Autoimmune Disease–Associated Haplotypes of BLK Exhibit Lowered Thresholds for B Cell Activation and Expansion of Ig Class‐Switched B Cells

Abstract

Objective. B lymphoid kinase (BLK) is associated with rheumatoid arthritis (RA) and several other B cellassociated autoimmune disorders. BLK risk variants are consistently associated with reduced BLK expression, but the mechanisms by which reduced expression alters human B cell function to confer autoimmune disease susceptibility are unknown. This study was undertaken to characterize the BLK risk haplotype and to determine associated B cell functional phenotypes involved in autoimmunity.Methods. The BLK risk haplotype association with RA (determined using whole-genome sequencing data) was confirmed in 2,526 RA cases and 2,134 controls. Peripheral blood mononuclear cells (PBMCs) from RA patients, healthy adults, and umbilical cord blood were used to study B cell functional phenotypes associated with the BLK risk genotype. Association of the BLK haplotype with B cell phenotypes was analyzed using cell culture and flow cytometry.Results. Two insertion/deletions were found on the RA risk haplotype in BLK, and the reduction in BLK expression associated with the risk haplotype was confirmed in primary B lymphocytes. Carriers of the RA-associated haplotype had evidence of lower basal B cell receptor (BCR) signaling activity, yet their B cells were hyperactivatable, with enhanced up-regulation of CD86 after BCR crosslinking and greater T cell stimulatory capacity. The number of isotype-switched memory B cells was also significantly increased in subjects carrying the risk haplotype.Conclusion. A major mechanism underlying the BLK association with autoimmune disease involves lowered thresholds for BCR signaling, enhanced B cell-T cell interactions, and altered patterns of isotype switching.

THU0003 CD4+ and B lymphocyte expression quantitative traits at rheumatoid arthritis risk loci in untreated early arthritis: implications for causal gene identification?

“…This was amply illustrated by Simpfendorfer et al, who, similar to our findings at the 8p23 locus, highlighted BLK transcript expression as subject to an eQTL in lymphocytes; however, the CD4+ T cell-specific effect was not sustained at the protein level in these cells. By measuring allelic expression imbalance, those authors went on to demonstrate a robust eQTL for both RNA and protein expression in naive/transitional B cell subsets isolated from umbilical cord blood, which was less evident in whole B cells, suggesting that disease risk is conferred during early B cell development rather than by CD4+ T cells (38), potentially via dysregulated B cell receptor signaling (39).…”

Review: Abnormal B Cell Development in Systemic Lupus Erythematosus: What the Genetics Tell Us

“…In a mechanism similar to that of the Lyp variant, this may contribute to autoimmunity by impairing central tolerance mechanisms. However, abnormalities in peripheral BCR signaling have also been described, with low baseline BCR activity at rest but enhanced responses on activation, heightened ability to stimulate T cells, and increased numbers of isotype‐switched memory B cells .…”

CD4+ and B Lymphocyte Expression Quantitative Traits at Rheumatoid Arthritis Risk Loci in Patients With Untreated Early Arthritis

“…This was amply illustrated by Simpfendorfer et al, who, similar to our findings at the 8p23 locus, highlighted BLK transcript expression as subject to an eQTL in lymphocytes; however, the CD4+ T cell–specific effect was not sustained at the protein level in these cells. By measuring allelic expression imbalance, those authors went on to demonstrate a robust eQTL for both RNA and protein expression in naive/transitional B cell subsets isolated from umbilical cord blood, which was less evident in whole B cells, suggesting that disease risk is conferred during early B cell development rather than by CD4+ T cells , potentially via dysregulated B cell receptor signaling .…”