Introduction

Hepatitis C virus (HCV) infection is a global health problem, affecting approximately 170 million people worldwide, and causing chronic hepatitis, liver cirrhosis and hepatocellular carcinoma with liver-related mortality in about 350,000 people/year. However, these data are underestimated do not taking into account the extrahepatic aspects that make the infection a systemic disease. Early after its discovery, it was shown that HCV is not only hepatotropic but also lymphotropic [1–3]. It was also shown that several extrahepatic manifestations (EHMs-HCV) can complicate HCV infection [4–6] including, first, lymphoproliferative and/or autoimmune disorders [7]. The present review will focus on the principal data available about the non-strictly hepatic manifestations of HCV, justifying the systemic nature of the consequences of such a chronic infection.

HCV and lymphoproliferative disorders

Mixed cryoglobulinemia

Mixed cryoglobulinemia (MC) is the most frequent, best known and strictly HCV-associated EHMs-HCV (>90 % MC HCV+ in some studies) [7–9]. MC may be defined a both autoimmune and B-lymphoproliferative disorder (LPD) that may evolve to a frank malignancy in about 8–10 % of cases [6, 10–14]. The definition refers to the presence of serum Igs that reversibly precipitate at low temperatures (<37 °C) (cryoglobulins, CGs) and are represented by circulating immune complex (CIC) typically consisting of an IgM RF (mono-oligoclonal in type II MC, or polyclonal in type III MC) and polyclonal Igs (most frequently IgGs) including anti-HCV antibodies [11]. MC has been generally reported, at least subclinically, in the majority of HCV patients, even if data may widely vary in different geographical areas (from <20 to >50 %) [4, 6, 15–17]. Only a minority of MC patients (5 to >30 %) shows a symptomatic MC or MC syndrome (MCS) (usually women aged more than 50 years), but even asymptomatic patients might develop MCS in the future [12]. The clinical manifestations of MCS are secondary to a systemic vasculitis characterized by the deposition of CGs in the vessels and can be classified as one of the CIC-mediated systemic vasculitis involving small-sized vessels. First described by Meltzer and Franklin [18] as a triad of purpura, fatigue and arthralgia, the various involvement of different organs and tissues (mainly skin, joints, renal, peripheral nerves) leads to variable clinical presentation and evolution [12, 19]. Palpable purpura (leukocytoclastic vasculitis) and petechiae most often affects the legs. Papules, ulcers, and livedo can also occur and can affect any skin site. A sicca syndrome, to be differentiated from the primary Sjogren syndrome (SS), occurs in MC and also in HCV patients without MC [20]. In the worst cases, a severe involvement of the kidney (see also below) and the peripheral and central nervous system is observed [6]. The peripheral neuropathy including mixed neuropathies (prevalently sensitive, axonal) is common in MC (80–90 % of cases), and also in HCV without MC [21]. Several studies, including personal observations, showed an association between MCS and severe liver damage. However, discordant data exist [22]. Circulating CGs, RF, and a low C4 represents the serum hallmarks of MC. In a large study, a significantly lower cumulative 10th-year survival, calculated from time of diagnosis, was scored in MCS patients versus the control population [12]. Various attempts at defining a clinical classification have been made since the 1990s and recently, preliminary classification criteria were developed according to the results of an international, European study [20], and then validated [23].

Concerning the therapeutic approach of MCS, the individuation of the viral etiology justified the use of antiviral drugs in addition to the classic symptomatic/pathogenetic therapies. Starting from the first pioneer studies [24, 25], the antiviral approach evolved following the modifications of anti-HCV therapy and, at present, the combination of pegylated IFN (Peg-IFN) plus ribavirin has been suggested as standard of care (SoC) for mild to moderate MCS [6, 26–28]. Evidence for the strong correlation between a complete viral eradication and resolution or consistent improvement of MCS has been produced and a role played by occult viral persistence in lymphatic cells shown [29, 30]. The effects of viral eradication in HCV-related LPDs confirm the etiopathogenetic role played by HCV. A regression of B-cell pathogenetic clones after viral eradication and new expansion in case of virological relapse was shown [31, 32]. For these reasons, the goal of therapy in HCV MCS should be to eradicate the virus. However, a cautious attitude could be suggested in consideration of the side effects possibly induced by IFN in a MC background (i.e., possible worsening of neuropathy, anemia). The introduction of new, more effective, direct anti-HCV drugs is of high interest, as also suggested by preliminary personal observations. For patients with severe vasculitis, including cases of renal insufficiency or intestinal ischemia, control of disease with potent immunosuppressive regimens (i.e., anti-CD20 Rituximab, RTX), with or without plasmapheresis, is usually required. The use of RTX in MCS was shown to be safe and useful in improving main manifestations, especially cutaneous [33–38]. Its use was initially limited to patients without advanced liver disease due to the risk of a hepatitis flare. However, in a study involving patients with MCS and severe liver disease, RTX was not only useful but also safe. Surprisingly, a consistent improvement of the cirrhosis syndrome was observed in some decompensated patients [35]. An IFN-based etiologic treatment in combination with RTX was also suggested to be suitable and effective in HCV MCS patients [39, 40].

Lymphoma and monoclonal gammopathies

The very close association between MC and HCV infection leads to the hypothesis that HCV may be involved in the pathogenesis of lymphoma as well [4, 7, 9]. A significant association between HCV and B-cell non-Hodgkin’s lymphoma (NHL) was initially reported in Italian subjects [41, 42] and confirmed in the large majority of studies [43, 44]. Some discordant data from northern countries [6] suggested the contribution of genetic factors. This association involves different histopathological types of B-cell NHL, the most strictly associated being the lymphoplasmacytic, marginal zone and diffuse large B-cell lymphoma [44]. A serum monoclonal gammopathy (MG), more frequently type IgMk and diagnosed as MG of uncertain significance (MGUS), was frequently observed in HCV patients, in most cases associated with a 2a/c genotype [45]. Clinical remission following effective antiviral therapy was especially observed in low-grade B-cell NHL and, in particular, in splenic marginal zone lymphoma (SMZL) [46]. Furthermore, the preventive value of viral eradication against the lymphomagenetic evolution of HCV infection was observed in some studies [47]. Consequently, the inclusion of antiviral therapy, alone or in combination with RTX, seems to be rational in the therapy of low-grade HCV-positive NHL [31, 46, 48]. In intermediate to high grade NHL, anti-HCV therapy may be helpful in prolonging the disease-free period [49]. Results of further, sufficiently large and controlled trials, will be precious in order to better standardize the therapeutic approach.

Pathogenesis of HCV-related LPDs

Available data suggest that HCV-related LPDs are the result of multiple and cooperating mechanisms and events belonging to three principal categories: an important and sustained activation of the B-cell compartment; an inhibition of B-cell apoptosis; genetic/epigenetic and environmental factors (Fig. 1).

Fig. 1
figure 1

Hypothetical pathogenetic cascade explaining the development of lymphoproliferative disorders (LPDs) during HCV chronic infection. HCV-related LPDs seem to be the result of multiple and cooperating mechanisms and events. Available data suggest that these latter belong to three principal categories: (1) an important and sustained activation of the B-cell compartment, possibly explained by several factors including the antigenic stimulation consequent to the chronic infection of liver/B cells, the E2-CD81 binding, a direct action of some HCV proteins, some cytokines (BAFF); (2) a prolonged B-cell survival/inhibition of B-cell apoptosis induced by t(14;18) translocation and other factors (possibly including different genetic aberrations and cytokines), (3) on a background of predisposing genetic-epigenetic and/or environmental factors conditioning the LPD clinical manifestation. The occurrence of additional genetic aberrations, favored by the abnormal B-cell survival, would lead to the frank malignancy. Refer to the text for a more detailed description

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A sustained antigenic stimulation due to the chronic HCV infection was shown to play an important predisposing role. Intrahepatic lymphatic follicle-like nodules are a peculiarity of HCV infection and sites of clonal B-cell expansion [11]. Isolated B cells have been shown to produce the WA cross-idiotype, a RF typical of MC [50]. Deeper analysis of BCR sequence and affinity showed a homology with anti-HCV protein. A more specific role of some viral antigens (NS3 and E2) in inducing HCV-related LPDs was proposed [51]. Interestingly, the HCV E2 protein was shown to be able to bind to the CD81 expressed on B cells leading to a lower B-cell activation threshold [52]. In addition, Igs isolated from a HCV NHL were able to bind E2 protein and a sequence homology was found between HCV E2 and IgG variable domains [53, 54]. The role played by the viral lymphotropism in the pathogenesis of HCV-related LPDs was initially suggested by the observation of significantly higher prevalence of PBMC or bone marrow cell infection in HCV+ patients with than without MCS [1, 55]; such observations were also confirmed by studies on in vivo and in vitro animal models [56, 57]. Interestingly, in an in vitro model, HCV-infected cells showed an increased rate of mutations of oncogenes and Ig genes, probably by the induction of an error prone DNA polymerase and the activation-induced cytidine deaminase (AID) [58]. This hypothesis was supported by the detection of abnormal levels of AID in B cells of HCV patients [59]. Concerning a possible direct role of viral proteins, the major interest was addressed to the effects of HCV core protein. Interestingly, in different transgenic models, the expression of HCV core was correlated with the development of lymphoma [60, 61]. Finally, an altered expression of some isoforms of the p53 family genes, the DNp63 and DNp73, previously described to be overexpressed in human cancers including lymphoma, was found in both B-cell lines expressing the HCV core protein and in primary B cells from patients with LPDs [62]. In the HCV-related lymphomagenetic process, an important step, possibly inducing “points of no-return”, is the occurring of chromosomal aberrations. In this context, the (14;18) translocation, causing the enhanced transcription of Bcl-2 protein and abnormal survival of B cells, was deeply described as strictly associated to type II HCV MC [10, 31, 63]. In addition, the regression of t(14;18)-positive B-cell clones after antiviral therapy with a new expansion of the translocated clones in patients who virologically relapsed was shown [29–32]. Furthermore, alterations of the ploidy grade, with a similar rate to the one characterizing the NHL, were recently described in HCV patients [64]. Several reports suggested the role of particular cytokines and chemokines, even if a profile typical for HCV LPDs is still not fully defined. These include IFNγ, TNFα, MIP-1α, MIP-1β, CXCL-10, CXCR-3, CXCL13 (BCA-1 or BLC), CXCL-11, IL-1α and osteopontin (OPN) [65–68]. A particular attention was focused on the role of B-cell activating factor, Baff or BLyS. In HCV patients, increased Baff serum levels have been found, especially in subjects with LPDs [69]. A particular genetic variant (−871 T) of Baff promoter was found with higher frequency in HCV patients with than without MC, strongly suggesting a key role of genetic factors in Baff overexpression [70, 71]. Finally, as already shown for other diseases, the role of microRNAs is possibly critical in the pathogenesis of HCV-related LPDs representing a field of interest, even if only a few data are nowadays available [72].

Renal pathology

Several renal manifestations have been associated with HCV infection, the most common being membranoproliferative glomerulonephritis (MPGN) [73–75]. HCV-associated membranous or proliferative glomerulonephritis or focal segmental glomerulosclerosis have been also described [22, 74].The strongest association was reported for cryoglobulinaemic MPGN (MC-MPGN) [6, 73, 74]. The presence of a renal involvement is one of the worst prognostic indices in the natural history of MCS [12] and in the etiologic treatment [76] even if its course can be variable. The urine analysis and creatinine determination should be suggested in HCV patients.

Endocrine pathology

The prevalence of thyroid disorders is generally high in HCV-positive patients [77] and most frequently represented by antithyroid peroxidase antibodies in female subjects [77]. Hypothyroidism has been frequently observed, especially in HCV MC [78], and an association with papillary thyroid carcinoma was also shown [79, 80]. The possibility that IFN treatment may unmask autoimmune thyroid disease must be also taken into account [81]. Overall, a careful monitoring of the thyroid in HCV patients should be suggested.

Interesting and still incompletely understood data exist about the relationship between HCV (especially type 3) infection and glucose metabolism. Several studies showed that HCV could lead to the development of type 2 diabetes mellitus, possibly as a result of HCV-induced metabolic disturbances [82]. However, discordant data exist [83] and the argument is still object of debate. Insulin resistance was observed in 30–70 % of HCV patients [84], improved after viral eradication [85] and was interpreted as the effect of several factors including proinflammatory cytokines/adipokines, advanced liver disease or HCV-related steatosis. Careful monitoring of glycaemia in patients with HCV infection is recommended.

Dermatological manifestations

Several dermatological manifestations have been associated with HCV infection including, first, the cryoglobulinemic vasculitis (see before), porphyria cutanea tarda (PCT) and lichen planus (LP). Others, including pyoderma gangrenosum, erythema nodosum, and urticaria, have been described in only anecdotal observations and need confirmation. The association between PCT and LP and HCV infection appeared strict in several studies mainly performed in southern Europe and Japan [4, 86–88]. However, it was suggested that the infection plays an indirect role, probably acting as a triggering factor in subjects predisposed due to genetic factors and/or liver disorders (hepatic iron overload, liver cirrhosis) [5, 87].

Cardiovascular manifestations

Recent studies suggest that HCV is involved in the development of dilated cardiomyopathy, hypertrophic cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy in addition to myocarditis. These associations are still object of debate [6].

The influence of HCV infection on aortic atherosclerosis risk was mostly evident in case of active viral replication and in some geographic areas [89, 90]. In an Italian study, HCV RNA sequences, including HCV replication intermediates, were found in the plaque tissue isolated from anti-HCV patients, including serum HCV RNA-negative subjects [89], suggesting a direct local action of the virus [89]. More recently, other studies performed on HCV mono-infected or HCV–HIV co-infected patients confirmed the link between carotid atherosclerosis and HCV chronic infection [91–93].

Rheumatologic disorders

Polyarthralgia is the most common rheumatologic symptom described in HCV-infected patients [5, 78]. HCV arthritis could be part of the MCS or be independent [94]. HCV-associated oligoarticular or polyarticular non-erosive arthritis can clinically mimic rheumatoid arthritis, although anti-cyclic citrullinated peptide (anti-CCP) antibodies and erosive joint changes are generally absent.

Central nervous system (CNS) abnormalities

Mild neurocognitive impairment, depression, anxiety and other psychiatric disorders have been frequently reported in HCV infection [95, 96]. An alteration of the neuro-endocrine-immune system response has been observed and could play a key role [96]. Serum tryptophan (TRP) and kynurenine levels were lower in HCV patients than in controls [97], but returned toward physiological levels after viral eradication and psychopathology improved, thus suggesting a direct viral role [98]. Overall, the CNS abnormalities observed during HCV infection greatly cooperate in the impairment of the quality of life of HCV patients with corresponding important direct and indirect costs [96].

In conclusion, HCV infection induces a systemic, complex disease whose varying manifestations deserve to be correctly recognized and managed. The possibility that HCV may be eradicated following effective antiviral therapy is important for both the therapeutic, preventive and pathogenetic points of view.