Potential conflict of interest: Nothing to report.
Hepatitis B virus infection: A strong case against ageism†
Article first published online: 28 SEP 2011
Copyright © 2011 American Association for the Study of Liver Diseases
Volume 54, Issue 4, pages 1477–1479, October 2011
How to Cite
Kroy, D. C., Lauer, G. M. (2011), Hepatitis B virus infection: A strong case against ageism. Hepatology, 54: 1477–1479. doi: 10.1002/hep.24604
- Issue published online: 27 SEP 2011
- Article first published online: 28 SEP 2011
- Manuscript Accepted: 26 JUL 2011
- Manuscript Received: 13 JUL 2011
Publicover J, Goodsell A, Nishimura S, Vilarinho S, Wang Z, Avanesyan L, et al. IL-21 is pivotal in determining age-dependent effectiveness of immune responses in a mouse model of human hepatitis B. J Clin Invest 2011;121:1154-1162. (Reprinted with permission.)
HBV is a noncytopathic hepadnavirus and major human pathogen that causes immune-mediated acute and chronic hepatitis. The immune response to HBV antigens is age dependent: viral clearance occurs in most adults, while neonates and children usually develop chronic infection and liver disease. Here, we characterize an animal model for HBV infection that recapitulates the key differences in viral clearance between early life and adulthood and find that IL-21 may be part of an effective primary hepatic immune response to HBV. In our model, adult mice showed higher HBV-dependent IL-21 production in liver, compared with that of young mice. Conversely, absence of the IL-21 receptor in adult mice resulted in antigen persistence akin to that of young mice. In humans, levels of IL-21 transcripts were greatly increased in blood samples from acutely infected adults who clear the virus. These observations suggest a different model for the dichotomous, agedependent outcome of HBV infection in humans, in which decreased IL-21 production in younger patients may hinder generation of crucial CD8+ T and B cell responses. These findings carry implications for therapeutic augmentation of immune responses to HBV and potentially other persistent liver viruses.
Despite availability of a highly effective recombinant vaccine, hepatitis B virus (HBV) infection remains a major global health problem, with more than 400 million individuals worldwide being chronically infected.1 There are striking age-related differences in the clinical outcome of acute HBV infection. Approximately 95% of infected adults clear HBV spontaneously, whereas more than 90% of exposed neonates and 30% of children (1-5 years of age) develop chronic infection.2 This implies significant differences in the immune response related to the age of the infected host, but what exactly these differences are remains mostly obscure.
One major obstacle for a better understanding of the immunological mechanisms underlying the different outcomes of HBV is the virtual impossibility to assess immune responses during HBV infection in neonates and young children, for a variety of practical and ethical reasons. Together with the rarity of adult infection resulting in viral persistence, this renders definition of immunological determinants of the outcome of human HBV infection an almost insurmountable scientific challenge. Thus, the introduction of an elegant mouse model that mirrors key immunological and clinical differences between early (childhood) and late (adulthood) HBV infection, by a group of scientists led by Jody Baron from the University of California at San Francisco, presents a welcome opportunity to better define the critical events leading to HBV control and persistence.
In the experiments reported by Publicover et al., two kinds of transgenic HBV antigen–expressing mice were studied: either constitutively expressing HBV envelope under an albumin promoter (HBVEnv mice) or actively replicating HBV DNA construct that allows viral replication and the release of infectious virions (HBVRpl mice). All mice were also crossed with RAG-1 knockout mice, resulting in animals expressing HBV antigens but lacking adaptive immune responses. Mice were adoptively transferred with syngeneic adult splenocytes, creating a scenario similar to primary exposure to HBV. Experiments were performed in both adult mice (8-10 weeks of age) and young animals (3-4 weeks of age) in order to explore potential immunological differences based on the maturity of the host.
Interestingly, the authors found significant clinical as well as immunological differences between adult and young animals that were reminiscent of many observations made in human HBV infection. Adult mice receiving splenocytes from equally mature mice displayed a robust inflammatory response and liver injury with elevated plasma alanine aminotransferase (ALT) levels, together with histopathological features reminiscent of HBV-associated portal and intraparenchymal inflammation and hepatic necrosis. In contrast, young animals that had a transfer of adult splenocytes showed only marginal signs of hepatic inflammation and no ALT rise. The lack of inflammation in the younger mice was not caused by a lack of functional adaptive immune cells, because the repertoire of the HBV-specific cytokine response by liver lymphoid cells was comparable in adult and young mice. However, there was a qualitative difference in the response, with adult mice producing produced significantly higher amounts of IL-4 and IFN-γ.
The murine model of HBV infection described by Publicover et al. also recapitulates key differences in human antibody responses at different ages (childhood versus adulthood). Adult HBVEnvRag mice adoptively transferred with adult splenocytes produced antibodies to hepatitis B surface antigen (HBsAg) that were detectable right after clearance of HBsAg from the blood, whereas young HBVEnvRag animals failed to clear HBsAg while displaying antibodies only against hepatitis B core antigen (HBcAg), but did not generate detectable HBsAg antibodies. With regards to the cellular immune response, adult HBVEnvRag recipients displayed more intrahepatic CD8+ T cells and T follicular helper (TFH) cells, whereas the number of liver resident CD4+ T cells, B cells, natural killer (NK) cells, natural killer T cells (NKT), and regulatory T cells (Tregs) was the same in young and mature animals. ELISpot analyses of intrahepatic T cells revealed stronger and broader HBV-specific T cell responses in the livers of adult versus young recipient mice, again reflecting human characteristics of HBV infection.3
Having established the potential of their new mouse model to interrogate the differences between neonatal/child and adult HBV infection, Publicover et al. further analyzed hepatic TFH cells, because this cell type was mainly present in adult mice, controlling HBV. The investigators focused on interleukin-21 (IL-21), because this cytokine is typically produced by TFH cells and has been shown to be critical for generating plasma cells and antibody isotype switching, for promoting CD8+ T cell expansion,4 and is capable of rescuing “exhausted” T cells in persistent infection.5-7 Eight days after the adoptive transfer, adult recipient mice—in comparison to young animals—displayed a significant increase of IL-21 messenger RNA levels in liver-derived CD4+ T cells and also an increase in the number of IL-21–producing hepatic lymphoid cells that were identified as TFH cells by analyzing the expression of chemokine receptor cysteine-X-cysteine receptor 5 (CXCR5). Consequently, adult mice exhibited a significantly higher number of plasma cells and plasma blasts that had differentiated into IgG1-, IgG2b-, and IgG3-expressing cells. Furthermore, Publicover et al. demonstrated that the IL-21 receptor on transferred splenocytes is essential for the generation of a robust immune response followed by viral clearance, because lack of IL-21 receptor on splenocytes led to the prevention of hepatitis during the peak of the adaptive immune response on day 7.
On the basis of these findings, the authors suggest that IL-21 production by intrahepatic TFH cells is critical for the generation of functional HBV-specific CD8+ T cell and B cell responses and thus is a key mechanism associated with clearance of HBV in adulthood, in stark contrast to the scenario in younger mice. This hypothesis was further supported by analysis of peripheral blood mononuclear cells from humans with acute self-limiting as well as established chronic HBV infection. As predicted by the mouse model of Publicover et al., HBV infection resulted in a significant seven-fold increase in IL-21 messenger RNA expression in peripheral blood mononuclear cells of patients who cleared the virus compared to that in patients with persistent viremia.
Together, the data convincingly identify IL-21 as a crucial cytokine produced by intrahepatic TFH cells in HBV infection, and demonstrate the utility of this novel murine model of HBV infection to study age-related differences in the immune response to HBV.
As important as it will be to harness the possibilities of this new mouse model of HBV infection, it will be almost equally important to define its potential limitations. One important difference to human infection, highlighted by the authors, is that in the transgenic mouse model, naive immune cells are suddenly confronted with hepatocytes expressing high levels of HBV antigens or virions, in contrast with the slower emergence of viral antigen in natural infection. Another important question to address will be whether there are any differences between the adaptive immune cells in younger versus older animals, because all experiments were performed with splenocytes from adult animals. Experiments conducted with splenocytes from young animals will establish whether solely age-related differences in the immunological environment, or also differences in the lymphoid cells themselves, are key to different outcomes of HBV infection.
We are looking forward to future studies that use the HBV model presented by Publicover et al., and are excited about the potential of this novel tool to investigate HBV infection, an important disease that continues to challenge clinicians and immunologists alike.