NF-B modulates B-cell-receptor editing to help prevent the development of self-reactive B cells.
When the B cell receptor (BCR) on an immature B cell recognizes self antigen, receptor editing occurs during which immunoglobulin light chain rearrangements continue in order to change the BCR specificity. Previous studies addressing the role of nuclear factor-B (NF-B) in B cell development have yielded conflicting results. Now, the results of a study by Cadera et al. point to a possible role for NF-B in receptor editing.
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The authors took advantage of the fact that NF-B directly regulates the activity of inhibitor of NF-B (IB). So, they looked at the activity of NF-B using a targeted mutant mouse in which -galactosidase (-gal) expression reports the activity of IB (IB+/lacZ mouse). Having first confirmed that known activators of NF-B also induced -gal expression, the authors looked at -gal expression during B cell development from bone marrow cells. Stage-specific -gal expression was observed, with a peak of expression in pre-B cells. Using real-time reverse transcription PCR to quantify various light chain locus transcripts, they found that -gal+ pre-B cells had increased light chain replacement or editing gene rearrangements compared with -gal- pre-B cells. Furthermore, retroviral infection of an IB super-repressor (which inhibits the activation of NF-B through the classical pathway) in primary bone marrow cell cultures resulted in diminished immunoglobulin light -chain (Ig) rearrangements and reduced Ig accessibility, suggesting that inhibiting NF-B can downregulate immunoglobulin light chain gene rearrangement. Various markers of receptor editing, including recombining sequence gene rearrangements (which are considered a hallmark of receptor editing) and secondary Iggene rearrangements, were observed in -gal+ pre-B cells and not -gal- pre-B cells, showing that IB expression correlates with receptor editing.
To confirm that NF-B has a role in receptor editing, IB+/lacZ mice were crossed with various BCR knock-in mice to generate mice expressing either a self-specific BCR or an innocuous BCR. In the two mouse lines expressing a self-specific BCR, in which the developing B cells are almost all undergoing receptor editing, most of the B cells expressed IB, as measured by -gal expression. By contrast, most of the B cells from the mice expressing an innocuous BCR, in which receptor editing was not needed, did not express the -gal reporter gene.
So, how might NF-B mediate the regulation of receptor editing? Transcripts of many NF-B target genes were quantified using reverse transcription PCR. This showed a fourfold increase in interferon-regulatory factor 4 (IRF4) transcripts in -gal+ pre-B cells compared with -gal- pre-B cells. This implies that NF-B could be acting through IRF4, which has previously been shown to have a role in pre-B cell development and receptor editing.
This study shows that although NF-B may not have an essential role in B cell development per se, it does seem to have a role in preventing the development of self-reactive B cells by modulating receptor editing.
Elaine Bell
References
Cadera, E. J. et al. NF-B activity marks cells engaged in receptor editing. J. Exp. Med.206, 1803–1816 (2009) | Article | PubMed |
B modulates B-cell-receptor editing to help prevent the development of self-reactive B cells.
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