We report that a subpopulation (10%) of the Mac-1 (CD1 1b/CD18) molecules on activated neutrophils mediates adhesion to ICAM-1 and fibrinogen. We describe a novel mAb (CBRM1/5) that binds to an activation-specific neoepitope on a subset of Mac-1 molecules on neutrophils and monocytes after stimulation with chemoattractants or phorobol esters but does not recognize Mac-1 on resting myeloid cells. CBRM1/5 immunoprecipitates a subpopulation of Mac-1 molecules from detergent lysates of neutrophils, binds to immunoaffinity-purified Mac-1, and localizes to the I domain on the alpha chain of Mac-1. Because CBRM1/5 recognizes a fraction of Mac-1 on activated neutrophils, but still blocks Mac-1-dependent adhesion to fibrinogen and ICAM-1, we suggest that only a small subset of Mac-1 molecules is competent to mediate adhesion.

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