Activation-dependent proteolytic degradation of polymorphonuclear CD11b

CD11b/CD18 is the principal integrin of polymorphonuclear (PMN) leucocytes and is involved in their adhesion, migration and phagocytosis. In quiescent cells, the receptor is stored in intracellular granules from where it is t ranslocated to the cell surface in response to a variety of stimuli. In thi s study, we demonstrated that strong stimulation of PMNs not only leads to the upregulation of CD11b surface expression, but also to the subsequent ti me-dependent apparent loss of this receptor, as detected by fluorescence-ac tivated cell sorting (FACS) using a monoclonal antibody (mAb) against an N- terminal CD11b epitope. This epitope loss was observed following either dir ect stimulation of protein kinase C (PKC) with phorbol 12-myristate 13-acet ate (PMA) or after multiple receptor stimulation using a combination of the agonist N-formylmethionyl-leucyl-phenylalanine (FMLP) and the priming agen ts granulocyte macrophage-colony stimulating factor (GM-CSF) and platelet f actor (PF) 4. However, upregulation following weak stimulation with FMLP al one was not followed by subsequent epitope loss of the receptor. The increa ses and subsequent decreases in CD11b expression induced by PMA were parall eled by an increase and a decrease in PMN adhesion to CD11b-specific ligand s, fibrinogen and intercellular adhesion molecule (ICAM)-1. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot ana lysis showed that this epitope loss of PMN CD11b was the result of proteoly tic degradation of the N-terminal region of the molecule. The use of a rang e of proteinase inhibitors indicated that this CD11b degradation involves a cell-associated serine proteinase. This is the first demonstration of the proteolytic alteration of CD11b in response to strong PMN stimulation. Give n the central role of CD11b/CD18 in all aspects of PMN function, this alter ation of the CD11b molecule and its effect on PMN adhesion are probably of considerable pathophysiological importance.