Tumor mutational burden (TMB) as a biomarker of response to immunotherapy in small cell lung cancer

Tumors avoid immune surveillance by inactivating various T-cell cytotoxic pathways that can promote cell death (1-3). Cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed death 1 (PD-1) are expressed primarily on cytotoxic T lymphocytes. In contrast, programmed death ligand 1 (PD-L1) is expressed on antigen-presenting lymphoid and non-lymphoid tissues, on tumor cells, and on virus-infected cells.