INTEGRIN-ASSOCIATED PROTEIN IMMUNOGLOBULIN DOMAIN IS NECESSARY FOR EFFICIENT VITRONECTIN BEAD BINDING

Integrin-associated protein (IAP/CD47) is physically associated with t he alpha(v) beta(3) vitronectin (Vn) receptor and a functionally and i mmunologically related integrin on neutrophils (PMN) and monocytes. An ti-IAP antibodies inhibit multiple phagocyte functions, including Arg- Gly-Asp (RGD)-initiated activation of phagocytosis, chemotaxis, and re spiratory burst; PMN adhesion to entactin; and PMN transendothelial an d transepithelial migration at a step subsequent to tight intercellula r adhesion. Anti-IAP antibodies also inhibit binding of Vn-coated part icles to many cells expressing alpha(v) beta(3). However, prior studie s with anti-IAP did not directly address IAP function because they cou ld not distinguish between IAP blockade and antibody-induced signaling effects on cells, To better determine the function of IAP, we have ch aracterized and used an IAP-deficient human cell line, Despite express ing alpha(v) integrins, these cells do not bind Vn-coated particles un less transfected with IAP expression constructs. Increasing the level of alpha(v) beta(3) expression or increasing Vn density on the particl e does not overcome the requirement for IAP. All known splice variants of IAP restore Vn particle binding equivalently, Indeed, the membrane -anchored IAP Ig variable domain suffices to mediate Vn particle bindi ng in this system, while the multiply membrane-spanning and cytoplasmi c domains are dispensable. In all cases, adhesion to a Vn-coated surfa ce and fibronectin particle binding through alpha(5) beta(1) fibronect in receptors are independent of IAP expression. These data demonstrate that some or, integrin ligand-binding functions are IAP independent, whereas others require IAP, presumably through direct physical interac tion between its Ig domain and the integrin.