Leishmania major disseminates in genetically susceptible BALB/c mice to cause fatal disease. Progressive infection has been linked to the failure of parasite-specific Th1, IFN-gamma-producing, CD4+ T lymphocytes to expand and direct macrophage activation and control of intracellular parasitism. In contrast, Th2 CD4+ cell expansion accompanies disease progression. Immunomodulation using CD4 cell depletion at the time of infection results in control of infection and Th1 CD4+ cell expansion. A Th1-like cell line, H1A, was established from the draining lymph nodes of an anti-CD4-pretreated BALB/c mouse infected with L. major, H1A was CD4, TCR(+)-alpha/beta, and released IL-2 and IFN-gamma in response to parasite Ag. A Th2-like cell line, U1A, was established from the lymph node cells of an infected BALB/c mouse that was also CD4, TCR(+)-alpha/beta but released IL-4 and IL-5 after stimulation. Mice with severe combined immunodeficiency were reconstituted with H1A and U1A before infection with L. major. Non-reconstituted mice were unable to restrict parasite growth. Mice reconstituted with H1A healed infection, whereas mice reconstituted with U1A suffered exacerbation of disease. Analysis of spleen cells by flow cytometry confirmed the reconstitution of CD4+ cells in both instances, and stimulation with mitogen established that the lymphokine profile of the donor cells had been maintained during 6 to 8 wk of infection. Histologic analysis of the lesions confirmed migration of donated cells to sites of infection. Neutralization of IFN-gamma in H1A-reconstituted mice and IL-4 in U1A-reconstituted mice reversed the disease phenotype mediated by the two cell lines. These data demonstrate the capacity of CD4+ T cells alone to modulate both positively and negatively the course of leishmaniasis in a lymphokine-dependent manner.

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