Impaired B cell maturation in mice lacking Bruton's tyrosine kinase (Btk) and CD40

Mutations in Bruton's tyrosine kinase (Btk) gene, in mice, result in reduced numbers and responses of peripheral B cells. Surface Ig-mediated signaling is defective in Btk mutant B cells as they do not proliferate upon slg cross-linking and lack thymus-independent (TI) type II responses. Signals through sIg and CD40 play a critical role in B cell maturation. To investigate the consequences of the lack of both Btk and CD40 on B cell development and function, mice were generated that were homozygous for targeted mutations in the Btk and the CD40 genes (BtkMCD40M). The CD40 mutation (CD40M) had a synergistic effect on the BtkM defects. In BtkMCD40M mice the number of B cells was reduced 3- to 4-fold compared to BtkM mice and mature B cells (IgMlow/IgDhigh) were virtually absent; serum levels of all Ig isotypes were diminished; and antibody responses to TI-I TI-II and thymus-dependent antigens were impaired. Furthermore, although wild-type BtkM and CD40M mice produced germinal centers in response to TI-I antigen, the BtkMCD40M mice did not. Maturational and functional B cell defects in BtkMCD40M mice may result from a combination of intrinsic B cell defects, lack of CD40L-dependent T cell help and microenvironmental defects. These data suggest that signals through Btk and CD40 are necessary for the production and maintenance of the mature B cell.