Shedding light on IL-33 in the eye
Authors: Sachin P. Gadani, Jonathan Kipnis
Journal: The Journal of Experimental Medicine (2016)

The Journal of Experimental MedicineThe assembly of immunoglobulin genes occurs in ordered waves during B cell development.The heavy chain (Igh) locus generally recombines first, and each allele has at most one chance to undergo a productive rearrangement.Subsequently, at the κ (Igk) locus, individual alleles can undergo sequential rounds of rearrangement, permitting different light-heavy chain combinations to be tested until a functional, nonself-reactive immunoglobulin is produced.The two waves of recombination are separated by a checkpoint governed by the pre-B cell receptor (pre-BCR), which enforces allelic exclusion at the Igh locus, triggers proliferation, and promotes Igk rearrangement.This raises the question: how do we mitigate the genomic damage that might occur if DNA cleavage and cell cycle entry were initiated simultaneously?In this issue, Bednarski et al. suggest a solution: an unexpected mechanism by which RAG-induced DNA double-strand breaks (DSBs) suppress pre-BCR signaling.The authors began by identifying genes that undergo RAG-dependent changes in expression as progenitors progress from the large pre-B stage to the small pre-B stage, at which Igk rearrangement begins.Of particular interest were the genes encoding RELB and p100, components of the transcription factor NF-κB2.Because RELB and p100 are induced by DSBs through the activation of ataxia-telangiectasia mutated (ATM), they seemed well positioned to act as a bridge between RAG and downstream transcriptional targets.Indeed, many of the genes regulated by RAG-dependent DSBs were targets of NF-κB2.One of these targets encodes the transcriptional repressor SPIC, which had been implicated as a negative regulator of B cell development.Bednarski et al. were able to situate RAG activity at the apex of a signaling cascade that induces SPIC through sequential activation of ATM and NF-κB2.SPIC was found to close the circle, inhibiting expression of the pre-BCR signal mediators SYK and BLNK by antagonizing PU.1 at their corresponding genes.The resulting extinction of pre-BCR signaling enforces cell cycle arrest in G1 while suppressing κ rearrangement.The ability of RAG-induced DSBs to suppress pre-BCR signaling may explain how κ rearrangement is punctuated in pre-B cells: (1) an initial round of κ rearrangement in small pre-B cells generates DSBs; (2) DSBs, acting through ATM, induce SPIC and turn down pre-BCR signaling, inhibiting κ rearrangement and enforcing G1 arrest; and (3) repair of the first wave of DSBs extinguishes ATM signaling and restores pre-BCR signaling, initiating another round of κ rearrangement.The model, which is attractive in its economy, may inform development of therapies that interrupt pre-BCR signaling in malignancies of pre-B cell origin, particularly acute lymphoblastic leukemias.Would interruption of the linkage between RAG DSBs and pre-BCR signaling promote genomic instability?Other mechanisms, notably activation of p53 and destruction of RAG-2 at the G1-to-S transition, are in place to segregate DSBs from S phase, but these may not be sufficiently genoprotective in the face of persistent pre-BCR signaling.In this regard, it will be interesting to assess genomic instability in SPIC-null mice, particularly in a p53-deficicient setting.