CD8A and neoplasm: Upon activation through the engagement of TCR on CD8+ T cells with neoantigens presented by MHC-1 on APCs and target cells, followed by secondary activation signal by CD28 and CD80 co-stimulatory signals, CD8+ T cells can directly kill tumor cells by releasing cytotoxic molecules, such as perforin and granzyme B. Perforin is a cytolytic pore-forming protein which allows entry of granzyme serine protease to activate the caspase cascade and eventually induces apoptosis of target cells.