Pathology is the study of the natural course of disease and can be divided into two branches: anatomic and clinical pathology. Clinical pathologists work to discover diagnostic signs of design by studying chemicals, cells, sputum, bone marrow, and urine. Anatomic pathologists study tissue samples from cell cultures, biopsies, surgery, and autopsy.

The study of HCV pathology, and of the anatomic branch in particular, has been complicated by the lack of an effective animal or cell culture model. Also, because HCV preferentially targets the liver, it is difficult to draw conclusions about how HCV behaves based on how it grows in non-liver cells. The variation in reaction profiles observed in HCV patients further complicates the elucidation of HCV pathology. This diversity of response is believed to be related to differences in HLA haplotypes, immune competence, and viral genotypes. Lastly, pathological studies have been hindered by the lack of a dependable test for disease progression short of invasive liver biopsy (see Treatment -- Diagnostic Tests).  Because of these complications, little is known with certainty about HCV pathology (3). What is understood has mostly been gleaned from indirect lines of evidence, and sometimes from live tissue biopsy.

Direct HCV Cytotoxicity:

The most important controversy over HCV pathology concerns its mode of tissue destruction, which is often manifested as total liver failure. Experiments have revealed the existence of both direct and indirect HCV cytotoxicity (1). There remains, however, much debate over which of these mechanisms is the primary mechanism of necrosis. Even greater uncertainty exists over the pathological mechanisms behind HCV's high incidence of hepatocellular carcinoma -- a rare form of cancer -- as well an assortment autoimmune reactions (1).

Nearly all support for direct HCV cytotoxicity has come from indirect lines of evidence. Most superficially, HCV is classified on the basis of homology as part of the Flaviviridae family. Other viruses belonging to this family cause diseases such as yellow fever, and have well-established routes for direct cellular damage (3). However, histologic examinations of HCV-infected livers have shown hepatocytes with gross cytoplasmic changes, but no accompanying inflammation (1). Although the virus has not been directly observed in these cells (HCV has never been visualized by electron microscopy), the cytoplasmic abnormalities that have been found are classically associated with virus-induced cell death.

Researchers reason that if HCV causes direct cell death, then patients with elevated levels of HCV RNA or HCV proteins should present with high incidence of necrosis and, therefore, chronic disease. So far, no such relationship has been demonstrated (3). PCR amplification to determine serum concentration of HCV RNA has shown no direct correspondence to nephron necrosis. Furthermore, transgenic animal models, with high levels of HCV antigens expressed in hepatocytes, demonstrate no associated cell death (2). Other studies have shown that hepatocyte cell death does occur when HCV concentration reaches certain critical levels, but within the majority of patients, such high levels of viremia are rare. Some severely immunocompromised HCV patients --  classified as "ultraquick progressors" -- have demonstrated extremely rapid liver failure. It is thought that this is the result of abnormally high levels of HCV viremia, resulting in direct cytotoxicity (1).

In conclusion, while it is likely that some degree of cell death results from direct HCV damage, current evidence points away from direct cytotoxicity, and towards the effects of an immune response, as the leading cause of HCV pathology.

Indirect Cytotoxicity -- The Immune Response:

HCV has been shown to stimulate all normal pathways of the immune response. Early in the immune response, HCV stimulates the effector cells of innate immunity including natural killer (NK) cells. NK cells are thought to contribute, at least in part, to the elevated levels of interferon (IFN) common in HCV patients. Soon after, the acquired immune response is stimulated and both HCV specific B and T cells are recruited (1). 

Humoral Immunity --  Antibody (Ab) production to both structural and nonstructural HCV proteins has been demonstrated in nearly all HCV patients (see Virology). In particular, a large gamut of Abs reactive to HCV envelope proteins E1 and E2 develop.  The clinical significance to these Abs has not been clearly demonstrated, but it is believed that hypervariable regions in the E1 and E2 proteins prevent these Abs from clearing the disease, and instead drive the antigenic drift of HCV (1). Further evidence supporting the ineffectiveness of Abs to clear infection has come form a set of experiments conducted by Farci et al. These researchers demonstrated  that chimpanzees that recovered from HCV infection could later be re-infected by both heterologous (different genotype) and autologous (same genotype) challenge. The criteria for 'recovery' in these studies involved an HCV RNA-negative status, and normal Alanine Aminotransferase levels (see Current Therapeutic Techniques).

There is speculation that autoreactive Abs may opsonize hepatocytes and stimulate both antibody dependent cytotoxicity (ADCC), as well as the complement (C') cascade. Either route of cell death presents possible explanations of hepatocyte necrosis observed in HCV biopsy. However, researchers have yet to find anti-HCV Ab that is reactive to cell membranes (1). Consequently, ADCC and C' pathways are considered unlikely, or at least largely insignificant. It is improbable that the humoral immune response results in the widespread liver necrosis observed in HCV patients. 

Lines of evidence do, however, support the hypothesis that some of the autoimmunity observed in correlation with HCV infection may result from humoral immunity. A subset of B-cells characterized by the CD5 surface molecule have been shown to undergo clonal expansion during HCV infection (see Glossary for the function of the CD5 molecule). CD5+ B-cells have been implemented by such autoimmune responses as rheumatoid arthritis, and may represent a possible cause of the observed autoimmune reactions (2). Furthermore, certain HCV antigens have been found to bear sequence homology to self antigens (see Virology). It is postulated that chronic exposure and activation of the humoral immune system to HCV, as well as its failure to clear the infection, could result in clonal expansion of self reactive antibodies (2). Specifically, auto-antibodies to the liver and the kidneys have been found in HCV patients (3).

Cell Mediated Immunity (CMI) --The study of cell-mediated immunity (CMI) in HCV infection has proven somewhat difficult due to the lack of a good animal model, and because peripheral blood T-cell levels are not reflective of liver T-cell concentrations.

The hepatitis C virus typically induces a strong T-cell reaction, generating both Th1 and Th2 responses. It is believed that the relative intensities of these two distinct responses is related to significant cell death, such as that observed in the livers of HCV-infected patients. In particular, unusually high amounts of Th1 cells have been linked to nephron necrosis. Patients with chronic HCV show high titers of the Th1 cytokines including IL-2, TNF-alpha, and IFN-gamma -- all of which implicate Th1 T-cells in cytotoxicity (1). Furthermore, it has been found that elevated levels of IFN-gamma mRNA and IL-2 mRNA in the liver correlated with fibrosis and inflammation; yet another indication that Th1 cytokines play some role in hepatocellular damage. Researchers have also been able to expand HCV reactive T-cells from liver biopsies. Nearly all of these lymphocytes have turned out to be activated cytotoxic T lymphocytes (CTLs). It has been hypothesized that insufficient clearance of the hepaititis C virus, in the presence of large scale CTL activation, can lead to tissue necrosis (2).

In addition to evidence identifying an aggressive Th1 response as an important cause of cell death, further evidence supports the Th2 response as being preventative against HCV progression (3). Studies have shown that 73% of all patients who contract HCV (measured by circulating HCV RNA), but present with no clinical signs of infection (indicated by normal liver biopsy and ALT levels), have high CD4+ responses to immunogenic core proteins such as NS4 (see Virology).  In contrast, only 10% of those who test positive for both HCV and clinical progression (indicated by necrosis in biopsy and elevated ALT levels) have strong CD4+ proliferative responses.  This correlation of CD4+ clonal expansion with non-progressive HCV suggests that Th2 CD4+ T-cells offer protection against cell damage (3). This relationship is consistent with classical dogma; it is understood that Th2 cytokines have a suppressive effect on the Th1 response. Future medical therapies for HCV may be able to make use of this observation by trying to induce a stronger Th2 response, and hence repressing the damaging effects of CTL activation.

References:

1.  Farci P, Alter HJ, Govindarajan S, et al: Lack of protective immunity against re-infection with hepatitis C virus.  Science 258:135, 1992.

2.  Fang JWS, Lau GKK, Marousis C, et al: Failure to detect cell surface expression of hepatitis C structural proteins using a vaccinia expression system.  Gastroenterology 106:A890, 1994

3.  Lau J, Nelson D, et al:  Pathogenesis of Hepatocellular Damage in Chronic Hepatitis C Virus Infection.  Clinics in Liver Disease, W.B. Saunders Company, 1997.