Natural killer (NK) cells are a type of lymphocyte that constitute a major component of the innate immune system Citation[1]. NK cells play an important role in the rejection of tumors and cells infected by viruses through direct or indirect cytolytic mechanisms. The target cells die by apoptosis or necrosis due to the release of two proteins: perforin and granzyme. In this process, proinflammatory cytokines, such as IFN-γ, and cytoplasmic granules are also released Citation[2].
In humans, NK cells express killer cell immunoglobulin-like receptors (KIRs) on their surface, which specifically recognize the HLA Class I antigens of the major histocompatibility complex present on target cells. The effector functions of NK cells are influenced by inhibitory receptors for self-HLA Class I ligands HLA-A, -B and -C, with HLA-C and Bw4 being the predominant ligands for most KIR receptors Citation[3]. The dimorphism in amino acids present in the a1 helix of the HLA-C molecules is characterized by Ser77/Asn80 and Asn77/Lys80 that define distinct allotypes of HLA-C (C1 and C2, respectively). The alleles encoding the C1 epitope include HLA-C*01, *03, *07 and *08, which interact with the KIR receptors 2DL2/3 and 2DS2. The alleles encoding the C2 epitope include HLA-C*02, *04, *05 and *06, which interact with the 2DL1 and 2DS1 receptors Citation[4,5]. The KIRs 3DL1 and 3DS1 interact with HLA-Bw4, and KIR 3DL2 recognizes HLA-A3 and -A11 Citation[6].
Several studies have pointed out that hepatitis C virus (HCV) persistence might be associated with a defective NK cell response Citation[7–10] and that therapy with IFN would re-establish NK cell activity Citation[11]. In the last few years, several studies highlighting the possible role of KIR receptors and ligands in persistence of HCV infection have been published in the scientific literature Citation[12–14].
In a study that appeared in Science in 2004, Khakoo et al. demonstrated that KIR2DL3 and homozygosity for its ligand C1 are associated with resolving HCV infection in patients with a low viral load Citation[12]. In another paper, the same group postulated that different genotypes confer either susceptibility or resistance to infectious diseases Citation[13].
Lopez Vazquez et al. identified a protective effect of HLA-Bw4I80 receptor 3DS1 against the development of hepatocellular carcinoma Citation[14]. Paladino et al. have shown that NK receptors are associated with the elimination of HCV. In particular, they found that the frequency of the KIR2DL3 gene was significantly higher in HCV RNA-positive patients with serum transaminase levels above the normal limit Citation[15]. In addition, they found that lower KIR2DL2 and 2DS2 frequencies were associated with increased frequencies of KIR2DS5 in subjects who cleared the virus.
In contrast to the studies described previously, Rauch et al. did not find any implication of KIR genes in resolving HCV infection. However, these results might be biased owing to a low sample size and/or the lack of information on patient viral load Citation[16].
Worldwide, the prevalence of HCV infection is 2% Citation[17] and nearly 30% of HCV-positive subjects develop complications, such as cirrhosis and hepatocellular carcinoma. For these individuals, liver transplantation is the best available treatment Citation[18]. Commonly, the criteria for liver allocation do not include donor/recipient HLA compatibility, the immunological factors being less relevant than the clinical ones. Nevertheless, recent reports suggest that alloreactivity of NK cells does exist Citation[19–21].
Does alloreactivity lead to liver rejection?
The role of NK cells in liver rejection is controversial. In animal models, it has been demonstrated that innate immunity (NK/IFN-γ) is activated after liver transplantation with consequent liver damage and inhibition of hepatocyte proliferation Citation[22].
In humans, a study has indicated that compatibility between the HLA-C ligand and KIR receptors would significantly reduce the number of rejection episodes Citation[20], while another study has evidenced that the expression of the C2 epitope is associated with host NK cell inhibition Citation[21]. Such inhibition would lead to an impaired graft outcome.
In contrast to these data, Ortel et al. concluded that, for liver transplanted patients under standard immunosuppressive therapy, NK alloreactivity did not have an impact on liver acute rejection Citation[19]. Tran Citation[23] and Hanvesakul Citation[21], in two independent studies, analyzed the possible correlation between the donor HLA-C2 epitope and liver survival and have arrived at divergent conclusions. Even though there were some notable differences between the two cohorts, it can be hypothesized that the alloreactivity of NK cells in liver transplantation is not relevant for all liver recipients and that the relative role of NK cells could be obscured by original disease or by the immunosuppressive treatment that the recipients underwent.
The analysis of the NK cell role in different subsets of patients would be an important area of study. In our paper, no correlation was found between the occurrence of rejection episodes and HCV progression Citation[24].
Do NK cells & their ligands affect the risk of severe HCV recurrence?
Following liver transplantation, HCV reinfection is universal Citation[25] and hepatitis occurs in most patients, with a 30% cumulative probability of progression to cirrhosis at 5 years following transplant Citation[18,26,27]. Its progression to liver fibrosis seems to be modulated by NK alloreactivity.
Patients with low pretransplant concentrations of NK CD56+ cells develop a more severe HCV recurrence than that of subjects with a high concentration Citation[28]. Moreover, a high viral load is correlated with impaired graft survival and function Citation[27]. Rosen has postulated that the pretransplant innate immune response is linked with post-transplant HCV recurrence Citation[28], while Golden-Mason has proposed a model in which NK cells might be the primary target for HCV immune-evasion strategies Citation[29].
We have shown how the presence of KIR2DL3 in liver transplant recipients correlated with the progression to liver fibrosis and that mismatching of HLA–KIR ligands favored the progression to fibrosis only in the presence of KIR2DL3 Citation[24]. We have also postulated that, in immunocompetent individuals, NK cells with the receptor KIR2DL3 would have a protective effect only during the first stage of the infection Citation[12] but not later when the viral load increases and the innate immune response is possibly overwhelmed Citation[30,31].
The recipient incapacity to eliminate HCV, would lead to liver damage caused by the immune response. In fact, HCV per se is not cytopathic and viral load does not correlate with the degree of liver fibrosis. However HCV induces the production of cytokines (e.g., IFN-γ, IL-12, IL-18 and IL-15) from hepatocytes that activate NK cells, which may result in liver damage.
The weak inhibitory receptor–ligand interaction, such as KIR2DL3–HLA-C1, allows a more rapid NK-cell activation than that of a strong inhibitory receptor–ligand interaction, such as KIR2DL2–HLA-C1 or KIR2DL1–HLA-C2 Citation[32]. The NK cells develop and activate CD4+ and CD8+ T cells. If persistent, this activation damages the liver due to the accumulation of these cells in liver parenchyma Citation[33].
The absence of the KIR2DL3 receptor and matching for the HLA-KIR epitopes in HCV-positive liver-transplant recipients might avoid a continuous activation of NK cells. Conversely, the NK cell receptor KIR2DL3 might modify cell activity in the presence of the mismatch, facilitating the progression to fibrosis.
Does the presence of a particular KIR haplotype favor the development of liver fibrosis?
So far, no correlation has been found between the different KIR haplotypes and liver fibrosis Citation[24]. However, because of the rapid evolution of KIR genes and haplotype diversity, the role of this parameter is difficult to assess.
Are there other factors that correlate with liver graft outcomes for HCV-positive patients?
In addition to NK-cell alloreactivity, other factors correlate with liver graft outcomes. Genotype 1b Citation[34] and cytomegalovirus-positive patients Citation[35] are at a greater risk of developing HCV recurrence. There is also some evidence that HLA-DRB1 donor/recipient matching plays a significant role; especially in the presence of HLA-DRB1*11 in the recipient which correlated with a decreased risk of HCV recurrence Citation[36]. Several clinical factors, including those related to the virus, the host and the donor, are probably implicated in post-transplantation HCV-related disease progression. For instance, a donor age of over 50 years is unanimously recognized as a detrimental factor in the progression to liver fibrosis Citation[37,38].
Is it possible to improve liver graft outcomes in HCV-positive recipients?
There is a compelling need for improving liver graft outcome in HCV-positive recipients. To pursue this goal, we have to take into consideration all factors already known and associated with donor and recipient clinico–pathological features. In addition, we have to consider the increasing evidence that NK cell alloreactivity may play an important role. Therefore, we should further investigate this factor in the current matching and selection of HCV-positive recipients of liver transplantation. If larger studies confirm that genetic components contribute to both NK cell-mediated control of HCV and to liver injury in the transplant setting, this will help to develop more appropriate allocation policies and to ensure the optimal use of the limited organs available.
Acknowledgements
The authors thank Ferruccio Bonino and Mario Scalamogna for their critical review of the manuscript.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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