes to shed light on clinical trials related to potential immune checkpoint inhibitors and to evaluate the availability of HEG1 in the differential diagnosis proposed in previous studies.

Materials and methods

Determination and preparation of cases

Our study included 69 cases selected from extrapleural pleurectomy, pleurectomy/decortication, pleural biopsy, mass excision material, peritoneal biopsy and orchiectomy materials diagnosed with MM, well differentiated papillary mesothelial tumor (WDPMT) and atypical mesothelial hyperplasia (AMH) between 2015 and 2021 in the Pathology Department of Dicle University, Faculty of Medicine. In addition, in order to compare HEG1 and VISTA expressions with MM cases, we included 3 lung adenocarcinomas in the pleura, 3 squamous cell carcinomas, 1 follicular variant papillary thyroid carcinoma, 1 type 2 papillary renal cell carcinoma, 1 clear cell renal cell carcinoma, 1 large cell carcinoma, 1 colorectal carcinoma, 1 breast carcinoma, 1 urothelial carcinoma, 1 sarcomatoid carcinoma, 1 large cell neuroendocrine carcinoma, 1 malignant melanoma metastasis, 1 myxo-inflammatory fibroblastic sarcoma, 1 solitary fibrous tumor and 1 Ewing sarcoma case.

Data showing the age, gender, tumor histological subtype, and prognostic values of the cases were obtained from the pathology reports of Dicle University, Faculty of Medicine, Department of Medical Pathology and the hospital automation system. For the study, the approval of the local ethics committee was obtained with decision No. 276 of Dicle University Faculty of Medicine on 25.03.2021.

Hematoxylin-eosin (H&E) and, diagnostic immunohistochemical markers and stained preparations of all materials were re-examined by two observers. H&E stained preparations and materials were divided into histological subtypes and grades according to the 2021 WHO classification.

Immunohistochemical study

The immunohistochemistry study was performed by an immunoperoxidase assay. In the study, for immunohistochemical markers for HEG1, VISTA and PD-L1, two 5 μm thick sections were taken for each case on adhesive slides (Isotherm, Germany) using a microtome device (Shandon, USA) from the blocks where routine tissue monitoring procedures were performed. HEG1 (Bioscience SB, USA, clone SKM9-2, monoclonal mouse/IgG antibody, catolog number: BSB-3728-7), VISTA (Cell Signaling Technology, USA, clone D5L5T, monoclonal rabbit mAb, catolog number: 54979S), PD-L1 (Ventana, clone SP142, rabbit primary antibody, catolog number:790-4860) primary antibodies were used. The immunohistochemical study was performed automatically on the Ventana BenchMark Ultra device (Roche Diagnostics, USA).

Immunohistochemical scoring

The immunohistochemical staining preparations were evaluated semi-quantitatively and subjectively by 2 pathologists under a light microscope. In HEG1, membranous staining for epithelioid MM, WDPMT and AMH, and membranous and cytoplasmic staining for sarcomatoid MM were considered positive [10]. The staining intensity in biphasic MM was scored according to the epithelioid component [11]. For HEG1, staining intensity (severity) and percentage staining (extent) levels were divided into four categories. The staining intensity was determined as 0: negative, 1: weak, 2: moderate, and 3: strong staining, and the staining percentage was determined as 0: 0%; 1: <25%; 2: 25-50%; 3: >50%. A coloring score was created by collecting the coloring intensity and coloring percentage. A staining score of ≤2 was considered negative, and a staining score of ≥3 was considered positive [12]. Positive HEG1 cytoplasmic staining of endothelial cells served as an internal positive control, and the absence of HEG1 staining in lymphocytes served as an internal negative control [11].

The expression of PD-L1 and VISTA in tumor cells was expressed as the tumor ratio score. The tumor ratio score was defined as the percentage of living tumor cells that showed positive staining relative to all living tumor cells present in the sample. Only membranous staining for PD-L1 expression and cytoplasmic and/or membranous staining for VISTA expression were considered positive. Staining at any intensity was considered positive for both markers [4]. The percentage of staining in tumor cells was scored for both VISTA and PD-L1 (from 0 to 100%). The cut-off value was accepted as 1% and 50% [13]. The percentage of staining of tumor-associated inflammatory cells was also scored for each antibody (0 to 100%). Staining in at least 1% of the cells was defined as positive, and staining in less than 1% of the cells or no staining was defined as negative [6].

Data analysis

IBM SPSS 18.0 for Windows (Statistical Package for Social Sciences) was used for the statistical evaluation of our research data. The frequency of the variables, the averages and the standard deviations of the numerical data were found. Measurement variables were presented as mean values with standard deviation (±SD), and categorical variables were presented as number and percentage (%). The conformity of continuous variables to the normal distribution was measured by the Kolmogorov-Smirnov test. In statistical evaluations, an independent t-test was used to evaluate the averages of normally distributed continuous variables between two groups, and those that did not comply with normal distribution were analyzed with Kruskal Wallis and Mann Whitney U test. The Chi-Square test was used for the statistical evaluation of categorical variables. Spearman’s test was used to evaluate the correlation of continuous variables; the hypotheses were bidirectional, and statistical significance was considered at the level of p < 0.05.

Results

In our study, 69 cases diagnosed as ‘MM, WDPMT, AMH’, sent to Dicle University Faculty of Medicine Department of Pathology between January 2015 and October 2021, were evaluated. Demographic, histopathological and immunohistochemical characteristics of the patients are given in Table 1.

Table 1. Distribution of mesothelial patients according to demographic, histopathological and immunohistochemical characteristics.

		n (%)
HISTOLOGIC SUBTYPE	Epithelioid	45 (65.2)
	Sarcomatoid	10 (14.5)
	Biphasic	9 (13.1)
	Well differentiated papillary mesothelial tumor	3 (4.3)
	Atypical mesothelial hyperplasia	2 (2.8)
AGE		62±10.94
SEX	F/M	30/39 (43/56)
LOCATION	Pleura	59 (85.5)
	Peritoneum	7 (10)
	Testis	3 (4.3)
HEG1	Negative	1 (%1.4)
	Positive	68 (98.6)
VISTA	Negative	6 (%8.7)
	Positive	63 (%91.3)
PD-L1	Negative	46 (%66.6)
	Positive	23 (33.3)

VISTA:V-domain Ig suppressor of T cell activation; PD-L1: programmed death ligand 1; HEG1: Sialylated protein HEG homolog 1.

Of the 69 cases in our study, 39 are men (56.5%) and 30 are women (43.5%). There was no significant relationship between gender and the immunohistochemical expression levels of HEG1, VISTA and PD-L1 (p = 0.255, p = 0.604, p = 0.732).

The youngest of the cases was 35 years old, and the oldest was 84 years old. The median age was 54 years and the mean age was 62 ± 10.94 years. There was no significant relationship between age and immunohistochemical HEG1, VISTA and PD-L1 expression levels (p = 0.354, p = 0.456, p = 0.151).

Immunohistochemical results

HEG1 expression

HEG1 expression was observed in 68 of 69 MM cases. The HEG1 expression status according to histological subtypes is given in Table 2. Although HEG1 expression was observed in all mesotheliomas, a statistically significantly lower HEG1 expression score was observed in sarcomatoid MMs (p = 0.043) (Figure 1). No statistically significant difference was observed between localization and HEG1 expression (p = 0.773). HEG1 expression was observed at higher scores in low-grade epithelioid MMs than in high-grade MMs (p = 0.049). HEG1 expression was not observed in any of the non-mesothelial tumors.

Figure 1. HEG1 expression in malignant mesothelioma subtypes. (A) Strong HEG1 expression in high-grade epithelioid malignant mesothelioma (immunoperoxidase, 100X). (B) Biphasic mesothelioma; strong HEG1 expression (arrow) in epithelioid areas, weak HEG1 expression (star) in sarcomatoid areas (immunoperoxidase, 100X). Scale bar = 500 µm.

Table 2. HEG1 Expression status according to tumor types.

HEG1 immunoreactivity
Tumor Type	Negative	Score 3	Score 4	Score 5	Score 6
Pleura	1	2	8	12	36
Epithelioid	1	1	4	7	25
Low grade	1	1	1	1	17
High grade	0	0	3	6	8
Sarcomatoid	0	1	4	3	2
Biphasic	0	0	0	1	7
WDPMP	0	0	0	1	0
AMH	0	0	0	0	2
Peritoneum	0	1	0	2	4
Epitheloid	0	1	0	1	2
Biphasic	0	0	0	0	1
WDMP	0	0	0	1	1
Testis	0	0	0	0	3
Pulmonary adenocarcinoma	3	0	0	0	0
SCC	3	0	0	0	0
Sarcomatoid carcinoma	1	0	0	0	0
Other carcinomas	8	0	0	0	0
Malignant melanoma	1	0	0	0	0
SFT	1	0	0	0	0
Ewing sarcoma	1	0	0	0	0
MIFS	1	0	0	0	0

WDMP: Well differentiated papillary mesothelial tumor; AMH: atypical mesothelial hyperplasia; SCC Squamous cell carcinoma; SFT: Solitary fibrous tumor; MIFS: Myxoinflammatory fibroblastic sarcoma; HEG1: Sialylated protein HEG homolog 1.

VISTA expression

VISTA expression was detected in 63 of the 69 MM cases. The VISTA expression status by histological subtypes is given in Table 3. The staining pattern was membranous in epithelioid MMs, and granular staining was noted in some cases. The staining pattern in sarcomatoid MM was in the form of cytoplasmic and/or membranous staining.

Table 3. VISTA and PD-L1 ımmunoreactivity in mesothelial lesion subgroups.

HISTOLOGIC SUBTYPE	IMMUNOHISTOCHEMICAL EXPRESSİON	VISTA		PD-L1
		FREQUENCY	%	FREQUENCY	%
Epithelioid MM	Negative	1	2	32	71
	%1-49	15	33	11	25
	%50-100	29	65	2	4
Sarcomatoid MM	Negative	4	40	2	20
	%1-49	5	50	4	40
	%50-100	1	10	4	40
Biphasic MM	Negative	1	11	6	67
	%1-49	6	67	3	33
	%50-100	2	22	0	0
WDMT	Negative	0	0	3	100
	%1-49	1	33	0	0
	%50-100	2	67	0	0
AMH	Negative	0	0	2	100
	%1-49	0	0	0	0
	%50-100	2	100	0	0
p	0,00

MM: Malignant mesothelioma; WDPMP: Well differentiated papillary mesothelial tumor; AMH: atypical mesothelial hyperplasia; VISTA: V-domain Ig suppressor of T cell activation; PD-L1: programmed death ligand 1.

The median VISTA score was higher in epithelioid MM (55%) (Figure 2(A)) than in biphasic MM (35%) (Figure 2(B)) and sarcomatoid MM (5%). There was a statistically significant decrease between histopathological subtype and immunohistochemical VISTA expression (p = 0.000).

Figure 2. VISTA and PD-L1 expression levels in malignant mesothelioma subtypes. (A) Strong membranous VISTA expression in epithelioid mesothelioma (immunoperoxidase, 100X). (B) Biphasic mesothelioma: Strong VISTA expression (arrow) in epithelioid areas, VISTA negative (star) in sarcomatoid areas (immunoperoxidase; 100X) C. Strong membranous PD-L1 positivity in sarcomatoid mesothelioma (immunoperoxidase, 100X). (D) Focal, weak PD-L1 positivity (arrow) in biphasic mesothelioma (immunoperoxidase, 100X). Scale bar = 500 µm.

The state of VISTA expression in epithelioid MM according to the histological degree is given in Table 4. There was no statistically significant relationship between histological degree and VISTA expression (p = 0.630).

Table 4. The state of VISTA and PD-L1 expressions in epithelioid malignant mesotheliomas according to the histological degree.

HISTOLOGIC GRADE	IMMUNOHISTOCHEMICAL EXPRESSION	VISTA		PD-L1
		FREQUENCY	%	FREQUENCY	%
Low Grade	Negative	1	4.7	20	95.3
	Positive	20	95.3	1	4.7
High Grade	Negative	0	0	7	41.1
	Positive	17	100	10	58.9
p		0.63		0.000

VISTA: V-domain Ig suppressor of T cell activation; PD-L1: programmed death ligand 1.

VISTA expression was observed in 3 tunica vaginalis and 4 peritoneal MM. There was no statistically significant difference between localization and VISTA expression (p = 0.385).

VISTA expression was not observed in any of the non-mesothelial tumors.

PD-L1 expression

PD-L1 expression was observed in 23 of the 69 MM cases. PD-L1 expression status according to histological subtypes is given in Table 3. PD-L1 expression was not observed in cases diagnosed with WDPMT and AMH. Membranous staining was observed in all positive cases. PD-L1 was expressed at a higher rate in sarcomatoid MM (Figure 2(C,D)), and a statistically significant relationship was detected between histopathological subtype and immunohistochemical PD-L1 expression (p = 0.02).

The status of PD-L1 expression in epithelioid MM according to histological degree is given in Table 4. Those with a high grade of pleural epithelioid MM show more frequent PD-L1 expression than those with a low grade (p = 0.000).

PD-L1 expression was not observed in any of the Tunica vaginalis MM. Two out of five peritoneal MM cases (40%) were positive for PD-L1. There was no statistically significant relationship between localization and PD-L1 expression (p = 0.425). Immunohistochemically, a negative relationship was found between VISTA and PD-L1 expression when compared (Table 5).

Table 5. Comparison of VISTA and PD-L1 expressions in mesothelial lesions.

VISTA
PD-L1	Negative	Moderate	Strong	Total
Negative	2	16	22	41
Moderate	2	7	9	17
Strong	2	3	1	6
Total	6	26	32	64

VISTA: V-domain Ig suppressor of T cell activation; PD-L1: programmed death ligand 1.

Discussion

MM is an aggressive disease that often affects the pleura, rarely the pericardium, the peritoneum, and the tunica vaginalis of the testis. MM is a disease characterised by an aggressive clinical course, with a 5-year survival rate of <5% in unresectable patients [1]. Although the approved front-line regimen for unresectable tumours is cisplatin-pemetrexed, results from clinical trials investigating the potential role of immune checkpoint inhibitors in treatment have recently been reported and have increased the interest in this topic [3,4].

Published studies mostly report MM in patients over the age of 60 [14]. In our study, the average age of the patients was 62 ± 10.94 y, in accordance with the literature. While male predominance is observed in pleural MM, peritoneal MM is observed more frequently in women [1]. Male predominance was seen in both locations in our series.

In the literature, the rate of pleural MM is 68-85%, and of peritoneal MM is 9-24%, while that of testicular MM is about 0.3-5% [15]. In agreement with the literature, 86% of our MM cases were located in the pleura, 8% in the peritoneum, and 4% in the testis.

According to the literature, the epithelioid type is seen in 60-80%, and the sarcomatoid and biphasic type is seen in 10-15% [1]. Our findings were consistent with the previous reports: 65% of the 69 patients in our series were epithelioid type, 14% sarcomatoid type, and 13% biphasic type MM.

It can be difficult to differentiate MM from some other malignancies, particularly non-small cell lung carcinomas. Although immunohistochemistry may be helpful in this differentiation, all of the mesothelial markers in routine use have varying degrees of cross-reactivity with many neoplasms, including non-small cell lung carcinomas. Therefore, multiple MM and carcinoma markers should be used in all cases for accurate diagnosis. It was shown in the literature that sialylated protein HEG homolog 1 (HEG1), a new mucin-like membrane protein, is a highly specific marker for MM, including sarcomatoid MM and desmoplastic MM. Gene silencing of HEG1 were found to significantly suppress the survival and proliferation of MM cells; thus, it was suggested that HEG1 may be a valuable target for function inhibition drugs [8,9].

PD-L1 has high expression in many carcinomas, including lung, thymus, breast, liver, colon and bladder carcinomas. Various tumor-infiltrating lymphocytes (TILs) also express PD-L1 and have an important role in reducing the antitumor immune response. Multiple studies have demonstrated varying degrees of PD-L1 expression in MM [16–18]. Although anti-PD-L1 therapy is used in pleural MM, the role of PD-L1 immunohistochemistry in predicting treatment response is not clear. Although several studies have reported that immunohistochemical PD-L1 expression in tumour cells does not correlate with response to immunotherapy, PD-L1 expression in tumour-infiltrating lymphocytes has been shown to be a positive prognostic factor in malignant pleural MM. Therefore, it is thought that the immune system may have an important function in malignant pleural MM [19]. PD-L1 expression has been detected more frequently in sarcomatoid type MM compared to other histological subtypes with both clone E1L3N and clone SP142 in pleural MM patients [16]. According to Eiji Matsumura et al. [18] PD-L1 (clone SP142) positivity was found to be higher in the group of patients diagnosed with non-epithelioid histology than in the group diagnosed with epithelioid histology. In our study consisting of 65 MM cases, PD-L1 expression was observed in 13 out of 45 epithelioid MM (24%), 8 out of 10 sarcomatoid MM (80%) and 3 out of 9 biphasic MM (22%). In our study, in accordance with the literature, we found more PD-L1 (clone SP142) positivity in the patient group diagnosed with non-epithelioid histology than in the group diagnosed with epithelioid histology. In our study, we also compared the expression of PD-L1 with the degree of epithelioid MM, which which to our knowledge had not been before in the literature, and found significantly higher PD-L1 expression in high-grade pleural epithelioid MM compared to low-grade ones. Chapel et al. [19] found higher PD-L1 expression in pleural MM compared to peritoneal MM. No significant relationship was found between tumor localization and PD-L1 expression in our study, but this issue should be studied with a wider series due to the low number of peritoneal MM cases. There are some studies in the literature that investigate PD-L1 expressions in MMS and benign mesothelial reactions. Our results are consistent with Derakhshan et al. [20], who suggested that strong-diffuse staining of PD-L1 may support the diagnosis of sarcomatoid MMa when the differential diagnosis is reactive mesothelial proliferation, but it will not be useful in distinguishing epithelioid MM from reactive mesothelial proliferation.

VISTA is expressed on myeloid cells and T lymphocytes. Overexpression of VISTA suppresses early T cell activation and proliferation and inhibits cytokine production. According to the Tumour Cancer Genome Atlas, there is strong immunohistochemical expression of VISTA in benign mesothelium and increased mRNA expression of VISTA in MM compared to other tumour types [13]. Some studies have investigated VISTA expression by PD-L1 in MM Muller et al. [13] and Chung et al. [4] found more frequent VISTA expression in epithelioid type MM and more frequent PD-L1 expression in sarcomatoid type MM compared to other histological subtypes. In our study, we also detected VISTA expression in epithelioid type MM more often than in other types. In addition, in our study, in accordance with these two studies, we observed a negative inverse relationship between VISTA and PD-L1 expression levels according to histological subtypes. Chung et al. [4] administered PD-L1 inhibitors to two patients diagnosed with epithelioid MM with high VISTA expression and no PD-L1 expression but did not observe any drug effect. According to the preliminary results of a phase II study, pembrolizumab activity has been demonstrated in MMs and this activity is independent of PD-L1 reactivity. In this cohort, no correlation was found between PD-L1 expression and response to anti-PD-1 therapy [21]. In another study, PD-L1 expression was not associated with response to nivolumab treatment in relapsed MM patients. Although PD-L1 immunohistochemistry has prognostic significance, it is not a predictive biomarker for immunotherapy response [22]. All data provide a strong rationale for investigating VISTA as a target of immunotherapy for MM. Considering the high frequency of VISTA expression in both tumour and tumour-associated inflammatory cells in MM, anti-VISTA therapies may be particularly effective in epithelioid malignant pleural MM. Preclinical studies of VISTA blockade have also shown encouraging results in this regard [23]. However, more research is needed to determine the effect of VISTA blockage in the treatment of MM, as well as the role of VISTA in an immunosuppressive setting.

According to Chung et al. [4] there was 94% VISTA positivity in epithelioid MM, while patchy cytoplasmic staining was observed in less than 5% of tumor cells in only 2 cases of non-small cell lung carcinomas. In another study [24], VISTA expression was detected by immunofluorescence staining in 21% of non-small cell lung carcinomas. While we observed VISTA expression in 44 of the 45 epithelioid MM in our study, we did not observe VISTA expression in any of the 19 non-mesothelial cases. Further studies in larger series are needed to determine whether VISTA is expressed in lung cancer cells. As suggested by Chung et al. [4], the specific expression of VISTA in the epithelioid MM may be of hope as a diagnostic marker, including the differential diagnosis of many tumors that have metastasized to the pleura, including MM and non-small cell lung carcinomas.

Researchers [8] have proposed that sialile HEG1 is an MM-related antigen, and HEG1 expression promotes the survival and proliferation of MM cells. According to Naso et al. [10], while epithelioid MM and sarcomatoid MM showed high-frequency HEG1 expression, none of 146 cases of non-small cell lung carcinoma showed HEG1 expression. Hiroshima et al. [11] showed high-frequency HEG1 expression in MM. While immunoreactivity was not observed in pulmonary adenocarcinomas in this study, cytoplasmic immunoreactivity was observed in 21.7% of pulmonary squamous cell carcinomas, and the sensitivity of membranous HEG1 expression to distinguish from all carcinomas was previously reported as 88.8%. In addition, the specificity for the differential diagnosis between MM and all carcinomas and pulmonary carcinomas was reported to be 92.3% and 98.7%, respectively [11]. In our study, HEG1 expression was observed in 63 of the 64 MM cases, 3 cases with WDPMT, and 2 cases with AMH diagnosis. Unlike the study of Hiroshima et al. [11], in our study, HEG1 expression was monitored in all sarcomatoid MM, but it was observed at a lower score compared to epithelioid MM. HEG1 expression was not observed in any of the non-mesothelial tumors. In our study, the sensitivity was found to be 98.4%, and the specificity was found to be 95%, thus, supporting the idea that HEG1 can be used in the differential diagnosis of MM.

Conclusions

Previous studies have shown that PD-L1 is associated with prognosis, and the results from this study supported these observations, as more frequent PD-L1 expression was detected in sarcomatoid type and high-grade epithelioid type MM. In contrast, VISTA expression was lower in sarcomatoid MM. In our study, PD-L1 and VISTA immune checkpoints showed encouraging changes for MM treatment, but this issue needs to be clarified with more detailed clinical studies. HEG1 is expressed at a high level in our MM cases, supporting previous studies that it can be used in the differential diagnosis of MM from other tumors.

Authors’ contributions

All authors contributed significantly to the work and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. MD and UA made substantial contributions to the conception and design of the work. MD, ANK, and UA were involved in project management, literature search, data acquisition, interpretation, drafting, and reviewing of the manuscript. ANK and UA were involved in visualization. All authors read and approved the final manuscript.

Acknowledgments

The study was reported as a presentation at the 35th European Congress of Pathology in 2023. [Dicleli M, Keleş A. E-PS-25-001 HEG1 expression in mesothelial tumours. In: 35th European Congress of Pathology – Abstracts. Virchows Arch. 2023;483 (Suppl 1), https://doi.org/10.1007/s00428-023-03602-w.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The data that support the findings of this study are available from the corresponding author, [UA], upon reasonable request.

Additional information

Funding

This study was supported by SBE.21.006 code number, was conducted at the Scientific Research Projects Unit of Dicle University.