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From the * Department of Medicine, Cornell University Medical College, New York 10021;
and Although implicated in the clinical expression of human visceral leishmaniasis, a disease-exacerbating T helper cell 2 (Th2)-associated immune response involving interleukin-4 (IL-4) and/
or IL-10 is not readily detectable in experimental visceral infection. To overcome this obstacle
to analyzing visceral Leishmania donovani in this relevant immunopathogenetic environment,
we sought a model in which a Th2 response induces noncuring infection. Four initial approaches were tested primarily in BALB/c mice which control intracellular L. donovani via an
IL-12- and interferon-
Department of Immunology, DNAX Research Institute of Molecular and Cellular
Biology, Palo Alto, California 94304
(IFN-
)-dependent Th1 mechanism: (a) modifying the cytokine milieu when the parasite is first encountered (treatment with exogenous IL-4 or anti-IL-12), (b)
providing sustained endogenous exposure to a Th2 cytokine (infection of IL-4 transgenic
mice), (c) increasing the parasite challenge inoculum, and (d) injecting heat-killed L. major promastigotes (HKLMP) to induce a cross-reactive Th2 response to live L. donovani. Only the last
approach generated a functional Th2-type response that induced disease exacerbation accompanied by inhibition of tissue granuloma assembly. In HKLMP-primed BALB/c mice, prophylaxis with anti-IL-4, anti-IL-10, or exogenous IL-12 (but not IFN-
) readily restored resistance. In primed mice with established visceral infection, treatment with either IL-12 or IFN-
also successfully induced antileishmanial activity. The results in this model (a) suggest that
rather than acting alone, IL-4 and IL-10 may act in concert to prevent acquisition of resistance
to L. donovani, (b) reemphasize the capacity of IL-12 to reverse early Th2-related effects, and (c)
demonstrate that Th1 cytokines (IL-12, IFN-
) have therapeutic action in an established systemic infection despite the presence of a disease-exacerbating Th2-type response.
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