Nonopsonic uptake of Mycobacterium avium complex by human monocytes and alveolar macrophages

The uptake of Mycobacterium avium complex (MAC) microorganisms by human peripheral blood monocytes (PBMs) and alveolar macrophages (AMs) is not well understood. We have previously shown, under opsonic conditions, that humoral factors are important in mediating the uptake of MAC by PBMs. However, the receptor-ligand interactions occurring under nonopsonic conditions remain unclear. We compared the uptake of untreated human PBMs and AMs in a serum-free medium with phagocytes treated to remove surface receptors. Removal of complement receptors CR1 and CR3, the Fc receptor (FcR), and the transferrin receptor (TfR) resulted in significantly lower levels of MAC uptake in serum-free medium by both PBMs and AMs. The addition of barley beta-glucan or mannan from Saccharomyces cerevisiae inhibited MAC uptake by untreated phagocytes in a dose-dependent manner. MAC uptake by PBMs or AMs was never completely abrogated by combining treatments (removal of CR1, CR3, FcR, and TfR and adding mannan or beta-glucan), indicating still-unknown mechanisms of uptake under nonopsonic conditions. We conclude that CR1, CR3, FcR, TfR, the mannose receptor, and possibly a separate beta-glucan-inhibitable receptor all may be involved in nonopsonic uptake of MAC by both PBMs and AMs.