The expression of programmed cell death ligand 1 (PD-L1) involves in the clinicopathologic characteristics and prognostic implications of testicular germ cell tumor (TGCT): a systematic review and meta-analysis

Introduction

Testicular germ cell tumor (TGCT), although accounting for approximately 1% of new male tumors annually, is the most prevalent tumor type among young men aged 15 to 40 years (1,2). Recently, the incidence among patients in their 40s is increasing (3). TGCT is derived from germ cell neoplasia in situ generated by delayed gonocytes, and it is categorized into two histological subtypes: seminoma and nonseminomatous germ cell tumor (NSGCT), consisting of embryonal carcinoma, choriocarcinoma, yolk sac tumor and teratoma (4). These subtypes can appear alone or in combination as mixed germ cell tumor (GCT). Seminoma, generally the predominant tumor type, is characterized by the elevated levels of human chorionic gonadotropin (HCG) and tends to show fewer metastatic events and a more favorable prognosis compared to NSGCT, in which alpha fetoprotein (AFP) and HCG are produced (2,5). Besides AFP, HCG and lactate dehydrogenase (LDH), genetic variants or other molecules, such as claudin 6 or programmed cell death ligand 1 (PD-L1)/programmed cell death 1 (PD-1) signaling protein, act as serum tumor biomarkers, playing pivotal roles in the diagnosis, treatment, and prognosis of TGCT (6,7).

TGCT is sensitive to radiotherapy and chemotherapy. The implementation of platinum-based chemotherapy in TGCT results in lower mortality rates compared to other tumor types, with a 5-year overall survival (OS) rate of up to 95% (8). However, despite radical orchiectomy and/or the combined treatment with radiotherapy or chemotherapy, approximately 10–15% of TGCT patients experience relapse, and salvage treatment fails to provide optimal outcomes for 50% of these individuals (9,10). Considering both the limited efficacy and the short- or long-term side effects associated with second-line and salvage therapies, alternative novel and effective treatment strategies are investigated. Compared to normal testicular tissues with macrophages as the predominant immune cell population, testicular tissues are primarily infiltrated by T cells in TGCT, thereby making immunotherapy possible (11). Recently, immunotherapy targeting immune checkpoint inhibitors such as PD-L1 or PD-1 is explored in the treatments of TGCT.

PD-1 is primarily expressed in lymphocytes and interacts with its ligand PD-L1, which can be found in tumor or immune cells. This interaction leads to the inhibition of T cell-mediated antitumor activity (12). Currently, the use of anti-PD-L1/PD-1, either alone or in combination with conventional radiotherapy or chemotherapy, demonstrates promising results in the treatments of various tumors, including lung cancer, kidney cancer, melanoma, hematologic tumors, as well as TGCT (7,13). Several case reports showed stable conditions or positive responses, such as decreased serum biomarkers and tumor volumes, in patients with relapsed/refractory TGCT following the administration of anti-PD-1 antibodies (14-16). Conversely, other evidence observed that the inhibition of PD-L1 failed to elicit antitumor effects in relapsed/refractory TGCT patients (17).

To a large extent, the effect of immunotherapy in TGCT depends on the expression of PD-L1 or PD-1. Some experimental studies revealed a higher prevalence of PD-L1 in TGCT specimens compared to normal testicular tissues, supporting the potential of immunotherapy targeting the PD-L1/PD-1 pathway in TGCT (18,19). While, other studies obtained the contradictory results showing the absent or lower expression of PD-L1 in TGCT patients (20-22). In addition, TGCT patients with higher expression of PD-L1 often achieved favorable treatment outcomes (18,19). However, other studies indicated that TGCT patients with lower PD-L1 expression had better prognosis (21). Therefore, in order to clarify the general expression of PD-L1 in TGCT, and its correlation with tumor pathology and prognosis, this meta-analysis was conducted. We present this article in accordance with the PRISMA reporting checklist (available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-2302/rc) (23).

Methods

This meta-analysis was registered on the International Platform of Registered Systematic Review and Meta-analysis Protocols (INPLASY, registration number: INPLASY2023120108). Ethical committee approval was not necessary since the analyzed data were derived from published articles. Any discrepancies between reviewers were resolved through discussion.

Search strategy

A comprehensive literature search was independently conducted by two reviewers (P.L. and W.P.) on the following databases: PubMed, Embase, Web of Science, Cochrane Library and China National Knowledge Infrastructure (CNKI), until May 6, 2024. The search terms utilized included “testis”, “testicle”, “testicular”, “cancer”, “carcinoma”, “tumor”, “neoplasm”, “programmed cell death ligand 1”, “programmed death ligand 1”, “PD-L1”, “PDL1”, “B7 homolog 1”, “B7-H1”, “B7H1” and “CD274”. For the studies included that required full-text retrieval, their references were scrutinized to identify additional relevant publications. An illustrative example of the search strategy performed in the PubMed database is shown in Appendix 1.

Eligibility criteria

Following a thorough evaluation of the title, abstract, and full-text of the retrieved literature by two reviewers (P.L. and W.P.), studies investigating the correlation between PD-L1 expression and TGCT were selected based on the Population, Intervention, Comparison, Outcomes and Study (PICOS) principal: (I) population: patients were diagnosed with TGCT; (II) intervention: PD-L1 expression in TGCT patients was assessed using immunohistochemistry; (III) comparison: the expression of PD-L1 was categorized into positive (high) and negative (low); (IV) outcomes: the study provided adequate information regarding the pathology or prognosis of TGCT patients; (V) study: cohort or case control studies.

Conversely, other factors were employed to exclude irrelevant studies, which encompassed: (I) failure to meet the inclusion criteria; (II) studies involving animal research, cell biology experiments, bioinformatic analyses, reviews, meta-analyses, comments, meeting abstracts, or case reports; (III) small sample sizes (n<15); (IV) duplicate studies or data; (V) unavailability of the full-text of the literature.

Data collection

The characteristics of the included studies were extracted independently by two reviewers (P.L. and Yuwei Zhong) and checked by another reviewer (M.Z.). Patients were divided into different subgroups depending on their specific situation. The useful data included (I) study information: authors’ name, publication year and country; (II) patient information: age, tumor type and stage, metastasis situation, International Germ Cell Cancer Collaborative Group (IGCCCG) risk classification, follow-up time and outcomes; (III) experiment information: staining method, PD-L1 antibody and cut-off value for PD-L1 positive; (IV) outcome information: total number of included patients and number of PD-L1 positive patients in each subpopulation. We collected and tabulated these data.

Quality assessment

The quality assessment of the included studies was carried out independently by two reviewers (Yuwei Zhong and Yonghong Zheng) using the Newcastle-Ottawa Scale (NOS), a tool containing eight items distributed across three domains: selection (0–4 scores), comparability (0–2 scores) and exposure or outcome (0–3 scores) (24). The NOS assigns a maximum score of 9, with studies scoring 6 or above considered to be of high quality.

Statistical analysis

The overall proportion of PD-L1 was calculated using the R package “meta” in R software version 4.3.0 for studies reporting a single proportion. To ensure appropriate data transformation, the Freeman-Tukey Double arcsine transformation method was applied (25). For statistical analysis, Review Manager 5.4 software (Revman, the Cochrane Collaboration, Oxford, UK) was utilized to compute heterogeneity, hazard ratio (HR), odds ratio (OR), and corresponding 95% confidence intervals (CIs), and generate forest plots. Heterogeneity among the studies was assessed using the Chi-squared test and I² statistic. If the P value of the Chi-squared test was less than 0.1 or I² exceeded 50%, significant heterogeneity was considered present, and a random-effects model was employed. Otherwise, a fixed-effects model was utilized (26). To explore the association between PD-L1 positivity and survival in TGCT patients, pooled HR with its 95% CI was determined. Additionally, the correlation between PD-L1 expression and clinicopathological characteristics in TGCT patients was analyzed using pooled OR and its 95% CI. Publication bias was assessed when the analysis included a minimum of five studies by utilizing Egger’s test, and sensitivity analysis was performed using the one-by-one elimination method through R programming (26). P<0.05 was considered statistically significant.

Results

Search results

Following the search strategy outlined above, a total of 992 articles were obtained from PubMed, Embase, Web of Science, Cochrane Library and CNKI databases. After removing duplicate studies, a preliminary screening of titles and abstracts was conducted. Non-original articles (n=427) and those not relevant to the scope of this study (n=268) were excluded. Subsequently, the full texts of the remaining studies were retrieved. Further exclusions were made based on specific criteria: utilization of duplicate patient samples (n=2), absence of dichotomization of PD-L1 expression into positive/high and negative/low groups (n=4), unavailability of analyzable data (n=2), and small sample size (n=3). Ultimately, a total of eight studies were included in this meta-analysis. The flow diagram illustrating the process of screening eligible publications for this study is presented in Figure 1.

Figure 1 The flow diagram for selecting the eligible studies. CNKI, China National Knowledge Infrastructure; PD-L1, programmed cell death ligand 1.

Study characteristics

All studies included in this meta-analysis were conducted from 2015 to 2024 in European and American countries except one study investigated Asian patients. With the exception of one study by Cierna et al., which involved specimens from nine patients with extragonadal GCT (21), the remaining studies focused on TGCT patients. The age of the patients ranged from 16 to 90 years. A total of 1,589 specimens were analyzed across these eligible studies. It is worth noting that both Cierna et al. and Chovanec et al. publications originated from the same research group. Consequently, there were 136 patients from the study by Chovanec et al. that were also included in the study conducted by Cierna et al. (21,27). Cierna et al. and Fankhauser et al. analyzed the expression of PD-L1 in tumor cells, while Chovanec et al., Sadigh et al. and Pęksa et al. examined its expression in tumor infiltrating immune cells (TIICs), such as lymphocytes and macrophages (18,21,27-29). Lobo et al., Möller et al. and Farahani et al. investigated PD-L1 expression in both tumor cells and TIICs (19,20,22). Immunohistochemistry was utilized in all these studies to determine the positivity of PD-L1, although different antibodies and cut-off values were employed. These variations contributed to the wide range of PD-L1 positive expression observed across different studies. The proportion of PD-L1 positive staining in tumor cells ranged from 0.0% to 69.6%, while in TIICs it ranged from 3.0% to 83.5%. Follow-up data on patient outcomes were available in four studies, where the prognostic value of PD-L1 expression was assessed using OS, progression-free survival (PFS), relapse-free survival (RFS), or other indicators. The characteristics of the studies included in this meta-analysis are presented in Table 1.

Furthermore, all included studies were either cohort or case-control studies and obtained a NOS score between 6 and 7, indicating good methodological quality (Tables 2,3).

Overall proportion of positive PD-L1 expression in TGCT

PD-L1 expression has been observed in various types of cancer, including melanoma, lung cancer, gastric carcinoma, as well as TGCT (30). By pooling the data on patients with positive PD-L1 staining, we determined that 31% of TGCT patients exhibited high PD-L1 expression in tumor cells (random-effects, 95% CI: 0.06–0.65) (Figure 2A). Additionally, PD-L1 was found to be overexpressed in TIICs in 41% of TGCT patients (random-effects, 95% CI: 0.16–0.69) (Figure 2B). All the studies included in this analysis showed significant heterogeneity.

Figure 2 Forest plots of overall percentage of PD-L1 positive expression in tumor cells (A) and TIICs (B) in TGCT patients. Axis represents the proportion of PD-L1 positive expression in TGCT. CI, confidence interval; PD-L1, programmed cell death ligand 1; TIICs, tumor infiltrating immune cells; TGCT, testicular germ cell tumor.

Association of positive PD-L1 expression with prognostic implications in TGCT

Despite the inclusion of four studies that followed up with TGCT patients, two different studies examined the relationship between PD-L1 expression in tumor cells and the prognosis of TGCT. One study found that TGCT patients who exhibited a lack of PD-L1 expression or lower PD-L1 expression in the primary tumor had a better PFS compared to those with positive or higher PD-L1 expression (negative/lower verse positive/higher, HR =0.40, 95% CI: 0.16–1.01, P=0.0081) (21). While, the other study reported no significant difference of RFS among TGCT patients showing PD-L1 positive or negative tumor cells (19).

The majority of studies primarily focused on the correlation between PD-L1 expression in TIICs and the prognostic outcomes of TGCT patients, particularly in terms of disease progression or relapse events. Notably, Pęksa et al. analyzed this correlation in two specific subgroups, namely seminoma and NSGCT (29). Therefore, we segregated this study into two distinct sub-studies: (I) seminoma and (II) NSGCT. The results demonstrated that TGCT patients with PD-L1⁺ TIICs showed a more favorable prognosis and experienced fewer progression or relapse events compared to those with PD-L1-negative or lower TIIC expression (positive/higher verse negative/lower, HR =0.21, 95% CI: 0.13–0.33, P<0.00001) (Figure 3). No significant heterogeneity was observed. Thus, a fixed-effects model was utilized for this analysis.

Figure 3 Forest plots of HR to evaluate the correlation between PD-L1 expression and event (progression or relapse) in TGCT. SE, standard error; IV, inverse variance; CI, confidence interval; HR, hazard ratio; PD-L1, programmed cell death ligand 1; TGCT, testicular germ cell tumor.

Association of positive PD-L1 expression with clinicopathologic characteristics in TGCT

Histology

Seminoma is generally considered to be less aggressive in comparison to NSGCT (31). There was no significant difference found in the expression of PD-L1 in tumor cells of seminoma and NSGCT (Figure 4A). However, there was a significant correlation between positive PD-L1 staining in immune cells and seminoma. Among 757 seminoma patients and 466 NSGCT patients, 69% and 41% of them exhibited PD-L1 positive immune cell infiltration, respectively. The odds of PD-L1⁺ TIICs in seminoma were 111% higher than those in NSGCT (95% CI: 1.57–2.84) (Figure 4B). Afterwards, we analyzed the relationship between PD-L1 expression and individual subtypes of NSGCT. The pooled OR results revealed that PD-L1 overexpression in tumor cells was more prevalent in choriocarcinoma (OR =11.11, 95% CI: 4.82–25.60) (Figure 4C), while there was no advantage of PD-L1⁺ TIICs in choriocarcinoma (Figure 4D), as well as PD-L1 positive tumor cells in embryonal carcinoma (Figure 4E). The odds of PD-L1⁺ immune cell infiltration in embryonal carcinoma were significantly higher by a 5.23-fold change compared to other NSGCTs (OR =6.23, 95% CI: 2.42–16.02) (Figure 4F). In addition, lower expression of PD-L1 was commonly observed in yolk sac tumor cells (OR =0.54, 95% CI: 0.32–0.91), not in TIICs (Figure 4G,4H). Similarly, lower expression of PD-L1 was significantly associated with teratoma not only in tumor cells (OR =0.17, 95% CI: 0.05–0.54) but also in TIICs (OR =0.07, 95% CI: 0.01–0.44) (Figure 4I,4J).

Figure 4 Forest plots of OR to evaluate the correlation between PD-L1 expression and histological characteristics in TGCT. Comparison of PD-L1 expression in tumor cells (A,C,E,G,I) or TIICs (B,D,F,H,J) between seminoma and NSGCT (A,B), choriocarcinoma and other types of NSGCT (C,D), embryonal carcinoma and other types of NSGCT (E,F), yolk sac tumor and other types of NSGCT (G,H), as well as teratoma and other types of NSGCT (I,J). NSGCT, nonseminomatous germ cell tumor; M-H, Mantel-Haenszel; CI, confidence interval; OR, odds ratio; PD-L1, programmed cell death ligand 1; TGCT, testicular germ cell tumor; TIICs, tumor infiltrating immune cells.

Stages

There were two articles examining the correlation between PD-L1 expression in tumor cells and tumor-node-metastasis (TNM) or S stages in TGCT, revealing that higher stage TGCT patients had a higher rate of positive PD-L1 staining (18,21). However, upon extracting data from studies conducted by Lobo et al. and Pęksa et al. (19,29), we discovered that stage I tumors in TGCT patients showed a higher presence of PD-L1⁺ TIICs compared to stage II and III tumors (OR =0.70, 95% CI: 0.42–1.15) (Figure 5A). In the study by Chovanec et al. (27), they compared data from stage S0, S1 and S2 tumors with stage S3 tumors in TGCT, therefore it cannot be included in this meta-analysis. However, similar results were obtained, showing a significant association between PD-L1 positivity in TIICs and low-stage tumors but not high-stage tumors (P value =0.03) (27). Opposed to these results, the infiltration of PD-L1⁺ immune cells into TGCT with pT2–4 tumors demonstrated an advantage over tumors with pT1 stage, although no significant statistical differences were observed (Figure 5B). In addition, positive expression of PD-L1 in TIICs was more common in tumors that did not develop lymph node metastases or develop lymphovascular invasion (LVI), albeit without significant statistical difference (Figure 5C,5D).

Figure 5 Forest plots of OR to evaluate the correlation between PD-L1 expression in TIICs and TNM stages (A), T stages (B), lymph node metastasis (C) and LVI (D) in TGCT. M-H, Mantel-Haenszel; CI, confidence interval; LVI, lymphovascular invasion; OR, odds ratio; PD-L1, programmed cell death ligand 1; TIICs, tumor infiltrating immune cells; TNM, tumor-node-metastasis; TGCT, testicular germ cell tumor.

IGCCCG risk classification

Based on the histological subtype, metastasis, and levels of AFP, HCG and LDH, TGCT can be categorized into good, intermediate and poor outcome groups according to the IGCCCG classification. These groups correspond to approximately 96%, 89%, and 67% of the 5-year OS rate (32,33). In this study, we conducted two comparisons due to the grouping differences between Chovanec et al. who combined the good and intermediate risk groups (27), and Lobo et al. who combined the poor and intermediate risk groups (19). Both comparisons revealed that positive expression of PD-L1 on TIICs was predictive of a favorable prognosis in TGCT (Figure 6). However, we did not analyze PD-L1 expression on tumor cells due to insufficient data, as only one study addressed this aspect, which reported contrary results: higher expression of PD-L1 on tumor cells predicted a worse prognosis in TGCT patients (21).

Figure 6 Forest plots of OR to evaluate the correlation between PD-L1 expression and IGCCCG risk classification in TGCT. Comparison of PD-L1 expression in TIICs between poor and good/IM group (A) as well as poor/IM and good group (B). M-H, Mantel-Haenszel; CI, confidence interval; OR, odds ratio; PD-L1, programmed cell death ligand 1; IGCCCG, International Germ Cell Cancer Collaborative Group; TGCT, testicular germ cell tumor; TIICs, tumor infiltrating immune cells; IM, intermediate.

Sensitivity analysis and publication bias

Sensitivity analysis was conducted to evaluate following comparisons: (I) the overall proportion of positive PD-L1 expression in TGCT as shown in Figure 2; and (II) the comparison of positive PD-L1 expression in TIICs between seminoma and NSGCT as showed in Figure 4B. All comparisons included more than five studies. Upon excluding each individual study from the overall analysis, no significant differences were observed (Figure S1). Furthermore, publication bias was assessed using Egger’s test for the aforementioned studies. Results indicated no evidence of publication bias in all of these studies, as P values were greater than 0.05 in all comparisons [P values of comparison (I) were 0.45 and 0.38, respectively; P value of comparison (II) was 0.37].

Discussion

PD-L1, also known as PD-L1, is a protein that can be expressed constitutively or induced in various cells within different tissues, including the testis (34). In the physiological state, PD-L1 released by Sertoli cells, myoid peritubular cells, and germ cells directly or indirectly suppresses T cell activation and proliferation, thereby contributing to the establishment of immune privilege in the testes (35). Several studies reported that PD-L1 expression is generally lower in normal testis (18,21,36). Upregulation of PD-L1 expression is frequently observed in specific types of testicular tumors, such as GCTs and testicular lymphoma (37-39). In this meta-analysis, we included a total of eight eligible studies based on our search strategy. Although additional studies also confirmed the significant upregulation of PD-L1 in TGCT, particularly in seminoma (40-43), we were unable to incorporate them into the final meta-analysis due to the lack of dichotomous expression data of PD-L1.

Immune cell infiltration is a crucial characteristic observed during the progression of TGCT (11). Among the infiltrating cells in TGCT, macrophages, CD8⁺ and CD45R0⁺ T cells, as well as dendritic cells, constituted the major population (44,45). High levels of TIICs were associated with a worse prognosis in TGCT, indicating an increased risk of tumor relapse (46). In addition to tumor cells, PD-L1 expression was also detected in TIICs in TGCT, such as lymphocytes and macrophages (27,28). The pooled results indicated that compared to the average of 31% PD-L1 positivity on tumor cells, PD-L1 was more frequently expressed on TIICs in TGCT, with an overall overexpression rate of 41%. Notably, Möller et al. reported the absence of PD-L1 positive staining on tumor cells, while up to 73% of TIICs in TGCT can express PD-L1 (20). However, Farahani et al. showed the contradictory results, in which only 3% of TIICs in TGCT patients presented PD-L1 (22).

Moreover, different conclusions were reported regarding the association of PD-L1 expression with patient outcomes in TGCT. One study suggested that higher expression of PD-L1 in tumor cells correlated with PFS and OS rates, while another study found no significant difference in RFS rates between TGCT patients with PD-L1⁺ and PD-L1⁻ tumor cells (19,21). In contrast, our comprehensive meta-analysis demonstrated that the presence of PD-L1⁺ TIICs was a significant predictor of favorable risk classification and prognosis, associated with reduced progression or relapse events. These findings suggest a contrasting role played by PD-L1 in tumor cells versus TIICs during the development of TGCT. One possible explanation for this discrepancy is that TGCT patients with PD-L1⁺ TIICs exhibit heightened sensitivity to various therapeutic interventions, resulting in more effective responses and consequently improving prognosis. Additionally, activated T cells can trigger negative feedback loops, leading to the development of adaptive immune resistance. Some TGCT patients may display limited or even absent immune cell infiltration, resulting in lower PD-L1 positivity rates in TIICs. However, these individuals may exhibit overexpression of PD-L1 in tumor cells, contributing to the divergent findings regarding PD-L1 expression in TIICs versus tumor cells. Similarly, elevated levels of PD-L1 expression in conjunction with TIICs are also associated with favorable prognostic stages in other types of tumors, including breast cancer, lung cancer or colorectal cancer (47).

The distribution of PD-L1 positive tumor cells and immune cells varies across different pathological subtypes of TGCT. Choriocarcinoma exhibited a higher prevalence of PD-L1⁺ staining in tumor cells, while seminoma and embryonal carcinoma displayed a higher frequency of PD-L1⁺ staining in TIICs. Remarkably, the levels of PD-L1⁺ tumor-associated macrophages were found to gradually decrease during the reprogramming process from pure seminoma to the seminoma component in mixed GCT, and subsequently to other NSGCT (43). Furthermore, irrespective of the PD-L1 expression pattern, its expression was significantly diminished in teratomas. These findings indicate that the PD-1/PD-L1 signaling pathway plays distinct roles among various subtypes of TGCT. Notably, PD-L1⁺ TIICs not only predicted the improved prognosis but also correlated with lower pathological grades.

The aforementioned observations strongly suggest that PD-L1 is prominently expressed in TGCT, particularly in TIICs, thereby providing a theoretical foundation and target support for the immunotherapy of TGCT. Simultaneously, the considerable variability in PD-L1 expression among patients highlights the necessity for personalized treatment approaches tailored to different individuals with TGCT. To date, immunotherapy outcomes for platinum-refractory TGCT are unsatisfactory. In a study involving fourteen TGCT patients experiencing relapse or lacking alternative options, no partial or complete responses were observed following PD-1 antibody treatment, despite only two of them were PD-L1 positive (48). Similarly, eight enrolled patients with relapsed or refractory TGCT received PD-L1 antibody immunotherapy but exhibited neither objective responses nor disease stabilization (17). However, certain case reports have described stable disease or partial response in PD-L1 positive TGCT patients following PD-1 antibody treatment (14,15). The existing clinical trial results do not definitively establish a correlation between PD-L1 expression and the efficacy of anti-PD-1/PD-L1 immunotherapy, warranting further validation through additional experiments.

In addition, the expression of PD-L1 in TIICs does not serve as a reliable individual predictor for tumor pathology and prognosis in TGCT. A study demonstrated that there was no significant difference in the 5-year event-free survival (EFS) rate between TGCT patients with PD-L1⁺ and PD-L1⁻ TIICs. However, when the data of PD-L1 and V-domain Ig suppressor of T cell activation (VISTA) were combined, lower expression of PD-L1 and VISTA on TIICs significantly correlated with a worse EFS rate in TGCT (29). Furthermore, the systemic immune-inflammation index (SII) is identified as a useful tool for predicting survival outcomes in TGCT patients. Combining SII with the expression of PD-L1 in TIICs provided a more accurate prediction of TGCT patient prognosis (49,50).

However, there were some limitations in this meta-analysis. Firstly, the included studies were of insufficient quality and quantity to generate robust and accurate data. It is better to focus on conducting large-scale studies in TGCT with comprehensive clinical and follow-up information in the future research. Secondly, in terms of population ethnicity, only one study included Asian population, and other studies included patients from Europe and America. Therefore, it is essential to investigate TGCT populations from other regions such as Asia and Africa. Thirdly, there was heterogeneity in the meta-analysis due to the use of different clones of PD-L1 antibodies from various companies. The variation in antibody affinity may lead to different staining results in immunohistochemical experiments. Additionally, discrepancies in the criteria used to determine PD-L1 positivity among different studies can also impact the results. For example, Pęksa et al. considered histoscores of 41 or higher as indicative of PD-L1 positivity, whereas the research group led by Chovanec et al. required a histoscore greater than 160 (27,29).

In summary, it is important to recognize the limitations of this meta-analysis, including the inadequate quality and quantity of studies, potential bias in population ethnicity, and heterogeneity in antibody selection and PD-L1 positivity criteria. Future research should address these limitations to enhance the reliability and generalizability of findings in TGCT. Despite the aforementioned limitations, this meta-analysis represents the first comprehensive investigation of the associations between PD-L1 expression and clinicopathological characteristics as well as prognosis in patients with TGCT.

Conclusions

Our findings revealed that PD-L1 was expressed in both tumor cells and TIICs in TGCT. Specifically, PD-L1 expression was observed in choriocarcinoma tumor cells in NSGCT, while yolk sac tumor and teratoma tumor cells exhibited lower or absent expression of PD-L1. Conversely, PD-L1 expression in TIICs was associated with seminoma and embryonal carcinoma, and was more commonly observed in TGCT patients with lower stages or without lymph node metastasis. Furthermore, positive PD-L1 expression in TIICs was found to be a favorable prognostic indicator in TGCT patients, as it correlated with a reduced likelihood of disease progression or relapse.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the PRISMA reporting checklist. Available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-2302/rc

Peer Review File: Available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-2302/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-2302/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are 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.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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