CD8A and neoplasm: Activation of the naïve, antigen-restricted CD8(+) CTLs first requires binding of the T cell receptor (TCR) to its cognate tumor antigen (TA) in complex with human leukocyte antigen (HLA) I. Although the engagement between a tumor antigen and a T cell receptor (TA-TCR engagement) is necessary, it is not sufficient for CTL activation and tumor cytolysis.