PDCD1 and neoplasm: In addition, low expression of MHC-I, which leads to a decrease of cytotoxic T-lymphocytes (TILs) infiltrating the SCLC tumor, and excess of regulatory T cells (Tregs), which can inhibit activation, expansion and effector functions of other T cells [62], may contribute to low response to immune checkpoint inhibitors (ICIs) (approximately 10% with anti-PD-1 monotherapy) [63, 64].