Two proteolytic pathways for down-regulation of the barrier molecule CD43 of human neutrophils

CD43, an anionic rod-like mucin molecule on white blood cells, is thought to provide a barrier that prevents interactions of other surface molecules and acts as negative regulator of cell function. As a correlate, CD43 is expected to be altered or down-regulated when blood cells are functionally activated. This study examines CD43 of blood neutrophils before and after treatment with known activating agents. Flow cytometry indicated that PMA and A23187, and to a much lesser extent, FMLP and IL-8, decrease neutrophil expression of CD43. Two separate mechanisms were identified for CD43 down-regulation. Both are proteolytic processes. PMA-induced down-regulation is a rapid process involving proteolysis at a minimum of two sites, one within the N-terminal distal region recognized by mAbs and the other at a membrane-proximal site. The PMA-induced protease, cd43' ase, is characterized by insensitivity to DFP, TLCK, leupeptin, pepstatin, and 1,10 phenanthroline (< 5 mM). PMA-induced CD43 down-regulation is extensive but never complete, terminating at approximately 10 min after down-regulating 65 to 85% of molecules, and thereby converting neutrophils from dense to sparse CD43 expression. The second CD43 down-regulation mechanism, although likely a regulated event in vivo, occurred slowly in this study in neutrophils incubated without additives; the process is not affected by PMA, involves the action of a DFP-sensitive protease, releases N-terminal mAb-reactive fragments of 52 kDa or 40 kDa and can be mimicked by exogenous neutrophil elastase. The complexity and apparent tight regulation described here for the two down-regulatory mechanisms are consistent with an important role for CD43 in preventing or dampening cell surface interactions of blood neutrophils.