Commitment of B lymphocytes to a plasma cell fate is associated with Blimp-1 expression in vivo

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional re pressor that is sufficient to trigger terminal differentiation in the B cel l lymphoma BCL-1, In this study, we have determined the expression pattern of Blimp-1 in vivo In primary and secondary lymphoid organs of humans and i mmunized mice. Blimp-1 is expressed In plasma cells derived from either a T -independent or T-dependent response in plasma cells that have undergone is otype switching and those resulting front secondary immunization, Blimp-1 i s also present in long-lived plasma cells residing in the bone marrow. Howe ver, Blimp-1 was not detected in memory B cells, This expression pattern pr ovides further evidence of a critical role for Blimp-1 in plasma cell devel opment, supporting earlier studies in cultured lines. Significantly, Blimp- 1 was also found in a fraction (4-15%) of germinal center B cells in murine spleen and human tonsils, Blimp-1 expression in the germinal center is ass ociated with an interesting subset of cells with a phenotype intermediate b etween germinal center B cells and plasma cells, In the mouse, Blimp-1(+) g erminal center B tells peak at day 12 postimmunization and disappear soon t hereafter, They are not apoptotic, some are proliferating, they express ger minal center markers peanut agglutinin or CD10 but not Bcl-6, and most expr ess CD138 (syndecan-1), IRF4, and cytoplasmic Ig, Together, these data supp ort a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.