| Abstract: | The β-amyloid protein associated with Alzheimer's disease (AD) has been well characterized biochemically; however, its primary biological function and mode of action in AD has not been determined. We have shown previously that β-amyloid (β25–35), in combination with interferon-γ (IFN-γ), can induce nitric oxide release from cultured hippocampal microglial cells. In the present study, binding of β-amyloid with the leukocyte integrin Mac-1, a cell surface receptor on microglia, was studied by observing (1) inhibition of β-amyloid (β25–35)-mediated release of nitric oxide from cultured microglial cells following exposure to monoclonal antibodies against Mac-1 (anti-CD18 and anti-CD11b) and (2) competitive binding of fluorochrome-labeled β25–35 with anti-CD18 or anti-CD11b using fluorescent flow cytometry. Wt.3 (anti-CD18 antibody) and OX42 (anti-CD11b antibody) were as effective as opsonized zymosan at inducing the release of nitric oxide from microglia. Furthermore, Wt.3 and OX42 acted synergistically to induce maximum nitric oxide release. An interaction between β-amyloid and CD18 of Mac-1 was evidenced by the suppressive action of β25–35 on Wt.3-mediated release of nitric oxide and the synergistic action between OX42 and β25–35 in inducing nitric oxide release from microglia. The tissue culture study was supported by competitive binding assays of fluorochrome-labeled β25–35 and Mac-1 antibodies (Wt.3 or OX42). The majority of microglial cells (71%) did bind biotinylated β-amyloid in the presence of cytochalasin B, suggesting that β-amyloid binding to microglia is a receptor-mediated event. Furthermore, pre-exposure to Wt.3, but not OX42, significantly decreased binding of biotinylated β25–35 to microglia. These findings suggest that CD18 of Mac-1 may play a role in β-amyloid-mediated release of nitric oxide. |
