GABP Transcription Factor Is Required for B Lymphocyte Maturation and Plasma Cell Development

B lymphocytes develop in the bone marrow and later encounter antigen in lymph nodes, where they complete their development as plasma cells or B memory cells. Several key transcription factors have been identified that are required for B cell development, including Pax5, BCL6, C-MYC, and others. GABP is a tetrameric ets transcription factor that includes the DNA-binding GABP alpha protein, and the unrelated protein, GABP beta, which contains multimerization and transcriptional activation domains. GABP plays key roles in cell cycle control and mitochondrial biogenesis. It is also required for lineage specific gene expression, and it was previously shown to control gene expression of the IL-7 receptor and Pax5, both of which are required for lymphocyte development. Disruption of mouse Gabpα caused cell cycle arrest in hematopoietic stem cells (HSC), profound loss of progenitor cells, and aberrant myeloid differentiation. We created a conditional knockout model of Gabpα in B lymphocytes by breeding mice with lox-P flanked Gabpa to mice that bear Cre recombinase knocked into the B-cell specific CD19 locus; the mice also carry the Rosa 26 lox-STOP-lox YFP transgene, which permits identification and isolation of individual Gabpα null cells, based on expression of YFP. Loss of Gabpα was highly lineage specific for B lymphocytes. Gabpa null mice were healthy and vigorous through young adulthood, but some developed rectal prolapse by nine months of age, and necropsy demonstrated thinning of the intestinal wall and loss of Peyer's Patches and other lymphoid tissue. We immunologically characterized mice between 6 and 8 weeks of age, in order to minimize secondary effects of the inflammatory process associated with rectal prolapse. There was no deletion of Gabpα in T lymphocytes, and no discernable effect on T-cell subpopulations. We observed a significant reduction in Gabpα null (YFP+) B cells, in comparison with the Gabpα replete (YFP-) B cells in bone marrow and spleen. Gabpα null cells contributed to the pro-B cell population, but there was a progressively reduced contribution of Gabpα null cells to later stages of B cell maturation. We detected no Gabpα null cells among mature naive IgD+/IgM+ B cells, indicating a profound block in B cell maturation in cells that lack Gabpα. Importantly, no YFP+ CD138+ cells were detected, indicating that Gabpα null cells could not contribute to plasma cell development. We conclude that Gabp is required for full B cell maturation and plasma cell development in mice, and that its deletion is associated with loss of Peyer's Patches and rectal prolapse. GABP was previously shown to regulate expression of IL-7R and Pax5, which are expressed in lymphoid progenitor cells long before activation of CD19 expression. Thus, failure of B cell development and plasma cell formation in this CD19-Cre Gabpα null model is independent of the effect of GABP on those other B cell factors, and indicates a new, critical role for GABP in later stages of B cell and plasma cell development. Although rectal prolapse has been observed in mice with T cell defects, this represents the first demonstration that B cell defects cause such a phenotype.