Visceral and cutaneous leishmaniasis comparative ultrastructure of host-parasite interactions.

The comparative ultrastructure of host-parasite interactions is described for the first time in patients with visceral (VL) and cutaneous (CL) leishmaniasis. In patients with VL, the parasite invades the bone marrow (BM) macrophages (Mcs) and neutrophils, while in patients with CL, the parasite invades the dermal fibroblasts in addition to Mcs. The skin Mcs seem to have more lethal effects on the parasite than the BM Mcs; this is possibly due to the presence of numerous melanosomes with acid phosphatase activity in the Mcs digestive vacuole. In patients with high level of VL parasitaemia, the parasite may induce the BM reticulocytes to phagocytose both the parasite and mature erythrocytes, i.e. lost recognition. In patients with low level of VLparasitaemia, the parasite may induce the BM Mcs to be haemophagocytic, i.e. temporarily mimick malignant histiocytosis until the course of treatment. In early stages of CL infection, the cellular infiltrate consists of the monocyte-macrophage system, plasma cells, lymphocytes and fibroblasts; while in the late stages, two types of epithelioid cells (ECs) are added to the infiltrate and are involved in the formation of tuberculous granulomas. Type I ECs thought to produce a granuloma factor, while type II ECs possibly precedes healing by fibrosis. However, the severity of host-parasite interactions seems to depend mainly on species of the parasite, the degree of parasitaemia, the type of infected tissue(s), and the variation of host tissue reaction against the parasite from one patient to another.