While natural killer (NK) cells earn their namesake through their primary function of cell cytotoxicity, they are also potent immune recruiters via secretion of interferon (IFN)-γ and tumor necrosis factor-α. In contrast to T or B cells, where target specificity is dependent upon genetic rearrangement, action of NK cells is determined by the integration of activating or inhibiting signals from a diverse repertoire of somatically encoded receptors. Astoundingly, >30 000 individual NK-cell phenotypes have been identified within a single human blood sample.1 Conserved across this multitude of receptor combinations, the ability to distinguish self from missing or injured self is central to the role of NK cells in solid organ transplantation outcomes. On the one hand, NK cells can promote allograft tolerance through elimination of donor antigen-presenting cells and alloreactive recipient effector T cells, and on the other hand, they can also promote acute and chronic allograft rejection through antibody-dependent cell-mediated cytotoxicity and recognition of damage molecules such as those induced during ischemia-reperfusion injury.2
Nowhere is this balancing act more evident than with the convergence of chronic viral infection and solid organ transplantation.3 Sequelae of hepatitis C virus (HCV) infection are leading global indications for liver transplantation.4 The degree of hepatic injury is dependent on the host immune response to HCV infection, and infection control depends on effective T-cell and NK-cell function. Consequently, chronic HCV antigen stimulation results in T-cell anergy and skewing of the NK-cell receptor repertoire. Further, persistent exposure to endogenous IFN-α induces long-standing alterations in NK-cell function marked by increased cytotoxicity and reduced capacity for cytokine production (Figure 1A).5 While studies showing skewed NK-cell phenotype in chronic HCV infection are well documented, it was not clear if the same phenomenon would happen after liver transplant in the context of removal of the infected organ and immunosuppression.
In this issue of Transplantation, Adenugba et al6 report that HCV-infected liver transplant recipients had similarly skewed NK-cell function with increased CD107a, a measure of cytotoxicity, and reduced IFN-γ capacity compared with HCV-uninfected liver transplant recipients. The authors demonstrated that in vitro IFN-α stimulation reproduced altered signal transducer and activator of transcription (STAT) signaling in NK cells from HCV-infected liver transplant recipients (Figure 1B). Specifically, in NK cells from HCV-infected liver transplant recipients, STAT1 and phosphorylated STAT1 were increased and phosphorylated STAT4 was decreased relative to other liver transplant recipients. Interestingly, this pattern of STAT4 signaling has been reported in circulating NK cells from all liver transplant recipients irrespective of HCV status and immunosuppression, suggesting that some degree of blunted IFN-γ capacity may be a marker of tolerance.7 While it is clear that NK cells are important mediators of operational tolerance after liver transplantation, the mechanism of this interaction has not been fully elucidated. Therefore, understanding how to contextualize these findings in terms of clinical outcomes remains challenging. Notably, while HCV infection in liver transplantation was shown to alter peripheral blood gene transcription signatures, it did not change the transcription of 13 NK-cell genes associated with tolerance, suggesting that the impact of HCV infection on NK-cell function may be independent of mechanisms of tolerance.8
Before the introduction of direct-acting antivirals (DAAs), liver transplant recipients with chronic HCV infection suffered from increased early graft damage relative to other recipients.9 Now, not only can chronic HCV-infected liver transplant recipients expect better outcomes with DAA therapy, but this therapy has also expanded the pool of HCV-infected donor livers to uninfected recipients. There are several effective DAA options, but current practice guidelines recommend ribavirin in some post–liver transplant regimens. The authors hypothesized that an interferon-free sofosbuvir-based DAA regimen with ribavirin would alter the function of NK cells from chronic HCV-infected recipients compared with interferon-free DAA alone. Regardless of the treatment type administered to chronic HCV-infected liver transplant recipients, there were reductions in phosphorylated STAT1 but not in cytotoxicity by CD107a after 12 wk of sustained viral response. This reproduces what others have demonstrated that despite elimination of constitutive IFN-α signaling from HCV clearance, some chronic NK-cell functional changes persist. However, in the chronic HCV-infected liver transplant recipients who received treatment with ribavirin, Adenugba et al6 show that there was an increase in frequency of IFN-γ capacity in NK cells, most notably in an activated population, which correlated to an increase in phosphorylated STAT4. These findings were notably absent in recipients who did not receive ribavirin as part of the HCV treatment protocol, suggesting an immunomodulatory effect of ribavirin independent of HCV clearance.
Should the finding of normalized NK cell IFN-γ capacity change management of HCV-infected liver transplant patients? Further study of the duration of these changes and their long-term impact on allograft tolerance is needed. These results are somewhat at odds with existing literature showing that NK-cell receptor diversity and some markers of NK-cell function may change slightly during DAA treatment but that the functional changes may not persist past 36 wk of sustained viral response.10 Further, NK-cell changes were not as profound in a group of subjects receiving a sofosbuvir and ribavirin regimen versus other DAA protocols. This suggests that host factors, such as immunosuppression or recipient killer immunoglobulin-like receptor and donor HLA match, may also contribute to NK-cell functional differences following HCV treatment.
As HCV-infected solid organs are increasingly being considered for HCV-uninfected recipients outside of the liver transplant setting, additional understanding of the mechanism of ribavirin immunomodulation is also needed before broader adoption. It may be that ribavirin exerts direct effects on NK cells or that it has secondary impact through NK cell–specific cytokines, such as interleukin-12 or interleukin-15, secreted from stromal cells. Therefore, tissue-specific ribavirin effects may have variable impacts on NK-cell function in other types of donor organs. Thus, while this study reveals that ribavirin modulates NK-cell function after liver transplant, further study is needed as to whether these impacts are transient and could alter long-term clinical outcomes.
D.R.C. thanks John R. Greenland for his input.
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