LAUSR

BOB.1/OBF.1 is a lymphocyte‐specific transcriptional co‐activator of octamer‐dependent transcription. It regulates the expression of genes important for lymphocyte physiology together with the Oct‐1 and Oct‐2 transcription factors. So far, BOB.1/OBF.1 has been studied in conventional knockout mice, whereby a function of BOB.1/OBF.1 in B but also in T cells was described. The main characteristic of BOB.1/OBF.1‐deficient mice is the complete absence of germinal centers. However, it is entirely unsolved at which stage of B‐cell development BOB.1/OBF.1 expression is essential for germinal center formation. Still, it is not known whether defects observed late in B‐cell development of BOB.1/OBF.1‐deficient mice are merely a consequence of defective early B‐cell development. To answer the question, whether BOB.1/OBF.1 expression is required before or during the process of germinal center formation, we established a mouse system, which allows the conditional deletion of BOB.1/OBF.1 at different stages of B‐cell development. Our data reveal a requirement for BOB.1/OBF.1 during both early antigen‐independent and late antigen‐dependent B‐cell development, and further a requirement for efficient germinal center reaction during complete B‐cell ontogeny. By specifically deleting BOB.1/OBF.1 in germinal center B cells, we provide evidence that the failure to form germinal centers is a germinal center B‐cell intrinsic defect and not exclusively a consequence of defective early B‐cell maturation.