Effector T cell dysfunction plays a vital role in the majority of chronic HBV infection and the severity of CHB progression [14, 15]. It has been reported that recovery of T cell function might reverse antiviral effect in host immunity and hinder the progression of chronic inflammation and fibrosis [16, 17]. However, little is known about the status of memory T cells and the related functional recovery after pegylated IFN- α treatment. During interferon treatment in chronic HBV infection, CD8+ and CD4+ T cell differentiation following virus clearance results in the formation of high quality, long-lived memory cells . Two key features of memory immunocytes are long-term persistence in the absence of antigen and rapid response upon re-exposure to the pathogen, which allow them to transfer to effector cells and confer protective immunity. This recall response is characterized by rapid elaboration of effector functions such as cytotoxicity, cytokine production, effector T cell proliferation accompanied by substantial increase in the number of activated T cells, and effector T cell migration to infection sites [19, 20].
In this study, we found that pegylated IFN- α treatment enhanced CD8 memory T cells recovery by down-regulating expression of inhibitory receptors on memory T cells in CHB patients. Our previous study  indicated that PD-1 and its ligand—programmed death ligand-1(PD-L1), expressed mainly on antigen present cells (APC)—were down-regulated in intrahepatic and peripheral lymphocytes after pegylated IFN- α treatment. As HBV antigen or viral load increases, these virus specific CD8+ T cells may express high levels of PD-1 in response to polyfunctional cell loss and effector cell dysfunction in a hierarchical manner. The pattern of inhibitory receptor co-expression and the frequency of receptors simultaneously expressed by the same CD8 memory T cell can substantially affect the severity of antiviral effector cell dysfunction [22, 23]. CD244 is a novel biomarker, which has been reported to be mainly expressed on nature killer cells, involved in partial exhaustion of activated T cells . Interestingly, PD-1 and CD244 co-expression facilitates the increased susceptibility to apoptosis of memory T cells, and reversing the expression of these receptors implies the recovery of antiviral immunity. In line with our previous study regarding CHB patients treated with interferon-α-2b, which attained down-regulation of inhibitory receptors accompanied with viremia control , we found that PD-1 and CD244 were down-regulated preferentially in CD8 memory T cells during pegylated IFN- α treatment. Dynamic down-regulation of inhibitory receptor expression on CD8 memory T cells indicates functional T cell recovery. However, these results were not observed on CD4 memory T cells, which may be due to limited samples and need further investigation.
Chemokines and their receptors are essential in memory T cell migration and homing and directing mature activated T cells to lymphoid tissues and other immune microenvironments that are suitable for their differentiation and function. The chemokine receptor CXCR4 belongs to the superfamily of G-protein-coupled receptors [26, 27]. A recent report has highlighted the role of CXCR4 as a prognostic marker in various types of cancer, including leukemia and breast cancer . CXCR4 and CCR5 are also co-receptors for HIV entry into human cells , but their roles in viral hepatitis have not yet been addressed. Formation of highly functional memory T cells accompany viral control during antiviral treatment. Once they encounter the viral antigens again, the populations of memory CD4 + and CD8+ T cells expand in lymphoid tissue and then immigrate to the periphery where they bind with their ligands and are then activated to take effect. The chemokine receptors that are highly expressed on memory T cells not only prepare them for life in the periphery but are also correlated with the outcome of antiviral treatment. CCR7+ T cmcells are capable of proliferating and transferring to CCR7- Tem cells after activation, while cytokine production are enriched in Tem cells . Interestingly, we found that CXCR4 was highly expressed on a subset of CD8 memory T cells in the responders, suggesting that memory T activation is accompanied with dramatic alterations in chemokine responsiveness. In our investigation, we found that CXCR4 was more highly expressed on CD8+ Tcm cells of the responders than on those of the non-responders, indicating that migration of Tcm (CD45RO + CCR7+) reverted after pegylated IFN-α treatment. Therefore, we postulate that higher co-expression of CCR7 and CXCR4 on memory T cells represents the potent mobility that can affect targets in antiviral treatment. These memory T cells with high chemokine receptor expression may serve to ensure a robust cycle of antigen-specific memory T-cell activation and proliferation. Unfortunately, CXCR4 and CCR7 tend to shed in culture in response to antigen stimulation, and it is hard to measure the effector cytokines expressed on memory T subset in this scenario.
CD127 is the α chain of the interleukin-7 (IL-7) receptor, and it’s mainly expressed on T cells. IL-7 is a survival cytokine of memory T cells and forms a feedback loop with its receptor expression in the immune milieu. Chronic viral infection induces a significant decrease in CD127 expression on CD8+ T cells [31, 32]. Studies elucidated that high CD127 expression indicates the enhanced function of memory T cells in viral clearance. Reported data strongly support that the rejuvenated cellular responses correlate with the homeostasis and proliferation of functional T cells, but the underlying mechanisms remain unclear [33–35]. In this study, we found that CD127 expression on CD8 memory T cells was significantly higher in the responders than in the non-responders, which indicated that proliferation took place predominately in the responders after pegylated IFN-α treatment and reinvigorated the T cells activation. However, the detailed mechanisms and immunological changes need further investigation. Meanwhile, CD127 expression was tightly correlated with HBV DNA load, suggesting that up-regulated CD127 expression in pegylated IFN-α treatment may be a novel biomarker to predict the outcome of antiviral treatment.
T cell dysfunction becomes progressively worse as viral load increases or inhibitory signals are upregulated. Bertoletti et al. indicated that interferon release lagged behind innate immune response in certain HBV infections. CD8 + memory T cells in the adaptive immune system take action upon encountering replicating HBV virus [36, 37]. Tcm proliferation and differentiation into effector T cells contribute to pathogen elimination by perforin-dependent cytolysis and secretion of other functional cytokines . In this study, the expression of effector molecules such as perforin and granzyme B on memory T cells were up-regulated, which enhanced the host antiviral activity by using exogenous interferon. But the perforin and granzyme B expressions on memory CD8+ T cells in responders were not higher than those in the non-responders at week 24. We speculate that the effector molecules synthetized in CD8 memory T cells mainly induce apoptosis and necrosis of virally infected cells, whereas memory T cells in the responders acquired viral control via the feedback of lower granzyme B and perforin expressions. IFN-γ expression on both CD4 and CD8 memory T cells was up-regulated after pegylated IFN-α treatment, indicating the potent generation of type 1 T helper cells and cytotoxic activity in the responders. These classical cell types are in charge of antiviral immune responses. IFN-γ expression in memory T cells was up-regulated dramatically after in vitro HBV core antigen re-challenging, reflecting the sensitive and potent capability of memory in the responders, which may predict long-term viral control after the treatment.
Taken together, we found that memory T cells recovered after pegylated IFN- α treatment via down-regulation of inhibitory receptors, up-regulation of chemokine and survival cytokine receptors, and enhanced production of effector molecules. Therefore, pegylated IFN- α regulates memory T cell functions during persistent chronic HBV infection. A better understanding of the characteristics and mechanisms responsible for memory T cell dysfunction and recovery during antiviral therapy helps one to develop sensitive immunological markers for predicting the outcome of antiviral treatment and vaccine approaches that reduce the disease burden of intractable chronic infections. These results were obtained from a small scale follow-up of CHB patients treated with pegylated IFN-α. A further study is needed with increased sample size and a longer period of follow-up.