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 of 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. Using an optimized LSR-seq high throughput method, we now show that AID-mediated LSR is evolutionarily conserved and also actively occurs in humans, providing an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR either focuses on the functional IgH allele or is bi-allelic, and its signature is mainly detected when LSR is ongoing while it vanishes from fully differentiated plasma cells or from “resting” blood memory B-cells. 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 although TLR ligation appeared the most efficient. Molecular analysis of breakpoints and junctions confirms that LSR is AID-dependent and reveals junctional sequences somehow similar to CSR junctions but with increased usage of microhomologies.
Highlights
Humoral immune responses and immunoglobulin (Ig) production rely on the selection of Bcells harboring antigen (Ag)-specific B-cell receptors (BCRs)
locus suicide recombination (LSR) actively targets the human IgH locus had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
While activation-induced cell death (AICD) pathways have been characterized in detail for T-cells, and notably involve FAS-induced apoptosis, they are less documented in B-cells
Summary
Humoral immune responses and immunoglobulin (Ig) production rely on the selection of Bcells harboring antigen (Ag)-specific B-cell receptors (BCRs) This selection implies proliferation and differentiation of those cells optimally binding Ag and elimination of the less efficient or inappropriately activated cells. Out-of-frame or other unfavorable V region mutations might result in BCR loss and promote apoptosis while more subtle cell fate decisions will arbitrate between death, short-term or long-term survival as memory lymphocytes or plasma cells [1,3] Such intra-GC cell fate choices are crucial since inappropriate survival or terminal differentiation of bystander cells producing useless Ig or Ig with increased affinity for self or environmental Ags might trigger auto-immunity, inflammation and disease. We show that the human IgH 3’RR super-enhancers include sequences ideally suited as recombination targets, and that LSR actively occurs in human lymphoid B-cells
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