The down-regulation of FcγRII and FcγRIIIB by N-formyl-methionyl-leucyl-phenylalanine (FMLP) depends on secretory events in human neutrophils

We have previously demonstrated that N-formyl-methionyl-leucyl-phenylalanine (FMLP) induces down-regulation of Fc gamma Rs on human neutrophils (PMN) modifying different Fc gamma R-dependent functions. The aim of this work was to assess the cellular mechanisms by which FMLP exerts this effect on Fc gamma Rs. The role of the microfilament and cytoskeletal apparatus in this process was evaluated using cytochalasin B (CB), an inhibitor of microfilament functions. The expression of Fc gamma RIIIB and Fc gamma RII after CB + FMLP treatment was drastically diminished when compared to FMLP-treated cells. Neutrophil degranulation induced by FMLP affect only 22% of the cells in response to FMLP. However, the Fc gamma Rs of the whole PMN population were reduced, suggesting that secretory products could be responsible for the down-regulation induced by FMLP or FMLP + CB. In fact, supernatants from FMLP-treated PMN also induced Fc gamma Rs down-regulation on naive neutrophils. Moreover, supernatants from FMLP + CB-treated PMNs exerted a higher effect. Data obtained from permeabilized PMN show that after FMLP treatment there is an intracellular depletion of both Fc gamma RIIIB and Fc gamma RII. In addition, the Fc gamma R down-regulation is abrogated by phenyl methyl sulfonyl fluoride (PMSF) but not by other protease inhibitors such as pepstatin, thiorphan, phosphoramidon and leupeptin, suggesting a role for serine protease(s) in this process. (C) 1999 Elsevier Science B.V. All rights reserved.