A plasma cell differentiation quality control ablates B cell clones with biallelic Ig rearrangements and truncated Ig production.

Aberrantly rearranged immunoglobulin (Ig) alleles are frequent. They are usually considered sterile and innocuous as a result of nonsense-mediated mRNA decay. However, alternative splicing can yield internally deleted proteins from such nonproductively V(D)J-rearranged loci. We show that nonsense codons from variable (V) Igκ exons promote exon-skipping and synthesis of V domain-less κ light chains (ΔV-κLCs). Unexpectedly, such ΔV-κLCs inhibit plasma cell (PC) differentiation. Accordingly, in wild-type mice, rearrangements encoding ΔV-κLCs are rare in PCs, but frequent in B cells. Likewise, enforcing expression of ΔV-κLCs impaired PC differentiation and antibody responses without disturbing germinal center reactions. In addition, PCs expressing ΔV-κLCs synthesize low levels of Ig and are mostly found among short-lived plasmablasts. ΔV-κLCs have intrinsic toxic effects in PCs unrelated to Ig assembly, but mediated by ER stress-associated apoptosis, making PCs producing ΔV-κLCs highly sensitive to proteasome inhibitors. Altogether, these findings demonstrate a quality control checkpoint blunting terminal PC differentiation by eliminating those cells expressing nonfunctionally rearranged Igκ alleles. This truncated Ig exclusion (TIE) checkpoint ablates PC clones with ΔV-κLCs production and exacerbated ER stress response. The TIE checkpoint thus mediates selection of long-lived PCs with limited ER stress supporting high Ig secretion, but with a cost in terms of antigen-independent narrowing of the repertoire.

AID-driven deletion causes immunoglobulin heavy chain locus suicide recombination in B cells.

Remodeling of immunoglobulin genes by activation-induced deaminase (AID) is required for affinity maturation and class-switch recombination in mature B lymphocytes. In the immunoglobulin heavy chain locus, these processes are predominantly controlled by the 3' cis-regulatory region. We now show that this region is transcribed and undergoes AID-mediated mutation and recombination around phylogenetically conserved switchlike DNA repeats. Such recombination, which we term locus suicide recombination, deletes the whole constant region gene cluster and thus stops expression of the immunoglobulin of the B cell surface, which is critical for B cell survival. The frequency of this event is approaching that of class switching and makes it a potential regulator of B cell homeostasis.

Cross talk between immunoglobulin heavy-chain transcription and RNA surveillance during B cell development.

Immunoglobulin (Ig) genes naturally acquire frequent premature termination codons during the error-prone V(D)J recombination process. Although B cell differentiation is linked to the expression of productive Ig alleles, the transcriptional status of nonfunctionally recombined alleles remains unclear. Here, we tracked transcription and posttranscriptional regulation for both Ig heavy-chain (IgH) alleles in mice carrying a nonfunctional knock-in allele. We show that productively and nonproductively VDJ-rearranged alleles are transcribed throughout B cell development, carry similar active chromatin marks, and even display equivalent RNA polymerase II (RNAPII) loading after B cell stimulation. Hence, these results challenge the idea that the repositioning of one allele to heterochromatin could promote the silencing of nonproductive alleles. Interestingly, the efficiency of downstream RNA surveillance mechanisms fluctuates according to B cell activation and terminal differentiation: unspliced nonfunctional transcripts accumulate in primary B cells, while B cell activation promotes IgH transcription, RNA splicing, and nonsense-mediated mRNA decay (NMD). Altogether, IgH transcription and RNA splicing rates determine by which RNA surveillance mechanisms a B cell can get rid of nonproductive IgH mRNAs.

Multiple RNA surveillance mechanisms cooperate to reduce the amount of nonfunctional Ig kappa transcripts.

Random V(D)J junctions ensure that the diversity of the Ig primary repertoire is adapted to the vast heterogeneity of Ags. In two-thirds of cases, recombination between variable segments induces a frameshift in the open reading frame and generates a premature termination codon. In B cells harboring biallelic V(D)J rearrangement of Ig genes, transcription is known to occur on both the functional and nonfunctional alleles, generating considerable amounts of primary transcripts with out-of-frame V regions. In this study, we analyzed in cell lines and primary B cells the RNA surveillance of nonfunctional Igkappa transcripts arising from nonproductive rearrangement. We demonstrated that splicing inhibition, nonsense-mediated decay and nonsense-altered splicing each have an individual partial effect that together associate into an efficient surveillance machinery, downregulating nonfunctional Igkappa mRNA. Moreover, we provide evidence that the RNA surveillance efficiency increases throughout B cell development. Whereas splicing inhibition remains constant in most cell lines, differences in nonsense-mediated decay and nonsense-altered splicing are responsible for the higher RNA surveillance observed in plasma cells. Altogether, these data show that nonfunctionally rearranged alleles are subjected to active transcription but that multiple RNA surveillance mechanisms eradicate up to 90% of out-of-frame Igkappa mRNA.