BCR Density Regulates IgE Germinal Center B Cell Survival

Introduction/Rationale: IgE antibodies mediate allergic disease, yet IgE⁺ B cells rarely persist in germinal centers (GCs). Low B cell receptor (BCR) density has been implicated in this instability, potentially due to three non-canonical polyadenylation sites downstream of the last IgE membrane exon, which are less efficient than the canonical secreted polyadenylation site (spA). However, how BCR density affects IgE GC survival remains unclear.

Methods: CRISPR-Cas9 gene editing was used to delete the secreted polyadenylation site (spA) at the IgE (IgEspAd) or IgG1 (IgG1spAd) locus. Mice were challenged intranasally with Alternaria alternata to elicit IgE responses. Flow cytometry was used to quantify GC B cells, plasmablasts, and surface BCR density, and to assess apoptosis using active caspase-3 staining. IgEspAd–wild-type mixed bone marrow chimeras were generated to distinguish the effects of BCR density from secreted IgE.

Results: Deletion of spA in IgEspAd mice abolished secreted IgE and increased surface BCR density, reducing IgE GC B cells and plasmablasts. IgG1spAd deletion greatly reduced secreted IgG1 but did not affect density. In IgEspAd–wild-type chimeras, IgEspAd-derived GC B cells showed reduced apoptosis, demonstrating that elevated BCR density promotes GC survival. Verigem mice, which retain secreted IgE while exhibiting increased BCR density, also showed reduced apoptosis and modestly increased IgE GC B cell numbers, supporting a role for secreted IgE in maintaining GC persistence. Ongoing 10x single-cell analysis will determine how enhanced IgE GC survival influences somatic hypermutation.

Conclusion: These findings identify BCR density as a key determinant of IgE GC B cell survival and define a molecular mechanism that constrains IgE responses. By linking BCR regulation to IgE GC B cell survival, this work highlights potential strategies to manipulate IgE immunity in allergy and immunotherapy.