Contributions of BCL3 to Humoral Immunity

Abstract

The inhibitors of NF-B (IB) are regulators of NF-B signaling that typically sequester NF-B subunits in the cytosol, thereby restraining the inflammatory response. Unlike other IB proteins, the atypical IB proteins, such as BCL3, are located in the nucleus; these proteins have the capacity to enhance as well as repress NF-B signaling. We identified in two siblings with recurrent infections, hypogammaglobulinemia and near absence of germinal centers in lymph nodes, who carried a novel homozygous variant in BCL3 affecting the canonical donor splice site after exon 2. Using CRISPR-Cas9 gene editing, previous members of the lab generated knock-in mice homozygous for the patients’ BCL3 variant (Bcl3ex2). Both the patients and mutant mice expressed ~5% of the normal level of full-length, WT BCL3 protein, generated through non-canonical splicing. In addition, alternative splicing generated a novel splice variant with a novel N-terminal sequence, followed by an intact WT sequence encoded by exons three through nine, that lacks the nuclear localization sequence. Bcl3ex2 mice exhibited intact development of transitional, follicular, and marginal zone B cells. However, they had hypogammaglobulinemia, an impaired antibody response to T-dependent antigens, impaired germinal center formation, and a paucity of follicular dendritic cells. This study identifies the first human primary immunodeficiency associated with defective BCL3 expression, thereby highlighting the role of BCL3 in the transcriptional regulation of T-dependent humoral immunity.