Expansion of CD56⁻ NK cells in chronic HCV/HIV-1 co-infection: Reversion by antiviral treatment with pegylated IFNα and ribavirin

Abstract

Co-infection with HCV and HIV-1 is a problem of increasing importance and the role of innate cellular immunity in this co-infection is incompletely understood. Here, we have observed sharply elevated numbers of CD56⁻CD16⁺ perforin^low NK cells in HCV/HIV-1 co-infected subjects on antiretroviral therapy. Interestingly, this expansion of unconventional CD56⁻ NK cells rapidly reverted when HCV was suppressed by IFNα and ribavirin treatment, and was not seen in mono-infected control groups. In vitro experiments suggested that this effect of treatment was due to suppression of HCV viremia rather than a direct effect of IFNα on these cells. In contrast, the conventional CD56⁺ NK cells were largely unchanged in subjects with high HCV loads, although they exhibited slightly decreased perforin expression. With delayed kinetics, the CD56^bright immuno-regulatory NK cell subset temporarily increased to supranormal levels in response to HCV treatment. In contrast to the NK compartment, the CD1d-restricted NKT cells were severely reduced by the co-infection and not restored by treatment. Together, our data suggest that the high HCV loads in HCV/HIV-1 co-infection alter the NK cell compartment in a way not observed in HCV mono-infection.

Introduction

Natural killer (NK) cells are important in cell-mediated innate immunity against viruses [1], [2], [3]. A role for NK cells in the defense against Hepatitis C Virus (HCV) infection is supported by the finding that the HCV E2 protein inhibits NK cell cytotoxicity and cytokine production by crosslinking the tetraspannin CD81 in what appears to be a mode of immune evasion [4], [5], [6]. In addition, HCV infection has been reported to induce changes in NK cell natural cytotoxicity receptor (NCR) expression and cytolytic function, although some of these changes remain controversial [7], [8], [9], [10]. On the cellular level, HCV efficiently interferes with type I interferon production as well as with signaling through interferon receptors [11], [12], [13].

In Human Immunodeficiency Virus (HIV)-1 infection, NK cells probably contribute to control of infection through the lysis of infected cells [14], [15], as well as suppression of viral entry via the production of CC-chemokines [16], [17], [18]. However, functional impairment in NK cell cytokine secretion and cytotoxicity has been observed in several studies [19], [20], [21], [22], [23]. Both acute and chronic untreated HIV-1 infection is associated with alterations in NK cell subset distribution, with depletion of CD56⁺CD16⁺ cells and expansion of CD56⁻CD16⁺ NK cells [20], [21], [22], [23], [24], [25], [26], [27]. These cells have been reported to have an impaired cytolytic function and cytokine production [27].

Natural killer T (NKT) cells are unconventional T lymphocytes, which are activated through their invariant and conserved αβ T cell receptor (TCR) that endows these cells with CD1d restriction rather than classical MHC restriction [28], [29], [30]. CD1d presents glycolipid antigens and NKT cells rapidly respond to antigen stimulation with high-level production of IFNγ and other cytokines [28], [29], [30]. NKT cells are susceptible to HIV-1, and infected subjects display a loss of these cells [31], [32], [33]. The impact of HCV infection on NKT cells remains controversial [34], [35], [36]. Interestingly, data from both experimental models and human trials indicate that there is crosstalk between NKT cells and NK cells, and that NK cell activation is one important pathway by which NKT cells influence the innate immune response [37], [38], [39], [40].

The HIV-1 and HCV epidemics are two of the most serious infectious disease problems the world is facing today, and a large and growing number of people are co-infected with the two viruses [41]. A few studies have addressed adaptive T cell responses to HCV and HIV-1 in co-infected subjects [42], [43], [44], [45]. Little is known, however, about the innate cell-mediated immunity, and how the two viruses may synergistically undermine the host innate immune response. From both a basic immunology perspective and from a clinical perspective it is important to determine how these infections may together induce changes in the immune system, to investigate the effects of treatment on immune mechanisms, and to eventually look for potential innate immune correlates with disease progression and treatment outcome [41], [46]. Here, we have investigated two cellular components of the innate immune system, the NK cells and the CD1d-restricted NKT cells in patients with HCV/HIV-1 co-infection who have effective suppression of their HIV-1 replication by antiretroviral treatment (ART). The results are important for the understanding of the innate immune system's role in HCV/HIV-1 co-infection, and suggest that a high level of HCV replication in co-infected subjects may drive changes that are not observed in HCV mono-infected patients. Moreover, our findings suggest a role for pegylated interferon α (peg-IFNα) and ribavirin treatment in restoring some of the changes to the innate cellular immune system in co-infected patients.