The establishment of early B cell tolerance in humans: lessons from primary immunodeficiency diseases.

Patients with primary immunodeficiency (PID) provide rare opportunities to study the impact of specific gene mutations on the regulation of human B cell tolerance. Alterations in B cell receptor and Toll-like receptor signaling pathways result in a defective central checkpoint and a failure to counterselect developing autoreactive B cells in the bone marrow. In contrast, CD40L- and MHC class II-deficient patients only displayed peripheral B cell tolerance defects, suggesting that decreased numbers of regulatory T cells and increased concentration of B cell activating factor (BAFF) may interfere with the peripheral removal of autoreactive B cells. The pathways regulating B cell tolerance identified in PID patients are likely to be affected in patients with rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes who display defective central and peripheral B cell tolerance checkpoints. Indeed, risk alleles encoding variants altering BCR signaling, such as PTPN22 alleles associated with the development of these diseases, interfere with the removal of developing autoreactive B cells. Hence, insights into B cell selection from PID patients are highly relevant to the understanding of the etiology of autoimmune conditions.