The transition of active CD8
+ T-cells into the exhausted phenotype (characterized by high levels of PD1, CTLA4, and TIM3) is a unique feature of viral persistence and disease progression (
Figure 1)
[11]. Moreover, in HBV-HCC, the HBV-mediated activation of the STAT3 pathway induced the expression of the oncogene SALL4, which consequently inhibited the expression of microRNA 200c
[12]. This microRNA 200c inhibition induced the expression of PDL1, which initiated CD8+ T-cell exhaustion
[12]. The high-resolution single-cell sequencing of human HBV-HCC confirmed the presence of two distinct T-cell subtypes within the tumor microenvironment: CD8+ resident memory T cells (T
RM) and Treg cells
[13]. T
RM cells expressed high levels of PD1 and were more suppressive and exhausted, while Treg cells expressed the immunosuppressive LAYN protein
[13]. HBV patients with the immune-tolerant phenotype showed a high susceptibility to develop HCC compared to immune active HBV patients, confirming the hypothesis that the presence of HBV-specific CD8
+T-cells that are functionally unable to remove the virus induced continuous inflammation followed by tumor development
[14]. In line with this, we have also shown evidence of HBV-DNA integration and clonal hepatocyte expansion in these patients considered immune-tolerant, indicating that hepatocarcinogenesis could be underway even in the early stages of HBV
[15]. Moreover, in those subjects deemed as having immune control, evidence of HBV-DNA integration is again present, with the upregulation of genes involved in carcinogenesis
[16]. The link between immunity and HBV DNA integration, however, remains tenuous, but with the use of novel technologies (e.g., tissue CyTOF and spatial transcriptomics) we can come to understand more about this relationship in the future. How functionally incompetent CD8 still leads to continuous inflammation is a complex process. Previous studies by Maini et al. have demonstrated that this may be an indirect pathway, where virus-specific CD8 T cells that were unable to control infection led to the recruitment of non-virus specific T cells to the liver, which could drive liver pathology
[17]. More recently, further studies have demonstrated that exhausted CD8 cells in human chronic infections and animal models are heterogenous and can be identified by phenotypic and transcriptional markers— e.g., PD1 and TCF1—differentiating them between early and terminally exhausted CD8 T cells
[17][18]. The balance of these heterogenous populations may also impact on the associated continuous inflammation. A further animal study demonstrated that the recovery of the immune response may be possible via the specific blockade of the check-point receptor TIGIT and overcoming T cell tolerance to HBsAg, but the consequences are increased liver inflammation and the development of liver tumors
[19]. Although the loss of CD4
+ T-cells in chronic HBV infection was associated with suboptimal HBV-specific CD8
+ T-cell function and viral persistence
[20], the number of circulating and liver-recruited CD4
+ CTLs increased in the early stages of human HBV-HCC
[21]. This T-cell population was reduced and malfunctioned with increasing disease stage, possibly due to the increased infiltration of Treg cells; this loss of CD4
+ CTLs was associated with poor patient outcomes
[21]. This finding supports the notion that the intra-tumoral immune profile is completely different from immunity developed in the corresponding benign liver disease
[22][23]. HBV-HCC is usually preceded by liver fibrosis and cirrhosis, which are characterized by the fibrogenic TGFβ pathway signature
[24]. The activation of the TGFβ pathway suppressed the expression of microRNA 34a, leading to the upregulation of the Treg-recruiting cytokine CCL22
[24]. Increased Treg recruitment in HBV-HCC patients was associated with poor prognosis and portal vein thrombosis
[24]. Treg cells suppressed the function of effector T-cells via the production of immunosuppressive cytokines, including, but not limited to, IL2, 1L10, and TGFβ
[25]. Compared to non-viral HCC, the Tregs detected in HBV-HCC uniquely expressed PD-1, which exhausted functional CD8+ T- cells (as marked by low granzyme A/B and perforin and blunted proliferation) within the tumor microenvironment
[13][26], explaining the reduced sensitivity of the non-viral HCC to PD1 treatment
[27].