Interaction of Leishmania gp63 with cellular receptors for fibronectin

The most abundant protein on the surface of the promastigote form of the pr otozoan parasites Leishmania spp, is a 63-kDa molecule, designated gp63 or leishmanolysin, Because gp63 has been shown to possess fibronectin-like pro perties, we examined the interaction of gp63 with the cellular receptors fo r fibronectin, We measured the direct binding of Leishmania to human macrop hages or to transfected mammalian cells expressing human fibronectin recept ors, Leishmania expressing gp63 exhibited modest but reproducible adhesion to human macrophages and to transfected CHO cells expressing alpha 4/beta 1 fibronectin receptors. In both cases, this interaction depended on gp63 bu t occurred independently of the SRYD sequence of gp63, because parasites ex pressing gp63,vith a mutated SRYD sequence bound to macrophages and alpha 4 /beta 1 receptor-expressing cells as well as did wild-type parasites. The c ontribution of gp63 to parasite adhesion was more pronounced when the assay s were performed in the presence of complement, suggesting that the recepto rs for complement and fibronectin may cooperate to mediate the efficient ad hesion of parasites to macrophages, The interaction of gp63 with fibronecti n receptors may also play an important role in parasite internalization by macrophages. Erythrocytes to which gp63 was cross-linked were efficiently p hagocytized by macrophages, whereas control erythrocytes opsonized with com plement alone bound to macrophages but remained peripherally attached to th e outside of the cell. Similarly, parasites expressing wild-type gp63 were rapidly and efficiently phagocytized by resting macrophages, whereas parasi tes lacking gp63 were internalized more slowly. This rapid internalization of gp63-expressing parasites was dependent on the beta 1 integrins, because pretreatment of macrophages with monoclonal antibodies to the beta 1 integ rins decreased the internalization of gp63-expressing parasites. These obse rvations indicate that complement receptors are the primary mediators of pa rasite adhesion; however, maximal parasite adhesion and internalization may require the participation of the beta 1 integrins, which recognize fibrone ctin-like molecules such as gp63 on the surface of the parasite.