OA13.05 Somatic Genetic Alterations and Immune Microenvironment in Malignant Pleural Mesothelioma

Abstract

The genomic landscape of malignant pleural mesothelioma (MPM) is not well understood. Advanced high-throughput sequencing technologies allow comprehensive characterization of genetic alterations. Knowledge of the somatic mutations and the immune microenvironment in patients with MPM will help to develop effective targeted therapies. We examined biopsy specimens from 12 MPM patients (8 epithelioid and 4 biphasic) that were removed during maximal cyto-reductive surgery. Specimens from 3 different sites (anterior, posterior and diaphragm, a total of 36 tissue samples) were studied through whole exome sequencing, T cell receptor (TCR) repertoire analysis of tumor-infiltrating T cells (TILs), and expression levels of immune-related genes. We also performed in silico prediction of potent neoantigens derived from non-synonymous somatic mutations in each specimen. For the comparison of tumor tissues from 3 different sites, we performed hierarchical clustering to assess the tumor heterogeneity and differences in immune environment. High mutation/neoantigen load was significantly correlated with higher clonal expansion of TILs (R=0.46) and high expression levels of immune-associated cytolytic factors, granzyme A (R=0.25) and perforin 1 (R=0.48), in tumor tissues. In the clustering analysis, heterogeneous MPM cases revealed unique neoantigens and clonotypes of TILs that were restricted to each of tumor site, suggesting infiltration of the neoantigen-specific T cells. Further sub-analysis according to histologic types showed that biphasic tumors had higher mutation/neoantigen load and stronger oligo-clonal T cell expansion (p=0.01) than epithelioid tumors. Our analysis demonstrated a significant correlation between somatic mutation/neoantigen load, clonality of TILs, and the immune-related tumor microenvironment in MPM. Our findings suggest that high mutation/neoantigen load in tumor cells might promote effective expansion and infiltration of functional (tumorocidal) T cells into the tumor bed. These findings provide a rationale for selecting MPM patients who can benefit from treatment with immune checkpoint blockades. This may accelerate development of the neoantigen targeting TCR-engineered T cell therapy for MPM.