Determining the Mechanisms of CD8+T cell responses to Hepatotropic Viruses
This web page was produced as an assignment for an undergraduate course at Davidson College.
Different priming methods were used to determine the effect of CD8+T cells which suggests new immunotherapeutic strategies against hepatitis B infection.
Noncytopathic viruses are viruses that establish very long-lasting infections and successfully can evade complete destruction(Heim et al. 2019). One example of a noncytopathic virus is hepatitis B virus (HBV). HBV replicates in liver cells and can cause acute or chronic infections. There are 300 million people worldwide that have a continuous infection with HBV and are at risk of developing chronic liver inflammation leading to cirrhosis and liver cancer(Zhang and Bevan 2011). Studies have shown that HBV-specific CD8+T cells are the main effector cells that contribute to immunological control(Heim et al. 2019). Priming is when a T helper cell makes first contact with an antigen. Priming of CD8+T cells can be prevented by the endothelial barrier that limits antigen recognition on epithelial cells(Bénéchet et al. 2019). Liver priming was thought to result in T cells becoming unresponsive or dysfunctional, but the mechanisms by which this occurs in HBV pathogenesis are not completely understood(Bénéchet et al. 2019).
In this paper the researchers are trying to determine the mechanism of the unresponsiveness of CD8+T cells in patients with HBV. In order to determine the mechanism, the researchers did three different experiments to determine the dynamics of CD8+T cells after priming the liver. Then genomic analysis was performed to determine the molecular determinants of HBV. Finally, they tested the effect of IL-2 treatment in HBV transgenic mice. For this article I’ll be talking about the dynamics of CD8+T cells after priming the liver.
The first experiment was analyzing HBV-specific CD8+T cells undergoing priming in a non-inflamed liver. There were two control groups and an experimental group. The control groups were wild-type mice and wild-type mice that was transduced with a lymphocytic vector that signals the HBV core protein to Kupffer cells and hepatic dendritic cells that are not usually infected by HBV. The experimental group was injecting CD8+T transgenic cells specific for the core protein of HBV known as Cor93 TN cells. These cells express a non-secretable version of the HBV protein in 100% of the hepatocytes. Antigen restriction was only applied to transgenic and wild-type lymphocytic vector mice. The results showed that in the transgenic mice group, antigen recognition on hepatocytes led to the generation of dysfunctional cells that produced little to no interferon(IFN) gamma and instead upregulated the inhibitory receptor PD-1. This indicates that depending on the antigen-presenting cell, the liver can support the development of either functional or dysfunctional CD8+ T cells. Images of the liver showed that clusters formed in wild-type mice transduced with the lymphocytic vector are dense, extravascular, and composed of largely immotile cells. In contrast, clusters formed in the transgenic mice are looser, intravascular, and composed of more motile cells. After 5-7 days clusters in the lymphocytic vector group disaggregate as the cells move out of the liver whereas clusters in the transgenic group remain in place. This possibly shows that antigens persist in the transgenic group compared to the lymphocytic group.
The second experiment was determining the fate of T cells that are primed with livers expressing low levels of HBV core antigen. The investigators first transferred Cor93 TN cells into wild-type mice which was previously injected with a low dose of a viral vector encoding the HBV core protein. Second, they transferred Cor93 TN cells into 3-4-week-old transgenic mice, which expresses only trace amounts of protein per hepatocyte. Reducing the amount of expressed antigen by more than 15-fold within individual hepatocytes did not affect the differentiation of intrahepatically primed CD8+ T cells. This experiment indicates that low expression of hepatocellular core antigen is not sufficient to induce effector differentiation.
In the final experiment, the researchers wanted to investigate the fate of Tn cells inside the liver primed by antigen presented by both hepatocytes as well as Kupffer cells and dendritic cells. This experiment was done by transferring Env28 and Cor93 TN cells into wild-type and transgenic mice transduced with vectors that either encode the HBV envelope alone or the HBV core protein and envelope. In wild-type mice, TN cells expanded and differentiated into IFN-gamma-secreting cells only when antigen was present. In transgenic mice, there was Env28 cell expansion and effector differentiation, but not Cor93 TN cells expansion and differentiation. This indicates that innate immune signals carried by the vectors are not sufficient to overcome Cor93 T cell dysfunction and dysfunctional Cor93 T cells do not produce soluble or membrane-bound mediators that inhibit Env28 T cell effector differentiation.
Overall, this paper portrayed the dynamics, genomic landscape, and functional consequences of CD8+ T cells undergoing intrahepatic priming. Hepatocellular presentation leads to a CD8+T cell dysfunction that is distinct from T cell alterations reported in other viral infections as such is not readily responsive to anti-PD-L1 treatment. The results reported in the paper should help to interpret the outcome of HBV studies and eventually inform the design of modified trials in select cohort patients. The investigators also determined that IL-2 can be used as a potential immunotherapeutic that can rescue CD8+ T cells rendered dysfunctional by hepatocellular priming. In the end IL-2 based strategies should be considered for treatment of chronic HBV infection. The paper does a really good job explaining how CD8+ T cells function in HBV patients using dynamic, genomic, and immunotherapeutic methods. The paper spent more time on studying the dynamics rather than the genomic and immunotherapeutic side of these T cells. If there was one thing this paper could have done differently, it would be to spend more time on the genomic side of CD8+ T cells to determine the cause of CD8+ T cell responses.
Nikhil Virani is currently enrolled at Davidson College. Contact him at nivirani@davidosn.edu
References
Bénéchet A. P., G. De Simone, P. Di Lucia, F. Cilenti, G. Barbiera, et al., 2019 Dynamics and genomic landscape of CD8 + T cells undergoing hepatic priming. Nature 574: 200–205. https://doi.org/10.1038/s41586-019-1620-6
Heim K., C. Neumann-Haefelin, R. Thimme, and M. Hofmann, 2019 Heterogeneity of HBV-Specific CD8+ T-Cell Failure: Implications for Immunotherapy. Front. Immunol. 10. https://doi.org/10.3389/fimmu.2019.02240
Zhang N., and M. J. Bevan, 2011 CD8+ T Cells: Foot Soldiers of the Immune System. Immunity 35: 161–168. https://doi.org/10.1016/j.immuni.2011.07.010
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