The leukocyte integrin p150,95 (CD11c/CD18) as a receptor for iC3b: Activation by a heterologous β subunit and localization of a ligand recognition site to the I domain

p150,95 is a member of the leukocyte integrin family of adhesion proteins. Compared with LFA-I and Mac-1 p150,95 is less well functionally characterized. Although p150,95 has complement receptor activity for iC3b and has been designated complement receptor type 4, transfected cells expressing p150,95 do not bind iC3b-sensitized cells. We report that cells cotransfected with a human p150,95 α subunit and a chicken, but not human, β subunit bind IgM-iC3b-coated erythrocytes, suggesting that interactions between the α and β subunits can regulate p150,95 adhesiveness. Furthermore, purified human p150,95 binds to cell-bound iC3b-coated erythrocytes. Because binding to iC3b by cellular and purified p150,95 is specifically abolished by mAbs that localize to the I domain of p150,95, we suggest that the I domain of the p150,95 α subunit is an important ligand recognition site for iC3b.