Involvement of CD44 exon v10 in B-cell activation

: The family of CD44 glycoproteins has been suggested to be involved in lymphocyte homing, maturation and activation. Using in vitro blocking studies with a monoclonal antibody, we here addressed the question of functional activity of CD44 variant exon v10 (CD44v10) in B‐cell activation. We became interested in this question by the observation that CD44v10 was transiently expressed on activated T cells, B cells and mono‐cytes as well as on a subpopulation of bone marrow cells. A potential ligand, as revealed by staining with a CD44v10 receptor globulin, was only detected on monocytes. Anti‐CD44v10 had no major impact on T‐cell activation and no influence on primed B cells, but interfered with the mounting of a primary B‐cell response to T‐independent and T‐dependent antigens. Addition of anri‐CD44v10 at different stages during the activation process revealed that CD44v10 was not engaged in B‐cell‐T‐cell interactions. The antibody exerted some effect on monocyte activation as defined by a slight decrease in IL‐1 production, but most efficiently inhibited antigen‐specific as well as mitogen‐induced B‐cell activation when present during the co‐culture of virgin B cells with monocytes. These findings, together with the observation that a CD44v10 ligand was only detected on monocytes but not on lymphocytes, point towards a requirement for CD44v10 in a B‐cell‐monocyte interaction. Furthermore, since activation of B cells by engagement both of the B‐cell receptor and of mitogen receptors was inhibited by anti‐CD44v10, the data suggest that a costimulatory function of CD44v10 proceeds independent of the B‐cell receptor.