Rheumatoid Factor-Producing Cd21(Low) Anergic Clonal B-Cells In Essential Mixed Cryoglobulinaemia: A Model For Autoantigen-Driven Pathogenesis Of Infectious And Non-Infectious Cryoglobulinaemias

Objective. Essential mixed cryoglobulinaemia (EMC) is a disorder of B-cells producing rheumatoid,factor (RF), and is clinically and immunologically similar to mixed cryoglobillinaemia (MC) related to hepatitis C virus (HCV-MC). We report here the first comprehensive analysis of B-cell donality, phenotype and function in EMC.Methods. The study population included 16 patients with EMC and 24 patients with HCV-MC. Molecular analysis was done for the detection of circulating clonal B cells and for B cell receptor sequencing. B-cell phenotype, proliferative response, apoptosis and ERK signalling were analysed by flow cytometry.Results. Molecular analysis of immunoglobulin genes rearrangements revealed circulating B-cell clones in about half of patients, on average of smaller size than those found in HCV-MC patients. Sequence analysis showed usage of the same stereotyped RE-encoding B-cell receptors frequently expressed in HCV-MC and in primary Sjogren's syndrome. B-cells with low expression of CD21 (CD21(low)) and unusual homing and inhibitory receptors were increased in EMC and in HCV-MC, but at a significantly lower extent in the former. The CD21(low) B-cells of EMC and HCV-MC patients shared functional features of exhaustion and anergy, namely reduced proliferation upon ligation of Toll-like receptor 9, high constitutive expression of phosphorylated ERK, and proneness to spontaneous apoptosis.Conclusion. Our findings suggest a common pathogenetic mechanism in EMC, HCV-MC and primary Sjogren's syndrome, consisting of autoantigen-driven clonal expansion and exhaustion of selected RE-producing B-cells. The more massive clonal expansion in HCV-MC may be due to co-stimulatory signals provided by the virus.