Changes in macrophage status in vivo during infection with and immunity to Leismania enrietti

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonspecific immunodepression and protective immunity in mice infected with Leishmania mexicana

C57BL/6 mice infected with Leishmania mexicana showed depression of the in vitro immunoglobulin M-plaque-forming cell response to sheep erythrocytes. Immunodepression was present 3 weeks after inoculation and was maximal at 11 weeks. Thereafter, there was a gradual return to normal immunoresponsiveness correlated with the resolution of lesions. At the time of maximal immunodepression, spleen cells from infected mice diminished the plaque-forming cell response to sheep erythrocytes of normal spleen cells. On the other hand, specific responses, as exemplified by protective immunity to a challenge infection and delayed hypersensitivity responses to parasite antigens, were apparently unaffected. These responses were both present in mice bearing primary lesions and were maximal in recovered mice. The significance of these findings is discussed in relation to a current hypothesis on parasite-induced immunodepression.

Growth of Leishmania donovani amastigotes in a continuous macrophage-like cell culture

The feastibility of using a macrophage-like murine tumor cell as a host for Leishmania donovani was investigated. This cell line, designated P388D1, rapidly phagocytized amastigotes and supported their intracellular replication. It can serve as a model "host" without the inherent limitations of primary macrophage cultures.