NF-κB activity marks cells engaged in receptor editing

Because of the extreme diversity in immunoglobulin genes, tolerance mechanisms are necessary to ensure that B cells do not respond to self-antigens. One such tolerance mechanism is called receptor editing. If the B cell receptor (BCR) on an immature B cell recognizes self-antigen, it is down-regulated from the cell surface, and light chain gene rearrangement continues in an attempt to edit the autoreactive specificity. Analysis of a heterozygous mutant mouse in which the NF-κB–dependent IκBα gene was replaced with a lacZ (β-gal) reporter complementary DNA (cDNA; IκBα+/lacZ) suggests a potential role for NF-κB in receptor editing. Sorted β-gal+ pre–B cells showed increased levels of various markers of receptor editing. In IκBα+/lacZ reporter mice expressing either innocuous or self-specific knocked in BCRs, β-gal was preferentially expressed in pre–B cells from the mice with self-specific BCRs. Retroviral-mediated expression of a cDNA encoding an IκBα superrepressor in primary bone marrow cultures resulted in diminished germline κ and rearranged λ transcripts but similar levels of RAG expression as compared with controls. We found that IRF4 transcripts were up-regulated in β-gal+ pre–B cells. Because IRF4 is a target of NF-κB and is required for receptor editing, we suggest that NF-κB could be acting through IRF4 to regulate receptor editing

Development of immunoglobulin k-chain-positive B cells, but not editing of immunoglobulin j-chain, depends on NF-jB signals A R T I C L E S

By genetically ablating IjB kinase (IKK)-mediated activation of the transcription factor NF-jB in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin k-chain-positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin j-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-jB signaling. During the first phase, in which NF-jB signaling is dispensable, predominantly j-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly k-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding j-chain. This second phase of development is dependent on NF-jB signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2. It is well established that the NF-kB family of transcription factors is critical to B cell physiology 1,2 . Activation of NF-kB by the alternative pathway, which is mediated by NF-kB-inducing kinase and the inhibitor of NF-kB kinase 1 (IKK1; A001170) downstream of interactions between B cell-activation factor of the tumor necrosis factor family (BAFF) and BAFF-receptor, is essential for mature B cell survival 3 . In addition, mature B cells depend on continuous signaling through the canonical NF-kB pathway, in which activation of the IKK complex, which consists of IKK1, IKK2 (A001172) and NF-kB essential modulator (NEMO; A001628), is central 1 . In contrast, the function of NF-kB signaling in B cell development remains unclear 1 and is indeed highly controversial. Initial experiments addressed that issue in mice lacking one or two individual NF-kB transcription factors. Whereas the generation of mature B cells is generally impaired in most of these mutant mice, the effects are often mild in B cell progenitors and it has remained unresolved whether these defects are B cell autonomous 2 . Notably, genetic ablation of the BAFF-receptor or IKK1 seems not to affect B cell development in the bone marrow, at least in terms of proportions of cells at the various developmental stages 1,3 ; the same is true for ablation of the canonical pathway by knockout of IKK2 or NEMO specifically in B cell

Development of immunoglobulin lambda-chain-positive B cells, but not editing of immunoglobulin kappa-chain, depends on NF-kappa B signals

By genetically ablating I kappa B kinase (IKK)-mediated activation of the transcription factor NF-kappa B in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin lambda-chain-positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin kappa-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-kappa B signaling. During the first phase, in which NF-kappa B signaling is dispensable, predominantly kappa-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly lambda-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding kappa-chain. This second phase of development is dependent on NF-kappa B signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2