NK cells controlling virus-specific T cells: Rheostats for acute vs. persistent infections

In vivo administration of a JAK3 inhibitor during acute SIV infection leads to significant increases in viral load during chronic infection

The studies reported herein are the first to document the effect of the in vivo administration of a JAK3 inhibitor for defining the potential role of NK cells during acute SIV infection of a group of 15 rhesus macaques (RM). An additional group of 16 MHC/KIR typed RM was included as controls. The previously optimized in vivo dose regimen (20 mg/kg daily for 35 days) led to a marked depletion of each of the major NK cell subsets both in the blood and gastro-intestinal tissues (GIT) during acute infection. While such depletion had no detectable effects on plasma viral loads during acute infection, there was a significant sustained increase in plasma viral loads during chronic infection. While the potential mechanisms that lead to such increased plasma viral loads during chronic infection remain unclear, several correlates were documented. Thus, during acute infection, the administration of the JAK3 inhibitor besides depleting all NK cell subsets also decreased some CD8⁺ T cells and inhibited the mobilization of the plasmacytoid dendritic cells in the blood and their localization to the GIT. Of interest is the finding that the administration of the JAK3 inhibitor during acute infection also resulted in the sustained maintenance during chronic infection of a high number of naïve and central memory CD4⁺ T cells, increases in B cells in the blood, but decreases in the frequencies and function of NKG2a⁺ NK cells within the GIT and blood, respectively. These data identify a unique role for JAK3 inhibitor sensitive cells, that includes NK cells during acute infection that in concert lead to high viral loads in SIV infected RM during chronic infection without affecting detectable changes in antiviral humoral/cellular responses. Identifying the precise mechanisms by which JAK3 sensitive cells exert their influence is critical with important implications for vaccine design against lentiviruses.

In efforts to define the potential role of innate immune effector mechanisms in influencing the course of SIV infection during the acute infection period, our lab utilized the in vivo daily administration of 20 mg/kg orally of a compound called Tofacitinib (a Janus kinase 3 inhibitor) to a group of 15 rhesus macaques starting at day −6 and until day 28 post intravenous SIVmac239 infection. An additional group of 16 similarly SIV infected rhesus macaques served as a placebo control. This drug targets the JAK/STAT pathway that is utilized by cells including the NK cell lineage, a major cell of the innate immune system. The dosage utilized was based on extensive previous PK studies that resulted in a marked depletion of the NK cells. Of interest while such drug administration had no effect on plasma viral loads during acute infection, such drug administration led to significant increases in plasma and gastro-intestinal tissues (GIT) viral loads during chronic infection. A series of phenotypic/functional studies were performed to determine the mechanisms for this delayed effect and the correlates identified. These data are the first to document the effect of JAK-3 inhibitor during acute SIV infection with implications for HIV vaccine design.

Distinct immune response in two MERS-CoV-infected patients: can we go from bench to bedside?

One year after the occurrence of the first case of infection by the Middle East Respiratory Syndrome coronavirus (MERS-CoV) there is no clear consensus on the best treatment to propose. The World Health Organization, as well as several other national agencies, are still working on different clinical approaches to implement the most relevant treatment in MERS-CoV infection. We compared innate and adaptive immune responses of two patients infected with MERS-CoV to understand the underlying mechanisms involved in the response and propose potential therapeutic approaches. Broncho-alveolar lavage (BAL) of the first week and sera of the first month from the two patients were used in this study. Quantitative polymerase chain reaction (qRTPCR) was performed after extraction of RNA from BAL cells of MERS-CoV infected patients and control patients. BAL supernatants and sera were used to assess cytokines and chemokines secretion by enzyme-linked immunosorbent assay. The first patient died rapidly after 3 weeks in the intensive care unit, the second patient still recovers from infection. The patient with a poor outcome (patient 1), compared to patient 2, did not promote type-1 Interferon (IFN), and particularly IFNα, in response to double stranded RNA (dsRNA) from MERS-CoV. The absence of IFNα, known to promote antigen presentation in response to viruses, impairs the development of a robust antiviral adaptive Th-1 immune response. This response is mediated by IL-12 and IFNγ that decreases viral clearance; levels of both of these mediators were decreased in patient 1. Finally, we confirm previous in vitro findings that MERS-CoV can drive IL-17 production in humans. Host recognition of viral dsRNA determines outcome in the early stage of MERS-CoV infection. We highlight the critical role of IFNα in this initial stage to orchestrate a robust immune response and bring substantial arguments for the indication of early IFNα treatment during MERS-CoV infection.

The yin-yang of KIR3DL1/S1: molecular mechanisms and cellular function

Killer Immunoglobulin-like Receptors (KIR) are a family of receptors expressed on natural killer (NK) and T-cell subsets. KIR3DL1 is a highly polymorphic receptor that binds to groups of HLAA and HLA-B allotypes that express the Bw4 epitope. The variation in KIR3DL1 allotypes manifests at a number of levels. Most dramatically, a common allelic variant encodes an activating rather than an inhibitory receptor (KIR3DS1). In addition, sequence variants can affect both the frequency of expression within the NK cell population and the intensity of expression on a given cell. KIR3DL1 polymorphism also influences the interaction with HLA-Bw4 molecules, due to contacts with the HLA molecule itself and sensitivity to the presented peptide. A body of evidence from genetic association studies supports the biological significance not only of the interaction of KIR3DL1 with HLA-Bw4 but also the functional variation seen with different KIR3DL1 and HLA allotypes. In this review, we discuss our current understanding of KIR3DL1 function and our recent insights from the structure of the KIR3DL1 in complex with HLA. In addition, we will summarize our current understanding of KIR3DS1, including its ligand specificity and its role in immune responses.

Therapeutic depletion of natural killer cells controls persistent infection

Persistent viral infections are associated with host and viral factors that impair effective antiviral immunity. Natural killer (NK) cells contribute to establishment of persistent lymphocytic choriomeningitis virus (LCMV) infection in mice through suppression of virus-specific T cell responses during the first few days of infection, but NK cell depletion during those early time points can enable severe T cell-mediated immune pathology and death of the host. Here we show that long after their peak in cytolytic activation, NK cells continue to support viral persistence at later times of infection. Delayed depletion of NK cells, 2 to 3 weeks after infection, enhanced virus-specific T cell responses and viral control. This enhancing effect of delayed NK cell depletion on antiviral immunity, in contrast to early NK cell depletion, was not associated with increased morbidity and mortality, and mice quickly regained weight after treatment. The efficacy of the depletion depended in part upon the size of the original virus inoculum, the viral load at the time of depletion, and the presence of CD4 T cells. Each of these factors is an important contributor to the degree of CD8 T cell dysfunction during viral persistence. Thus, NK cells may continuously contribute to exhaustion of virus-specific T cells during chronic infection, possibly by depleting CD4 T cells. Targeting of NK cells could thus be considered in combination with blockade of other immunosuppressive pathways, such as the interleukin-10 (IL-10) and programmed death 1 (PD-1) pathways, as a therapy to cure chronic human infections, including those with HIV or hepatitis C virus. IMPORTANCE Persistent virus infections are a major threat to global human health. The capacity of viruses, including HIV and hepatitis C virus, to overwhelm or subvert host immune responses contributes to a prolonged state of dampened antiviral immune functionality, which in turn facilitates viral persistence. Recent efforts have focused on therapeutics that can restore the effector functions of these functionally exhausted virus-specific T cells in order to expedite viral clearance. Here we establish that natural killer (NK) cells actively contribute to immune dysfunction and viral persistence at later stages of infection. This previously undescribed mechanism of immune suppression during chronic infection provides a vital clue for the design of novel therapeutic strategies targeting NK cell immunosuppressive activity in order to restore immune function and enhance viral control in chronically infected individuals.

Pathogenesis of chronic viral hepatitis: differential roles of T cells and NK cells

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections account for 57% of cases of liver cirrhosis and 78% of cases of primary liver cancer worldwide and cause a million deaths per year. Although HBV and HCV differ in their genome structures, replication strategies and life cycles, they have common features, including their noncytopathic nature and their capacity to induce chronic liver disease, which is thought to be immune mediated. However, the rate of disease progression from chronic hepatitis to cirrhosis varies greatly among infected individuals, and the factors that regulate it are largely unknown. This review summarizes our current understanding of the roles of antigen-specific and nonspecific immune cells in the pathogenesis of chronic hepatitis B and C and discusses recent findings that identify natural killer cells as regulators of T cell function and liver inflammation.

Natural killer cells: multifaceted players with key roles in hepatitis C immunity

Natural killer cells (NKs) are involved in every stage of hepatitis C viral (HCV) infection, from protection against HCV acquisition and resolution in the acute phase to treatment-induced clearance. In addition to their direct antiviral actions, NKs are involved in the induction and priming of appropriate downstream T-cell responses. In the setting of chronic HCV, overall NK cell levels are decreased, subset distribution is altered, and changes in NK receptor (NKR) expression have been demonstrated, although the contribution of individual NKRs to viral clearance or persistence remains to be clarified. Enhanced NK cell cytotoxicity accompanied by insufficient interferon-γ production may promote liver damage in the setting of chronic infection. Treatment-induced clearance is associated with activation of NK cells, and it will be of interest to monitor NK cell responses to triple therapy. Activated NK cells also have anti-fibrotic properties, and the same hepatic NK cell populations that are actively involved in control of HCV may also be involved in control of HCV-associated liver damage. We still have much to learn, in particular: how do liver-derived NKs influence the outcome of HCV infection? Do NK receptors recognize HCV-specific components? And, are HCV-specific memory NK populations generated?