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Corresponding authors. Addresses: Toronto Center for Liver Disease & Schwartz Reisman Liver Research Centre, Toronto General Hospital Research Institute, University Health Network, 200 Elizabeth Street, Toronto, Ontario, M5G 2C4, Canada.
Department of Immunology, University of Toronto, Toronto, Ontario, CanadaToronto Center for Liver Disease & Schwartz Reisman Liver Research Centre, Toronto General Hospital Research Institute, University Health Network, Toronto, Ontario, Canada
Department of Immunology, University of Toronto, Toronto, Ontario, CanadaAjmera Transplant Centre & Schwartz Reisman Liver Research Centre, Toronto General Research Institute, University Health Network, 200 Elizabeth Street, Toronto, Ontario, M5G 2C4, CanadaDepartment of Laboratory Medicine and Pathobiology, University of Toronto, Medical Sciences Building, 1 King's College Circle, Toronto, Ontario, M5S 1A8, Canada
The liver provides a unique niche of lymphocytes enriched with a large proportion of innate-like T cells. However, the heterogeneity and functional characteristics of the hepatic T-cell population remain to be fully elucidated.
Despite recent advances in our understanding of the cellular heterogeneity of the human liver, the role of liver resident lymphocytes and in particular, intrahepatic T cells remains an important open question with implications in liver health and disease. Despite early reports of the liver promoting T cell apoptosis,
T cells are increasingly understood to be functional in the liver. However, the functions of intrahepatic T cells are altered in disease states such as suppression of T cell function during endotoxin challenge,
which may contribute to delayed antigen clearance. Studies such as these underscore the phenotypic complexity of intrahepatic T cells, which remains to be fully examined, a topic Koh et al., address through a multi-omic approach with extensive functional validation
in their paper in this issue of Journal of Hepatology (Fig. 1).
Fig. 1Liver sinusoidal T cellswith natural killer cell (NK)-like properties: CD56hiCD161-CD8+T-cells characterized byNK-likeactivation, including the expression of perforin, granzyme, interferon (IFN)-ɣ and the potential to degranulate. These cells are expanded in Hepatitis B virus (HBV)-associated liver disease and exert NKG2C-mediated NK-like effector functions with a restricted T cell receptor (TCR) repertoire and weak TCR engagement. These liver sinusoidal T cells are highly responsive to innate cytokines, particularly interleukin (IL)-12/IL-18 and IL-15.
To perform these examinations, the authors obtained liver perfusates and biopsies from healthy living donor liver transplant donors and transplant recipients being transplanted for hepatitis B virus (HBV)-associated chronic liver disease. The authors employed cellular indexing of the transcriptomes and epitopes by sequencing (CITE-seq) analysis along with T cell receptor sequencing (TCR-seq) to examine the transcriptional programs and clonotypic diversity of liver sinusoidal lymphocytes in healthy and HBV-related disease, validating their transcriptional findings with ex vivo phenotypic and functional experiments. This approach revealed a distinct sinusoidal CD56hiCD161-CD8+ T-cell population characterized by natural killer cell (NK)-like activation, including the expression of perforin, granzyme, IFN-ɣ and the potential to degranulate. This population, which was enriched in the liver versus the blood, was expanded in HBV-associated liver disease and exerted NKG2C-mediated NK-like effector functions in the absence of TCR stimulation. Koh and colleagues further showed that these intrahepatic CD56hiCD161-CD8+ T-cells possessed a restricted TCR repertoire and exhibited enhanced responses to innate cytokines, particularly interleukin (IL)-12/IL-18 and IL-15. The authors assert that these liver-resident CD56hiCD161-CD8+ T-cells may share features with virtual memory T cells (TVM), recently described in mice.
Importantly, these liver sinusoidal CD56hiCD161-CD8+ T cells exerted NK-like, TCR-independent cytotoxic activity through NKG2C ligation whereas previous reports demonstrated that the NK receptor, NKG2D, served only as a co-stimulatory receptor for TCR-mediated activation.
An important analysis from this study was a formal comparison of immune cells isolated from liver perfusions and immune cells isolated from enzyme-digested liver biopsy tissue. To perform this comparison, the authors isolated liver sinusoidal mononuclear cells from matched liver perfusates and biopsy tissues obtained from 20 individuals with or without HBV-associated chronic liver disease. The authors then comprehensively compared cellular compositions and phenotypes between liver sinusoidal and intrahepatic immune cells. In terms of cellular composition, the authors found a significant correlation in the percentages of TEM, TEMRA, CD57+, and CD56hiCD161- cells between liver sinusoidal and intrahepatic CD8+ T cells. Regarding phenotype, the authors found that the frequency of perforin+ cells, granzyme B+ cells, and granulysin+ cells was similar between the liver sinusoidal CD56hiCD161-CD8+ T cells and the intrahepatic CD56hiCD161-CD8+ T cells. This is an important evaluation that reinforces the notion that liver sinusoidal T cells reflect that of the intrahepatic immune environment.
Regarding activation of intrahepatic T cells, these data add to a growing body of literature that intertwines antigen-specific, TCR-mediated activation with bystander T cell activation. Antigen-specific tolerance occurs when naive T cells are primed in the absence of inflammation.
However, inflammation that activates antigen presenting cells and provides the third signal for effective T cell priming also induces bystander effector T cell activation. IL-15 and transforming growth factor (TGF)-β coordinate to differentiate CD8+ T cells towards a tissue resident phenotype, capable of producing high levels of IL-2,
Phenotypic and functional differences of HBV core-specific versus HBV polymerase-specific CD8+ T cells in chronically HBV-infected patients with low viral load.
However, IL-2 can coordinate with IL-12 to induce the activation of a large hepatotoxic CD8+ T cell population that can non-specifically kill hepatocytes through a Fas ligand-dependent mechanism.
Similarly, hepatotoxic antigen non-specific CD8+ T cell populations that contribute to inflammation and disease stage have been observed in Hepatitis Delta virus infection.
While the HBV-related liver disease samples included in the study by Koh et al. originated from end-stage disease, they support the role of TCR-independent bystander T cell activation in progression of liver disease. This work raises the question of whether the observed cytotoxic potential of the intrahepatic NK-like T cells translates to hepatocyte killing and in what context is this killing triggered. Bystander T cell activation presents a significant challenge for new therapies aiming to achieve cure for chronic hepatitis B where the inflammatory profiles distinguishing antiviral events from inflammation-driving pathogenic liver damage appear similar.
The inflammatory cytokine profile associated with liver damage is broader and stronger in patients with chronic hepatitis B compared to patients with acute hepatitis B.
In conclusion, the findings of Koh et al., provide new insights into the phenotype and functional characteristics of liver sinusoidal lymphocytes in the healthy liver and in the context of HBV infection. They propose a model in which liver sinusoidal CD56hiCD161-CD8+ T cells with NK-like properties appear to be prone to bystander activation through hyper responsiveness to innate cytokines including IL-12/IL-18 and IL-15 in a TCR-independent manner. Although IL-15 is known to be released by activated intrahepatic macrophages,
whether this bystander activation is mediated directly through innate immune activation or via activated parenchymal cells with the capacity to secrete innate cytokines has yet to be determined. Additionally, the zonated location of these NK-like T cells in the hepatic sinusoid remains to be examined in health and disease. In terms of the clinical translational potential of these data, a key question that remains is how to therapeutically target bystander T cell activation for the desired outcome.
Financial support
This work was supported by a Canadian Institutes of Health Research grant (grant number 162098 to S.A.M and grant number 180525 to A.J.T) and an NSERC Discovery Grant (grant number RGPIN 2018 05958 (to S.A.M)).
Conflict of interest
The authors declare no conflicts of interest that pertain to this work.
Please refer to the accompanying ICMJE disclosure forms for further details.
Supplementary data
The following are the supplementary data to this article:
Phenotypic and functional differences of HBV core-specific versus HBV polymerase-specific CD8+ T cells in chronically HBV-infected patients with low viral load.
The inflammatory cytokine profile associated with liver damage is broader and stronger in patients with chronic hepatitis B compared to patients with acute hepatitis B.
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