Constitutive and induced phosphorylation of the α- and β-chains of the CD11/CD18 leukocyte integrin family: Relationship to adhesion-dependent functions

We sought to determine whether the activation event which renders the CD11/CD18 leukocyte integrin/Leu-CAM glycoproteins capable of promoting cell to cell adhesion was associated with the induced posttranslational modification of phosphorylation. In neutrophils, two species of α-chains, a predominant CD11b 165-kDa subunit and a minor 150-kDa CD11c subunit were found to be constitutively phosphorylated. However, the 95-kDa CD18 common β-chain was not phosphorylated in resting cells but became strongly phosphorylated in cells incubated with PMA. The β-chain was phosphorylated in a dose-related manner within 1 min of the addition of PMA, reached maximal intensity between 4 to 10 min, and remained fully phosphorylated at 30 min. The similarities of the kinetics of homotypic aggregation induced by PMA to the time course of phosphorylation suggest that phosphorylation may be relevant to at least this type of Leu-AM-dependent adhesion. In contrast, in the presence of FMLP which induces aggregation with different kinetics than PMA, no phosphorylation of the common β-chain was observed over a time interval of from 30 s to 10 min further emphasizing the apparent differences in the two modes of activation to an adhesive state. The phosphorylated species on neutrophils were readily detected by immunoprecipitation with each CD18 mAb and most but not all CD11b mAb which otherwise precipitated ¹²⁵I-labeled CD11b species suggesting that the CD11b α-chain labelled with ³²P may differ slightly from the ¹²⁵I-labeled species in terms of its recognition by certain CD11b mAb. In mononuclear cells, similar constitutive phosphorylation of the CD11a and CD11c α-chains was observed that remained unchanged in the presence of either FMLP or PMA. As was demonstrated in neutrophils, phosphorylation of the CD 18 β-chains of mononuclear cells was not constitutive but was induced in the presence of PMA and not FMLP. Taken together these data suggest the existence of specific recognition sites on β-chains for a regulatory kinase-phosphatase system.