Cutting Edge: Novel Priming of Tumor-Specific Immunity by NKG2D-Triggered NK Cell-Mediated Tumor Rejection and Th1-Independent CD4+ T Cell Pathway

NKG2D is an activation receptor on NK cells and has been demonstrated as a primary cytotoxicity receptor for mouse NK cells. Primary rejection of class I-deficient RMA-S lymphoma cells expressing the NKG2D ligand, retinoic acid early inducible-1beta, was critically dependent upon NK cell perforin and occurred independently of T cells. NKG2D-triggered NK cell rejection of RMA-S-retinoic acid early inducible-1beta tumor primed a secondary tumor-specific T cell response mediated by both CD4+ and CD8+ T cells in the effector phase. Surprisingly, during the priming phase, CD4+ T cells, but not CD8+ T cells, were also required to generate this secondary T cell immunity; however, T cell priming was independent of Th1 cytokines, such as IFN-gamma and IL-12. These data imply a novel pathway for priming T cell immunity, that is, stimulated upon NK cell-mediated cytotoxicity of NKG2D ligand-expressing tumor cells, dependent upon CD4+ T cells in the primary phase, and independent of conventional Th1-type immunity.