Extracellular calcium results in a conformational change in Mac-1 (CD11b/CD18) on neutrophils. Differentiation of adhesion and phagocytosis functions of Mac-1.

Mac-1 is a leukocyte integrin involved in multiple adhesion phenomena and also in the phagocytosis of particles that are bound via CR1 and the IgG FcR. We examined the divalent cation requirements for the Mac-1-dependent processes of adhesion and receptor-mediated phagocytosis. With phorbol ester stimulation, both processes occurred in the absence of extracellular Ca+2. In Ca+2-containing buffer, IB4, an anti-beta 2 mAb, inhibited both adhesion and CR1-mediated ingestion. In the absence of Ca+2, IB4 no longer had any effect on ingestion, although it continued to inhibit adhesion to protein-coated plastic completely. This demonstrates that the role of Mac-1 in adhesion is distinct from its role in phagocytosis. In addition, 1) IB4 did not change intracellular Ca+2 homeostasis; 2) as little as 300 nM free extracellular Ca+2 could restore the ability of IB4 to inhibit ingestion; and 3) in the absence of extracellular Ca+2, Mac-1 was more susceptible to enzymatic cleavage. Together these data suggest a Ca+2-dependent conformational change in Mac-1, which allows distinction of the roles of Mac-1 in phagocytosis and adhesion.