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

Area of Science:

• Immunology
• Molecular Biology
• Developmental Biology

Background:

• Progenitor B cell (ProB) to precursor B cell (PreB) transition relies on the immunoglobulin heavy chain checkpoint (IgHCC).
• Productive immunoglobulin heavy chain (IgHC) assembly into the PreB cell receptor (PreBCR) signals this transition and enforces allelic exclusion.
• Nonsense-mediated decay (NMD) degrades non-productive immunoglobulin heavy chain messenger RNA (IgHR), obscuring its role in IgHCC.

Purpose of the Study:

• To investigate the distinct roles of IgHC protein versus stable IgHR in the IgHCC.
• To determine whether IgHR alone can initiate PreB cell development.
• To clarify the molecular mechanisms underlying immunoglobulin heavy chain allelic exclusion.

Main Methods:

• Generation of a novel mouse model (Igh^m/+) with a premature stop codon in the Igh leader exon to prevent NMD and transmembrane domain formation.
• Utilized sensitive sandwich Western blot to detect read-through translation of Igh^m/+ mRNA.
• Employed RNA sequencing (RNA-Seq) to analyze gene expression and PreB cell markers.

Main Results:

• Read-through translation of Igh^m/+ mRNA was detected, producing low levels of IgHC protein.
• This low IgHC protein successfully initiated PreB cell markers associated with PreBCR signaling.
• A knock-in allele producing stable IgHR but no detectable IgHC failed to induce PreB cell development.

Conclusions:

• The immunoglobulin heavy chain checkpoint (IgHCC) is regulated at the level of IgHC protein expression, not stable IgHR.
• IgHC protein, even at low levels, is sufficient to trigger key events in PreB cell development.
• Stable IgHR alone cannot drive PreB cell development, challenging previous assumptions about its role in allelic exclusion.