Interaction of leishmania metacyclics with macrophages.

Metacyclics of L. major and putative metacyclics of L. m. mexicana survived better in explanted murine macrophages than promastigotes from mid-log phase cultures. The latter forms, however, attached in greater numbers to macrophages and, in the case of L. major, also more became intracellular. Only a small percentage of the macrophages infected with amastigotes exhibited a respiratory burst (as detected by nitroblue tetrazolium (NBT) reduction), whereas this occurred with most of the macrophages infected with either metacyclic or non-infective promastigotes of L. major. Approximately half of the macrophages infected with L. m. mexicana promastigotes reduced NBT. The results suggest that avoidance of the oxygen metabolite arm of the host cell's microbicidal activity is not the main survival strategy for metacyclics entering macrophages. Metacyclics of L. major, however, were found to be less sensitive than mid-log phase promastigotes to hydrogen peroxide and also human serum; properties which may aid their survival.