Chronic hepatitis C virus (HCV) infection is currently the second most common viral infection worldwide, with approximately 170 million people infected . Although the number of newly infected individuals is gradually decreasing as a result of the appropriate screening of blood and blood-derived products, a substantial number of patients who acquired the infection before 1990 present with evidence of chronic liver disease and its associated complications.
Rapidly accumulating data over the past 15 years have provided a clearer picture of the prevalence, natural history and morbidity/mortality of this chronic infection [2,3]. Its prevalence is approximately 2% worldwide, with significant geographical variations, whereas its incidence in high-risk populations including intravenous drug users and individuals with multiple sexual partners remains high. HCV infection is characterized by a high rate of chronicity (50–80%) indicating the ability of the virus to overcome the initial host immune response [2,3]. Approximately one fifth of chronically infected patients develop significant chronic liver inflammation that progressively leads to cirrhosis and hepatocellular carcinoma . The recent introduction of combination therapeutic schemes including new (ribavirin) and improved forms of established (pegylated interferon-α) antiviral agents offer for the first time the chance of a cure (viral eradication) in more than half of chronically mono-infected individuals .
Soon after the discovery of HCV as the major etiological agent of non-A non-B chronic hepatitis, it was appreciated that this RNA virus was uniquely associated not only with chronic liver inflammation but also with an array of extrahepatic manifestations . In the majority of these associated extrahepatic conditions, the pathogenetic mechanisms that are operating are considered autoimmune in origin.
In this review, the available data on the pathogenesis, prevalence and characteristic features of the clinical and laboratory extrahepatic autoimmune manifestations in patients with chronic HCV infection will be summarized.
Overview of extrahepatic manifestations
In Table 1 the conditions and laboratory autoimmune manifestations that have been associated with chronic HCV infection are shown. For some of these conditions a strong link has been established based on epidemiological and clinical observations over the past 15 years, whereas for others the supporting data are limited or conflicting.
Among the numerous extrahepatic manifestations of chronic HCV infection that have been reported so far, cryoglobulinemia and its clinical sequelae hold the strongest association . The circulating cryoglobulins that are detected in patients with chronic HCV infection are mixed cryoglobulins, composed of polyclonal IgG and monoclonal (type II) or polyclonal (type III) IgM antibodies with rheumatoid factor activity . Although cryoglobulinemia can be observed in a variety of autoimmune, infectious and neoplastic disorders, chronic HCV infection is by far the most common cause [9,10]. Cryoglobulins are found in approximately half (45–55%) the patients with chronic HCV infection [9,10].
The unique viral–host interactions that culminate in the generation and sustained production of these autoantibodies have not been delineated. Certain observations regarding their presence in chronic HCV infection can be made. First, cryoglobulin production in HCV infection is closely related to disease duration. Chronically infected cryoglobulinemic HCV patients have a much longer disease duration compared with HCV patients without cryoglobulinemia [11,12]. Moreover, there have not so far been any reports of cryoglobulinemia in patients with acute hepatitis C. It thus appears that chronic antigenic stimulation is required for the emergence of specific B cells that produce cryoglobulins. Second, in patients with type II cryoglobulinemia the majority of monoclonal IgM rheumatoid factors (80%) share a common idiotype, termed WA. This feature indicates an antigen-driven clonal expansion of B cells. Intrahepatic B-cell clonal expansions have recently been demonstrated in the liver microenvironment of HCV-infected patients and appeared to correlate significantly with symptomatic cryoglobulinemia . Third, in the majority of patients cryoglobulinemia remains asymptomatic . A number of studies from Europe and north America have demonstrated that symptomatic cryoglobulinemia and cryoglobulinemic vasculitis is present in less than 10% of patients with chronic HCV infection . In a recent case–control study of male USA veterans, cryoglobulinemia was found in only 0.6% of hospitalized patients with chronic HCV infection (n = 34 204) . In the majority of cases with symptomatic cryoglobulinemia that have been reported in the literature type II circulating cryoglobulins were present .
The critical host or virological factors that determine the appearance of symptoms in this group of patients are currently under intense investigation. In a recent study , patients with mixed cryoglobulinemia were in general older, with longer disease duration, higher levels of rheumatoid factor and cryoglobulins as well as soluble IL-2 receptor levels compared with asymptomatic cryoglobulinemic patients and patients with liver disease only.
The clinical syndrome of mixed cryoglobulinemia and the associated cryoglobulinemic vasculitis results from tissue deposition of immune complexes containing cryoglobulins and the subsequent localized host immune response . The most common clinical manifestations of HCV-associated mixed cryoglobulinemia are shown in Table 2. Non-specific symptoms such as fatigue, arthralgias, myalgias and weakness are prevalent in this group of patients (40–90%, Table 2) [9,10,18–20]. At the same time, it should be noted that these complaints are common in unselected HCV populations without cryoglobulinemia (15–25%)  (M. O'Neil, L.H. Calabrese, unpublished data).
The main targeted organs in patients with mixed cryoglobulinemia are the skin, kidneys and nervous system . Intermittent purpura of the lower extremities is the most common clinical finding (70–90%) and is usually the presenting manifestation of HCV-associated cryoglobulinemic vasculitis. A leukocytoclastic vasculitis of the small vessels with immune complexes containing HCV RNA deposited in the vessel walls is detected in skin biopsies of these patients. Leg ulcers, nodules, urticaria or more rarely digital necrosis can also occur .
In one third of patients with mixed cryoglobulinemia, symptomatic renal involvement occurs, mainly in the form of membranoproliferative glomerulonephritis (MPGN) . A number of other less common nephropathies have been reported, including membranous nephropathy, focal segmental glomerulosclerosis and messangial proliferative glomerulopathy . MPGN represents a late manifestation of mixed cryoglobulinemia and presents usually as hypertension, hematuria, proteinuria and renal insufficiency. The cumulative incidence of chronic renal insufficiency has not been clearly defined, but it has been estimated to occur in 15% of cases .
An immune-complex mediated glomerulonephritis with predominant renal deposition of cryoglobulins containing monoclonal rheumatoid factor (type II) is the presumed underlying pathogenetic mechanism of HCV-associated MPGN. The attempt to detect HCV RNA and HCV-encoded proteins in kidney lesions from these patients has given conflicting results .
The prevalence of nerve involvement in patients with mixed cryoglobulinemia ranges between 20 and 35% [24,25]. Although both the peripheral and the central nervous system can be affected, the vast majority of patients present with peripheral neuropathy either in the form of symmetrical distal polyneuropathy (80%) or mononeuritis multiplex (10%) . Biopsies from involved nerves or muscles reveal inflammation of small or medium size vessels with or without associated necrosis [24,25]. Although HCV RNA has been detected in these lesions, replicative virus has not been isolated. Central nervous system involvement has rarely been reported in patients with mixed cryoglobulinemia .
Approximately 10% of patients with chronic HCV infection complain of sicca symptoms (xerostomia or xeropthalmia) . Biopsies of salivary glands in these patients reveal a mild lymphocytic infiltration (grade I–II) by B and T lymphocytes. In contrast to primary Sjögren's syndrome, the more specific anti-Ro/SS-A or anti-La/SS-B autoantibodies are usually absent (3% of cases), whereas there is no association with known HLA class II phenotypes . In a recent study, Ramos-Casals et al.  showed that patients with HCV-associated sialadenitis more often had swelling of the parotid glands, circulating cryoglobulins and low complement levels compared with patients with primary Sjögren's syndrome. A close relationship between HCV-associated sialadenitis and cryoglobulinemia was also shown in older studies .
The pathogenesis of HCV-associated lymphocytic sialadenitis has not yet been elucidated. A direct pathogenetic role for HCV has been hypothesized based on the resemblance of salivary lesions from a transgenic model overexpressing HCV envelope proteins to HCV-associated human sialadenitis . To date the search for replicating virus or virus-encoded proteins in human salivary glands has provided conflicting results .
Porphyria cutanea tarda
A possible link between chronic HCV infection and porphyria cutanea tarda (PCT) is based mainly on epidemiological data showing an increased prevalence of HCV infection in unselected populations of patients with PCT (∼45%) . On the other hand, two studies of large cohorts of HCV-infected patients from France and the USA showed that the prevalence of PCT was lower than 1% [15,21]. Furthermore, there are no convincing experimental data so far to support a direct pathogenetic role for HCV in PCT.
Oral lichen planus
Lichen planus is detected in 0.3–4% of patients with chronic HCV infection [15,21]. Its frequent association with chronic liver diseases in general has raised doubts about its relation to chronic HCV infection. Recent histological and experimental in-vitro findings suggest that this lesion may result from a localized cellular immune response to HCV. HCV RNA has been detected at low levels in lymphoproliferative lesions by different investigators, whereas at the same time Pilli et al.  were able to demonstrate HCV-specific CD4 and CD8 T-cell responses in these inflammatory lesions.
The role of chronic HCV infection in the development of lymphoproliferative disorders currently remains one of the most debatable issues . The majority of lymphoproliferative disorders that have been reported in chronically infected HCV patients are B-cell non-Hodgkin's lymphomas (NHL) [7,33]. Data supporting an etiopathogenetic role for HCV come from two critical observations: first, a number of epidemiological studies have shown an increased prevalence of HCV infection among patients with B-cell NHL (13% in a recent metanalysis) ; second, in certain well-documented cases of lymphomas, antiviral treatment was associated with the regression of NHL [34,35].
On the other hand, the arguments against the involvement of HCV in the pathogenesis of B-cell NHL stems from large prospective studies that failed to show such an association [36,37]. Furthermore, a number of case–control studies of B-cell NHL in different countries did not show any association with chronic HCV infection .
It has been hypothesized that HCV can participate in the transformation of normal B cells to neoplastic lymphocytes by two mechanisms. The first involves a direct lymphomagenic role for HCV replicating in normal B cells. Replicating negative strands of HCV RNA have rarely been documented in normal B cells from patients with HCV infection and mainly in cases in which HIV co-infection was present [38,39]. The second proposed process begins with chronic antigenic stimulation of the immune system by HCV, which in certain predisposed individuals and under the influence of additional factors (e.g. t(14,18) translocation, c-myc mutation, etc.) can lead years later to the emergence of neoplastic B cell clones . So far, accumulating data support the second hypothesis. Further studies involving detailed molecular clonal analysis of the transformed B cells as well as carefully designed prospective studies of large HCV cohorts are expected to answer these important questions about the role of HCV in B-cell NHL.
One of the laboratory hallmarks of chronic HCV infection is the frequent detection of various autoantibodies (Table 3) [21,40–42]. Rheumatoid factor and cryoglobulins are the most commonly encountered autoantibodies, detected in 40–70% of patients with chronic HCV infection (Table 3). Non-organ-specific autoantibodies such as antinuclear, anti-smooth muscle and antimitochondrial antibodies are detected less frequently (1–30%), whereas organ-specific antibodies such as antithyroid antibodies are found in less than 10% of cases [21,40,41].
The frequent appearance of organ and non-organ-specific autoantibodies indicates the profound effect of HCV in the host immune system. A direct effect of HCV on various cells of the host immune machinery has been shown including B cells , T lymphocytes , dendritic cells  and natural killer cells [46,47]. Although this interaction appears to be primarily responsible for the frequent transition of acute to chronic infection, at the same time it may be responsible for the emergence of immune cells with autoreactive potential. Despite the frequent detection of these autoantibodies their clinical significance, with the exception of cryoglobulins, is not known.
Autoimmune cytopenias including autoimmune thrombocytopenia, autoimmune hemolytic anemia and autoimmune neutropenia are increasingly reported in patients with chronic HCV infection . Although cytopenias are not uncommon during the course of this chronic infection, other non-immune mechanisms may be involved including hypersplenism and antiviral drug complications. Nevertheless, in certain cases autoimmune mechanisms are operating. Ramos-Casals et al.  recently reviewed 35 cases of autoimmune cytopenias in patients with chronic HCV infection who had not received antiviral treatment. In the majority of cases, cytopenias were severe and were associated with increased mortality. It should be noted that in approximately 70% of cases other immunological abnormalities were present (hypocomplementemia, cryoglobulinemia, the presence of antinuclear antibodies) indicating immune dysregulation in this group of patients.
In conclusion, the propensity of chronic HCV infection to be associated with numerous extrahepatic immunological manifestations is unique and can be matched only with similar manifestations of another chronic viral infection that targets the host immune system, i.e. HIV infection. The exact mechanisms that lead to this immune dysregulation in a number of systems outside the liver, which is the main target organ of chronic HCV infection, still remain unclear. Regardless of their pathogenesis, the appearance of clinical or laboratory manifestations such as unexplained fatigue, arthralgias or arthritis, sicca symptoms, purpura, skin ulcers, polyneuropathy, Raynaud's phenomenon, the presence of positive rheumatoid factor, cryoglobulins or other autoantibodies should alert the physician to a possible underlying HCV infection. Appropriate testing for HCV infection could identify the correct diagnosis and offer the chance of a cure.
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