This web page was produced as an assignment for an undergraduate course at Davidson College.
Mixed Cryoglobulinemia
This
image shows the foot of a patient that has been suffering from mixed
cryoglobulinemia for 12 years. Purpura (rash due to blood vessel damage)
and the discoloration caused by chronic inflammation is clearly visible.
Gangrene infection is visible on the second toe. This image was obtained
from the Online Archives of Rheumatology
and permission has been requested. Click on the image to see the page from
which it was obtained.
Mixed cryoglobulinemia (MC) is a chronic autoimmune disorder that is almost always associated with chronic liver inflammation due to hepatitis C virus (HCV) infection. It is a combination of an immune complex disorder and a lymphoproliferative disorder, with distinct symptoms caused by each aspect of the disease. MC is believed to develop when chronic liver inflammation causes B cells to grow out of control and produce excessive amounts of antibodies, especially anti-IgG antibodies known as rheumatoid factors. The B-1 (CD5+) cells are believed to be important in the production of rheumatoid factors found in MC patients (Newkirk 2002). MC is a relatively rare disorder, affecting only 13-54% of HCV infected individuals. Despite its rarity, it is of significant interest because it is one of the only autoimmune disorders so clearly associated with viral infection (Della Rossa et al. 2001). The most effective current treatments are antiviral medications such as IFN-a (Gross 1999).
Symptoms of Mixed Cryoglobulinemia
The above image shows one of the less severe symptoms of MC, the purplish rash known as purpura. The image was obtained from the Johns Hopkins Vasculitis Center webpage and permission has been requested. Click on the image to view the site from which it was obtained.
The above image shows the electon microscopy image of the kidney of a patient with MC. The image was obtained from the Johns Hopkins Vasculitis Center webpage and permission has been requested. Click on the image to view the site from which it was obtained.
The most common symptom of mixed cryoglobulinemia (MC) is purpura, which is a purple spotted rash caused by internal bleeding. It is predominantly found in the extremities and is present in over 90% of patients. The other two extremely common symptoms are weakness (experienced by 89% of patients) and joint pain (experienced by 83% of patients). More serious but rare disorders include peripheral nerve damage, liver damage, kidney damage (membranoproliferative glomerulonephritis due to accumulation of immune complexes), and skin ulcers. The disease can also lead to B cell non-Hodgkin’s lymphoma and hepatocellular carcinoma, which are found in only 7.5% and 2.4% of patients, respectively (Ferri, Zignego, and Pileri 2002).
The most important laboratory indicators of MC are the immune complexes of rheumatoid factor and polyclonal IgG immunoglobulin that precipitate out of an affected patient’s cooled blood serum (Ferri, Zignego, and Pileri 2002). Rheumatoid factors are autoantibodies that bind to the Fc region of IgG (Newkirk 2002). The IgG molecules of the immune complex are often specific for hepatitis C viral antigens. These antigens precipitate out of solution as part of the immune complex (Agnello 1995). The two molecules interact with a low affinity at body temperature (37 degrees Celcius). As the temperature decreases, strengthening chemical interactions cause the complexes to increase in size until they precipitate out of solution at 4 degrees Celcius. MC is also indicated by abnormally low levels of complement in the blood (Ferri, Zignego, and Pileri 2002).
Risk Factors
The most important risk factor for mixed cryoglobulinemia (MC) is hepatitis C virus (HCV) infection. Anti-HCV antibodies are present in 70 to 100% of patients with MC. Nevertheless, only 13-54% of HCV infected patients develop the disorder (Della Rossa et al. 2001). Women are more likely than men to develop the disease: Women account for 63% of all HCV-associated MC and 71% of type II MC (types I, II, and II will be distinguished in the next section). HCV infected patients carrying the HLA DR11 allele are more likely to develop MC, whereas patients carrying the HLA DR7 allele seem to be protected against developing MC (Newkirk 2002). Also, there is evidence that other environmental factors may play a role. MC is most prevalent in southern Europe and its frequency diminishes in more northern locations. Its worldwide prevalence is not known (Ferri, Zignego and Pileri 2002).
Disease Development
The exact mechanism by which MC arises from HCV infection is unknown. MC symptoms fall into two distinct categories: those due to the accumulation of immune complexes (membranoproliferative glomerulonephritis, purpura, etc.) and those due to excessive lymphoproliferation (liver damage, lymphomas, etc.). Immune complexes accumulate because of the excessive amounts of IgG antibodies and rheumatoid factors in the blood due to chronic inflammation. The IgG and rheumatoid factors form immune complexes capable of activating complement (Newkirk 2002). Lymphoproliferation may be caused by a variety of factors, including surface proteins on HCV that stimulate B cell activation, molecular mimicry, and natural genetic mutations that become more likely with increasing cell division. In many ways the immune complex disorder is a result of the excessive lymphoproliferation, but the two aspects of the disease can become self-sustaining (Ferri, Zignego, and Pileri 2002).
Rheumatoid factors (RFs) are antibodies against the Fc (constant region) of
IgG antibodies. An image of an RF fab fragment bound to an IgG Fc region
is at the left. Click on the image to go to the protein data bank site where
I got it. RFs are usually of the IgM isotype, especially in MC, but can be of
any isotype. RFs are produced by a subset of B cells known as B-1 or CD5
B cells. CD5 normally downregulates autoantibody production, and the pathology
found in MC may be due in part to defective CD5 signalling. B-1 cells
are found in the germinal centers that form in the liver as a result of HCV
infection and the chemical environment in these centers probably contributes
to the affinity and production levels of RFs in MC. The chemical environment
will include the cytokines secreted by Th1 or Th2 cells that have been selected
to fight the infection as well as gender-biased hormones. These hormones
may explain the high incidence of MC in women as opposed to men. The normal
role of RFs in immune response is unclear, but they are produced transiently
in response to almost all infectious agents. Their main role is probably
to aid in immune complex clearance by making complexes larger and activating
complement. In MC, they are a major component of the cryoglobulins that
precipitate out of patients blood serum, an important laboratory indicator of
the disease (Newkirk 2002).
Cryoglobulins are classified into three groups. Type I cryoglobulins are composed of only monoclonal antibodies and are found mostly in patients with lymphoid tumors. Type II cryoglobulins are composed of a mix of polyclonal IgG and monoclonal IgM RFs. Type III are polyclonal IgG and IgM RFs. Types II and III are classified as mixed cryoglobulinemia and can both be produced as a result of infectious agents (Ferri, Zignego, and Pileri 2002). Immune complexes in the blood can increase blood viscosity and accumulate in the nephron and capillaries to cause membranoproliferative glomerulonephritis and purpura (Della Rossa et al. 2001).
Lymphoproliferation is responsible for liver damage (due to chronically inflamed
germinal centers), cancers, and possibly nerve damage in MC patients.
The mechanism by which HCV may cause lymphoproliferation is still being researched,
but there are several theories. The first is based on the finding that
the HCV protein E2 binds CD81, which is part of the B cell coreceptor
along with CD21 and CD19. This theory argues that E2-CD81 binding reduces
the threshold necessary for B cell stimulation. The threshold reduction results
in an overreactive B cell that could produce excessive levels of antibody and
proliferate. Since B-1 cells, which are implicated in RF production, are
thymus independent, this theory seems especially plausible for explaining excessive
levels of RFs in MC patients. The second theory argues that HCV activates
B cells specific for antibody:HCV protein complexes that are constantly available
due to the chronic infection. These B cells would be constantly stimulated
to divide while overproducing antibodies. The final theory is that the
prolonged division cycle induced by chronic infection naturally increases the
likelihood that mutations will occur that may produce excessive proliferation
(Ferri, Zignego, and Pileri 2002).
Treatment
The secondary goal of MC treatment is alleviation of the symptoms, which requires immunosuppression. Non-steroidal anti-inflammatory drugs are used to treat minor symptoms such as purpura, weakness, and joint pain. Steroids are prescribed for more serious symptoms such as sensory neuropathy and glomerulonephritis. A low-antigen content diet has also shown promise for treatment of liver disease. It is thought that the diet works by decreasing the amounts of large ingested substances that compete with immune complexes for clearance by the mononuclear phagocytic system. If the diet reduces the concentration of these substances, the immune complexes may be more efficiently cleared. The most seriously life threatening conditions, such as acute progressive glomerulonephritis, motor neuropathy, and hyperviscosity syndrome, are treated with plasma exchange therapy accompanied by immunosuppressive drugs. This treatment rapidly decreases the number of circulating immune complexes (Della Rossa et al. 2001).
Sources
Agnello V. The Aetiology of Mixed
Cryoglobulinaemia Associated with Hepatitis C Virus Infection. Scand J Immunol
1995;42:179-185.
Della Rossa A, Tavoni A, Baldini
C, Bombardieri S. Mixed Cryoglobulinemia and Hepatitis C Virus Association: Ten
Years Later. IMAJ 2001 June;3:430-434.
Ferri C, Zignego AL, Pileri SA.
Cryoglobulins. J Clin Pathol 2002; 55:4-13.
Gross WL. New Concepts in treatment protocols for severe systemic vasculitis. Curr Opin Rheumatol 1999; 11:41-46.
Janeway, CA Jr, Travers P, Walport M, Shlomchik MJ. Immonobiology 5. New York: Garland Publishing, 2001.
Newkirk MM. Rheumatoid Factors: Host Resistance or Autoimmunity? Clinical Immunology 2002 Jul;104(1):1-13.
taught by Dr. Malcolm Campbell
