Nemo-Dependent, ATM-Mediated Signals from RAG DNA Breaks at Igk Feedback Inhibit V κ Recombination to Enforce Igκ Allelic Exclusion

The Cyclin D3 Protein Enforces Monogenic TCRβ Expression by Mediating TCRβ Protein-Signaled Feedback Inhibition of Vβ Recombination

In jawed vertebrates, adaptive immunity depends on the process of V(D)J recombination creating vast numbers of T and B lymphocytes that each expresses unique Ag receptors of uniform specificity. The asynchronous initiation of V-to-(D)J rearrangement between alleles and the resulting protein from one allele signaling feedback inhibition of V recombination on the other allele ensures homogeneous receptor specificity of individual cells. Upon productive Vβ-to-DβJβ rearrangements in noncycling double-negative thymocytes, TCRβ protein signals induction of the cyclin D3 protein to accelerate cell cycle entry, thereby driving proliferative expansion of developing αβ T cells. Through undetermined mechanisms, the inactivation of cyclin D3 in mice causes an increased frequency of αβ T cells that express TCRβ proteins from both alleles, producing lymphocytes of heterogeneous specificities. To determine how cyclin D3 enforces monogenic TCRβ expression, we used our mouse lines with enhanced rearrangement of specific Vβ segments due to replacement of their poor-quality recombination signal sequence (RSS) DNA elements with a better RSS. We show that cyclin D3 inactivation in these mice elevates the frequencies of αβ T cells that display proteins from RSS-augmented Vβ segments on both alleles. By assaying mature αβ T cells, we find that cyclin D3 deficiency increases the levels of Vβ rearrangements that occur within developing thymocytes. Our data demonstrate that a component of the cell cycle machinery mediates TCRβ protein-signaled feedback inhibition in thymocytes to achieve monogenic TCRβ expression and resulting uniform specificity of individual αβ T cells.

Poor-Quality Vβ Recombination Signal Sequences and the DNA Damage Response ATM Kinase Collaborate to Establish TCRβ Gene Repertoire and Allelic Exclusion

The monoallelic expression (allelic exclusion) of diverse lymphocyte Ag receptor genes enables specific immune responses. Allelic exclusion is achieved by asynchronous initiation of V(D)J recombination between alleles and protein encoded by successful rearrangement on the first allele signaling permanent inhibition of V rearrangement on the other allele. The ATM kinase that guides DNA repair and transiently suppresses V(D)J recombination also helps impose allelic exclusion through undetermined mechanisms. At the TCRβ locus, one Vβ gene segment (V31) rearranges only by inversion, whereas all other Vβ segments rearrange by deletion except for rare cases in which they rearrange through inversion following V31 rearrangement. The poor-quality recombination signal sequences (RSSs) of V31 and V2 help establish TCRβ gene repertoire and allelic exclusion by stochastically limiting initiation of Vβ rearrangements before TCRβ protein-signaled permanent silencing of Vβ recombination. We show in this study in mice that ATM functions with these RSSs and the weak V1 RSS to shape TCRβ gene repertoire by restricting their Vβ segments from initiating recombination and hindering aberrant nonfunctional Vβ recombination products, especially during inversional V31 rearrangements. We find that ATM collaborates with the V1 and V2 RSSs to help enforce allelic exclusion by facilitating competition between alleles for initiation and functional completion of rearrangements of these Vβ segments. Our data demonstrate that the fundamental genetic DNA elements that underlie inefficient Vβ recombination cooperate with ATM-mediated rapid DNA damage responses to help establish diversity and allelic exclusion of TCRβ genes.