Locus Suicide Recombination actively occurs on the functionally rearranged IgH allele in B-cells from inflamed human lymphoid tissues

Abstract

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control by the 3' regulatory region (3'RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining Sµ to "like-switch" DNA repeats that flank the 3' super-enhancer can thus accomplish so-called "locus suicide recombination" (LSR) in mouse B-cells. We now show that AID-mediated LSR also actively occurs in humans, and provides an activationinduced cell death pathway in multiple conditions of B-cell activation. LSR deletions either focus on the functional IgH allele or are bi-allelic, since they can only be detected when they are ongoing and their signature vanishes from fully differentiated plasma cells or from "resting" blood memory B-cells, but readily reappears when such memory Bcells are re-stimulated in vitro. Highly diversified breakpoints are distributed either within the upstream (3'RR1) or downstream (3'RR2) copies of the IgH 3' super-enhancer and all conditions activating CSR in vitro also seem to trigger LSR. AUTHOR SUMMARYClass switch recombination, initiated by the activation-induced deaminase enzyme rearranges immunoglobulin (Ig) genes in order to replace expression of IgM by IgG, IgA or IgE. A variant form of this event, locus suicide recombination (LSR), was previously reported in mouse B-lymphocytes and simply deletes all functional Ig constant genes, thus terminating B-cell function. This study first demonstrates that the structure of the human Ig heavy chain locus provides an ideal target for LSR, and is thus actively (but transiently) affected by this deletional process at the activated B-cell stage. LSR then yields recombined genes that do not support B-cell survival and which thus become undetectable among long-lived memory B-cells or plasma cells. INTRODUCTIONHumoral immune responses and immunoglobulin (Ig) production rely on the selection of B-cells harboring antigen (Ag)-specific B-cell receptors (BCRs). This selection implies not only proliferation and differentiation of those cells optimally binding Ag but also elimination of the less efficient or inappropriately activated cells. The latter can be accomplished through various pathways leading to anergy, death-by-neglect or activation-induced cell death (AICD).While AICD pathways have been characterized in detail for T-cells, and notably involve FAS-induced apoptosis, they are less documented in B-cells. A major and unique feature of mature B-cells during Ag-driven responses, is their ability to reshape their genome, and more specifically Ig genes, after activation-induced deaminase (AID)-dependent modifications. Somatic hypermutation (SHM) within germinal centers (GC) can yield cells with higher affinity V d...

Concerted variation of the 3′ regulatory region of Ig heavy chain and Gm haplotypes across human continental populations

“…The same study showed that disease‐associated SNPs occur in superenhancers of disease‐relevant cells and that this occurs more frequently in superenhancers than in typical enhancers. Recent evidence has also been provided for a role of 3′RRs in the so‐called locus suicide recombination occurring during B cells maturation (Dalloul et al, ).…”

Mechanism and regulation of class switch recombination by IgH transcriptional control elements

“…1B). LSR targeted the two 3'RRs, occurred at low frequency in healthy individuals, and was AID-dependent as judged by the absence of LSR in AID-deficient patients (Dalloul et al, 2019).…”

“…LSR reportedly occurred at levels approaching classical CSR as detected by PCR/Southern blot on excised episomal circles (Péron et al, 2012), though apparently not by more sensitive techniques (Meng et al, 2014;Qian et al, 2014). Recently, LSR was also detected at the human IgH locus undergoing CSR, suggesting conservation of the process between humans and mice (Dalloul et al, 2019). The human IgH locus contains two 3'RRs that include switch-like repeats:…”

“…The presumed bias of LSR towards the expressed allele (Dalloul et al, 2019) may point to specific mechanisms. Alternatively, LSR may be a byproduct of classical CSR.…”

Increased frequencies of IgH locus suicide recombination points on Chronic Lymphocytic Leukemia with low rate of AID related somatic mutations, cMYC overexpression and short telomeres

“…LSR results in the complete deletion of the cluster of IgH constant genes. When LSR hits the active IgH locus, it induces the loss of BCR expression and the death of the B-cell [9][10][11] .…”