Increased surface expression of CD11b/CD18 (Mac-1) is not required for stimulated neutrophil adherence to cultured endothelium.

The mechanism whereby the human neutrophil membrane heterodimer, CD11b/CD18 (Mac-1, Mo1), mediates neutrophil adherence is not known. We studied the role of CD11b/CD18 surface expression in the promotion of neutrophil adhesiveness. We found that phorbol myristate acetate (PMA), calcium ionophore (A23187), and FMLP caused a three- to sevenfold increase in surface expression of both CD11b (alpha M) and CD18 (beta) as assayed by binding of MAbs 60.1 (anti-CD11b) and 60.3 (anti-CD18). Increased binding of MAbs was temporally associated with the promotion of neutrophil aggregation and adherence to cultured endothelial monolayers. Pretreatment of neutrophils with the anion channel-blocking agent, DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid), inhibited the increased surface expression of CD11b and CD18 after stimulation by PMA, A23187, or FMLP and resulted in nearly complete inhibition of neutrophil aggregation. However, pretreatment with DIDS did not diminish either PMA-, A23187-, or FMLP-stimulated neutrophil adherence to endothelial monolayers. We also observed that stimulation of granule-depleted neutrophil cytoplasts by PMA, A23187, or FMLP induced aggregation and adherence to endothelial monolayers without increasing surface expression of CD11b or CD18. We conclude that the increased surface expression of CD11b/CD18 that occurs after stimulation is neither sufficient nor necessary for enhanced adherence to endothelium. Moreover, though both are CD11b/CD18-dependent, the mechanisms involved in neutrophil aggregation are different from those involved in neutrophil adherence to endothelium.