Abstract
Visceral leishmaniasis (VL) is a fatal disease of the internal organs caused by the eukaryotic parasite Leishmania. Control of VL would best be achieved through vaccination. However, this has proven to be difficult partly because the correlates of protective immunity are not fully understood. In contrast, protective immunity against nonfatal cutaneous leishmaniasis (CL) is well defined and mediated by rapidly recruited, IFN-γ-producing Ly6C(+)CD4(+) T cells at the dermal challenge site. Protection against CL is best achieved by prior infection or live vaccination with Leishmania major, termed leishmanization. A long-standing question is whether prior CL or leishmanization can protect against VL. Employing an intradermal challenge model in mice, we report that cutaneous infection with Leishmania major provides heterologous protection against visceral infection with Leishmania infantum. Protection was associated with a robust CD4(+) T cell response at the dermal challenge site and in the viscera. In vivo labeling of circulating cells revealed that increased frequencies of IFN-γ(+)CD4(+) T cells at sites of infection are due to recruitment or retention of cells in the tissue, rather than increased numbers of cells trapped in the vasculature. Shortly after challenge, IFN-γ-producing cells were highly enriched for Ly6C(+)T-bet(+) cells in the viscera. Surprisingly, this heterologous immunity was superior to homologous immunity mediated by prior infection with L. infantum. Our observations demonstrate a common mechanism of protection against different clinical forms of leishmaniasis. The efficacy of leishmanization against VL may warrant the introduction of the practice in VL endemic areas or during outbreaks of disease.