Transcription factor NF-kappa B regulates Ig lambda light chain gene rearrangement.

The tissue- and stage-specific assembly of Ig and TCR genes is mediated by a common V(D)J recombinase complex in precursor lymphocytes. Directed alterations in the accessibility of V, D, and J gene segments target the recombinase to specific Ag receptor loci. Accessibility within a given locus is regulated by the functional interaction of transcription factors with cognate enhancer elements and correlates with the transcriptional activity of unrearranged gene segments. As demonstrated in our prior studies, rearrangement of the Igkappa locus is regulated by the inducible transcription factor NF-kappaB. In contrast to the Igkappa locus, known transcriptional control elements in the Iglambda locus lack functional NF-kappaB binding sites. Consistent with this observation, the expression of assembled Iglambda genes in mature B cells has been shown to be NF-kappaB independent. Nonetheless, we now show that specific repression of NF-kappaB inhibits germline transcription and recombination of Iglambda gene segments in precursor B cells. Molecular analyses indicate that the block in NF-kappaB impairs Iglambda rearrangement at the level of recombinase accessibility. In contrast, the activities of known Iglambda promoter and enhancer elements are unaffected in the same cellular background. These findings expand the range of NF-kappaB action in precursor B cells beyond Igkappa to include the control of recombinational accessibility at both L chain loci. Moreover, our results strongly suggest the existence of a novel Iglambda regulatory element that is either directly or indirectly activated by NF-kappaB during the early stages of B cell development.

An intact NF-kappa B signaling pathway is required for maintenance of mature B cell subsets.

Members of the NF-kappaB/Rel transcription factor family are expressed constitutively during B cell development and are further induced by mitogen activation. Mice harboring germline disruptions in individual NF-kappaB subunits exhibit distinct defects in B lymphocyte activation and survival. However, the role of NF-kappaB in the production and maintenance of B cell subsets has been difficult to dissect in these knockout animals due to functional impairment of other immune cells. To directly address the cell autonomous requirements for NF-kappaB in humoral immune compartments, transgenic mice were generated that express a transdominant form of Ikappa-Balpha in B lineage cells. Whereas expression of the inhibitor had only modest effects on basal or LPS-induced levels of NF-kappaB, transgenic B cells were significantly impaired for cellular proliferation and NF-kappaB induction in response to B cell receptor (BCR) crosslinking. Furthermore, the trans-dominant inhibitor produced a dose-dependent reduction in the population of mature splenic B cells. This cellular defect was more pronounced in long-lived B lymphocyte subsets that recirculate to the adult bone marrow. Together, these results indicate that BCR-mediated signaling must maintain NF-kappaB levels above a stringent threshold for proper regulation of B cell homeostasis.

Transcription factor NF-kappaB regulates inducible Oct-2 gene expression in precursor B lymphocytes.

The POU transcription factors Oct-1 and Oct-2 regulate the activity of octamer-dependent promoters, including those that direct transcription from rearranged immunoglobulin genes. Unlike Oct-1, which is constitutively expressed in many cell types, Oct-2 expression is restricted primarily to B lymphocytes and can be induced in precursor B cells by stimulation with bacterial lipopolysaccharide (LPS). However, the precise factors that mediate this induction mechanism remain unknown. In the present study, we monitored Oct-2 expression in cells arrested for the activation of NF-kappaB, an LPS-responsive member of the Rel transcription factor family. Despite stimulation with LPS, disruption of the NF-kappaB signaling pathway in precursor B cells led to the loss of inducible Oct-2 DNA binding activity in vitro and the suppression of Oct-2-directed transcription in vivo. This biochemical defect correlated with a specific block to Oct-2 gene expression at the level of transcription, whereas the expression of Oct-1 was unaffected. The finding that Oct-2 is under NF-kappaB control highlights an important cross-talk mechanism involving two distinct transcription factor families that regulate B lymphocyte function.

Corepression of RelA and c-rel inhibits immunoglobulin kappa gene transcription and rearrangement in precursor B lymphocytes.

Multiple members of the NF-kappa B/Rel protein family are induced during B cell differentiation and have been implicated in transcriptional activation of the immunoglobulin kappa (Ig kappa) locus. Despite these findings, normal numbers of Ig kappa + B lymphocytes are produced by mice bearing targeted mutations in individual NF-kappa B/Rel genes. In the present study, precursor B lymphocytes were engineered to express a trans-dominant form of I kappa B alpha that simultaneously impairs the c-Rel and RelA transactivating subunits of NF-kappa B. This dual block in NF-kappa B/Rel signaling led to potent inhibition of germline Ig kappa transcription and rearrangement, whereas recombinase activity was unaffected. These findings suggest that c-Rel and RelA serve compensatory functional roles in the developmental mechanisms that govern Ig kappa gene assembly.