Binding of the 68-kilodalton protein of Mycobacterium avium to α(v)β₃ on human monocyte-derived macrophages enhances complement receptor type 3 expression

Attachment to and uptake by host cells are important early events in the pathogenesis of intracellular organisms such as Mycobacterium avium. Monocyte-derived macrophages (MDM) are known to express multiple surface receptors that play a role in binding to and uptake of M. avium. These include complement receptor type 3 (CR3), fibronectin receptor, mannose receptor, and transferrin receptor. In addition to these, we have previously reported that the integrin receptor α(v)β₃ also plays a role in binding to M. avium in a nonopsonic environment. Further, we have shown that a 68-kDa surface protein of M. avium binds to human monocytes and plays a role in attachment of M. avium to MDM. The present study provides direct evidence that this protein mediates attachment of M. avium to MDM by binding to α(v)β₃. Using the technique of cell surface enzyme-linked immunosorbent assay, we have shown that the M. avium 68-kDa protein inhibits the binding of monoclonal antibodies (MAb) against α(v)β₃ to MDM compared to control proteins such as ovalbumin and laminin (P < 0.05). Dual-labeling studies were performed to demonstrate that after phagocytosis, α(v)β₃ is present along with M. avium in phagosomes of M. avium-infected MDM. In addition, we have demonstrated that this interaction between α(v)β₃ and the M. avium 68-kDa protein resulted in enhancement of CR3 expression, which is known to play a role in complement-mediated uptake of M. avium. Attachment of MDM to wells coated with the M. avium 68-kDa protein resulted in a twofold increase in CR3 expression compared to attachment of MDM to wells coated with ovalbumin. This enhancement was completely inhibited by pretreatment of MDM with MAb against α(v)β₃. In summary, M. avium binds to MDM via α(v)β₃ with the help of the M. avium 68-kDa protein, and this ligation enhanced the expression of CR3 on MDM. Since CR3 has been known to play a role in M. avium uptake, enhanced expression of this receptor mediated by M. avium-α(v)β₃ interaction indicates a complex mechanism of communication among different receptors that participate in M. avium attachment and uptake. These findings add to current understanding of the roles played by multiple receptor-ligand systems in uptake and pathogenesis of intracellular pathogens such as M. avium.