ENDOCYTOSIS OF BETA-2 INTEGRINS BY STIMULATED HUMAN NEUTROPHILS ANALYZED BY FLOW-CYTOMETRY

Flow cytometry and fluorescently labeled monoclonal antibodies were used to investigate endocytosis of human neutrophil beta2 integrins following cellular activation. CD18 initially present on the cell surface cycled in two phases after exposure to formyl peptide or platelet-activating factor. The first phase lasted 3 min at 37-degrees-C; after a lag, CD18 was specifically internalized at approximately 20%/min. Subsequently a second phase was detectable consisting of exponential reduction of internal fluorescence with a half-time of approximately 2 min, representing probe reexpression. At peak endocytosis approximately 40% of CD18 was internalized. All of the internalized CD18 was associated with alpha(M) (CR3), no endocytosis Of alpha(L) (LFA-1) was observed. When neutrophils were stimulated with phorbol esters or calcium ionophore, CD18 was internalized much more slowly (t1/2 = 5 min) and probe was not reexpressed. Endocytosis of CD18 may participate in regulating neutrophil adhesiveness, removing activated receptors, or permitting receptor recycling.