Autoimmune disease-associated haplotypes of BLK exhibit lowered thresholds for B cell activation and expansion of Ig class-switched B cells

Objective: B lymphoid kinase (Blk) is associated with rheumatoid arthritis (RA) and several Other B cell-associated 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.