Fc receptor-like proteins and their role in B-cell responses and autoimmune diseases

• Fc receptor-like (FCRL) proteins regulate B-cell responses. • FCRL4 and/or FCRL5 expression is enriched in activated CD27 − memory B cells. • Increased expression of FCRL4/FCRL5 is implicated in the pathology of multiple autoimmune diseases. Fc receptor-like (FCRL) proteins constitute a receptor family that displays overlapping yet distinct features compared to classical Fc receptors. In a healthy immune system, FCRL proteins play a role in promoting and regulating the immune response through their immunoreceptor tyrosine-based motifs. FCRL proteins are expressed mainly by B cells, suggesting a primary role in B cell responses. For several autoimmune diseases, studies have shown that particularly FCRL4, which binds to dimeric IgA, and FCRL5, which binds to IgG, may have a role in disease pathology and prognosis. These proteins and their transcripts are often enriched in blood and/or affected tissue of patients with a systemic or organ-specific autoimmune disease like rheumatoid arthritis, Sjögren’s disease, systemic lupus erythematous, Graves’ disease or myasthenia gravis. The expression of FCRL4 and FCRL5 appears to be disease- and context-specific, and influenced by the tissue microenvironment. Yet, the functions of FCRL proteins are still incompletely understood, and more mechanistic studies are necessary to unravel the contribution of FCRL4 and FCRL5 to pathogenic B cell responses occurring in autoimmune diseases.