The Ig heavy chain protein but not its message controls early B cell development

Abstract

Development of progenitor B cells (ProB cells) into precursor B cells (PreB cells) is dictated by immunoglobulin heavy chain checkpoint (IgHCC), where the IgHC encoded by a productively rearrangedIghallele assembles into a PreB cell receptor complex (PreBCR) to generate signals to initiate this transition and suppressing antigen receptor gene recombination, ensuring that only one productiveIghallele is expressed, a phenomenon known asIghallelic exclusion. In contrast to a productively rearrangedIghallele, theIghmessenger RNA (mRNA) (IgHR) from a nonproductively rearrangedIghallele is degraded by nonsense-mediated decay (NMD). This fact prohibited firm conclusions regarding the contribution of stableIgHRto the molecular and developmental changes associated with the IgHCC. This point was addressed by generating theIghTer5H∆TMmouse model fromIghTer5Hmice having a premature termination codon at position +5 in leader exon ofIghTer5Hallele. This prohibited NMD, and the lack of a transmembrane region (∆TM) prevented the formation of any signaling-competent PreBCR complexes that may arise as a result of read-through translation across premature Ter5 stop codon. A highly sensitive sandwich Western blot revealed read-through translation ofIghTer5Hmessage, indicating that previous conclusions regarding a role ofIgHRin establishing allelic exclusion requires further exploration. As determined by RNA sequencing (RNA-Seq), this low amount of IgHC sufficed to initiate PreB cell markers normally associated with PreBCR signaling. In contrast, theIghTer5H∆TMknock-in allele, which generated stableIgHRbut no detectable IgHC, failed to induce PreB development. Our data indicate that the IgHCC is controlled at the level of IgHC and notIgHRexpression.