Journal of Hepatology
Volume 48, Issue 1 , Pages 8-11, January 2008

Hepatitis C core protein – The “core” of immune deception?☆

University of Massachusetts Medical School, Department of Medicine, 364 Plantation Street, LRB 215, Worcester, MA 01605-2324, USA

published online 07 November 2007.

Associate Editor: M.U. Mondelli

Article Outline

 

The unique feature of the hepatitis C virus (HCV) is that it causes chronic infection in the majority of patients. This has been largely attributed to the inability of the host immune system to clear the initial HCV infection [1]. Robust HCV-specific CD4+ and CD8+ T cell activation is associated with clearance of an acute infection while in chronic infection HCV-specific T cell clones are present to many viral determinants but with low frequency and are functionally ineffective. Additional abnormalities in immune responses in chronic HCV infection include inadequate activation of the innate immune system including excessive pro-inflammatory cascade in monocytes and altered dendritic cell (DC) functions (Reviewed in [2]). Innate immune pathways through engagement of pattern recognition receptors such as Toll-like Receptors (TLR) and helicases, expressed in cells of the classical immune system as well as in hepatocytes, are also affected by HCV infection, where Type I IFN production is inhibited by interaction of HCV with these intracellular virus recognition receptors [2] (Fig. 1).

The HCV genome encodes ∼3010 amino acid single polyprotein which is processed by cellular and viral proteases into non-structural (core, envelope (E) proteins 1 and 2) and non-structural (NS) proteins (NS2, NS3, NS4 and NS5) [3]. The HCV core is an RNA-binding dimeric alpha-helical protein with membrane preferences that participates in the formation of the viral nucleocapsids (Reviewed in [4]). In heterologous expression systems such as HCV replicons, core is found in the endoplasmic reticulum (ER), at the surface of lipid droplets and in the nucleus [4]. Interaction of HCV core protein with a wide variety of cellular proteins has been reported to influence host cell functions; however, the effects of intracellular versus membrane localized HCV core protein are unknown. It has been proposed that HCV core protein affects cell signaling, apoptosis, carcinogenesis, and lipid metabolism [4], [5], [6], [7], [8], [9].

In this issue of the journal, Zimmermann et al. report that HCV core protein impairs the in vitro priming of T cell responses by dendritic cells and hepatocytes [10]. In addition to professional antigen presenting cells, hepatocytes have been suggested to present antigens, thus providing an additional layer of complexity to pathogen-specific immunity [11]. T cells make direct contact with hepatocytes through cytoplasmic extensions penetrating the endothelial fenestrations that perforate the LSECs [12]. The expression of molecules required for T cell activation, such as MHC class I and ICAM-1, is polarized on hepatocytes to the perisinusoidal cell membrane, thus maximizing the opportunity for interactions with circulating lymphocytes [9]. Further, hepatocytes induce apoptosis of activated T cells in ICAM-1-, lectin receptors- and alternative co-stimulatory molecules (PD-1/PD-L1)-dependent manner [13]. It is currently believed that such close interactions between hepatocytes and immune cells contribute to “tolerogenic” environment in the liver [1]. To date, it is unclear if the accessory capacity of hepatocytes is comparable to that of professional antigen presenting cells, such as dendritic cells. Further, it is unclear if HCV infection breaks or enhances the tolerogenic environment in the liver. Zimmermann et al. found defective in vitro priming of CD8+ T cells by mouse hepatoma cells (Hepa1–6 cells) that expressed HCV core protein. After contact with HCV core-expressing Hepa1–6 cells, CD8+ T cells showed reduced intracellular IFNα and TNFα but increased IL-10 levels. Similar defects in CD8+ T cell priming were found by isolated hepatocytes from HCV core protein-expressing transgenic mice and by primary mouse hepatocytes after treatment with recombinant HCV core protein. The defects in CD8+ T cell priming correlated with reduced expression of MHC class I in the HCV core-expressing hepatocytes. Decreased antigen-specific CD8+ T cell responses have been described in human HCV disease; however, it remains unclear what is the role of HCV-infected hepatocytes in induction of CD8+ T cell priming.

Alterations in human dendritic cell functions have been found in chronic HCV infection and previous studies demonstrated that HCV core protein alone could reproduce many abnormalities of the HCV-infected patients’ DCs [14]. Most reports with human cells showed reduced accessory capacity of HCV core protein-conditioned myeloid DCs in mixed lymphocyte reaction (Reviewed in [2]). Decreased T cell stimulatory capacity, overproduction of the immunoregulatory cytokine IL-10 and deficiency in co-stimulatory molecules were detected in myeloid dendritic cells (mDC) of patients with HCV infection by some investigators [14], [15], [16], [17], while others failed to identify any mDC abnormalities [18]. The current study in mouse myeloid DCs demonstrates significant defects in DC functions after HCV core treatment including decreased antigen presentation capacity and decreased activation of CD4+ T cells. This was associated with reduced surface expression of MHC class I and class II, and the co-stimulatory molecules CD80, CD86 and PD-L1. Expression of the co-stimulatory molecules paralleled the decreased secretion of IL-12 and IL-6, suggesting that inadequate combination of events at antigen presentation may be impaired during HCV infection. Similar defects were found in human DCs when generated in the presence of HCV core protein [14], [15], [16], [17]. Further, decreased IL-12 and increased IL-10 production was found after HCV core protein treatment in human DCs [14].

In addition to the myeloid DC dysfunctions, plasmacytoid dendritic cell capacity to produce IFNα and to activate T cells is also impaired in chronic HCV infection [19]. A recent study demonstrated that HCV core protein can inhibit IFNα production by human PDCs and this effect is mediated through HCV core-induced IL-10 and TNFα production by monocytes [19]. HCV core protein not only inhibited IFNα production but also resulted in increased PDC apoptosis through induction of monocyte-derived IL-10 and TNFα [19]. Previous studies suggested that different subsets of dendritic cells accumulate in the liver during HCV infection [20], [21]. Further, hepatocyte-conditioned immune cells produce both pro- and anti-inflammatory cytokines [22]. These cytokines, in turn, shape the dendritic cells in an autocrine and paracrine fashion [2], [14], [15]. The study by Zimmermann et al. suggests that HCV core protein affects both hepatocytes and dendritic cells. In this context, it remains to be determined if the HCV core-induced defects of hepatocytes and dendritic cells simply co-exist and co-participate in impaired HCV-specific immunity. Alternatively, it is plausible that hepatocytes may play a central role in directing the functional capacity of dendritic cells and thus driving the outcome of the HCV-specific immune response.

Dendritic cells are pivotal in induction of pathogen-specific T cell priming [1], [2], [15], [16], [17], [18]. Optimal T cell activation requires expression of MHC and co-stimulatory molecules, and specific cytokine environment. Therefore, it is not unexpected that Zimmerman et al. identified reduced CD4+ T cell activation by DC that showed reduced MHC class II and co-stimulatory molecule expression after HCV core treatment. Decreased T cell proliferation was associated with reduced IFNγ production while increased number of cells showed intracellular staining for IL-10. Such cytokine milieu is unfavorable for T cell activation, since IL-10 is a negative regulator of both T lymphocytes and antigen presenting cells [23]. IL-10 expression in T cells is often associated with a phenotype that is referred to as regulatory T cells [23], [24]. Regulatory T cells (Tregs) play a central role in the maintenance of immunological tolerance in the periphery and are key in viral clearance [25], [26]. An increased frequency of Tregs during HCV infection was previously reported [27]. Tregs limit proliferation of HCV-specific T lymphocytes and are protective against HCV-associated immune autoimmune activation [28]. Tregs of HCV-infected patients are CD4+CD25+FoxP3+ and display CD45ROhigh, CD45RAlow, CD27high, CD28high, CD62Lhigh, CD95high, and CD69low; they require contact with effector T cells to execute their inhibitory capacities and produce immunomodulatory cytokines IL-10 and TGFβ [29], [30], [31]. The mechanisms of Treg expansion during HCV infection are currently unknown. The study by Zimmermann et al. suggests that HCV core protein may exert its inhibitory actions on dendritic cells to induce a phenotype that will promote induction of IL-10-producing T cells. The authors speculated that T cells activated by HCV core protein-expressing dendritic cells and hepatocytes may present regulatory functions, however, further characterization of the IL-10-producing T cells with regard to markers of Tregs, functional characteristics and antigen specificity was beyond the scope of the current study. It is possible that such amplification of the “tolerogenic loop” in the liver is created as an attempt to control the viral infection. Indeed, several groups of investigators reported the existence of heterogeneous population of virus-specific IL-10-producing T cells, which prevented liver damage during chronic HCV infection [29], [30], [31], [32], [33], [34]. Although IL-10 production by T cells is a hallmark of regulatory T cells, further investigation of the hypothesis that Treg induction by DCs is a mechanism for HCV-induced immune defects is yet to be completed.

Interestingly, the negative effect of HCV core protein on antigen presenting cell function was present whether the HCV core protein was expressed intracellularly with an expression vector, expressed as a transgene in hepatocytes of the HCV core transgenic mice, or administered exogenously to hepatocytes in the form of recombinant protein, suggesting that an effect of HCV core on both intracellular and extracellular pathways may be involved. The mechanisms by which the HCV core protein interacts with the cellular machinery of hepatocytes and/or dendritic cells to cause the defects in accessory cell functions reported in this study remain elusive. Extracellularly administered recombinant HCV protein is quickly taken up by most cells and enters the intracellular compartment [35]. Unlike in human DCs, monocytes/macrophages or T cells, where activation of TLR2 and complement receptor C1qR was indicated in HCV core-induced immune defects [8], [9], [36], [37], Zimmermann et al. found no evidence for TLR2 or C1qR involvement for the role of TLR2 or C1qR in murine cells by using antibody blocking approach. Species-specific differences in HCV core protein recognition have been reported, where murine macrophages utilized only TLR6, while human macrophages used either TLR1 or TLR6 as TLR2 co-receptor in HCV core protein-induced activation [37]. Thus, mechanistic investigations of HCV-related defects of antigen presenting cells in a mouse model and their validation in human system are needed.

Taken together, the study by Zimmermann et al. provides a new insight into HCV core protein-induced cellular mechanisms for undermining adaptive immune responses. Dissection of the HCV-induced defects of dendritic cells and hepatocytes and their co-operations are needed. It remains to be determined if the effects of HCV core-induced suppression of antigen presenting cells are limited to HCV-specific effector cells or extend beyond the antigen specificity. Such investigations are imperative since the inhibition of HCV-specific immune response is not associated with generalized immunosupression in the majority of HCV-infected patients [1]. The role of HCV-infected hepatocytes in inhibition of CD8+ T cell activation in the context of local liver microenvironment is emergent. Finally, the concept that myeloid dendritic cells may result in induction and/or expansion of regulatory T cells in HCV infection deserves further exploration.

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☆ A.D. declares that she has nothing to disclose regarding funding or conflict of interest with respect to this manuscript. G.S. declares that she has nothing to disclose regarding conflict of interest with respect to this manuscript. NIH funded study.

PII: S0168-8278(07)00582-X

doi:10.1016/j.jhep.2007.10.005

Journal of Hepatology
Volume 48, Issue 1 , Pages 8-11, January 2008

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