EPITOPE CLEAVAGE BY LEISHMANIA ENDOPEPTIDASE(S) LIMITS THE EFFICIENCYOF THE EXOGENOUS PATHWAY OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS I-ASSOCIATED ANTIGEN PRESENTATION

The activation of CD8(+) T cell responses is commonplace during infect ion with a number of nonviral pathogens. Consequently, there has been much interest in the pathways of presentation of such exogenous antige ns for major histocompatibility complex class I-restricted recognition . We had previously shown that Leishmania promastigotes transfected wi th the ovalbumin (OVA) gene could efficiently target OVA to the parasi tophorous vacuole (PV), with subsequent recognition by class II-restri cted T cells. We now report the results of studies aimed at evaluating the PV as a route of entry into the exogenous class I pathway. Bone m arrow-derived macrophages can present soluble OVA (albeit at high conc entrations) to the OVA(257-264)-specific T cell hybridoma 13.13. In co ntrast, infection with OVA-transfected Leishmania promastigotes failed to result in the stimulation of this hybridoma. This appeared unrelat ed to variables such as antigen concentration, parasite survival, and macrophage activation status. These results prompted an analysis of th e effects of promastigotes on class I peptide binding using RMA-S cell s and OVA(257-264) Our data indicate that the major surface protease o f leishmania, gp63, inhibits this interaction by virtue of its endopep tidase activity against the OVA(257-264) peptide. The data suggest tha t this activity, if maintained within the PV, would result in loss of the OVA(25-264) epitope. Although we can therefore draw no conclusions from these studies regarding the efficiency of the PV as a site of en try of antigen into the exogenous class I pathway, we have identified a further means by which parasites may manipulate the immune repertoir e of their host.