Integrin CR3 mediates the binding of nonspecifically opsonized Borrelia burgdorferi to human phagocytes and mammalian cells

Like other pathogens, the spirochete Borrelia burgdorferi, the agent of Lyme disease, possesses multiple pathways for cell binding; adhesion to phagocytic cells is of particular interest since it reportedly occurs even in the absence of specific antibodies. This study sets out to investigate how B. burgdorferi binds to human polymorphonuclear leukocytes (PMNs) when an exogenous complement is added and how the CR3 complement receptor, known as Mac-1 or alpha(m) beta(2) integrin, is involved in the binding process. Experiments performed on PMNs and CHO Mac-1-expressing cells demonstrate that binding is inhibited by monoclonal anti-iC3b site antibodies, fibrinogen, and N-acetyl-D-glucosamine. These findings, which are not present with non-Mac-transfected CHO cells, indicate that the integrin alpha(m) beta(2) acts as a receptor for spirochetes in nonimmune phagocytosis; furthermore, binding occurs on different domains of the CD11b subunit, involving the iC3b site and the lectin domain. The interaction of B. burgdorferi with alpha(m) beta(2) integrin adds a novel pathway to Borrelia-phagocyte binding; not only does this binding affect the early stages of phagocytosis, but also it can influence the effector intracellular mechanisms which are activated by the beta(2) integrin, as are the cytotoxic mechanisms.