Macrophage-parasite interaction in the lesions of cutaneous leishmaniasis. An ultrastructural study

Localized cutaneous infections with Leishmania, which demonstrate complex host-parasite interactions, were studied ultrastructurally in 16 patients at phases ranging from onset to resolution. In the early lesions the host macrophages were 1) heavily parasitized and vesiculated, 2) undifferentiated, or 3) large and active, with fewer organisms. Progressive activation and epithelioid transformation of incoming monocytes was associated with the elimination of parasites. Killing and degradation appeared to take place simultaneously within the phagolysosome, but lysosomal fusion did not prevent survival into the activated cell stage. Host cell lysis, the alternative mechanism of parasite elimination, was accomplished following contact of the macrophage with plasma cells or its engulfment by a large granular cell. Lysis was either sporadic, proceeding from the periphery, or total in a central mass; and in each case macrophage lysis was preceded by connective tissue damage. The externalized parasites appeared to enhance both the activation and lytic processes, but degraded extracellular organisms were associated with dendritic-like cells more than with macrophages. This needs further study.