Clinicopathologic Features of Thymoma With the Expression of Programmed Death Ligand 1

Thymic epithelial tumor medical treatment: A narrative review

Thymic epithelial tumors, a malignancy originating in the thymus, are the commonest primary neoplasm of the anterior mediastinum; however, among thoracic tumors, they have a relatively low incidence rare. Thymic epithelial tumors can be broadly classified into thymic carcinoma and thymoma. As the cornerstone of thymic tumor treatment, surgery is the preferred treatment for early-stage patients, whereas, for advanced unresectable thymic tumors, the treatment is chemoradiotherapy. Targeted therapy is less effective for thymic tumors. Moreover, the use of immune checkpoint inhibitors as another effective treatment option for advanced unresectable thymic tumors, particularly thymomas, is limited owing to immune-related adverse effects. Here, we have summarized all pertinent information regarding chemotherapy, especially preoperative neoadjuvant chemotherapy, and chemotherapy in combination with other treatments, and reviewed the effectiveness of these procedures and recent advances in targeted therapy. In addition, we analyzed the efficacy and safety of immune checkpoint inhibitors in thymic epithelial tumors, to provide a holistic treatment view.

Depiction of the Genetic Alterations and Molecular Landscapes of Thymic Epithelial Tumors: A Systematic Review and Meta-Analysis

Thymic epithelial tumors (TETs), consisting of thymomas, thymic carcinomas (TCs), and thymic neuroendocrine tumors, are rare diseases. Surgery remains the prime option in resectable and early-stage TETs, while chemotherapy, targeted therapy, and immunotherapy are also potential treatment modalities. However, the inadequate comprehension of the molecular landscape of TETs impedes the exploitation of such therapies. Hence, we conducted a meta-analysis which includes 21 studies reporting on genomic alterations in TETs and 14 studies reporting on PD-L1 expression levels, respectively. The pooled estimated rates of the most frequently mutated genes and PD-L1 expression levels were analyzed using the R software. We uncovered that the pooled estimated overall mutation rate is 0.65 ([0.49; 0.81]), and the top three genes with highest mutation frequency in thymomas and TCs are GTF2I (0.4263 [0.3590; 0.4936]), TP53 (0.1101 [0.0000; 0.2586]), and RAS (0.0341 [0.0104; 0.0710]), and TP53 (0.1797 [0.0732; 0.3203]), CDKN2A (0.0608 [0.0139; 0.1378]), and TET2 (0.0318 [0.0087; 0.0639]), respectively. A uniform GTF2I mutational rate in thymomas and TP53 mutational rate in thymic squamous cell carcinomas (TSCCs) are also observed. The pooled estimated expression level of PD-L1 is 0.71 ([0.59-0.81]). This systematic review provides an overview of the gene alteration landscape and PD-L1 expression levels in TETs, discovers several potential confounding factors that may contribute to the high heterogeneity, and facilitates deeper investigations into the elucidation of the molecular landscape of TETs.

Immunotherapy in thymic epithelial tumors: tissue predictive biomarkers for immune checkpoint inhibitors

Thymic epithelial tumors (TETs) are rare malignant neoplasms arising in the thymus gland. Nevertheless, TETs, including thymomas (TMs), thymic carcinomas (TCs), and thymic neuroendocrine neoplasms (TNENs), are the most common mediastinal malignancies overall. A multidisciplinary approach is required for the appropriate diagnostic and therapeutic management of TETs. To date, the main therapeutic strategies are largely depended on the stage of the tumor and they include surgery with or without neoadjuvant or adjuvant therapy, represented by platinum-based chemotherapy, radiotherapy or chemoradiotherapy. Immune checkpoint inhibitors (ICIs) are ongoing under evaluation in the advanced or metastatic diseases despite the challenges related to the very low tumor mutation burden (TMB) and the high incidence of immune-related adverse events in TETs. In this regard, predictive impact of tissue biomarkers expression such as programmed cell death ligand-1 (PD-L1), and other emerging biomarkers, as well as their optimal and shared interpretation are currently under evaluation in order to predict response rates to ICIs in TETs.

Molecular and Functional Key Features and Oncogenic Drivers in Thymic Carcinomas

Thymic epithelial tumors, comprising thymic carcinomas and thymomas, are rare neoplasms. They differ in histology, prognosis, and association with autoimmune diseases such as myasthenia gravis. Thymomas, but not thymic carcinomas, often harbor GTF2I mutations. Mutations of CDKN2A, TP53, and CDKN2B are the most common thymic carcinomas. The acquisition of mutations in genes that control chromatin modifications and epigenetic regulation occurs in the advanced stages of thymic carcinomas. Anti-angiogenic drugs and immune checkpoint inhibitors targeting the PD-1/PD-L1 axis have shown promising results for the treatment of unresectable tumors. Since thymic carcinomas are frankly aggressive tumors, this report presents insights into their oncogenic drivers, categorized under the established hallmarks of cancer.

Immunotherapy of thymic epithelial tumors: molecular understandings and clinical perspectives

Immunotherapy has emerged to play a rapidly expanding role in the treatment of cancers. Currently, many clinical trials of therapeutic agents are on ongoing with majority of immune checkpoint inhibitors (ICIs) especially programmed death receptor 1 (PD-1) and its ligand 1 (PD-L1) inhibitors. PD-1 and PD-L1, two main immune checkpoints, are expressed at high levels in thymic epithelial tumors (TETs) and could be predictors of the progression and immunotherapeutic efficacy of TETs. However, despite inspiring efficacy reported in clinical trials and clinical practice, significantly higher incidence of immune-related adverse events (irAEs) than other tumors bring challenges to the administration of ICIs in TETs. To develop safe and effective immunotherapeutic patterns in TETs, understanding the clinical properties of patients, the cellular and molecular mechanisms of immunotherapy and irAEs occurrence are crucial. In this review, the progress of both basic and clinical research on immune checkpoints in TETs, the evidence of therapeutic efficacy and irAEs based on PD-1 /PD-L1 inhibitors in TETs treatment are discussed. Additionally, we highlighted the possible mechanisms underlying irAEs, prevention and management strategies, the insufficiency of current research and some worthy research insights. High PD-1/PD-L1 expression in TETs provides a rationale for ICI use. Completed clinical trials have shown an encouraging efficacy of ICIs, despite the high rate of irAEs. A deeper mechanism understanding at molecular level how ICIs function in TETs and why irAEs occur will help maximize the immunotherapeutic efficacy while minimizing irAEs risks in TET treatment to improve patient prognosis.

Surgical, Radiation, and Systemic Treatments of Patients With Thymic Epithelial Tumors: A Systematic Review

INTRODUCTION

Thymic epithelial tumors are rare and are classified as thymoma, thymic carcinoma, and thymic neuroendocrine tumors. The objective of this systematic review was to evaluate the treatment options for patients with thymic epithelial tumors.

METHODS

This systematic review was developed by Ontario Health (Cancer Care Ontario)'s Program in Evidence-Based Care and by the Lung Cancer Disease Site Group. MEDLINE, EMBASE, and the Cochrane Library were searched for studies comparing surgical, radiotherapy, or systemic treatments against any combination of these treatments in patients with thymic epithelial tumors. Meta-analyses were conducted with clinically homogenous studies.

RESULTS

A total of 106 studies were included, mainly from observational studies. There was an overall survival benefit with postoperative radiotherapy for patients with thymic carcinoma (hazard ratio = 0.65, 95% confidence interval: 0.47-0.89) and for patients with thymoma (hazard ratio = 0.70, 95% confidence interval: 0.59-0.82), especially for those with a high risk for mortality. Patients with thymic carcinoma or thymoma had a response to chemotherapy. Selection bias affected the results for studies that evaluated neoadjuvant chemotherapy or minimally invasive surgical techniques. Furthermore, the overall survival benefit found for adjuvant chemotherapy may have been confounded by the administration of postoperative radiotherapy.

CONCLUSIONS

For patients with thymoma or thymic carcinoma, the literature is of low quality and subject to bias. There were overall survival benefits with postoperative radiotherapy. The results of this systematic review were used to inform treatment recommendations in a clinical practice guideline. Future large-scale prospective studies that control for confounders are needed.

Expression of Programmed Cell Death 1 Ligand 2 in Patients With Thymoma and Thymomatous Myasthenia Gravis

OBJECTIVES

This study aimed to examine the expression of programmed cell death 1 ligand 2 (PD-L2) in thymoma and thymomatous myasthenia gravis (MG).

METHODS

The records of 70 patients with thymoma receiving surgical resection between January 2017 and December 2018 were retrospectively reviewed. Thymoma PD-L2 expression was evaluated by immunohistochemistry staining. Associations between PD-L2 expression and clinicopathologic features were examined.

RESULTS

PD-L2 expression was positive in 41 patients (58.6%) and negative in 29 patients (41.4%). Of them, 33 had thymomatous MG. Patients with MG were more likely to be 50 years of age or younger (69.70% vs 35.14%); have more World Health Organization (WHO) type B thymomas (84.85% vs 64.86%); have tumors of smaller size (4.09 ± 2.33 cm vs 6.47 ± 2.42 cm); have positive PD-L2 expression (78.79% vs 40.54%); and have a higher percentage of PD-L2-positive cells, higher PD-L2 expression intensity, and score (all P < .05). Positive PD-L2 expression was associated with more type B thymomas, higher Masaoka-Koga stage, smaller tumor size, ectopic thymus, and MG (all P < .05). Factors significantly associated with MG were age under 50 years, tumor size less than 5 cm, and positive PD-L2 expression (all P < .05).

CONCLUSIONS

Thymoma PD-L2 expression is significantly associated with thymomatous MG and WHO histologic types B2 and B3.

Transcriptomic and Mutational Analysis Discovering Distinct Molecular Characteristics Among Chinese Thymic Epithelial Tumor Patients

INTRODUCTION

Thymic epithelial tumors (TETs) are malignancies arising from the epithelium of the thymic gland, rare but with relatively favorable prognosis. TETs have different pathological subtypes: thymomas and thymic carcinoma, and they show different clinical characteristics regarding prognosis, pathology, and molecular profiles, etc. Although some studies have investigated the pathogenesis of TETs, more molecular data is still needed to further understand the underlying mechanisms among different TETs subtypes and populations.

METHODS

In this study, we performed targeted gene panel sequencing and whole transcriptome sequencing on the tumor tissues from 27 Chinese TET patients, including 24 thymomas (A, AB, and B subtypes) and 3 thymic squamous cell carcinomas. We analyzed the genetic variations and differentially expressed genes among multiple TET subtypes. Moreover, we compared our data with the published The Cancer Genome Atlas (TCGA) TET data on both the genetic and transcriptomic levels.

RESULTS

Compared with the TCGA TET genomic data, we found that NF1 and ATM were the most frequently mutated genes (each with a frequency of 11%, 3/27). These mutations were not mutually exclusive, since one B1 thymoma showed mutations of both genes. The GTF2I mutation was mainly enriched in subtype A and AB thymomas, consistent with the previous reports. RNA-seq results unveiled that the genes related to thymus development (FGF7, FGF10 and CLDN4) were highly expressed in certain TET subtypes, implicating that the developmental process of thymus might be linked to the tumorigenesis of these subtypes. We found high expression of CD274 (PD-L1) in B2 and B3 thymoma samples, and validated its expression using immunohistochemistry (IHC). Based on the expression profiles, we further established a machine learning model to predict the myasthenia gravis status of TET patients and achieved 90% sensitivity and 70.6% specificity in the testing cohort.

CONCLUSION

This study provides the first genomic and transcriptomic analysis of a Chinese TET cohort. The high expression of genes involved in thymus developmental processes suggests the potential association between tumorigenesis of TETs and dysregulation of developmental pathways. The high expression of PD-L1 in B2 and B3 thymomas support the potential application of immunotherapy on certain thymoma subtypes.

Induction Strategy for Locally Advanced Thymoma

Surgery remains cornerstone for the management of thymoma. Complete surgical resection (R0), is recognized as the constant and significant factor for prognosis. However, in locally advanced (Masaoka-Koga stages III-IVa) thymomas, achieving R0 resection remains challenging due to local-regional invasion of the disease. Induction treatment, with the aim of reducing bulky tumor mass, offers new strategy to facilitate totally surgical resection. Herein, we reviewed recent progress and provided a comprehensive overview of induction strategy in locally advance thymoma.

The Expression of PD-L1 and B7-H4 in Thymic Epithelial Tumor and Its Relationship With Tumor Immune-Infiltrating Cells

Background

PD-L1 and B7-H4 have been reported to be expressed in various malignancies and are considered as promising prognostic factors and potential immunotherapy targets.

Methods

We analyzed the correlation between the expression of PD-L1 and B7-H4 transcriptomes and clinicopathological characteristics in 121 TET patients from The Cancer Genome Atlas (TCGA) database. The immune-infiltration levels in the TET microenvironment were estimated using ssGSEA and quanTiseq algorithms. We collected 80 TET cases from 2008 to 2015. PD-L1、B7-H4、FOXP3 and CD163 protein expression in tumor tissues were detected by immunohistochemistry.

Results

TCGA database showed PD-L1 mRNA levels can predict the OS (P = 0.018) and DFS (P = 0.033) of TET patients. B7-H4 mRNA levels were positively related to the World Health Organization (WHO) pathological classification (P = 0.003) but not correlated with patient prognosis. Immune infiltration analysis showed PD-L1 is positively correlated with Tregs and M2 macrophages, B7-H4 is positively correlated with Tregs. Patients with high PD-L1 and Tregs or M2 macrophages, high B7-H4 and Tregs had a worse prognosis. Immunohistochemistry showed PD-L1 expression was positively correlated with the WHO pathological classification and Masaoka stage (P = 0.025, 0.003) and high PD-L1 expression can predict the poor OS of patients (P = 0.043); B7-H4 was also positively correlated with WHO pathological classification and Masaoka stage (P = 0.036, 0.049). However, B7-H4 expression did not correlate with patient prognosis. Evaluation of co-expression patterns showed TET patients with a high-grade WHO pathological classification harbored a 44.4% co-expression of PD-L1 and B7-H4. In addition, we found the expression level of PD-L1 is positively correlated with FOXP3 and CD163 (P = 0.004, P = 0.029) and B7-H4 is positively correlated with FOXP3 (P = 0.037). High PD-L1 combined with High FOXP3 and High CD163, High B7-H4 combined with High FOXP3 can be used to predict the poor prognosis of TET patients (P = 0.026, 0.031, 0.028, respectively).

Conclusion

PD-L1 and B7-H4 were related to the aggressiveness of TET and their expression level can indicate the suppressive immune microenvironment. Combined with FOXP3 and CD163, PD-L1 and B7-H4 can indicate a poor prognosis of TET.

Comparison of PD-L1 immunohistochemical assays and the significance of PD-L1 expression in thymoma

Thymoma is a relatively rare malignancy, which is categorized as thymic epithelial tumor but known as the most common pathology that is developed in the anterior mediastinum. Complete resection is recommended for localized tumors and usually favorable prognosis can be obtained. However, poor survival period has been reported in unresectable cases exhibiting extensive invasion or distant metastasis, as effective chemotherapeutic regimens are restrained. We previously assessed expression of programmed death ligand 1 (PD-L1) and programmed death 1 (PD-1) and discussed their prospective application in the immunotherapy of thymic epithelial tumors. After our publication, additional studies using reliable PD-L1 antibodies, which are currently administered to predict efficacy of PD-1/PD-L1 blockade therapy were performed and further characterized PD-L1 in thymoma. Herein, recent knowledge in relation to the significance of PD-L1 expression in thymoma is reviewed based on recent findings using qualified PD-L1 clones. Most studies coherently found high expression of PD-L1 on the cell membrane and cytoplasm of tumor epithelial cells in accordance with previous reports, which is a predictive marker for effectiveness of anti-PD-1/PD-L1 drugs, even when approved PD-L1 antibodies were employed. On the other hand, PD-L1 expression on tumor infiltrating immune cells remains to be sufficiently determined. High PD-L1 expression can be expected in cases with high grade histological subtypes, such as type B2/B3 thymomas, or those with advanced stages III or IV of the disease. Interestingly, the level of PD-L1 expression was found to be upregulated after chemotherapy compared with that before, which could be explained by immunogenic cell death. The prognostic impact of PD-L1 expression in thymoma might be found only when thymic carcinoma patients were excluded. Furthermore, it also could be identified when we analyzed thymomas completely resected, distinct from biopsy and incompletely resected cases.

Prognostic and clinicopathological roles of programmed death-ligand 1 (PD-L1) expression in thymic epithelial tumors: A meta-analysis

Background

Programmed death‐ligand 1 (PD‐L1) is one of the immune checkpoint proteins, and plays an important role in the progression and microenvironment of cancer. PD‐L1 expression has been associated with poor survival in many cancers. Several studies have also shown an association between PD‐L1 expression and the prognosis of patients with thymic epithelial tumors (TETs). In this study, we systematically evaluated the prognostic and clinicopathological roles of PD‐L1 expression in TETs.

Methods

We searched the literature through PubMed, Embase and Cochrane library and chose the eligible studies, and subsequently performed a meta‐analysis to evaluate the prognostic and clinicopathological roles of PD‐L1 expression in TETs.

Results

Six of the 75 articles found in the literature were selected. PD‐L1 expression was significantly related to unfavorable overall survival (hazard ratio 1.52, 95% confidence interval [CI]: 1.01–2.30, P = 0.046) in TETs. PD‐L1 expression was significantly associated with male gender (odds ratio [OR] 1.55, 95% CI: 1.08–2.22, P = 0.017) and higher Masaoka stage (OR 3.93, 95% CI: 2.44–6.32, P < 0.001).

Conclusions

PD‐L1 expression was correlated with unfavorable prognosis in TETs, indicating PD‐L1 expression could help determine the prognosis of TET patients.

Downregulation of CYLD promotes IFN-γ mediated PD-L1 expression in thymic epithelial tumors

OBJECTIVES

Recent genomic studies suggest the biological significance of the　cylindromatosis (CYLD) gene in thymic epithelial tumors (TETs). CYLD is a crucial regulator of immune response, and we previously reported that CYLD mutation is associated with high PD-L1 expression in thymic carcinoma. Therefore, we wanted to explore the role and mechanism of CYLD in regulating PD-L1 expression in TETs.

MATERIALS AND METHODS

The role of CYLD in PD-L1 expression was assessed by knockdown of CYLD in TET cells upon stimulation with interferon gamma (IFN-γ), tumor necrosis factor-α (TNF-α) or polyinosinic-polycytidylic acid (poly I:C). The molecular mechanism was investigated through analysis of downstream molecules in the STAT1/IRF1 pathway. Moreover, the clinical correlation between low CYLD and high PD-L1 expression, and the clinical impact of CYLD expression were evaluated in tissue microarrays of 105 TET cases.

RESULTS

CYLD knockdown significantly enhanced the expression of PD-L1 in presence of IFN-γ stimulation in most TET cell lines. However, this phenomenon was not observed in presence of TNF-α stimulation. CYLD knockdown upregulated IFN-γ mediated activation of the STAT1/IRF1 axis, which in turn induced PD-L1 expression. Interestingly, we found a significant association between low CYLD expression and ≥ 50 % PD-L1 expression (p = 0.001). In addition, the average proportion of tumor cells exhibiting PD-L1 staining was significantly higher in the low CYLD expression group (24.7 %) than in the high CYLD expression group (5.2 %) (p = 0.005). There was no correlation between CYLD expression and the frequency of pre-existing paraneoplastic auto-immune diseases. In advanced stages (III/IV), the low CYLD expressing group had numerically worse survival than the high CYLD group (log-rank p = 0.089).

CONCLUSIONS

Our findings provide insight into the mechanism of regulation of PD-L1 expression by CYLD in TET cells. Tumors with low CYLD expression could be potential targets for PD-1/PD-L1 inhibitors.

PD-L1 expression level in different thymoma stages and thymic carcinoma: a meta-analysis

BACKGROUND

The immune checkpoint ligand, programmed cell death 1 ligand 1 (PD-L1), is expressed in various tumors and associated with response to drugs that target programmed cell death protein 1. Previous studies have estimated the level of PD-L1 expression among different stages of thymoma and thymic carcinoma to evaluate its potential use as a diagnostic factor; however, its varying expression level has been problematic. We conducted this meta-analysis of published literature to evaluate PD-L1 expression in thymomas and thymic carcinomas.

METHODS

We analyzed 12 studies that included 320 patients with type A/AB/B1 thymoma, 225 patients with type B2/B3 thymoma, and 180 patients with thymic carcinoma.

RESULTS

No difference in PD-L1 expression level was found between the B2/B3 vs C groups (odds ratio [OR], 0.67; 95% confidence interval [CI], 0.26, 1.76; p = 0.42). However, the heterogeneity was very high (I² = 78%), and a significant difference was found between groups A/AB/B1 and B2/B3 (OR, 0.22; 95% CI, 0.12, 0.41; p < 0.001), with a relatively low heterogeneity (I² = 55%).

CONCLUSION

PD-L1 positivity might be a useful factor to differentiate type A/AB/B1 thymoma from type B2/B3 and thymic carcinoma. This result might be valuable for potential anti PD-L1 treatment in thymoma and thymic carcinoma.

Immune biomarkers in thymic epithelial tumors: expression patterns, prognostic value and comparison of diagnostic tests for PD-L1

Background

Immunotherapy is currently under investigation in B3 Thymoma (TB3) and Thymic Carcinoma (TC). PD-L1 expression has been evaluated on a limited number of patients with selected antibodies. We aimed to analyze cohort of TB3 and TC with a panel of antibodies to assess the prevalence of PD-L1 expression, its prognostic value and to set up a reproducible test.

Methods

We retrospectively studied 103 patients samples of FFPE histologically confirmed TB3 (n = 53) and TC (n = 50) by expert pathologists within the RYTHMIC national network. We compared PD-L1, PD1, CD8 and PD-L2 expression and performed correlation with tumor types and patients outcomes. Four PD-L1 antibodies were tested, three of them validated as companion tests in lung cancer, one tested on two automates on whole section of tumors. We evaluated the percentage and intensity of both epithelial and immune stained cells.

Results

TB3 epithelial cells had a higher and more diffuse expression of PD-L1 than TC regardless the antibodies tested (p < 0.0001). Three out of four antibodies targeting PD-L1 tested on the DAKO autostainer gave similar staining. Concordance between antibodies was lower for PD-L1 staining on immune cells with no significant difference between TB3 and TC except on E1L3N antibody. PD-L2 antibody stained no tumor epithelial cells. High PD-L1 expression was correlated with a better overall survival for TB3 and was not correlated with tumor staging.

Conclusion

Frequent PD-L1 expression, particularly in TB3, paves the way for immunotherapy in TET (Thymic Epithelial Tumor). Otherwise, we have set up three reproducible LDT (laboratory-developed test) for four PD-L1 antibodies.

PD-L1 Expression and Tumor-Infiltrating Lymphocytes in Thymic Epithelial Neoplasms

Thymic epithelial tumors (TETs) are rare malignant mediastinal tumors that are difficult to diagnose and treat. The programmed death 1 (PD-1) receptor and its ligand (PD-L1) are expressed in various malignant tumors and have emerged as potential immunotherapeutic targets. However, the immunobiology of TETs is poorly understood. We evaluated PD-L1 expression and the presence of tumor-infiltrating lymphocytes (CD8 and CD3 expression) in surgical TET specimens from 39 patients via immunohistochemistry and determined their relation to clinicopathological parameters. Cases with membranous reactivity of the PD-L1 antibody in ≥1% of tumor cells were considered positive. Positive PD-L1 expression was observed in 53.9% of cases. Histologically, PD-L1 expression was positive in 2/6 type A, 2/6 type AB, 3/9 type B1, 4/4 type B2, 5/6 type B3, and 5/8 type C TET cases. Thus, the number of cases with PD-L1 expression and the percent expression of PD-L1 were significantly higher in more aggressive thymomas (type B2 or B3). CD3+ and CD8+ tumor-infiltrating lymphocytes were diffusely and abundantly distributed in all cases. These data suggest that a PD-1/PD-L1 blockade is a promising treatment for TETs, with more beneficial treatment effects for aggressive thymomas such as type B2 or B3.

Clinicopathologic Significance and Immunogenomic Analysis of Programmed Death-Ligand 1 (PD-L1) and Programmed Death 1 (PD-1) Expression in Thymic Epithelial Tumors

Objectives: Thymic epithelial tumors (TETs) are rare malignant tumors that exhibit heterogeneous histology and clinical behavior. As immune check point inhibitors, drugs targeting anti-programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) have shown remarkable results against many cancers; thus, the importance of PD-1/PD-L1 immunohistochemistry as a predictive or prognostic biomarker has grown. However, limited data on PD-L1 and PD-1 expression in TETs have been reported; moreover, these results have been variable. Here, we examined the expression of PD-1/PD-L1 proteins in TETs and analyzed the clinicopathologic significance of this expression.

Patients and Methods: A tissue microarray was constructed using 368 samples of TETs, each in triplicate. Immunohistochemistry for PD-L1 (SP263 assay) and PD-1 in TETs and CD8 in thymic carcinoma (TC) was performed; next, correlations with clinicopathologic characteristics were analyzed. PD-L1^high was designated as ≥50% of tumor proportion score; PD-1^high and CD8^high were defined as ≥5% and 1% of tumoral immune cells, respectively.

Results: The cohort consisted of 308 patients with thymomas and 60 patients with TC. PD-L1 positivity was identified in 90.6% (328/362, ≥1%) of TETs, PD-1 expression of intra-/peritumoral T cells was identified in 53.6% (194/362) of TETs and CD8 positivity was identified in 11% (7/60, ≥1%) of TC. Of the 362 patients, 141 (39.0%) exhibited high PD-L1 expression (PD-L1^high). The PD-L1^high thymoma group was correlated with high Masaoka-Koga stage (p < 0.001), type B3 histology (p < 0.001), and myasthenia gravis (p < 0.001). This group exhibited poor overall survival (OS, p = 0.003, log-rank) and worse disease-free survival (DFS, p = 0.042, log-rank). No survival differences were detected between PD-L1^high and PD-L1^low groups in TC. Additionally, there was no correlation between PD-1 expression and survival in patients with TETs. Multivariate analysis revealed that PD-L1^high expression was an independent poor prognostic factor (p = 0.047, HR 2.087, 95% CI, 1.009–4.318) in thymomas.

Conclusions: To our knowledge, this is the largest study on TETs published in English literature. This study provides useful information regarding the prognosis of and potential therapeutic options for patients with TETs.

Immune checkpoints in thymic epithelial tumors: challenges and opportunities

Thymic malignancies are rare mediastinal cancers, classified according to the World Health Organization's histopathologic classification which distinguishes thymomas from thymic carcinomas. One key consideration when discussing immunotherapy for thymic epithelial tumors is that one-third of patients diagnosed with thymomas present at the time of diagnosis with autoimmune disorders, the most common being myasthenia gravis. The first step in the understanding of autoimmunity in thymic epithelial tumors is to distinguish true autoimmune disorders from paraneoplastic syndromes; besides pathophysiology, clinical correlates, impact on oncological management and survival may differ strongly. Autoimmune disorders are related to a deregulation in the physiological role of the thymus (i.e. to induce central tolerance to tissue self-antigens) through control of differentiation and subsequent positive and negative selection of immature T cells; from a clinical standpoint, in thymomas, once autoimmune disorders are present, they may not regress significantly after thymectomy. PD-L1 expression, while observed in more than 90% of epithelial cells of the normal thymus with a medullar tropism respecting Hassall's corpuscles, has also been identified in thymomas and thymic carcinomas using various immunohistochemistry protocols. Immune checkpoint inhibitors of the PD-1/PD-L1 axis have been assessed in advanced and metastatic thymic epithelial tumors, mainly thymic carcinomas. Several case reports have been published, and four trials have assessed the efficacy and safety of these inhibitors. Immunotherapy is not standard given the frequent occurrence of severe autoimmune disorders, and clinical trials are ongoing.

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Thymic malignancies are rare tumors.

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Advanced, metastatic and refractory thymic tumors may be treated with chemotherapy.

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Autoimmune disorders, such as myasthenia gravis, are observed in 30% of cases.

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PD-L1 expression is a hallmark of thymic epithelial cells.

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Immunotherapy is not standard, given the occurrence of autoimmune disorders.