Current Review & Update on Hepatitis C & HIV/HCV Cection
Pathogenesis of HCV
Pathogenesis of HCV
Chronic infection with both HIV-1 and HCV are both characterized by dynamic equilibrium between virus production and clearance. Daily virus production appears high for both viruses: the estimated daily virion production for HIV-1 is 9.3 log 10 10.2 log 10 and for HCV is 11.6 log 10 13.0 log 10 (as much as billions of HIV virions and trillions of HCV virions).
Virology: Hepatitis C Genetic Diversity and High Level of Viral Replication Pose a Challenge
At the AASLD HCV Conference recently held in Chicago (June 2001), HCV virology and immunology were discussed by Charles Rice, (Rockefeller University), Particia Farci (University of Cagliari, Italy), and Barbara Reherman (NIH). Rice and Farci discussed hepatitis C viral diversity. Within an individual, the virus exists as a quasispecies (i.e.a heterogeneous population of mutated viral forms). Virus replication is a dynamic process with virion half life of 2-3 hours and a high level of virus replication which contributes to genetic diversity, HCV RNA occurs via synthesis of a complemen- tary negative-strand RNA intermediate and is error prone resulting in the generation of a large number of variants. Such high replica- tion rates and the resulting genetic diversity pose important challenges to the development of an effective vaccine and to effective treatment. Evidence is accumulating to suggest that the diverse genetic nature of HCV may have important biologic & clinical implications for viral persistence, for drug resistance and for vaccine failure. Genetic variability is a critical mechanism for the virus to persist in the host (human) possibly by escaping the immune system. During the acute phase of the infection, approximately 15-20% of individuals will clear HCV, but the factors that determine whether the virus is cleared or persists have not been elucidated. Recent evidence suggests that one important factor is viral diversity during acute infection, such that an increase in diversity is associated with progression to chronicity, while self-limited clearance is associated with a virus that remains stable. These data indicate that the early events of the virus-host interaction may determine the outcome of HCV infection. Farci also stated that patients who achieve a response to interferon therapy with sustained HCV clearance exhibit a significant decrease in viral diversity and in the number of viral strains, with a similar pattern to that seen in patients who spontaneously clear the virus. By contrast, patients who do not respond to therapy show persistence of the original dominant viral strains, suggesting that resistant strains are already present prior to therapy.
The immune response to HCV is yielding important new information regarding host/viral interactions. A broad and strong anti-HCV specific CD4+immune response is an important determinant of recovery during the acute phase of HCV and in the prevention of severe HCV recurrence after hepatic transplantation. Vigorous HCV-specific CD8 immunity distinguishes individuals with self-limited HCV infection from individuals with chronic HCV infection. Both CD4+and CD8+responses to HCV structural proteins (core, E1,E2) are important determinants of a successful outcome to therapy. HIV may have a deleterious effect on HCV specific immune responses in coinfected patients, which may be one of the reasons why higher CD4+T cell counts and lower HCV viremia have been associated with improved responsiveness to interferon. See the section below on the effect of HAART on HCV.
Pathogenesis:The central pathogenic mechanisms, whether direct viral cytotoxicity (direct killing of cell by infected virus) or the host s immune response, have not been conclusively established for either virus, although each mechanism has been hypothesized to be important in each viral infection.Infection with HIV results in progressive immune dysfunction, secondary to the continued loss of CD4+T cells, and the eventual onset of opportunistic infections, as a consequence of immunodeficiency. HCV is a hepatotropic virus, and the principle adverse consequence of HCV infection is the development of liver failure. The symptoms associated with end stage HIV infection result directly from pathogenic infection (i.e. pulmonary symptoms with pneumocystis carinii pneumonia or odynophagia from esophageal candidiasis). In contrast, end stage liver disease in HCV results from hepatic fibrosis,a response that can occur to a variety of insults including viral hepatitis,autoimmune attack of hepatocytes, alcohol abuse, drugs, or metabolic diseases due to an overload of iron or copper. The normal liver contains an epithelial component (hepatocytes), an endothelial lining, tissue macrophages (Kupffer cells and perivascular mesenchymal cells (stellate cells). Stellate cells are the key fibrogenic cell; they become activated, transition from quiescent cells into pro- liferative, fibrinogenic, and contractile myofibroblasts, in response to hepatic injury of any etiology. TGF-b is the major stimulus for stellate cell production of fibrin.
Several studies have evaluated the determinants of hepatic fibrosis in HCV monoinfected and HIV/HCV coinfected individuals. Poynard et al (1997) found that age greater than 40 years at the time of HCV acquisition, alcohol consumption of 50g or more and male sex are independently associated with accelerated hepatic fibrosis. Benhamou et al (1999) found that HIV seropositivity and severe immunosuppression (CD4 cell count <200 cells/mm 3, and alcohol consumption were all associated with a higher fibrosis progression rate in HIV/HCV coinfected individuals. Puotietal (2001) recently reported an independent association between CD4+cell count <500 cells/mm 3 and the presence of fibrous septa (odds ratio,3.2;P =0.037). Powell et al (2000) demonstrated an association between individuals who inherit high TGF- b1 and angiotensinogen (AT)-producing genotypes and the development of progressive hepatic fibrosis.
Immunology:Strong CTL Response May Be Needed for Viral Eradication
Barbara Reherman reported that prospective analysis of the cellular immune responses of patients with symptomatic, self-limited hepatitis C demonstrate strong Th1 dominated T cell helper and cytotoxic T cell (CTL) responses in the blood. This cellular immune response was targeted against virus epitopes in all structural and nonstructural proteins of the virus. This is similar to findings in HIV. In other patients, lack of such a response or an inability to maintain it for an adequate time was associated with chronic infection. In chimpanzees the CTL response in the liver cells was stronger when virus was self-contained compared to the development of chronic infection.
Rehermann also reported that HCV specific cellular immune responses are maintained for decades after HCV recovery sometimes in the absence of HCV-specific antibodies. These type of responses are relevant to the question of whether they may prevent persistent infection after re-exposure to HCV.
It appears that the immune system and its relationship to HCV is obviously not well understood. Reherman reported that in chronic HCV, virus specific T cells were present at low frequencies in the blood, but could be found. HCV specific T cells were more frequently found in the liver. She said the reasons for the relative weakness of the cellular immune response, that is unable to clear the virus, yet strong enough to contribute to chronic inflammatory liver injury are not known. General immune tolerance or immuno-suppression are unlikely to be the cause of HCV persistence since in the absence of liver cirrhosis, the majority of chronically infected patients display normal immune responses against other viruses.
It appears that HIV has a negative impact on the immune response to HCV in coinfected persons. HIV may further impair the immune response to HCV and this may contribute to HIV accelerating HCV progression. The effect of HIV on HCV progression appears very complicated. Some persons have end-stage liver disease or cirrhosis with high CD4s and undetectable viral load.
Reherman reported that HCV specific antibodies are generally detected 7 to 31 weeks after infection and that they are targeted against epitopes in all viral proteins (the immune system mounts an antibody response to HCV). Several studies have described neutralizing antibodies that are targeted against the envelope proteins. However, this may be considered a misnomer since these anti-bodies really do not neutralize the virus. Recovery from HCV has also been described in the absence of an antibody response to envelope proteins and HCV persistence (chronic infection) without sequence changes in the envelope proteins. Therefore, progression to persistent infection is most likely a multifactorial process depending on many aspects of virus-immune system interactions.
Immune Based Therapy
Ray Chung (Massachusetts General Hospital/Harvard Medical School) suggested at the AASLD meeting in Chicago based on preliminary data that immune therapy may be a viable goal. HCV-specific cellular immune responses might help to contain the virus, but it's also been suggested that the immune response might contribute to liver damage. In analyzing the CD8 mediated immune response, Chung identified and reported on two individuals with a strong and persistent cellular immune response against HCV during chronic infection. Both persons also showed control of viral replication and no evidence of active liver disease. Chung concluded that this suggests that an effective, strong immune response can control HCV and may not cause liver damage, and that these findings provide evidence for a rationale to consider immunotherapy in chronic HCV. In HIV we have been looking for immunotherapy without success but maybe we can have better luck in HCV.
Does HAART Affect Liver Disease Progression in HCV/HIV Coinfected Patients?
The short answer is that the Sterling data described below suggests HAART may accelerate HCV progression if the number of CD4 cells increases sufficiently and HIV is suppressed. Other studies do not address the question as directly but suggest that HAART does not accelerate HCV.
An unanswered question is how HAART affects the liver of the HCV/HIV coinfected person. Opinions on this are mixed. Some doctors and researchers feel that HAART may increase ALT, liver inflammation, and virus activity, but that this may not have much long term clinical significance on fibrosis progression. Obviously, the other side is that the affect of HAART on HCV liver disease may be to accelerate progression. A small French study previously reported that patients taking HAART did not have any faster HCV progression than patients with HCV alone. This has not been confirmed, although other researchers have reported findings suggesting the same.
Although the Sterling study (Richard K. Sterling, MCV/VCU, Richmond, VA, DDW May 2001) discussed below has flaws in its design and analysis, as most studies do, it presents interesting findings. The study is small (n=39), but Sterling finds that patients with undetectable or low viral load and CD4 cell counts above 200 are more likely to have cirrhosis than patients with lower CD4 cell counts or higher viral loads. As an explanation for this observation, Sterling suggests that superior or optimal immune restoration from HAART may adversely impact on HCV disease severity. Another possible explanation is that patients with high CD4's and undetectable viral load were more adherent, and therefore had more HIV drug exposure.
Obviously, the patients with hi CD4s & low viral loads (n=29) received treatment for HIV and were doing well with therapy. As Sterling reports, the mean viral load was 1.3 log and the mean CD4 cell count was 552. The mean viral load in the patients with CD4s <200 was 2.7 log viral load and the mean CD4 cell count was 143 (n=9). The patients with high CD4 cell counts (>200) had been infected with HIV for a longer period of time (10 vs 7.8 years). However, there was no difference in the level of ALT, the percent with normal ALT, serum HCV RNA, the percent with high HCV RNA, and total HAI (Hepatitic Activity Index) comparing those individuals with CD4 cell counts above and below 200 cells/mm3.
Coinfected patients with >200 CD4 cells were more likely to have cirrhosis (21% vs 0%) than patients with <200 CD4 cells. Although the percent with advanced fibrosis was similar in both groups, no case of cirrhosis was seen in those individuals with low CD4 cell counts. Also, patients with undetectable viral load were more likely to have cirrhosis (25% vs 5%).
The authors suggest that the more advanced fibrosis seen in those with higher CD4 cell counts and HIV RNA below assay detection suggests that immune function may be an important determinant in HCV disease severity. Immune restoration with effective HIV therapy may adversely impact on the severity of HCV disease in HIV/HCV coinfected individuals.
In Maribel Rodriguez study (Maribel Rodriguez, Fundacion Gastroenterologica de Diego, San Juan, PR; Jose F. Rodriguez, Univ of Puerto Rico, San Juan, PR, DDW May 2001), she finds that liver disease progression is faster in HCV/HIV coinfected patients than in HCV monoinfected patients despite the fact that alcohol use was increased in the HCV monoinfected patients (97% vs 65%). She reported that these results could be explained by the higher hepatitis C viral load at baseline (5.67 log vs 5.13 log, ie 467,000 copies/ml vs 132,000 copies/ml) observed in the co-infected patients when compared to the mono-infected patients. I think its important to bear in mind a limitation of this study. In speaking to the author she said that some patients had undetectable HIV and some patients did not. This creates a question about the findings. If the patients with undetectable viral load or high CD4 cell counts progressed more quickly, as Sterling found, and since Rodriguez did not separate the data based on that its hard to conclude coinfected progress more quickly. It may be that only those individuals with low CD4 cell counts and high viral loads progress more quickly. As well, in speaking to the study authors, Rodriguez expressed concern that HAART related elevated lipids, insulin resistance, elevated glucose, and other HIV or HAART related negative effects on the liver may worsen fibrosis.
Now, if you look at the study from San Diego described below, they found that having HCV did not negatively impact on the survival of patients with well managed HIV. But this study does not report whether these patients had cirrhosis or not as the study above addresses. So, I dont think the two studies necessarily contradict each other. These patients may have had cirrhosis but may not have died.
At the HIV Retrovirus Conference (Feb 2001), Torriani from San Diego and Spanish researchers (abstract 575) looked at the development of hepatoxicity after starting ART (antiretroviral therapy) for 94 HCV/HIV coinfected patients compared to 94 HIV infected patients. They reported hepatotoxicity was correlated with immunologic (CD4) and virologic (HIV viral load) response to ART. 37/94 (39%) of HCV/HIV coinfected vs 10/94 (11%) with HIV had >2 fold increase in ALT, leading to medication substitutions in 8 patients and cessation of ART in 2 HIV/HCV patients compared to no substitutions or discontinuations in the group with HIV monoinfection. The authors reported hepatoxicity was seen at all time points in coinfected patients, although Im unsure how they defined hepatotoxicity.
A group from Jacksonville, FL. reported on the impact of HAART on HCV disease in HCV/HIV coinfected patients (Abhijit Roychowdhury, Div of GI, Univ of Florida Health Science Ctr, Jacksonville, FL, DDW May 2001). They compared histologic progression by biopsy in HCV/HIV coinfected patients who received HAART to a group that did not receive any medication. The authors concluded that HAART may play a role in slowing HCV disease progression as the French research group found. However, these authors did not analyze their results taking into consideration various covariables as the Sterling group did. The HAI inflammatory activity score in the HIV/HCV coinfected patients who received HAART was slightly less than the patients who did not without HAART. They also reported HCV RNA level and HAI inflammation activity scores were slightly decreased in HIV/HCV patients on HAART, but there was no difference in HAI fibrosis scores when compared with the control HIV/HCV and HCV patients, suggesting that HAART may play a role in slowing HCV disease progression. They also reported that after being on HAART for 2 years or more, ALT (hepatoxicity) was more likely to increase.
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