A re‐evaluation of the effects of X‐linked immunodeficiency (xid) mutation on B cell differentiation and function in the mouse

CBA/N (xid) mice have a point mutation in Bruton's tyrosine kinase (btk), which results in their failure to respond to T‐independent type 2 (TI‐2) antigens, and to several B cell mitogens [most notably anti‐immunoglobulin (Ig)] in vitro. They have reduced numbers of peripheral (B2) B cells, which are regarded as being phenotypically and functionally immature. We show here that adult CBA/N mice in fact have two distinct B cell populations: some 60% of the cells are CD23+ HSAlo sIgDhi and hence resemble recirculating, follicular (RF) B cells from normal mice, except that they are sIgMhi. The remaining 40% of xid B cells are CD23− HSAhi sIgD−/lo and resemble immature transitional (TR) B cells. TR B cells from xid mice do not synthesize DNA when cultured with lipopolysaccharide (LPS), whereas those from normal mice do so. Only the RF cells from either xid or normal mice proliferate in response to ligation of CD40. In neonatal normal mice the emergence of mitogen responsiveness followed the chronological sequence LPS → anti‐CD40 → anti‐Ig ≈ anti‐CD38. The same developmental sequence was seen with B cells from xid mice (for LPS and anti‐CD40), but it occurred at a significantly slower tempo and this correlated with the later appearance of RF‐type cells. TR xid B cells express very low levels of bcl‐2 and we conclude that these cells resemble very immature (bone marrow) B cells, rather than normal transitional cells. We, therefore, propose that the xid mutation imposes a multistage brake on B cell differentiation in the mouse. The available data suggest that btk is required for the positive selection of B cells throughout their differentiation in the periphery. This in turn implies that low level signaling via surface Ig is needed throughout this process in order for peripheral B cells to become functionally mature.