Identification of an early activation antigen (Bac-1) on human B cells

We have produced a monoclonal antibody, Bac-1, that appears to identify a novel antigen on activated human B cells. The Bac-1 antigen can be detected between 8 to 16 hr, as well as transferrin receptors (T9), after activation of small resting B cells with phorbol myristic acetate, anti-IgM antibody, Staphylococcus aureus Cowan I, or Epstein-Barr virus. The expression of the Bac-1 antigen precedes that of IL 2 receptors (Tac-1). Peak expression of the Bac-1 antigen was observed on day 3 after activation, and decreased thereafter. The Bac-1 antigen was present on a minor subpopulation of relatively large B cells isolated from blood samples, and on "preactivated" B cells of heterogeneous size isolated from spleens and tonsils. It was not detected on bone marrow pre-B cells, blood small B cells, or plasma cells, nor was it expressed by resting or activated T cells or nonlymphoid cells. Certain B cell neoplasms and B lymphoblastoid cell lines were Bac-1+, but neoplastic cells of non-B lineage were Bac-1-. With immunoperoxidase staining, Bac-1+ cells were detected predominantly in the germinal centers of tonsil sections. The Bac-1 antigen on activated B cells was destroyed by protease treatment and was enhanced by neuraminidase treatment, suggesting that the Bac-1 antibody detects a cell surface molecule via an antigenic determinant which is partially obscured by neighboring sialic acid residues. The reactivity pattern of Bac-1 differs from the patterns of cellular reactivity reported for other monoclonal antibodies with specificity for activated human B cells.