2 Role of RAG1 and RAG2 in B-cell development | 35 tivity. Interestingly, it has also been shown that in Btk/Slp65-deficient mouse pre-B cells, malignancies can also arise independently from V(D)J recombination169. Additionally, BCR signaling plays a crucial role in initiating processes such as clonal deletion and receptor editing, which are essential for removing autoreactive B cells. As opposed to the cells with competent BCRs, the immature B cells with a self-reactive BCR lacking CD19, which functions as a BCR co-receptor, show attenuated PI3K activation. Also, the self-reactive immature B cells show increased tonic BCR signaling leading to re-expression of RAGs in the effort of the self-reactive B cell to attempt to “edit” the autoreactive BCR and escape apoptosis24. B cells undergoing receptor editing internalize the BCR, which leads to a decrease in PI3K/AKT signaling170. Already in the 1980s, the notion was put forward that NF-kB is a critical trans-acting factor that mediates the B-cell-specific expression of Igk genes171. The NF-kB transcription factor family consists of five proteins: RelA (also known as p65), RelB, c-Rel, p100, and p150. The p100 and p150 are precursors, which after proteolysis produce p50 and p52 subunits172. The p50 subunit was shown to suppress Rag1 and Rag2 expression in mice. Members of NF-kB were shown to associate with the RAG locus and this transcriptional regulation was demonstrated to be limiting for Igl gene recombination173. Another study showed that crosslinking of the BCR by self-antigen signals through NF-kB drove the IRF4 expression and thereby increased the accessibility of Igl to RAG1/2 endonuclease and enabling receptor editing162. Besides the role in receptor editing and allelic exclusion, the NF-kB signaling pathway was shown to have an important role in RAG1/2 regulation and B-cell development. Single-gene knock-in/knock-out experiments, involving the manipulation of a single gene within the NF-kB pathway occasionally yielded conflicting outcomes likely attributed to redundancies existing between the classical and alternative pathways. However, even more sophisticated mouse models targeting both pathways or simultaneously deleting several NF-kB components, did not always provide consistent results, all of which make studying NF-kB a challenging task, as illustrated by the following examples: mice expressing a dominant negative form of IkBa showed reduced numbers of pre-B cells, probably due to the altered expression of PAX5 and IRF4174. The deletion of p100 led to the activation of the alternative NF-kB pathway through a process involving the liberation of NF-kB dimers. Normally, p100 functions as an inhibitor of the alternative pathway by sequestering NF-kB dimers, particularly RelB/p52 complexes, in an inactive form within the cytoplasm. However, upon deletion of p100, this inhibition is alleviated. Such constitutive activation of the alternative NF-kB through p100 deletion showed arrested B-cell development at pro-B cell stage175. However, in other studies, the constitutive activation of the alternative NF-kB pathway 176 or the classical NF-kB pathway 177 showed no impact on B-cell development in bone marrow. TNF receptor-associated factors 2 and 3 (TRAF2 and TRAF3) participate in the NF-kB pathway, namely, they inhibit the alternative NF-kB pathway by targeting NF-kB-inducing kinase (NIK) for ubiquitin-mediated degradation and thereby preventing the p100
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