Leishmania infection of human skin fibroblasts in vitro: absence of phagolysosomal fusion after induced phagocytosis of promastigotes, and their intracellular transformation.

Cellular interactions between human skin fibroblasts and promastigotes of two leishmanial species were studied in vitro by light and electron microscopy. Fibroblasts were found to become infected by the species with a history of causing mucocutaneous infection, but not by that of the visceral type or Leishmania donovani. Scanning and transmission electron microscopy revealed that promastigotes of the invasive species entered fibroblasts flagellum-end first through pseudopodia-like structures formed on the host cell surface, reminiscent of "induced phagocytosis." Ingested promastigotes became lodged in vacuoles that did not fuse with secondary lysosomes prelabeled with an electron-dense marker for identification. Transformation of promastigotes into amastigotes occurred among those located within host cells and was influenced by the ambient temperature. Intracellular parasite populations gradually decreased during a 3-week period in vitro, although dividing forms were occasionally seen at all incubation temperatures (32--37 degrees C). There was evidence that viable amastigotes were liberated by cytolysis and/or exocytosis of some infected cells. It is postulated that invasion of non-phagocytic cells by promastigotes and their subsequent transformation therein may allow them to escape from the often fatal consequence of direct confrontation with mononuclear phagocytes, and may be a survival mechanism associated with this parasite stage during the early host-parasite interaction in natural infection.