CR1, the C3b receptor, mediates binding of infective Leishmania major metacyclic promastigotes to human macrophages.

The role of complement receptors on monocyte derived human macrophages in phagocytosis of infective (MP) and noninfective (LP) developmental stages of Leishmania major promastigotes was studied. We compared binding of these specific developmental stages to MO after preincubation in fresh or heat-inactivated serum. Although LP do not require fresh serum for attachment, MP were dependent on serum C opsonization for entry. Inhibition of CR1 substantially abolished binding of the infective MP. In contrast, inhibition of iC3bR (CR3 and p150,95) had no effect on MP binding. Inhibition of both iC3bR, however, did block binding of nonopsonized LP. Attachment of LP to CR3 was blocked by fluid phase addition of mAb OKM1 and M1/70, which inhibit complement-independent binding to CR3, but not by mAb OKM10 which inhibits iC3b binding to this receptor. After fresh serum pretreatment of LP, however, only simultaneous inhibition of CR3 and CR1 effectively blocked their attachment. Addition of mannan did not inhibit attachment of either promastigote stage. Both opsonized and nonopsonized LP trigger a respiratory burst in MO, possibly via the C independent site in CR3, whereas the CR1-mediated uptake of MP does not generate a respiratory burst. The use of this receptor by MP may facilitate their subsequent intracellular survival.