Binding of the 68-kilodalton protein of Mycobacterium avium to alpha(v)beta3 on human monocyte-derived macrophages enhances complement receptor type 3 expression.

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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 alpha(v)beta3 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 alpha(v)beta3. 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 alpha(v)beta3 to MDM compared to control proteins such as ovalbumin and laminin (P < 0.05). Dual-labeling studies were performed to demonstrate that after phagocytosis, alpha(v)beta3 is present along with M. avium in phagosomes of M. avium-infected MDM. In addition, we have demonstrated that this interaction between alpha(v)beta3 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 alpha(v)beta3. In summary, M. avium binds to MDM via alpha(v)beta3 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-alpha(v)beta3 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.

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