Mechanisms of immunity to leishmaniasis. II. Significance of the intramacrophage localization of the parasite.

The effect of initiating leishmanial infection in guinea-pigs with organisms contained within macrophages has been examined. Infection of animals in this way resulted in the development of metastatic disease with inocula 2 logs lower than required when free parasites were injected. The macrophage localization was found to protect the parasite from innate resistance, and, at certain times, from mechanisms of acquired immunity. Despite this, initiation of infection with parasites secluded in macrophages did result in the development of specific cell-mediated and humoral immunity. The results indicate that protection of the parasite by the macrophage contributes to the development of metastatic disease. Furthermore, it was revealed that metastatic disease can devlop in the face of acquired mechanisms of resistance. The possibility that non-healing diffuse leishmaniasis is the cause rather than the result of the suppressed immunological reactivity associated with this disease is discussed.