Somatic hypermutations confer rheumatoid factor activity in hepatitis C virus-associated mixed cryoglobulinemia.

Chronic hepatitis C virus (HCV) infection is a major global health burden, affecting [{"type":"entrez-nucleotide","attrs":{"text":"H11011","term_id":"875831","term_text":"H11011"}}H11011]170 million persons worldwide, and it is the leading indication for liver transplantation in the US and Europe (1). Up to 20% of individuals with chronic HCV develop serious liver-related complications, such as cirrhosis and hepatocellular carcinoma. Although hepatocytes are the primary target for HCV infection, extrahepatic manifestations of HCV infection occur in up to 40% of patients (2). One of the most prominent of these manifestations is a small vessel vasculitis associated with mixed cryoglobulinemia (MC) (3). MC is characterized by the aberrant clonal expansion of B cells that produce rheumatoid factor (RF)–like IgM (4,5). This RF forms immune complexes (ICs) containing polyclonal HCV– specific IgG and HCV RNA; these complexes in turn deposit on the vascular endothelium of organs such as skin, kidneys, and peripheral nerves, eliciting a complement C1q–mediated vasculitis (6). HCV is also associated with an increased risk of B cell non-Hodgkin's lymphoma (NHL) (7), frequently of the low-grade marginal zone (MZ) or mucosa-associated lymphoid tissue (MALT) subtypes. The continued presence of HCV is necessary for abnormal B cell lymphoproliferation in both HCV-associated MC (HCV-MC) and NHL, since eradication of HCV typically results in resolution of both diseases (8). However, it remains unclear why B cells undergo clonal proliferation during chronic HCV infection. It is likely that HCV-induced B cell lymphoproliferation arises from chronic antigenic stimulation of an initially limited pool of preexisting autoreactive B cells. Persistently high levels of HCV-containing ICs may stimulate the proliferation of RF-bearing B cells, but the precise antigen(s) and stimulatory mechanisms have remained elusive. A striking feature of these expanded B cells is their preferential use of the Ig gene segments VH1-69 and Vκ3-20 (5,9,10), which together frequently encode RF of the Wa cross-reactive idiotype (11). We have previously shown that these antibodies have low-to-moderate levels of somatic hypermutation (SHM), and our phylogenetic analyses have suggested that they have acquired SHM as a result of antigen-directed affinity maturation (5). Expansions of B cells expressing VH1-69/Vκ3-20–encoded RFs have been found in other diseases without clearly defined infectious etiologies, including primary Sjogren's syndrome (SS) (12), chronic lymphocytic leukemia (CLL) (13,14), and non–HCV-related MALT NHL (12). It has been proposed that a VH1-69–restricted repertoire has been shaped by evolution to recognize common antigens and that continued proliferation predisposes to clonal B cell expansion (15). Moreover, it has been suggested that polyreactive IgM “natural antibodies” rapidly form a first line of defense against invading pathogens, as they broadly recognize microbial epitopes independently of SHM. Interestingly, some humoral immune responses to microbial infections are associated with autoreactivity. For example, it has recently been shown that anti–human immunodeficiency virus (anti-HIV) neutralizing antibodies are polyreactive to both HIV and self antigens (16). Whereas the HIV reactivities of these (highly mutated) antibodies depend upon SHM, their self reactivities do not. This implies that in the case of HIV, antibodies are positively selected on the basis of germline-encoded polyreactivity, and that this polyreactivity is retained throughout affinity maturation despite SHM. Given the above, a central question in HCV-MC pathogenesis is what causes the expansion of autoreactive B cells, and whether RF activity is present in the germline or arises as a result of SHM. A finding that HCV-related RFs depend upon SHM for their activity would strongly suggest that during MC pathogenesis, certain VH1-69/Vκ3-20–expressing B cells may have an initial low affinity for IgG, but this affinity increases as a result of IgG-directed SHM. Such a finding would imply that IgG coupled to HCV, not HCV by itself, is a major driver of B cell lymphoproliferation. This result has implications not only for HCV-MC, but also for other IC-mediated diseases.