32 | Chapter 2 tively regulated by Gfi1b, Ebf1, and c-Myb141,142. The chromosomal architecture of the Rag locus is further summarized in great detail elsewhere143. Signaling pathways controlling RAG expression The many signaling pathways and their interplay at the different stages of B-cell development are still the subject of active investigations. In this section, important signaling pathways that affect Rag expression are summarized, amongst which are IL7 receptor (IL7R) signaling, pre-B cell receptor (pre-BCR) signaling, and B-cell receptor (BCR) signaling. IL7R signaling Developing B cells in the bone marrow receive proliferative signals from stromal cells. Prior to the gene recombination, the processes in early pro-B cells are controlled mainly by the IL7 receptor (IL7R) and two tyrosine kinase receptors: c-kit and Flt3, activating downstream signaling pathways that are interconnected. The binding of stem cell factor present on BM stromal cells causes dimerization and transphosphorylation of the c-kit receptor, which leads to an activation of various downstream pathways, including phosphatidylinositol-3 kinase (PI3K) and mitogen-activated protein kinase (MAPK), which are signaling pathways that promote survival and proliferation144. Similar to c-kit, Flt3 also dimerizes upon binding of the Flt3-ligand (Flt3L) and activates Ras and PI3K pathways, further stimulating the proliferation of developing B cells145. Upon binding of IL7 to the IL7a chain, the IL7a and IL7g subunits heterodimerize, which results in an activation and transmission of the signals through pathways including the Janus kinase/signal transducer and activator of transcription (JAK/STAT) and the PI3K/AKT pathway. In developing B cells, the PI3K/AKT activation inhibits forkhead box O1 (FOXO1) and forkhead box O3 (FOXO3) transcription factors through AKT-mediated phosphorylation,146,147 which causes their nuclear export, sequestration, or degradation.148 B-cell specific deletion of Foxo1 using the conditional CD19-Cre mouse model showed only a partial block in B-cell development, suggesting a certain degree of redundancy with other transcription factors. However, the peripheral lymphoid organs from these mice clearly showed an aberrant representation of B-cell subsets, and even B220+ B cells without surface Ig were detected147,149. Similarly, FOXO3a deficient mouse shows a lower number of pre-B cells but not a complete block146. IL7R activation triggers association of JAK1 with JAK3. The activated tyrosine kinases phosphorylate in return the a-subunit of IL7R, which creates binding sites for STAT protein. Amongst the 7 different STAT proteins, the STAT5a and STAT5b isoforms mediate the IL7R specifically in developing lymphocytes150. STAT5 conveys the survival signals in developing B cells by activating pro-survival factors such as B-cell lymphoma 2 (Bcl2), B-cell lymphoma extra-large (Bcl-xl), and myeloid cell leukemia sequence 1 (Mcl1)151. In addition, STAT5 increases the expression of Ccnd3, encoding for Cyclin D3, which drives prolifera-
RkJQdWJsaXNoZXIy MTk4NDMw