LEISHMANIA-AMAZONENSIS PROMASTIGOTES EVADE COMPLEMENT KILLING BY INTERFERING WITH THE LATE STEPS OF THE CASCADE

During their growth in vitro, promastigotes of Leishmania amazonensis undergo differentiation from complement-susceptible to complement-resi stant forms. Here, we demonstrate that both forms bind comparable amou nts of C3 on their surfaces, with the predominant molecule species bei ng the haemolytically active C3b. Likewise, equivalent amounts of C9 a re deposited on both forms of promastigotes. However, while C9-bearing complexes are exposed on the cell surface of resistant promastigotes, they are cryptic in the susceptible stage of the parasites. The membr ane fraction of complement-resistant promastigote lysates has the abil ity to inhibit complement-mediated haemolysis, blocking C9, but not C3 deposition to complement-activating complexes. Moreover, the membrane fraction of complement-resistant promastigote lysates can inhibit the late steps of guinea-pig erythrocyte lysis much more efficiently than complement-susceptible ones. Our results indicate that L. amazonensis promastigotes evade complement killing by inhibiting the cytolytic pa thway of the complement cascade.