CD274 and neoplasm: However, several mechanisms of immune resistance have been identified in SCLC, such as downregulation of major histocompatibility complex (MHC) antigens I and II, low PD-L1 expression, increase in regulatory T-cells, and poor tumor infiltration of effector T-cells, partially due to a lack of vasculature surrounding the tumor [36].These features likely diminish the response in SCLC compared to the observed immunotherapeutic response in NSCLC.