Dendritic cells and NK cells stimulate bystander T cell activation in response to TLR agonists through secretion of IFN-alpha beta and IFN-gamma.

Recognition of conserved features of infectious agents by innate pathogen receptors plays an important role in initiating the adaptive immune response. We have investigated early changes occurring among T cells after injection of TLR agonists into mice. Widespread, transient phenotypic activation of both naive and memory T cells was observed rapidly after injection of molecules acting through TLR3, -4, -7, and -9, but not TLR2. T cell activation was shown to be mediated by a combination of IFN-alphabeta, secreted by dendritic cells (DCs), and IFN-gamma, secreted by NK cells; notably, IFN-gamma-secreting NK cells expressed CD11c and copurified with DCs. Production of IFN-gamma by NK cells could be stimulated by DCs from TLR agonist-injected mice, and although soluble factors secreted by LPS-stimulated DCs were sufficient to induce IFN-gamma, maximal IFN-gamma production required both direct contact of NK cells with DCs and DC-secreted cytokines. In vitro, IFN-alphabeta, IL-18, and IL-12 all contributed to DC stimulation of NK cell IFN-gamma, whereas IFN-alphabeta was shown to be important for induction of T cell bystander activation and NK cell IFN-gamma production in vivo. The results delineate a pathway involving innate immune mediators through which TLR agonists trigger bystander activation of T cells.