Leishmania tropica: suggestive evidences for the effect of infectious dose on pathogenicity and immunogenicity in an experimental model.

Leishmania (L.) tropica is a causative agent of cutaneous and occasionally visceral or viscerotropic leishmaniasis in humans. The dose of parasites influences the course and outcome of disease in some Leishmania species. The effect of parasite dose on L. tropica infection in an experimental model was studied in the current paper. High and low doses of L. tropica were used for ear infection of BALB/c mice and lesion development, parasite load, and cytokine responses were assessed. L. major infection was used for comparison. Pre-infected mice were challenged in the footpad by a fixed high dose of L. tropica, and immune response and protection level were evaluated. High dose L. tropica infection in comparison to low dose results in higher lesion diameters, higher load of parasite in draining lymph node, higher levels of interferon-γ and interleukin-10, dissemination of parasite to spleen, and induction of protection against further L. tropica challenge. Comparison of L. tropica with L. major showed that L. tropica results in lower lesion diameters, more potential for growth in lymph nodes at early phases of infection, parasite dissemination to spleen, lower levels of IL-10, and a permanent lower cytokine response against low parasite dose in comparison to high dose. Our findings suggest that for L. tropica infection, only the high dose results in visceralization of the parasite and protection against further challenge of L. tropica. Therefore, the parasite dose may be an important factor in pathogenesis and immunity in L. tropica infection.