Comparison of receptors required for entry of Leishmania major amastigotes into macrophages.

We investigated the mechanisms of entry of amastigotes of Leishmania major from two different sources into macrophages by comparing their use of the Fc receptor (FcR), complement receptor type 3 (CR3), and mannose-fucose receptor (MFR). Amastigotes were obtained from BALB/c mice and SCID mice. FcR involvement was examined by opsonizing L. major with parasite-specific immunoglobulin G (IgG). Antiparasite IgG did not alter the uptake of amastigotes from BALB/c mice since these amastigotes had antibody bound to their surface: IgG1 was the most predominant antibody, followed by IgG2b, IgM, and IgG2a. However, opsonization with antiparasite IgG enhanced the entry of amastigotes that lacked antibody on their surface, namely, amastigotes obtained from SCID mice or from macrophages infected in vitro. These results indicate that the FcR is important for amastigote entry into macrophages. Down-modulation of FcRs onto immune complexes, however, did not reduce the entry of amastigotes containing surface-bound IgG into macrophages. Monoclonal antibodies against the CR3 inhibited the entry of amastigotes from either BALB/c or SCID mice into J774A.1 macrophage-like cells. Simultaneous blocking of FcR and CR3 further increased the inhibition of phagocytosis. Treatment of macrophages with soluble mannan or down-modulating the MFR onto mannan-coated coverslips had no effect on the entry of amastigotes from BALB/c or SCID mice. Thus, the MFR does not appear to be used by amastigotes of L. major. We show that ingestion of amastigotes appears to occur primarily through the FcR and CR3; however, additional receptors may also participate in the uptake of amastigotes.