Colocalization of the IL-12 Receptor and FcgammaRIIIa to Natural Killer Cell Lipid Rafts Leads to Activation of ERK and Enhanced Production of Interferon-gamma

Natural killer (NK) cells express an activating receptor for the Fc portion of IgG (FcgammaRIIIa) that mediates interferon (IFN)-gamma production in response to antibody (Ab)-coated targets. We have previously demonstrated that NK cells activated with interleukin-12 (IL-12) in the presence of immobilized IgG secrete 10-fold or more higher levels of IFN-gamma as compared with stimulation with either agent alone. We examined the intracellular signaling pathways responsible for this synergistic IFN-gamma production. NK cells costimulated via the FcR and the IL-12 receptor (IL-12R) exhibited enhanced levels of activated STAT4 and Syk as compared with NK cells stimulated through either receptor alone. Extracellular signal-regulated kinase (ERK) was also synergistically activated under these conditions. Studies with specific chemical inhibitors revealed that the activation of ERK was dependent on the activation of PI3-K, whose activation was dependent on Syk, and that sequential activation of these molecules was required for NK cell IFN-gamma production in response to FcR and IL-12 stimulation. Retroviral transfection of ERK1 into primary human NK cells substantially increased IFN-gamma production in response to immobilized IgG and IL-12, while transfection of human NK cells with a dominant-negative ERK1 abrogated IFN-gamma production. Confocal microscopy and cellular fractionation experiments revealed that FcgammaRIIIa and the IL-12R colocalized to areas of lipid raft microdomains in response to costimulation with IgG and IL-12. Chemical disruption of lipid rafts inhibited ERK signaling in response to costimulation and significantly inhibited IFN-gamma production. These data suggest that dual recruitment of FcgammaRIIIa and the IL-12R to lipid raft microdomains allows for enhanced activation of downstream signaling events that lead to IFN-gamma production.