Defining the regulation of transitional B cell development and the expression of late transitional stage marker genes CD21 and CD23 by transcription factor networks and BAFF signaling

Transitional development of B cells starts when surface IgM expressing immature B cells from the bone marrow emigrate into the peripheral lymphoid organ spleen. These transitional type 1 / T1 cells mature into transitional type cell T2 and finally differentiate into mature follicular /FM and marginal zone/MZ B cells. The transitional stage is a point of control for BCR induced positive and negative selections in the periphery and its dysregulation leads to autoimmune disorders. The central goal of this dissertation is to better understand the transcription factor networks and signals regulating this critical window of B cell development. In this study, we have established a comprehensive account of transcriptional mediators necessary for maturing, transitional B cell development as well as for their progression into the FM/MZ peripheral B cell pool. A candidate gene approach was followed to evaluate the role of potential positive regulators in B cell maturation primarily by utilizing knockout mouse models. Analyses of CIITA, Th-POK and Mef2c deficient mice were described. We showed that, Mef2c deficiency delays B cell development and directly regulates the transcription of the target gene CD23. In addition, novel gene targeted conditional deletion mouse models were generated for transcription factors Zfp318 and Spi-C. BAFF, the critical cytokine for transitional stage was analyzed for it's downstream transcriptional mediators using a BAFF receptor mutant mouse model. Our data demonstrated intact NF-kB activation and nuclear function in absence of BAFF/BAFF-R coupling. Characterization of the in vivo transcription factor complexes on transitional stage marker genes CD21/Cr2 and CD23/Fcer2a demonstrated a shared network of factors. Evaluation of BAFF signaling in the transcriptional modulation of CD21/CD23 suggested no direct regulation and showed BAFF as a fitness determinant of transitional subsets, thus promoting the expression of B cell specific genes. In summary, this work detailed the transcriptional mediators of transitional B cell development, analyzed transcription factor networks for marker genes CD21 and CD23, evaluated the role of BAFF signaling and finally, analyzed the role of candidate transcription factors in the regulation of splenic B cell maturation by utilizing knockout mouse models and by generating new gene targeted mice.