In-vitro interaction of human macrophages with Pneumocystis carinii.

Pneumocystis carinii is an important opportunistic pathogen in patients with compromised cell-mediated immunity. T-cell and macrophage function are believed to be of prime importance in defence against this organism. The present ultrastructural study is aimed at the analysis of the interaction between human macrophages and P. carinii in vitro. Adherent peripheral blood mononuclear cells from healthy volunteers were exposed in vitro to Pneumocystis derived from lungs of steroid-treated rats. The macrophages were harvested at different intervals and studied by transmission and scanning electron microscopy. The material used for inoculation of macrophages was of identical morphology to previously described P. carinii. When mixed with Pneumocystis in vitro, the macrophages appeared to move towards the organism, extended pseudopods and ingested trophozoites and cysts. Within 24 h, intracellular Pneumocystis underwent progressive degeneration inside macrophage vacuoles. This study highlights the possible role of macrophages in host defence against P. carinii.