Sitia G, Aiolfi R, Di Lucia P, Mainetti M, Fiocchi A, Mingozzi F, et al. Antiplatelet therapy prevents hepatocellular carcinoma and improves survival in a mouse model of chronic hepatitis B. Proc Natl Acad Sci U S A. 2012;109:E2165-E2172. (Reprinted with permission.)
Chronic infection with hepatitis B virus (HBV) is a major risk factor for the development of hepatocellular carcinoma (HCC). The pathogenesis of HBV-associated HCC involves both viral and host factors. The latter include a functionally inefficient CD8+ T-cell response that fails to clear the infection from the liver but sustains a chronic necroinflammatory process that contributes to the development of HCC. According to this scenario, amelioration of immune-mediated chronic liver injury may prevent HCC. Because platelets facilitate immune-mediated liver injury by promoting the hepatic accumulation of virus-specific CD8+ T cells, we evaluated the long-term consequences of antiplatelet therapy in an HBV transgenic mouse model of chronic immune-mediated necroinflammatory liver disease that progresses to HCC. Treatment with aspirin and clopidogrel during the chronic phase of the disease diminished the number of intrahepatic HBV-specific CD8+ T cells and HBV-nonspecific inflammatory cells, the severity of liver fibrosis, and the development of HCC. Antiplatelet therapy improved overall survival without causing significant side effects. In contrast, the same antiplatelet regimen had no antitumor effect when HCC was induced nonimmunologically by chronic exposure to a hepatotoxic chemical. The unprecedented observation that antiplatelet therapy inhibits or delays immune-mediated hepatocarcinogenesis suggests that platelets may be key players in the pathogenesis of HBV-associated liver cancer and supports the notion that immune-mediated necroinflammatory reactions are an important cause of hepatocellular transformation during chronic hepatitis.
Hepatocellular carcinoma (HCC) is the fifth most frequent tumor and the third cause of cancer-related death worldwide.1 Among the etiologic factors, chronic liver infection by hepatitis B virus (HBV) accounts for more than 50% of HCC cases, identifying HBV as the major carcinogen for this cancer type.1 Potential mechanisms whereby HBV can promote liver cancer development have been hypothesized, and both viral (insertional mutagenesis, expression of viral gene products with oncogenic properties) and host (antiviral immune response) factors seem to play an important role in this process.1-3 Generally, HCC develops in HBV carriers in a context of long-standing immune-mediated chronic hepatitis.2, 3 In the latter condition, a functionally inefficient virus-specific response mediated by CD8+ T cells that is unable to eliminate HBV from the liver takes place. Consequently, this ineffective response facilitates the persistence of low-level hepatocellular destruction, thus triggering compensatory hepatocellular proliferation and exposing proliferating hepatocytes to mutagens generated by the inflammatory reaction.2-4 Ultimately, these repetitive cycles of cell death and proliferation induce an irreversible damage to liver parenchyma, leading to cirrhosis, fibrosis, and, eventually, HCC.2-4
Although anti-HBV vaccinations implemented in infants have led to a striking drop in the incidence of liver cirrhosis and HCC, over 400 million individuals remain HBV infected worldwide and at risk of developing HCC.5 Thus, a better understanding of HCC pathogenesis associated with HBV infection is required for the development of novel, effective chemopreventive and therapeutic strategies against this deadly disease.
Major contributions in the elucidation of the role of HBV in hepatocarcinogenesis come from studies performed in mouse models. Transgenic (TG) mice producing hepatotoxic quantities of HBV large-envelope polypeptide displayed hepatocellular injury with regenerative hyperplasia, chronic inflammation, oxidative DNA damage, and aneuploidy that inexorably led to HCC.2, 3 These studies demonstrated that HBV might promote hepatocarcinogenesis in the absence of insertional mutagenesis or genotoxic chemicals.2, 3, 6 Subsequently, a TG mouse (lineage 107-5, inbred B10D2), in which 100% of the hepatocytes express nontoxic quantities of HBV large, middle, and small envelope proteins, was generated.7 This mouse is immunologically tolerant to hepatitis B surface antigen, displaying no evidence of liver disease during the lifetime, unless its immune system is destroyed and replaced with bone marrow and spleen cells from syngeneic non-TG donors previously immunized to produce an efficient virus-specific CD8+ T-cell response.7 As a result, these mice develop acute and chronic hepatitis, finally giving rise to large HCCs.7 Noticeably, the investigators previously identified platelets as prominent cells involved in biologic consequences of viral hepatitis in this model.8 Indeed, using additional mouse models of acute HBV infection (i.e., HBV-replication–competent TG mice), they showed that platelets are detectable within CD8+ T-cell-containing hepatic necroinflammatory foci, and their depletion significantly ameliorated the severity of liver disease.8 In platelet-depleted animals, the profound reduction of liver damage was paralleled by a proportional decrease in intrahepatic accumulation of virus-specific CD8+ T cells. Transfusion of healthy platelets in platelet-depleted mice restored accumulation of CD8+ T cells and severity of disease, indicating that—upon activation—platelets contribute to liver disease and viral clearance by promoting the hepatic recruitment of virus-specific CD8+ T cells.8
To further delineate the role of platelets in HBV-dependent chronic liver damage and carcinogenesis, Sitia et al. investigated the effect of antiplatelet therapy (using aspirin, clopidogrel, or the combination of the two drugs) in HBV TG mice (lineage 107-5).9 The investigators employed a large-scale experiment consisting of a total of 540 mice. Treatment began 30 days after induction of hepatitis and continued indefinitely. A remarkable reduction of liver injury was detected in TG mice subjected to antiplatelet treatment with either aspirin or clopidogrel, when compared to vehicle-treated mice, and the combination of the two drugs further prevented liver damage.9 In long-term follow-up, TG mice receiving aspirin/clopidogrel combination exhibited a less-pronounced liver injury, as a result of decreased accumulation of HBV-specific CD8+ T cells and secondarily recruited HBV-nonspecific inflammatory cells. Also, aspirin/clopidogrel-treated mice displayed reduced compensatory hepatocyte proliferation and severity of liver fibrosis.9 In terms of tumor prevention, at days 270 and 450, 30% and 65% of vehicle-treated mice had one or more large HCCs, respectively, whereas significantly smaller tumors developed in only 5% and 20% of aspirin/clopidogrel-treated animals. By day 510, 75% of vehicle-treated mice were found dead as a consequence of HCC burden, and remaining mice were euthanized because of tumor-induced cachexia. In contrast, only 20% of aspirin/clopidogrel-treated mice had died and none showed signs of cachexia at the same time point. Also, 66.7% of aspirin/clopidogrel-treated mice were HCC free, and remaining mice developed small tumors by day 600. Finally, aspirin/clopidogrel therapy was applied to a mouse model of chemically induced hepatocarcinogenesis. Noticeably, combination therapy was unable to prevent hepatocarcinogenesis in the latter model, excluding a pathogenetic role for platelets in this setting.9
The landmark study by Sitia et al. further underlines the role of platelets in HBV-associated hepatocarcinogenesis, providing both crucial information in terms of HCC pathogenesis and potentially important implications for treatment options. As concerns the pathogenesis of chronic liver damage and HCC, the investigators convincingly showed that platelets facilitate hepatic accumulation of virus-specific CD8+ T cells. The investigators hypothesize that the inflammatory response in the liver might induce changes of the vessel wall, promoting platelet adhesion and activation. Once activated, platelets might interact with virus-specific CD8+ T cells, presumably allowing their egression from the bloodstream and penetration in the liver parenchyma, with consequent pathogenic functions. Moreover, activated platelets might favor CD8+ T-cell division. Furthermore, platelets might play yet-unrecognized role(s) on liver injury and carcinogenesis. Mounting evidence supports the importance of platelets in immunomodulation and synthesis of proteins with multiple functions.10 In particular, platelets contain the largest amount of transforming growth factor beta (TGF-β) in the body,10 and they might stimulate TGF-β secretion in the setting of HBV chronic infection, thus inducing a fibrogenic environment prone to liver fibrosis and HCC development.11
In terms of treatment, a chemopreventive role of antiplatelet therapy against the development of liver fibrosis and HCC is envisaged. Different from costly nucleoside inhibitors that reduce HBV proliferation, but might not reduce HCC occurrence,1 antiplatelet therapy represents a remarkably inexpensive, potentially effective and well-tolerated option for HBV chronic carriers. Furthermore, because HCV- and alcohol-associated HCC most often develop in a context of chronic inflammation,1 antiplatelet therapy could also be highly beneficial for these individuals.
Although the data presented by Sitia et al. are of pivotal importance in understanding HBV-associated hepatocarcinogenesis, several issues remain to be addressed before antiplatelet treatment can be applied in a clinical setting. First, the applicability of the antiplatelet regimen might be limited by the increased risk of bleeding in patients affected by chronic liver diseases. However, recent observations indicate the presence of a procoagulant imbalance in individuals with chronic liver disease,12 suggesting that antiplatelet therapy might be beneficial, both in terms of HCC and thrombosis prevention, at least in some patient subgroups. Second, this mouse model does not provide information in terms of liver cirrhosis, in keeping with the notion that rodents seldom develop this pathological condition. Third, the TG mouse model used for the investigation supports neither infection nor viral replication, and, as such, the effect of antiplatelet therapy on these parameters could not be monitored. Thus, it is possible that a reduced hepatic accumulation of HBV-specific CD8+ T cells consequent to antiplatelet therapy may have a negative effect on the extent to which CD8+ T cells control liver viral load. Also, because the TG mouse model used for the investigation does not overexpress the full viral genome, this might lead to an underestimation of the role of genes with oncogenic potential in HCC development in HBV chronic carriers. Concomitant use of nucleoside inhibitors might be helpful in limiting both HBV replication and HBV-related oncogenic properties in patients subjected to antiplatelet therapy.