The Involvement of Complement Receptor Type 3 (CR3;CDll b/CD18) in Neutrophil Adhesion and Opsonin-Independent Phagocytosis
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𝛽2 integrins, such as complement receptor type 3 (CR3;CDll b/CDI8), relay proinflammatory signals from GPI-linked proteins through exodomains to the cytoskeleton. Recent studies suggested that CR3 interacts physically and functionally with glycosylphosphatidylinositol (GPI)-linked membrane proteins. Our studies tested the hypothesis that CR3 can physically associate with CD 14, a GPI-linked membrane protein, under certain conditions. The CD 14 molecule is a membrane binding site for lipopolysaccharide (LPS), an endotoxin. Although CD14 lacks transmembrane and cytoplasmic sequences, it activates CR3-mediated neutrophil adhesion and cytokine release. Using resonance energy transfer microscopy, we show that LPS in the presence of serum or LPS binding protein (LBP) triggers formation of CD 14-CR3 complexes. The kinetic studies in this report show that CD 14-CR3 complexes dissociate as neutrophils attach to substrates. We speculate that LPS-charged CD14 enhances CR3-mediated adhesion by directly binding to CR3. Additionally, we have tested the role of lectin-like interactions, with particluar emphasis on CR3, in phagocytosis of non-opsonized E. coli by neutrophils. A recent study revealed specific saccharides which inhibit lectin-like sites on CR3. In addition to this, another study, focusing on GPI-linked membranes, recognized CD 14 to be involved in the uptake of nonopsonized gram-negative bacteria. With reference to these two studies, we speculated that an interaction may take place between a GPI-linked receptor and CR3, in which a lectin-like interaction may be necessary for signaling transmission to be successful. We show the saccharides N-acetyl-D- glucosamine (NADG) and 𝛽-Glucan have the ability to affect phagocytosis, dramatically reducing internalization of non-opsonized gram-negative bacteria. Data presented in this report suggests that NADG and 𝛽-Glucan most likely bind to the lectin-like site on CR3 and block the activation signal for phagocytosis, which could possibly be initiated by a GPI-anchored protein.