LOSS OF VIRULENCE IN LEISHMANIA-DONOVANI DEFICIENT IN AN AMASTIGOTE-SPECIFIC PROTEIN, A2

Leishmania donovani is the etiologic agent of fatal visceral leishmani asis in man, During their life cycle, Leishmania exist as flagellated promastigotes within the sandfly vector and as nonflagellated amastigo tes in the macro phage phagolysosomal compartment of the mammalian hos t, The transformation from promastigotes to amastigotes is a critical step for the establishment of infection, and the molecular basis for t his transformation is poorly understood. To define the molecular basis for amastigote survival in the mammalian host, we previously identifi ed an amastigote stage-specific gene family termed ''A2.'' In the pres ent study, we have inhibited the expression of A2 mRNA and A2 protein in amastigotes using antisense RNA and show that the resulting A2-defi cient amastigotes are severely compromised with respect to virulence i n mice. Amastigotes that did survive in the mice had restored A2 prote in expression, These data demonstrate that A2 protein is required for L. donovani survival in a mammalian host, and this represents the firs t identified amastigote-specific virulence factor identified in Leishm ania. This study also reveals that it is possible to study gene functi on in Leishmania through the expression of antisense RNA.