The role of DEAD-box helicase 1 in humoral immune responses

Abstract

A key characteristic of humoral immune responses is the ability of B-cells to recognise an endless array of antigens through their immunoglobulin (Ig) genes and secret these as soluble antibodies to help combat invading pathogens. Primary diversification occurs through VDJ recombination during B-cell development, whereas secondary diversification occurs during ongoing immune responses through the introduction of affinity enhancing point mutations in Ig variable genes, a process termed Somatic Hypermutation (SHM). Additionally, to tackle the variety of pathogens we may encounter, B-cells can substitute their Ig constant regions by Class Switch Recombination (CSR). CSR is a DNA deletional and recombinational event, in which the RNA-binding protein DEAD-box helicase 1 (DDX1) is implicated in targeting the mutagenic enzyme Activation-Induced Cytidine Deaminase (AID) to Ig loci to initiate CSR. The aim of this project was to interrogate further the role of DDX1 during humoral immune responses, to understand whether DDX1’s impact on B-cell biology extends beyond its role in CSR. To this end, we characterised DDX1’s contribution by utilising multiple conditional knock-out models. We focused on characterising the formation and function of Germinal centres (GC); microanatomical structures where affinity enhancement and B-cell clonal expansion occurs, following T-cell dependent immunisation. We discovered that GC-specific ablation of DDX1 resulted in reduced GC size with a reduction in antigen-specific B-cells, affinity maturation and clonal expansion, despite sufficient T-cell mediated selection. Further interrogation at the single-cell level pointed to a defect in post-transcriptional control of gene expression in GC B-cells upon DDX1-depletion. Furthermore, alongside a reduction in CSR, depletion of DDX1 prior to B-cell activation resulted in diminished generation of GC and extrafollicular B-cell responses. Although DDX1-deficient B-cells show comparable activation in vitro, these B-cells were severely impaired in proliferation, with indication that efficient translation in activated B-cells requires DDX1 expression. Finally, we demonstrate the validation of a novel mouse model which will enable characterisation of the contribution of DDX1’s ATP-dependent helicase activity on the GC phenotypes we have observed. Together, we have demonstrated that DDX1’s role in humoral immunity extends much further than its characterised role in CSR, with the identification of a novel role for DDX1 in modulating mRNA translation in activated B-cells.