INTERLEUKIN-12 IS EFFECTIVE TREATMENT FOR AN ESTABLISHED SYSTEMIC INTRACELLULAR INFECTION - EXPERIMENTAL VISCERAL LEISHMANIASIS

When administered at or near the initiation of experimental intracellular infection caused by Leishmania major, Toxcoplasma gondii, or Cryptococcus neoformans, treatment with the immunoregulatory cytokine interleukin 12 (IL-12), induces protective antimicrobial activity. In contrast, once infections are established, IL-12 exerts considerably less or no effect in the face of a suppressive Th2 cell-associated response (L. major) or rapidly progressive fatal infection (T. gondii), To test the efficacy of IL-12 in an established intracellular protozoal infection but under quite different immunologic conditions (Th1 cell response, acquired resistance), L. donovani-infected BALB/c mice were treated starting 2 wk after challenge coincident with the onset of the Th1 cell response. In this environment, 7 d of IL-12 treatment reduced liver parasite burdens by 47%, an effect comparable to that induced by exogenous interferon (IFN) gamma. The in vivo mechanism responsive to IL-12 was complex, and required both CD4(+) and CD8(+) T cells as well as natural killer cells and the action of multiple endogenous antileishmanial cytokines (IFN-gamma, IL-2, tumor necrosis factor alpha). Early treatment with IL-12 before the expression of the Th1 cell response was also effective and induced an accelerated, near-cure response via an IFN-gamma-dependent mechanism. These results extend the antimicrobial-inducing capacity of IL-12 beyond prophylaxis by indicating that IL-12 can exert clear-cut therapeutic activity in an established intracellular infection.