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 intr acellular parasites, and to some degree by B lymphocytes. IL-12 induce s cytokine production, primarily of IFN-gamma from NK and T cells, act s as a growth factor for activated NK and T cells, enhances the cytoto xic activity of NK cells, and favors cytotoxic T lymphocyte generation . In vivo IL-12 acts primarily at three stages during the innate resis tance/adaptive immune response to infection: 1. Early in the infection , IL-12 is produced and induces production from NK and T cells of IFN- gamma, which contributes to phagocytic cell activation and inflammatio n; 2. IL-12 and IL-12-induced IFN-gamma favor Th1 cell differentiation by priming CD4(+) T cells for high IFN-gamma production; and 3. IL-12 contributes to optimal IFN-gamma production and to proliferation of d ifferentiated Th1 cells in response to antigen. The early preference e xpressed in the immune response depends on the balance between IL-12, which favors Th1 responses, and IL-4, which favors Th2 responses. Thus , IL-12 represents a functional bridge between the early nonspecific i nnate resistance and the subsequent antigen-specific adaptive immunity .