Association of the putative B-lymphocyte surface molecule B7.3 with a protein kinase activity.

Monoclonal antibody BB1 was previously shown to recognize the CD80 protein as well as a putative B7.3 molecule, both ligands of the important T-cell receptors CD28 and CTLA-4. We report that BB1 coprecipitated from detergent lysates of human B- and pre-B-cell lines a protein kinase activity, as detected by solid phase immunoprecipitation followed by in vitro kinase assay. As other monoclonal antibodies to CD80 did not co-precipitate any kinase activity, it seems likely that this protein kinase is associated with the so far poorly characterized putative B7.3 molecule.