Immunobiology of essential mixed cryoglobulinemia

Introduction Mixed cryoglobulinemia (MC) is characterized by the production of monoclonal (type II MC) or polyclonal (type III MC) rheumatoid factors (RF), which form with endogenous IgG cold-precipitable immune complexes that cause small-vessel vasculitis and multi-organ damage. Hepatits C virus is the causative agent in 90% of MC patients, usually characterized by the expansion of an anergic B cell subpopulation called CD21low B cells. Only a minority of the patients has idiopathic or essential MC (EMC) and the B cell population has been scarcely investigated so far. Objective: to characterize the phenotypical and functional proprieties of B cells in EMC and compare them with those of HCV-related MC and from healthy donors. Method The B cell phenotype and function was studied in 13 patients with EMC and compared to 24 patients with HCV-MC. The proliferative response of B cells was investigated through the CFSE assay, the intracellular pERK content was measured by the BD Phos-Flow system and apoptosis was measured through annexin/7AAD staining. All the analyses were performed by flow-cytometry. Results EMC patient showed significant lower absolute numbers of circulating B cells compared to HCV-MC (mean ± SD: 185/mm3 ± 236 vs 529/mm3 ± 795). Interestingly percentages and absolute numbers of CD21low B cells were significantly higher in EMC compare to HD but lower than HCV-MC patients. Similarly to CD21low B cells found in HCV MC, CD21low B cells in EMC proliferated poorly in response to TLR9 stimulation, displayed dysregulated pERK signaling and were apoptosis prone. Conclusion Similar features of virus-specific exhaustion and anergy induced by continual antigenic stimulation observed in B cells expanded in HCV-MC are found in B cells EMC. Our findings open the question of a possible role of a still yet unknown antigen responsible for the development of EMC.