Function and clinical use of interleukin-12.

Interleukin-12 is a heterodimeric cytokine produced by phagocytic cells, professional antigen-presenting cells such as dendritic cells and skin Langerhans cells, and B cells. Interleukin-12 production is induced by bacteria, intracellular pathogens, fungi, viruses, or their products in a T-cell-independent pathway or a T-cell-dependent pathway, the latter mediated through CD40 ligand-CD40 interaction. Interleukin-12 is produced rapidly after infection and acts as a proinflammatory cytokine eliciting production of interferon gamma, by T and natural killer cells, which activates phagocytic cells. The production of interleukin-12 is strictly regulated by positive and negative feedback mechanisms. If interleukin-12 and interleukin-12-induced interferon gamma are present during early T-cell expansion in response to antigen, T-helper type-1 cell generation is favored and generation of T-helper type-2 cells is inhibited. Thus interleukin-12 is also a potent immunoregulatory cytokine that promotes T-helper type-1 differentiation and is instrumental in the T-helper type-1-dependent resistance to infections by bacteria, intracellular parasites, fungi, and certain viruses. By inhibiting T-helper type-2 cell response, interleukin-12 has a suppressive effect on allergic reactions; by promoting T-helper type-1 responses it participates in the immunopathology responsible for several organ-specific autoimmune diseases. Viruses inducing a permanent or transient immunodepression, such as HIV and measles, may act, in part, by suppressing interleukin-12 production. Because of its ability to enhance resistance to several infectious diseases and to act as an adjuvant in vaccination, and because of its powerful antitumor effect in vivo, interleukin-12 is currently in clinical trials in cancer patients and HIV-infected patients, and it is being considered for therapeutic use in other diseases.