Mechanism of varlilumab antitumor activity. Interaction of CD27 and CD70 (A) and varlilumab and the Ag-specific TCR (B) in the immune activation of effector T cells. (C) Interaction of CD27-expressing tumor cells with varlilumab on natural killer (NK) cells for a cytolytic response. APC, antigen-presenting cell; MHC, major histocompatibility complex.