Mixed cryoglobulinemia (type II or type III), cryoglobulinemic vasculitis.

The term cryoglobulinemia refers to the presence in the serum of one (monoclonal cryoimmunoglobulinemia) or more immunoglobulins (mixed cryoglobulinemia), which precipitate at temperatures below 37°C and re-dissolve on re-warming 12. This is an in vitro phenomenon (Fig. 1), the actual mechanism(s) of cryoprecipitation remains obscure, it could be secondary to intrinsic characteristics of both mono- and polyclonal immunoglobulin (Ig) components, it can be caused as well by the interaction among single components of the cryoprecipitate 1234567891011121314.

Cryoglobulinemia is usually classified into three subgroups 4 according to Ig composition (Table 1): type I cryoglobulinemia is composed of only one isotype or subclass of immunoglobulin. Both type II and type III mixed cryoglobulins are immune complexes composed of polyclonal IgGs, the autoantigens, and mono- or polyclonal IgMs, respectively; the IgMs are the corresponding autoantibodies with rheumatoid factor (RF) activity 3456. With more sensitive methodologies, i.e. immunoblotting or 2-dimensional polyacrylamide gel electrophoresis, type II mixed cryoglobulins frequently shows a microheterogeneous composition; in particular, oligoclonal IgM or a mixture of polyclonal and monoclonal IgM can be detected 3. This particular serological subset, termed type II-III mixed cryoglobulinemia (MC), could represent an intermediate, evolutive state from type III to type II MC. Moreover, type II-III MC could fit together the most recent molecular studies showing the presence of oligoclonal B-lymphocyte proliferation in the liver and bone marrow biopsies from MC patients 3. In two third of type II MC, a cross-idiotype WA monoclonal RF (firstly isolated from the serum of a patient with Waldenström's macroglobulinemia) has been demonstrated 14.

Type I cryoglobulinemia is almost invariably associated with well-known hematological disorders and is frequently asymptomatic per se; similarly, circulating mixed cryoglobulins are commonly detected in a great number of infectious or systemic disorders 1234567891011121314. On the contrary, 'essential' MC represents a distinct disorder 3456, which can be classified among systemic vasculitides 345678910. Cryoglobulinemic vasculitis (Fig. 2) is secondary to vascular deposition of circulating immune-complexes, mainly cryoglobulins, and complement, with the possible contribution of both hemorheological and local factors 3456. Due to its clinical and histological features, MC is classified in the subgroup of small vessel systemic vasculitides, which also includes cutaneous leukocytoclastic vasculitis and Henoch-Schonlein purpura 310.

The leukocytoclastic vasculitis is the histopathological hallmark of MC (Fig. 2). It may involve small- and medium-sized vessels and may be responsible for multiple organ involvement. The term 'cryoglobulinemic vasculitis' is frequently used as synonym; it better focus on the typical histopathological alterations responsible for cutaneous and visceral organ involvement 310.

The prevalence of MC presents great geographic heterogeneity; the disease is more common in Southern Europe than in Northern Europe or Northern America 345678910111213. It is considered to be a rare disorder, however, yet there are no adequate epidemiological studies regarding its overall prevalence. Given its clinical polymorphism, a single manifestation (skin vasculitis, hepatitis, nephritis, peripheral neuropathy, etc.) is often the only apparent or clinically predominant feature, so that MC patients are often referred to different specialties (Fig. 3). A correct diagnosis might thus be delayed or overlooked entirely. Consequently, the actual prevalence of MC might be underestimated.

For the same reasons, the clinical pattern of the MC syndrome may vary largely among patients series referred to different tertiary care facilities 36891011121314 (Fig. 3).

It has been estimated that low levels of circulating mixed cryoglobulins can be detected in over 50% of HCV-infected individuals, while overt cryoglobulinemic syndrome develops in about 5% 31516. Because of the wide diffusion of HCV infection worldwide, a growing incidence of HCV-related MC can be expected, especially in underdeveloped countries where HCV in the general population is rather prevalent 317.

The prevalence of 'essential' MC is reported as approximately 1:100,000 but this term is now referred to a minority of MC patients. The female-to-male ratio is 3:1.

According to its first description 5, MC syndrome is characterized clinically by a triad -purpura, weakness, arthralgias- and by a series of pathological conditions, including chronic hepatitis, membranoproliferative glomerulonephritis (MPGN), peripheral neuropathy, skin ulcers, diffuse vasculitis, and, less frequently, by lymphatic and hepatic malignancies 345678910111217181920212223. The clinical pattern of cryoglobulinemic vasculitis is comparable in patients with type II or type III MC 3. The prevalence of MC manifestations reported in Table 2 regards an Italian patient population referred to our university-based division of rheumatology. Patients' recruitment at different specialist centers, as well as the racial composition among patients' series may be responsible for the variable prevalence of MC symptoms reported in the literature 34567891014.

The presenting symptoms of MC largely vary among patients with cryoglobulinemia. At the initial observation, MC shows different clinico-serological patterns, varying from apparently isolated serum mixed cryoglobulins to complete cryoglobulinemic syndrome (Fig. 4). This is a combination of serological findings (mixed cryoglobulins with RF activity and frequent low C4) and clinico-pathological features (purpura, leukocytoclastic vasculitis with multiple organ involvement) 678910142425. Asymptomatic serum mixed cryoglobulins can be found in some chronically HCV-infected individuals 31516; a condition that may precede for years or decades the clinical onset of the disease. On the other hand, some patients show typical cryoglobulinemic syndrome, without serum cryoglobulins, the hallmark of the disease (Fig. 4). This is generally a transient phenomenon due to the wide variability of the percentage of cryoprecipitable immune-complex 326. Repeated cryoglobulin determinations are necessary for a correct diagnosis in these subjects.

Skin lesions represent the most frequent manifestations of the MC 678910142425. Orthostatic purpura is generally intermittent, the dimension and diffusion of purpuric lesions largely varied, from sporadic isolated petechias to severe vasculitic lesions, often complicated by torpid ulcers of the legs and malleolar areas (Fig. 2). In a significant proportion of patients, repeated episodes of purpura may lead to stable, often confluent areas of ochreous coloration on the legs (Fig. 2). Cutaneous manifestations, in particular orthostatic purpura and ulcers, are the direct consequence of vasculitic alterations with the possible contributions of various co-factors, in particular chronic venous insufficiency, physical stress, mainly the prolonged standing, and/or muggy weather. In addition, the contribution of hemorheological disturbances due to high cryocrit levels may also be taken in account 27. In this respect, the purpuric outbreaks are frequently observed late in the afternoon when highest cryocrit levels are generally observed, often in concurrence with prolonged standing 24.

MC patients commonly report arthralgias, while clear clinical signs of arthritis (usually mild, non-erosive oligoarthritis) are relatively rare 11122528

Almost half MC patients complain of xerostomia and xerophthalmia, however, only a few cases meet the current criteria for the classification of primary Sjogren's syndrome (see 'differential diagnosis') 111225.

Peripheral neuropathy may frequently complicate the clinical course of the MC, in the majority of cases as mild sensory neuritis 24252930. The common symptoms are paresthesias with painful and/or burning sensations in the lower limbs, often with nocturnal exacerbation. The patient quality of life may be severely compromised because of the chronicity of these symptoms along with their scarce sensibility to therapeutic attempts. In a minority of cases the peripheral neuropathy may be complicated by severe sensory-motor manifestations, which usually appeared abruptly, often as asymmetric mononeuritis; in some patients, it may complicate the alpha-interferon treatment, possibly in predisposed subjects 2930313233. Dysarthria and hemiplegia expression of central nervous system involvement are rarely reported 2534; it is often difficult to distinguish these symptoms from the most common atherosclerotic manifestations.

Since HCV infection represents the underlying disorder of the MC in the vast majority of cases, overt chronic hepatitis, generally with mild-to-moderate clinical course, can be observed at any time during the natural history of the disease 2425. Chronic hepatitis was found in a great number of patients (Table 2), evolving to cirrhosis in 1/4, while only 7 patients developed hepatocellular carcinoma. In few cases, especially in combination with renal involvement, liver involvement became a life-threatening complication. On the whole, the clinical course and the prognostic value of this manifestation seem to be less severe if compared to HCV-related chronic hepatitis without MC syndrome 2425; similarly, hepatocellular carcinoma less frequently complicates MC syndrome compared to the whole population of HCV-positive individuals 2425. These differences are quite intriguing, but very difficult to fully explain. It is possible that the light consumption of alcohol and/or the relatively low prevalence of HCV genotype 1b 2425 may explain, at least in part, the rather benign clinical course of liver involvement and its scarce prognostic relevance in MC series.

Membranoproliferative glomerulonephritis type 1 is another important organ involvement, which may severely affect the prognosis and survival of the disease 242534353637. MC-related nephropathy is a typical immune-complex-mediated glomerulonephritis, although other immunological mechanisms have also been hypothesized 24253839.

Widespread vasculitis involving medium-small sized arteries, capillaries and venules with multiple organ involvement may develop in a small proportion of patients 3610242540. This extremely severe complication may involve the skin, kidney, lungs, central nervous system, and gastrointestinal tract. In rare cases intestinal vasculitis may suddenly complicate the disease, often in patients with renal and/or liver involvement; pain simulating an acute abdomen is the presenting symptom of intestinal vasculitis. A timely diagnosis and aggressive steroid treatment are necessary for this life-threatening complication.

Interstitial lung involvement has been anecdotally observed in MC syndrome as well as in patients with isolated HCV infection 325414243. Almost invariably, lung involvement in MC is characterized by subclinical alveolitis, as demonstrated by means of bronco-alveolar lavage in unselected patient series 44; this condition may predispose to harmful infectious complications and, in rare cases, it may lead to clinically evident interstitial lung fibrosis. The hyperviscosity syndrome, due to high levels of serum cryoglobulins is another rare clinical manifestation of MC 27. Generally, there was no relationship between the severity of clinical symptoms, such as glomerulonephritis, skin ulcers, or diffuse vasculitis and the serum levels of cryoglobulins and/or hemolytic complement 32425. Low complement activity is almost invariably detectable in MC, with the typical pattern of low or undetectable C4 and normal or relatively normal C3 serum levels, regardless the disease activity (Table 2). Moreover, in vitro consumption of complement can be also observed due to the anti-complement activity of some cryoimmunoglobulins 3. Of interest, a sudden increase in C4, raised to abnormally high levels, can be observed in MC patients developing a B-cell lymphoma 45. The lack of correlation between circulating cryoglobulin levels and the severity/activity of MC manifestations might be explained on the basis of different hypotheses: the pathogenic role of other non-cryoprecipitable immune-complexes, their intrinsic capacity to activate the complement, and/or the in situ formation of immune-complexes, with a relative concentration of HCV virions 3142425.

Some endocrinological disorders are significantly more frequent in MC patients compared to the general population 3121324254647484950. The most common thyroid disorders are autoimmune thyroiditis, subclinical hypothyroidism, and thyroid cancer; while hyperthyroidism is less frequent, generally as reversible complication of interferon treatment 51525354. Moreover, a statistically increased incidence of diabetes mellitus has been observed in HCV-positive patients with and without MC syndrome compared to the general population 555657. Finally, HCV-positive males with or without cryoglobulinemic vasculitis may develop erectile dysfunction, attributable to hormonal and/or neuro-vascular alterations 49.

B-cell lymphoma is the most frequent neoplastic manifestation complicating MC, often as late manifestation of the MC syndrome 5859606162636465. This complication may be related to peripheral B-lymphocyte expansion and to lymphoid infiltrates observed in the liver and bone marrow of MC patients 19395859. In particular, these infiltrates have been regarded by some authors as "early lymphomas", since they are sustained by lymphoid components indistinguishable from those of B-cell chronic lymphocytic leukemia/small lymphocytic lymphoma (B-CLL) and immunocytoma (Ic) 3122559. However, unlike frank malignant lymphomas, they tend to remain unmodified for years or even decades and are followed by overt lymphoid tumors in about 10% of cases 3. These characteristics justify the proposed term of "monotypic lymphoproliferative disorder of undetermined significance (MLDUS)" 3122559. Interestingly enough, type II MC-related MLDUS has its highest incidence in the same geographic areas where about 30% of patients with 'idiopathic' B-cell lymphomas also display HCV-positivity, and where an increased prevalence of HCV genotype 2a/c has been observed in both MC and lymphomas 3122559. The type II MC-associated MLDUS presents two main pathological patterns; namely, the B-CLL-like and the Ic-like 3122559. In the clinical practice, it is not rare to observe the appearance of malignant B-cell lymphomas in patients with mild MC clinical course, sometimes unexpectedly during a routine evaluation. It is possible to observe a sudden decrease or disappearance of serum cryoglobulins and RF, sometimes associated with abnormally high levels of C4, as presenting symptom of complicating B-cell malignancy 40.

Other neoplastic manifestations, i.e. hepatocellular carcinoma or papillary thyroid cancer, are less frequently observed 311245059. In this light, the MC can be regarded as a pre-neoplastic disorder 61; consequently, a careful clinical monitoring is recommendable, even in the presence of mild MC syndrome 311.

Since the first description of MC syndrome 5, chronic hepatitis has been reported as frequent manifestation appearing during the clinical course of the disease 34561124; therefore, a possible role for hepatotropic viruses in the pathogenesis of the disease has long been suggested 312. A role for hepatitis B virus (HBV) has been firstly investigated 66; however, HBV viremia is rarely recorded, while anti-HBV antibodies largely varied among different MC patient populations 3. It can be estimated that HBV can represent a causative factor of MC in less than 5% of individuals (Fig. 5).

Soon after the discovery of hepatitis C virus (HCV) as the major etiologic agent of non-A-non-B chronic hepatitis 67, a possible role of HCV infection in MC was proposed independently by two pioneering reports 6869 showing a significantly higher prevalence of serum anti-HCV antibodies compared to the general population. This hypothesis was definitely demonstrated in 1991, when the presence of HCV RNA was detected by means of polymerase chain reaction (PCR) in 86% of Italian MC patients 70. Successively, an increasingly number of studies including clinico-epidemiological observations, as well as both histopathological and virological investigations (HCV RNA detection by PCR and/or in situ hybridization) have been definitely established the preeminent role of HCV in the pathogenesis of MC 891011121314212559717273747576. The prevalence of serum anti-HCV antibodies and/or HCV RNA in MC patients ranged from 70% to almost 100% among different patient populations 311121325. Given the striking association between MC and HCV infection, the term 'essential' is now referred to a minority of MC patients (in Italy <5%, Fig. 5) 311121325.

Because of the frequent association between MC and HCV, the behavior of MC is closely linked to the natural history of HCV chronic infection 31112132425. However, MC is also the result of concomitant genetic and/or environmental factors, which remain largely unknown (Fig. 6). HCV has been recognized to be both a hepato- and lymphotropic virus, as suggested by the presence of active or latent viral replication in the peripheral lymphocytes of patients with type C hepatitis or MC 37377. HCV is an RNA virus without reverse trascriptase activity; therefore viral genome cannot integrate in the host genome 63. Probably, HCV may exert a chronic stimulus of the immune-system, through different viral proteins such as core protein 92263. Chronic stimulation of the lymphatic system exerted through viral epitopes, autoantigen production, and/or molecular mimicry mechanism has been suggested by the presence, in HCV-positive patients, of anti-GOR antibodies, which are cross-reactive autoantibodies directed to both HCV core and a nuclear antigen named GOR 35963. Other Authors suggested that HCV, in association with very low-density lipoprotein (VLDL), might induce a T-independent primordial B-cell population producing monoclonal immunoglobulin with WA idiotype 14. In turn, HCV-VLDL complexes may trigger RF production as consequence of somatic mutations of WA clones; the possible evolution to B-cell lymphomas might be the consequence of the accumulation of stochastic genetic aberrations 59. Chronic stimulation of the B-cell by HCV epitopes may produce the expansion of some B-cell subpopulations with favorable and/or dominant genetic characteristics. This hypothesis recalls the pathogenetic role of Helicobacter pylori in MALT lymphoma of the stomach, for which different evolutive phases are requested 59.

Another important pathogenetic factor is the interaction between HCV E2 protein and CD81 molecule, a quite ubiquitary tetraspannin present on B-cells surface 78; the consequence may be the strong and sustained polyclonal stimulation of B-cell compartment (Fig. 6). A following pathogenetic step of HCV-related autoimmune-lymphoproliferative disorders may be the t 1418 translocation observed in B-cells of HCV-infected individuals 6379. Even if not definitely confirmed 8081, the t 1418 translocation might lead to abnormally elevated expression of Bcl-2 protein with consequent inhibition of apoptosis and abnormal B-cell survival. Interestingly, the relevant prevalence of t 1418 translocation in patients with only type C hepatitis (about 37–38%), become particularly high in patients with HCV-related cryoglobulinemic syndrome, ranging 85% in type II MC 63. It is possible to hypothesize that during chronic HCV infection, several factors (including the interaction between HCV E2 protein and CD81 molecule, the high viral variability, and the persistent infection of both hepatic and lymphatic cells) may favor a sustained and strong B-cell activation (Fig. 6). This latter may in turn favors the apparition of t 1418 translocation and Bcl-2 overexpression; the consequent B-lymphocyte expansion is responsible for autoantibody production, including the cryoglobulins 3111263777879. In addition, the prolonged B cell survival may represent a predisposing condition for further genetic aberrations, which may lead to frank B-cell malignancy as late complication of MC syndrome 11202482.

Of interest, HCV-driven lymphoproliferation may explain the pathogenetic role of HCV infection also in 'idiopathic' B-cell lymphomas 1159646580. This association was firstly described in unselected Italian patients with 'idiopathic' B-cell lymphomas 64 and successively confirmed by different epidemiological and laboratory studies, mainly in the same geographical areas where HCV-associated MC is commonly found 65.

Given its biological characteristics, HCV may be involved in a wide number of autoimmune and lymphoproliferative disorders 3111263777879. Fig. 6 summarizes the main causative factors -infectious, toxic, genetic, and/or environmental- that are potentially involved in the pathogenesis of MC 111279828384. These factors, alone or in combination, may trigger two multistep pathogenetic processes, not mutually exclusive, responsible for MC and other HCV-related disorders. The first one produces a 'benign' poly-olygoclonal B-lymphocyte proliferation responsible for organ- and non-organ-specific autoimmune disorders, including the immune-complex-mediated cryoglobulinemic vasculitis; the second one is characterized by different oncogenetic alterations, which ultimately may lead to malignant complications 1182. Comparable pathogenetic mechanisms could be also hypothesized for HCV-negative MC, this intriguing clinical subset might be correlated to other infectious agents or associated to some well-known autoimmune/rheumatic or lymphoproliferetive disorders (Fig. 5, 6) 13. Th1-mediated tissue alterations are probably responsible for some endocrine disorders, i.e. diabetes type 2 and thyroid dysfunction, in HCV-associated MC patients 57. Our preliminary data suggest that HCV infection of thyrocytes, as well as of beta cells, may upregulate CXCL10 gene expression and secretion (as shown in human hepatocytes); the consequent recruitment of Th1 lymphocytes that secrete IFN-γ and TNF-α may in turn induce CXCL10 secretion by infected cells, thus perpetuating the immune cascade responsible for these endocrine disorders. All the above pathogenetic mechanisms could be also hypothesized for HCV-negative MC, this intriguing clinical subset might be correlated to other infectious agents or associated to some well-known autoimmune/rheumatic or lymphoproliferetive disorders (Fig. 5, 6) 13.

MC with or without overt clinical syndrome has been reported in patients with a great number of infectious agents, usually as anecdotally observations 9. A significant prevalence of MC has been observed in patients with human immunodeficiency virus (HIV) infection, with and without HCV co-infection 85. HIV alone may exert a continuous antigenic stimulation of B-lymphocytes; these latter may be responsible for type III MC production earlier in the course of HIV infection. In some patients the B-cell disorder may evolve to monoclonal MC with typical clinical syndrome. As observed for HCV infection, the prevalence of other virus-related MC seems to show a variable prevalence among patient's series from different geographical areas 86. Moreover, a number of clinico-epidemiological studies revealed a heterogeneous distribution of different HCV-related extra hepatic manifestations, including some autoimmune disorders such as 'primary' Sjogren's syndrome 878889909192939495969798.

Cryoglobulinemic syndrome may share a number of etiopathogenetic events and clinical features with both autoimmune diseases such as autoimmune hepatitis, Sjögren's syndrome, and polyarthritis, and B-cell lymphomas 311122528858687888990919293949596979899100. Therefore, a differential diagnosis should be carefully done in all patients with MC syndrome (Fig. 7, see also the paragraph 'differential diagnosis'); a correct disease classification may decisively affect the overall clinico-therapeutic approach and prognosis.

There are no available diagnostic criteria for MC; in 1989 the Italian Group for the Study of Cryoglobulinemias has proposed preliminary criteria for MC classification 101. A revised version of these criteria (Table 3), including clinico-pathological and virological findings, has been successively proposed 3. Circulating mixed cryoglobulins, low C4, and orthostatic skin purpura are the hallmarks of the disease; moreover, leukocytoclastic vasculitis, involving medium- and, more often, small-sized blood vessels (arterioles, capillaries, and venules) is the typical pathological finding of involved tissues. It is easily detectable by means of skin biopsy of recent vasculitic lesions (within the first 24–48 hours) 3111225.

The detection of serum mixed (IgG-IgM) cryoglobulins is necessary for a correct classification of MC syndrome (Table 3, Fig. 1). Unfortunately, there are no universally accepted methodologies for cryoglobulin measurements, but simple standardized indications are often sufficient for testing cryoglobulinemia 3923. Since cryoglobulins present a high thermal instability, the measurement of Ig cryoprecipitate should be performed immediately in the same place where the blood is sampled. For a correct evaluation of serum cryoglobulins, it is necessary to avoid false-negative results due to Ig cold precipitation also at room temperature. For this purpose, the first steps (blood sampling, clotting, and serum separation by centrifugation) should be always carried out at 37°C and the cryocrit determination and cryoglobulin characterization at 4°C (after 7 days). The serum with cryoglobulins should be tested for reversibility of the cryoprecipitate by rewarming an aliquot at 37°C for 24 hours. Moreover, cryocrit determinations (percentage of packed cryoglobulins referred to total serum after centrifugation at +4°C; Fig. 1) should be done on blood samples without anticoagulation to avoid false-positive results due to cryofibrinogen or heparin-precipitable proteins. Without the above relatively simple precautions, not only will the quantities of cryoglobulins measured be incorrect, but the test may completely fail to even detect cryoglobulins.

After isolating and washing the cryoprecipitate, the identity of cryoglobulin components can be determined by immunoelectrophoresis or immunofixation. These analysis must be performed at 37°C to avoid precipitation and hence loss of the cryoglobulin during the procedures. More sophisticated methodologies, such immunoblotting or two-dimensional polyacrylamide gel electrophoresis, can be proposed for laboratory investigations 102103104. While the detection of serum cryoglobulins is fundamental for the diagnosis of MC, the levels of serum cryoglobulins usually do not correlate with the severity and prognosis of the disease 311122425. Very low levels of cryocrit, often difficult to quantify, can be associated with severe and/or active cryoglobulinemic syndrome; on the contrary, high cryocrit values may characterize a mild or asymptomatic disease course. In rare cases, very high cryocrit levels, possibly associated to cryogel phenomenon, can be associated to classical hyperviscosity syndrome 242527. On the contrary, a sudden decrease or disappearance of serum mixed cryoglobulins, with or without abnormally high levels of C4, can be regarded as alarming signal of complicating B-cell malignancy 45.

Table 4 summarizes the clinico-serological work-up at patient's first evaluation in order to correctly classify the MC syndrome and to identify possible overlapping disorders (see next paragraph), and/or comorbidities. The prevalence of these latter, in particular atherosclerosis, may be correlated with the disease duration, as well as with cumulative side effects of prolonged treatments. Moreover, the diagnosis and monitoring of the major MC manifestations is fundamental for timely treatments, especially for life-threatening liver, renal, and/or neoplastic complications 311122425.

The term 'essential' MC was originally referred to autonomous disease when other well known systemic, infectious or neoplastic disorders have been ruled out by means of a wide clinico-serological work-up 5. However, in some patients a definite diagnosis may be difficult because of the clinical polymorphism of the disease. Moreover, the association of MC with HCV infection may further complicate the differential diagnosis with other immunological HCV-related disorders: there is a frequent clinico-pathological overlapping among different HCV-related disorders. Cryoglobulinemic syndrome can represent a crossroads between some autoimmune diseases (autoimmune hepatitis, Sjögren's syndrome, polyarthritis, glomerulonephritis, thyroiditis, type 2 diabetes, etc.) and malignancies (B-cell lymphomas, hepatocellular carcinoma) 311122528878889909192939495969798. It is possible to observe in the same patient a slow progression from mild HCV-associated hepatitis to various extrahepatic manifestations (arthralgias, sicca syndrome, Raynaud's phenomenon, RF positivity, etc.), and ultimately to overt MC syndrome with typical clinico-serological manifestations. In only a minority of MC patients a malignancy may develop, generally after a long lasting follow-up period 311122425. Therefore, a careful patient evaluation is necessary for a correct diagnosis of MC syndrome, particularly with regards to other RF-positive, systemic rheumatic disorders such as rheumatoid arthritis (RA) and primary Sjögren's syndrome (pSS) (Fig. 7).

While arthralgias are one of the most frequent symptoms, clear signs of sinovitis are quite rare. Generally, patients develop mild, non-erosive oligoarthritis 2890, often sensitive to low doses of corticosteroids with or without hydroxychloroquine. In contrast, a rheumatoid-like polyarthritis is more frequent in patients with HCV-related hepatitis without MC syndrome 28. In patients with HCV-associated MC and symmetrical, erosive polyarthritis the diagnosis of overlapping MC/RA syndrome can be suspected. In these cases, the detection of serum anti- cyclic citrullinated peptide antibodies, markers of classical RA, may represent a useful diagnostic tool 9192.

Almost half MC patients complain of sicca syndrome; however, only a few cases meet the current criteria for the classification of pSS. MC and pSS may share various symptoms: xerostomia and/or xerophthalmia, arthralgias, purpura, RF and serum cryoglobulins, and the possible complication with B-cell lymphoma 311122587939495969798. However, a careful patient clinical assessment is usually sufficient for a correct diagnosis in the large majority of cases by considering some important findings: histopathological alterations of salivary glands and specific autoantibody pattern (anti-RoSSA/LaSSB) of pSS are rarely found in MC patients; conversely, HCV infection, cutaneous leukocytoclastic vasculitis, and visceral organ involvement (renal, liver) are seldom recorded in primary Sjögren's syndrome (Fig. 7). Given the above considerations, it has been recently proposed that the presence of HCV infection per se should be considered exclusion criteria for the classification of pSS 99100. In rare cases in which the differential diagnosis may result very difficult, particularly in HCV-negative individuals, it might be correct to classify the disorder as overlap syndrome. However, patients with MC/pSS overlap syndrome are often characterized by more severe clinico-prognostic evolution; they show a significant low rate of anti-RoSSA/LaSSB along with a high prevalence of mixed cryoglobulinemia, hypocomplementemia, systemic autoimmune manifestations, and complicating lymphomas (94, 97, 98). In the clinical practice, this particular condition could be better regarded as vasculitic syndrome with relevant implications on the patient monitoring and treatment 112425.

Finally, autoimmune hepatitis, the old 'lupoid' hepatitis, may be associated to HCV infection, mainly in the same countries where HCV-associated MC is more frequently found 88. Moreover, it may share with MC syndrome a number of extrahepatic symptoms, including serum mixed cryoglobulins 89. The differential diagnosis between these two conditions may be problematic: some typical features of MC (leukocytoclastic vasculitis, hypocomplementemia, glomerulonephritis), as well as the presence of serum autoantibodies commonly found in autoimmune hepatitis (anti-smooth muscle antibodies) can be taken into account.

Considering its complex etiopathogenesis and clinical polymorphism, the treatment of MC syndrome is particularly challenging. For a correct therapeutic approach, three important factors (Fig. 6), namely, the HCV infection, the presence of autoimmune disorder, and the possible neoplastic complications should be considered 325. Following the main steps of the etiopathogenetic process, from HCV infection to B-lymphocyte proliferation, and lastly to cryoglobulinemic vasculitis (Fig. 8), the disease can be treated at different levels by means of etiologic, pathogenetic, and/or symptomatic therapies. Since HCV represents the main causative factor of the disease by exerting a chronic stimulus on the immune-system 3891011121424, an attempt at HCV eradication by alpha-interferon treatment should be done in all cases of HCV-associated MC 3111224105106107108109. However, the beneficial effect observed with this drug is often transient and possibly associated with important immune-mediated side effects such as peripheral sensory-motor neuropathy, thyroiditis, and rheumatoid-like polyarthritis 51525354. Probably, in predisposed subjects alpha-interferon (which is both antiviral and immunomodulating agent) can trigger or exacerbate some pre-existing, often subclinical, symptoms. Unfortunately, there are no available parameters for predicting this harmful complication; thus, alpha-interferon therapy should be avoided at least in those patients with clinically evident peripheral neuropathy. On the whole, the usefulness of alpha-interferon treatment in MC patients is limited by the low rate of responders and frequent side effects. The association of alpha-interferon and ribavirin might achieve the eradication of HCV infection in a higher number of treated subjects, even if the long-term effects are inconstant and often unpredictable 3111224105106107108109110111112113114. After careful patient's evaluation of contraindications and/or possible side effects, a standard treatment with alpha-interferon and ribavirin 111 should be attempted in MC patients with prevalent clinico-pathological findings of chronic hepatitis (Fig. 9, 10). Controlled clinical trials might establish the actual usefulness of antiviral therapy in HCV-associated MC and better evaluate the predictive markers of response to treatment (HCV genotypes, host HLA alleles?). Hopefully, with the rapid growth of molecular biology a vaccine against HCV might be available in the near future. A vaccine-based therapy 115 with recombinant HCV proteins in HCV-infected individuals could be able to prevent the progression of viral infection and possibly to interrupt the self-perpetuating autoimmune mechanism underlying the MC.

In patients with 'essential' MC the immunosuppressive treatment, i.e. cyclophosphamide or rituximab, is still the first-line intervention (Fig. 9, 10). For HCV-associated MC, immunomodulating/immunosuppressive treatments should be considered, especially in patients with more severe complications. These treatments include steroids, low-antigen-content (LAC) diet, plasma exchange, and immunosuppressors 3111224116117118119120121122. In particular, both traditional plasma exchange and double-filtration plasma exchange are able to markedly reduce the levels of circulating immune-complex, especially the cryoglobulins 24121122. Oral cyclophosphamide (50–100 mg/day for 2–6 weeks) during the tapering of apheretic sessions can reinforce the beneficial effect of plasma exchange; moreover, it can prevent the rebound phenomenon that may be observed after the aphaeresis discontinuation 24. Plasma exchange is particularly useful in severe MC complications such as active membranoproliferative glomerulonephritis (Fig. 8, 9, 10).

LAC-diet is a particular dietetic treatment that can improve the clearance circulating immune-complexes by restoring the activity of the reticulo-endothelial system, overloaded by large amounts of circulating cryoglobulins 122123. LAC-diet has been designed to reduce the input of alimentary macromolecules crossing the mucosal barrier of the gut; some foods, particularly dairy products and eggs, present a potential antigenic activity, and consequently might be involved in the pathogenesis of some diseases in humans. The reduction of the alimentary input of macromolecules directed to mononuclear phagocytic system may improve its function in those conditions characterized by abnormal endogenous production of immune-complexes responsible for organ damage, i.e. mixed cryoglobulinemia and other immune-complex mediated diseases. An impaired function of mononuclear phagocytic system has been also demonstrated in patients with IgA nephropathy 123. Given the above pathogenetic considerations, LAC-diet has been usefully employed in patients with clinically mild MC symptoms and IgA nephropathy (reduction of 24-hour proteinuria). Usually, LAC-diet and/or low dosage of steroids (6-methyl-prednisolone 2–4 mg/day) may be sufficient to improve mild manifestations (arthralgias, sporadic purpura, etc.) of the MC (Fig. 8, 9, 10); patients with mild-moderate symptoms, such as palpable purpura, are particularly sensitive to the smallest variations of daily steroid dosage (1–2 mg).

In clinical practice, MC treatment should be tailored for the single patient according to the severity of clinical symptoms (Fig. 9, 10). Therefore, patients with severe vasculitic manifestations must be promptly treated with high doses of steroids and/or plasma exchange and/or cyclophosphamide or rituximab. Some recent clinical studies suggested that sequential or combined antiviral/immunosuppressive treatment could represent a rather useful therapeutic strategy 124125. The rationale of such aggressive therapies could be particularly indicated in MC patients with major clinical manifestations and partial/transitory remission after standard treatments 25124125. On the contrary, clinically asymptomatic patients usually do not need any treatment, even in the presence of high levels of cryocrit. In all cases, a careful clinical monitoring of the disease is mandatory, with particular attention to neoplastic complications. Preliminary studies suggested the synergic affect of some therapies in association with antiviral treatment, namely the cyclosporine A or tumor necrosis factor inhibitors, in the setting of chronic hepatitis type C 126127128. These agents may contribute to viral clearance and/or to improve the immune-mediated hepatic and extrahepatic inflammatory manifestations; future clinical trials might evaluate their potential usefulness even in MC, possibly in selected patient subsets.

The natural history of MC is not predictable and strongly depends on concomitant diseases and complications and response to treatment. Morbidity due specifically to cryoglobulinemia may also be significant (infections, cardiovascular diseases, progressive renal failure, advanced neuropathy). The overall prognosis is worse in patients with renal disease, liver failure, lymphoproliferative disease, and malignancies. Mean survival is estimated to be approximately 50–60% at 10 years after diagnosis 24. Careful monitoring of life-threatening MC complications (mainly nephropathy, widespread vasculitis, and B-cell lymphoma or other malignancies) should be carried out in all MC patients.

MC was first described as distinct disorder in 1966 5; 25 years later the discovery of the strong association between MC and HCV infection 311122570 represented a decisive contribute for a better understanding of the etiopathogenetic mechanisms responsible for the disease, and consequently for an adequate therapeutic strategy.

However, the following points remain still to be clarified:

1- Since the etiopathogenesis of HCV-associated MC syndrome is a multifactorial and multistep process, two important aspects need further investigations: a) HCV may represent the simple triggering factor or it could also contribute to self-perpetuating mechanism of the disease: b) nature and role of other etiopathogenetic co-factors;

2- The etiopathogenesis of 'essential' MC syndrome;

3- The actual role of HCV eradication on the natural history of HCV-associated MC syndrome;

4- The usefulness of sequential or combined antiviral/immunesuppressive treatments compared to traditional therapeutic approach.

Ig: immunoglobulin; RF: rheumatoid factor; MC: mixed cryoglobulinemia; HCV: hepatitis C virus; MPGN: membranoproliferative glomerulonephritis; B-CLL: B-cell chronic lymphocytic leukemia; Ic: immunocytoma; MLDUS: monotypic lymphoproliferative disorder of undetermined significance; HBV: hepatitis B virus; PCR: polymerase chain reaction; VLDL: very low-density lipoprotein; MALT: mucosa-associated lymphoid tissue; HIV: human immunodeficiency virus; RA: rheumatoid arthritis; pSS: primary Sjögren's syndrome; LAC-diet: low-antigen-content diet.

The author declares that they have no competing interests.