Hepatitis C: Pathogenesis, Virology, and Immunology
Disclosures
Adrian M. Di Bisceglie, MD, FACP
Introduction
Several themes emerged during this year's meeting of the American Association
for the Study of Liver Diseases that related to the pathology, virology, and
immunology of hepatitis C. This report explores these prominent topics and
places them in relevant clinical context.
The Association Between Steatosis and Hepatitis C
It has been recognized for some time that hepatic steatosis is frequently
present in patients with chronic hepatitis C, perhaps more so than in other
forms of hepatitis. The reasons for the latter are not clear. Certainly,
infection with hepatitis C virus (HCV) genotype 3 has been suggested as a risk
factor for hepatic steatosis, but this finding occurs even in patients infected
with other genotypes.
Patton and colleagues[1] conducted a comprehensive assessment of the role of
host and viral factors in the development of steatosis with HCV in a large
patient population. All patients (n = 576) with chronic hepatitis C who were
enrolled in a single-center database in the United States were included and were
naive to treatment at the time of their biopsy. These investigators found that
16% of this patient population had grade 2 or 3 steatosis based on results of
liver biopsy. According to stepwise logistic regression analysis, the only host
factor associated with steatosis was body mass index, whereas viral factors
included HCV genotype 3 and viral load.[1] It appears that hepatic steatosis is
associated with more severe hepatic fibrosis,[2] linked to increased serum
levels of c-peptide and insulin as markers of hyperinsulinism. Hyperinsulinism
has been closely linked with the development of nonalcoholic fatty liver
disease.
Sustained virologic response to antiviral therapy was found to result in a
decrease in hepatic steatosis, independent of weight loss; however, there was
controversy as to whether the latter occurred to a greater extent in patients
infected with HCV genotype 3. Infection with HCV genotype 3 appears to cause
hepatic steatosis to a greater extent than with other genotypes.[3,4]
Other Factors Effecting the Natural History of Hepatitis C
Role of Alcohol Consumption
The outcome of chronic HCV infection is quite variable, with some individuals
having rapidly progressive liver disease and others seemingly having
nonprogressive hepatitis. Alcohol consumption is well known to exacerbate the
chronic liver injury associated with HCV infection, although the mechanisms by
which this occurs are not known. The quantities of alcohol typically reported
are not sufficient to result in liver injury alone.
Sartori and colleagues[5] found that moderate alcohol intake was associated with
increased oxidative stress in patients with chronic hepatitis C. They measured
the products of lipid peroxidation in serum of patients with chronic hepatitis C
who consumed varying degrees of alcohol and compared them with those in normal
controls. Levels of these products were increased even in patients who consumed
moderate amounts of alcohol (up to 50 g per day).
Another interesting effect of alcohol consumption was highlighted by a study
among US Veterans with chronic hepatitis C.[6] The study authors found that the
proportion of patients at their institution who were seropositive for anti-HCV
but negative for HCV RNA (implying that these individuals had cleared HCV
infection) was lower among those with heavy alcohol consumption compared with
those with minimal or moderate consumption. These findings suggest that alcohol
may impair the immune response to HCV infection, thereby increasing the rate of
chronicity. The investigators did not address the issue of how ongoing alcohol
consumption might lead to more severe hepatic fibrosis in those with chronic HCV
infection.
Chemokine and Chemokine Receptor Polymorphisms
An important study of chemokine and chemokine receptor polymorphisms in patients
with hepatitis C failed to confirm a previously reported association between the
CCR5-delta 32 mutation, HCV infection, and increased viral loads. This mutation
has been very convincingly linked with susceptibility to HIV infection and, in a
preliminary report,[7] had been found to be linked with susceptibility to HCV
infection as well.
Promrat and coworkers[8] from the National Institutes of Health evaluated a
series of 5 polymorphisms in chemokines and chemokine receptors, including the
CCR5-delta 32 mutation, in a cohort of 339 patients with chronic HCV infection
and more than 2000 healthy blood donor controls. The frequency of mutations was
the same among patients with chronic HCV infection and controls. It was
interesting to note that a mutation in the RANTES promoter (which can activate
several chemokine receptors) correlated with a less severe degree of hepatic
inflammation, a finding that needs to be validated in further studies.
HIV Coinfection
It has been suggested that HIV infection may result in more severe liver disease
among patients with HCV infection. This concept has been called into question by
Bonacini and colleagues,[9] who followed a cohort of 474 patients with HIV
infection. Of these patients, 233 were seropositive for HCV RNA and 73 were
positive for hepatitis B surface antigen. In fact, patients with HIV infection
and no viral hepatitis had the highest all-cause mortality rates. When this
analysis was corrected for CD4 cell count, survival curves of the HIV alone,
HIV/HCV, and HIV/hepatitis B virus (HBV) groups were superimposable. However,
the rate of liver-related death was higher in patients with either HIV/HCV
coinfection or HIV/HBV coinfection.
Thus, the picture with HIV/HCV coinfection remains unclear. Certainly, these 2
infectious entities are often found together. The frequency of liver mortality
appears to be greater in individuals with coinfection than in those with HIV
infection alone, but the mechanism by which this occurs is the subject of much
speculation. The role of antiviral therapy directed against HIV-induced liver
damage requires further study.
Hepatitis C and the Brain
There is growing interest in the effect of HCV infection on the brain. It
appears that patients with hepatitis C suffer from mild, cognitive impairment
and an excess of psychiatric problems such as depression. The causes for the
latter are not known, but possible explanations include the effects of substance
abuse (frequently associated with HCV infection), presence of a premorbid
condition that may lead to an increased rate of substance abuse and hence to HCV
infection, or, possibly, the direct effects of HCV infection. In patients with
advanced liver disease, the development of hepatic encephalopathy may complicate
the picture even more.
To examine the hypothesis that HCV infects the brain, Laskus and colleagues[10]
studied cerebrospinal fluid from a small series of patients with HCV infection.
Many of these individuals were coinfected with HIV and had spinal tap done for
various clinical indications such as aseptic meningitis. They studied both the
fluid itself and cells found in that fluid, searching for the negative strand of
HCV (viral replicative intermediate) and for HCV quasispecies variability.
Findings from these samples were compared with those of the peripheral blood.
These investigators found an association with HCV-RNA positivity in the
cerebrospinal fluid of HCV-infected persons, particularly in association with
the cellular compartment. The presence of the negative strand of HCV suggested
active HCV replication rather than contamination from the peripheral blood. In
patients harboring different strains of HCV in serum and peripheral blood
mononuclear cells (PBMCs), as determined by phylogenetic analysis, cerebrospinal
fluid-derived strains were more closely related to those found in PBMCs than in
serum. The study authors proposed that PBMCs could carry the virus into the
central nervous system and provide a mechanism for HCV neuroinvasion.
Forton and colleagues[11] were able to identify brain-specific variants of HCV
from brain tissue of HCV-infected individuals collected at autopsy. These
variants had discrete genomic mutations in the internal-ribosomal entry site
(IRES) region of the genome, suggesting that the IRES may be important in
promoting replication of the virus in cells other than hepatocytes.
Newer Diagnostic Tests for HCV Infection
The performance of several newer diagnostic tests for HCV infection was
discussed during these meeting proceedings.
A newly developed qualitative assay for the detection of HCV RNA has a lower
limit of detection of 5.3 IU/mL of serum and appeared to have excellent
performance characteristics when evaluated by Schiff and coinvestigators.[12]
Thus, this assay may have a role in clinical practice for detecting low levels
of viremia, such as those that occur during antiviral therapy or when other
assays are negative.
A new assay is now available to detect HCV core antigen in serum of infected
individuals. This test has the advantages over PCR-based assays of simplicity,
short turnaround time, and low false-positivity rates, but does not quite have
the sensitivity of the PCR-based studies.[13] This novel assay may have a role
in monitoring the effect of antiviral therapy.[14]
In the setting of well-established diagnostic assays with good sensitivity and
specificity, the role of these new assays needs to be further defined.
Predictors of Cirrhosis
Progression of liver disease due to hepatitis C is marked by progression of
hepatic fibrosis. The degree of fibrosis is best assessed by liver biopsy.
However, the search for a noninvasive means of assessing hepatic fibrosis has
been a "holy grail" of hepatology for years.
Based on data from the HALT-C (Hepatitis C Antiviral Long-term Treatment Against
Cirrhosis) trial, Lok and colleagues[15] developed a clinical model to predict
cirrhosis in patients with chronic hepatitis C. They found that the use of
routine laboratory tests such as platelet count, aspartate
aminotransferase/alanine aminotransferase ratio, alkaline phosphatase level, and
prothrombin time give a very good idea of the presence of cirrhosis, but are not
yet accurate enough to replace the role of liver biopsy.
Myers and colleagues, in collaboration with Poynard,[16] evaluated the value of
a panel of serum fibrosis markers to predict the presence of cirrhosis. This
panel included alpha-2-macroglobulin, apolipoprotein A1, haptoglobin, gamma
glutamyl transferase, and total serum bilirubin. Evaluation of this test was
conducted using proprietary methods and the calculated score increased with
worsening fibrosis and could predict cirrhosis with 87% sensitivity and a
negative predictive value of 98%. It is clinically important to determine the
presence of cirrhosis, not only because these patients are at greater risk of
liver disease progression and therefore require aggressive antiviral therapy,
but also because these individuals are targets for screening for esophageal
varices and hepatocellular carcinoma.
References
1.. Patton H, Behling C, Patel K, et al. Body mass index, HCV genotype 3, and
HCV RNA levels are associated with steatosis severity in patients with chronic
hepatitis C. Hepatology. 2002;36:265A. [Abstract #408]
2.. Johnsson J, Hickman IJ, Clouston AD, et al. Fibrosis in chronic hepatitis
C correlates significantly with circulating C-peptide and insulin levels.
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3.. Kumar D, Farrell GC, George J. Sustained viral response improves hepatic
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following successful antiviral treatment for chronic hepatitis C infection.
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http://www.medscape.com/viewarticle/444782
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