Kinetics of the intracellular differentiation of Leishmania amazonensis and internalization of host MHC molecules by the intermediate parasite stages

The establishment of Leishmania in mammals depends on the transformation of metacyclic promastigotes into amastigotes within macrophages. The kinetics of this process was examined using mouse macrophages infected with metacyc lic promastigotes of L. amazonensis. The appearance of amastigote character istics, including large lysosome-like organelles called megasomes, stage-sp ecific antigens, high cysteine protease activity and sensitivity to L-leuci ne methyl ester, was followed over a 5-day period. Megasomes were observed at 48 h but probable precursors of these organelles were detected at 12 h p .i. The promastigote-specific molecules examined were down-regulated within 5 to 12h after phagocytosis whereas the amastigote-specific antigens studi ed were detectable from 2 to 12-24 h. An increase in the cysteine protease activity and in sensitivity to L-leucine methyl ester of the parasites was detected from 24 h. The data indicate that at 48 h p.i., parasites exhibit several amastigote features but that complete differentiation requires at l east 5 days. The appearance of megasomes or of megasome precursors and the rise in cysteine protease activity correlate quite well with the capacity o f parasites to internalize and very likely degrade host MHC molecules. The fact that internalization by the parasites of host cell molecules occurs ve ry early during the differentiation process argues for a role of this mecha nism ill parasite survival.