Endocytosis of beta 2 integrins by stimulated human neutrophils analyzed by flow cytometry.

Flow cytometry and fluorescently labeled mono- clonal antibodies were used to investigate endocytosis of human neutrophil f32 integrins following cellular activa- tion. 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 mm at 37#{176}C; af- ter a lag, CD18 was specifically internalized at approxi- mately 20%/mm. Subsequently a second phase was detec- table consisting of exponential reduction of internal fluorescence with a half-time of approximately 2 mm, representing probe reexpression. At peak endocytosis ap- proximately 40% of CD18 was internalized. All of the in- ternalized CD18 was associated with aM (CR3); no en- docytosis of aL (LFA-1) was observed. When neutrophils were stimulated with phorbol esters or calcium iono- phore, CD18 was internalized much more slowly (ti/2 = 5 mm) and probe was not reexpressed. Endocytosis of CD18 may participate in regulating neutrophil adhesive- ness, removing activated receptors, or permitting recep- tor recycling. J. Leukoc. Rio!. 53: 462-469; 1993.