Immune aberrations in thymic epithelial tumors

Thymic epithelial tumors (TETs) are rare cancers that arise from cortical or medullary thymic epithelial cells and are associated with unique clinical, genomic and immunological features that include a poorly understood predisposition towards autoimmunity. The risk for autoimmune disease can be traced back to the physiological role of the thymus in T cell development. Recent studies have revealed key differences in the immune cell composition and tumor immune microenvironment of thymomas and thymic carcinomas (TCs). Thymomas contain a greater proportion of immature double positive T cells whereas TCs have larger numbers of differentiated single positive T cells. The proportion of B cells, dendritic cells, macrophages and regulatory T cells vary depending on TET histology. Thymomas exhibit higher immune cell infiltration whereas TCs are characterized by a pronounced stromal signature. Histology-based differences in T cell receptor diversity and in the cytokine profile of the tumor microenvironment have also been described. Programmed cell death-ligand 1 expression is variable in TETs and confounded by physiological expression in the normal thymus. Knowledge of the immune cell composition and the tumor immune microenvironment of TETs is essential to understand the risks of paraneoplastic autoimmunity and treatment-related toxicity in the era of immunotherapy. This review synthesizes recent advances in thymic biology and tumor immunology to frame the immune landscape of TETs.