Defective B cell receptor-mediated responses in mice lacking the Ets protein, Spi-B

Spi-B is a hematopoietic-specific Ets family transcription factor closely related to PU.1. Previous gene targeting experiments have shown that PU.1 is essential for the production of both lymphocytes and monocytes. We have now generated mice with a null mutation at the Spi-B locus. Unlike PU.1 mutant mice, Spi-B(-/-) mice are viable, fertile and possess mature B and T lymphocytes. However, Spi-B(-/-) mice exhibit severe abnormalities in B cell function and selective T cell-dependent humoral immune responses. First, although Spi-B(-/-) splenic B cells respond normally to lipopolysaccharide stimulation in vitro, these B cells proliferate poorly and die in response to B cell receptor (surface IgM) cross-linking. Secondly, Spi-B(-/-) mice display abnormal T-dependent antigenic responses in vivo and produce low levels of antigen-specific IgG₁, IgG₂, and IgG(2b) after immunization. Finally, Spi-B(-/-) mice show a dramatic defect in germinal center formation and maintenance. In contrast to wild-type animals, germinal centers in Spi-B(-/-) mice are smaller and short-lived with significantly increased numbers of apoptotic B cells. Taken together, these results demonstrate that Spi-B is essential for antigen-dependent expansion of B cells, T-dependent immune responses and maturation of normal germinal centers in vivo.