ANTIGEN PRESENTATION BY LEISHMANIA MEXICANA-INFECTED MACROPHAGES - ACTIVATION OF HELPER T-CELLS BY A MODEL PARASITE ANTIGEN SECRETED INTO THE PARASITOPHOROUS VACUOLE OR EXPRESSED ON THE AMASTIGOTE SURFACE

Leishmania are protozoan parasites which invade mammalian macrophages and multiply as amastigotes in phagolysosomes (parasitophorous vacuole s). Using L. mexicana and bone marrow-derived macrophages (BMM), the q uestion is addressed whether infected BMM induced to express major his tocompatibility complex class II molecules can present defined antigen s to specific T helper type 1 cells. As a model antigen, a membrane-bo und acid phosphatase (MAP), a minor protein associated with intracellu lar vesicles in amastigotes, was either overexpressed at the surface o f the parasites or overexpressed in a soluble form leading to antigen secretion into the parasitophorous vacuole. Presentation of MAP epitop es by these three types of amastigotes was then compared for macrophag es containing live parasites or amastigotes inactivated by drug treatm ent. It is shown that surface-exposed and secreted MAP can be efficien tly presented to T cells by macrophages harboring live amastigotes. Th erefore, the parasitophorous vacuole communicates by vesicular membran e traffic with the plasmalemma of the host cell. The intracellular MAP of wild-type cells or the abundant lysosomal cysteine proteinases are not or only inefficiently presented, respectively. After killing of t he parasites, abundant proteins such as overexpressed MAP and the cyst eine proteinases efficiently stimulate T cells, while wild-type MAP le vels are not effective. We conclude that intracellular proteins of int act amastigotes are not available for presentation, while after parasi te inactivation, presentation depends on antigen abundance and possibl y stability. The cell biological and possible immunological consequenc es of these results are discussed.