Cure of Progressive Murine Leishmaniasis: Interleukin 4 Dominance Is Abolished by Transient CD4⁺ T Cell Depletion and T Helper Cell Type 1–selective Cytokine Therapy

Progressive infection with Leishmania major in susceptible BALB/c mice is mediated by interleukin (IL)-4–producing T helper cell type 2 (Th2) CD4⁺ T cells that, once established, become resistant to Th1-deviating therapies with recombinant (r)IL-12 and/or neutralizing anti–IL-4 antibodies. We sought to restore protective immunity in advanced leishmaniasis by depletion of Th2-biased CD4⁺ populations and by cytokine-directed reconstitution of Th1 cellular responses during lymphocyte recovery. Treatment with cytolytic GK1.5 anti-CD4 mAb alone did not reverse disease in 3 wk–infected BALB/c mice, but GK1.5 combined with anti–IL-4 antibody and intralesional rIL-12 cured cutaneous lesions in 80% of mice and established a Th1-polarized cytokine response to L. major antigen protective against reinfection. The curative effects of GK1.5 were not replaced by cytotoxic anti-CD8 monoclonal antibody 2.43 or nondepleting anti-CD4 mAb YTS177, confirming that depletion of CD4⁺ cells was specific and essential for therapeutic effect. Finally, combined CD4⁺ depletion and IL-4 neutralization were curative, indicating that neither increased parasite burden nor altered accessory cell function independently biased towards Th2 reconstitution in advanced leishmaniasis. Advanced leishmaniasis can be cured by T cell depletion and cytokine-directed recovery of Th1 cellular responses, suggesting novel interventions for other immune-mediated diseases and identifying distinct roles for CD4⁺ T cell and non-T cell in the maintenance of Th2 and Th1 phenotypes.