Metacyclogenesis of Leishmania (Viannia) braziliensis in vitro: evidence that lentil lectin is a marker of complement resistance and enhanced infectivity.

Axenic culture in modified Grace's medium was used to induce metacyclogenesis of Leishmania (Viannia) braziliensis in vitro. Morphological characteristics, lectin agglutination profiles, susceptibility to complement lysis, and infectivity in vivo were compared between metacyclic promastigotes and promastigotes in mid-log phase growth. Short, arrow-like promastigotes and round, oval promastigotes were defined as putative metacyclic forms on the basis of being highly motile and free swimming, with a small cell body and long flagellum. These forms increased during metacyclogenesis to > 80% whereas long-bodied, slender promastigotes and intermediate slender promastigotes declined progressively. Lentil lectin selectively agglutinated L. braziliensis after the induction of metacyclogenesis, whereas concanavalin A, wheat germ agglutinin and peanut agglutinin similarly agglutinated metacyclic promastigotes and mid-log phase promastigotes. Metacyclic promastigotes survived in 7.5%-20% human serum whereas mid-log phase promastigotes did not. Five hundred metacyclic promastigotes were highly infective to hamsters whereas 500 mid-log phase promastigotes rarely caused any lesion. Specific agglutination by lentil lectin should allow purification of metacyclic organisms for standardization of immunoprotection and challenge experiments.