Interleukin-12: A Proinflammatory Cytokine with Immunoregulatory Functions that Bridge Innate Resistance and Antigen-Specific Adaptive Immunity

Interleukin-12 (IL-12) is a heterodimeric cytokine produced mostly by phagocytic cells in response to bacteria, bacterial products, and intracellular parasites, and to some degree by B lymphocytes. IL-12 induces cytokine production, primarily of IFN-γ, from NK and T cells, acts as a growth factor for activated NK and T cells, enhances the cytotoxic activity of NK cells, and favors cytotoxic T lymphocyte generation. In vivo IL-12 acts primarily at three stages during the innate resistance/adaptive immune response to infection: 1. Early in the infection, IL-12 is produced and induces production from NK and T cells of IFN-γ , which contributes to phagocytic cell activation and inflammation; 2. IL-12 and IL-12- induced IFN-γ favor Thl cell differentiation by priming CD4 + T cells for high IFN-γ production; and 3. IL-12 contributes to optimal IFN-γ production and to proliferation of differentiated Th l cells in response to antigen. The early preference expressed in the immune response depends on the balance between IL-12, which favors Thl responses, and IL-4, which favors Th2 responses. Thus,IL-12 represents a functional bridge between the early nonspecific innate resistance and the subsequent antigen-specific adaptive immunity.