NK cell subsets and dysfunction during viral infection: a new avenue for therapeutics?

Abstract

In the setting of viral challenge, natural killer (NK) cells play an important role as an early immune responder against infection. During this response, significant changes in the NK cell population occur, particularly in terms of their frequency, location, and subtype prevalence. In this review, changes in the NK cell repertoire associated with several pathogenic viral infections are summarized, with a particular focus placed on changes that contribute to NK cell dysregulation in these settings. This dysregulation, in turn, can contribute to host pathology either by causing NK cells to be hyperresponsive or hyporesponsive. Hyperresponsive NK cells mediate significant host cell death and contribute to generating a hyperinflammatory environment. Hyporesponsive NK cell populations shift toward exhaustion and often fail to limit viral pathogenesis, possibly enabling viral persistence. Several emerging therapeutic approaches aimed at addressing NK cell dysregulation have arisen in the last three decades in the setting of cancer and may prove to hold promise in treating viral diseases. However, the application of such therapeutics to treat viral infections remains critically underexplored. This review briefly explores several therapeutic approaches, including the administration of TGF-β inhibitors, immune checkpoint inhibitors, adoptive NK cell therapies, CAR NK cells, and NK cell engagers among other therapeutics.

Keywords: CMV; HIV; NK cell dysfunction; NK cells; SARS-CoV-2; hepatitis C; immunotherapy; influenza.

Figure 1

The activating and inhibitory receptors of NK cells. NK cell activation depends on a balance of activating and inhibitory signals using receptors that primarily bind HLA to distinguish between virally infected cells and uninfected cells in a non-antigen specific mechanism. Activating receptors include NKG2C, NKG2D, NKG2E, NKp30, NKp44, NKp46, NKp80, DNAM-1, and KIR receptors with short cytoplasmic tails. Inhibitory receptors include NKG2A, PD-1, LAG-3, TIGIT, TIM-3 and KIR receptors with long cytoplasmic tails excluding KIR2DL4 which is activating. The KIR receptor family contains both activating and inhibitory receptors, as well as receptors that can act as either. Similarly, 2B4 can also provide either an activating or inhibitory signal.

Figure 2

The CD56^neg NK cell subset increases during chronic viral infections. This NK cell subset has been observed to expand in the setting of HIV, chronic HCV, EBV, and chronic CMV. The subset has substantially impaired cytolytic capabilities due to alterations in its receptor repertoire, particularly due to decreased expression of activating receptors like NKp30 and NKp46 in addition to the increased expression of the inhibitory KIR receptor KIR2DL2 (52). Interestingly, the expression of inhibitory KIR receptors KIR2DL1 and KIR3DL1 remain comparable between CD56⁺ and CD56^neg NK cells. In HIV, this subset has been found to be more expressive of TGF-β, which contributes to the exhaustion of other NK cells.

Figure 3

Self clearing HCV infections are associated with a more highly differentiated NK cell repertoire compared to chronic HCV infections. Following acute HCV infection, two outcomes may follow. In roughly a quarter of cases, HCV infection is spontaneously cleared whereas roughly three quarters of cases become chronic. Spontaneously cleared cases are associated with a more a more differentiated NK cell phenotype with increased prevalence of NKp30, CD57 and KIR expression as well as decreased expression of NKG2A. Chronic HCV cases are associated with diminished NK cell expression of NKp46, NKp30, and CD16 along with increased expression of NKG2A. NKG2A inhibitory signaling has been identified as a key source of NK cell exhaustion in HCV infection.

Figure 4

Acute CMV Infection and Subsequent Reactivation is Associated with an Expansion in NKG2C⁺ memory-like NK cells. These cells are phenotypically mature, long lasting, and exhibit enhanced functional potential leading them to be coined adaptive or memory like NK cells. This population is phenotypically defined by the expression of NKG2C and is associated with a high level of expression of CD57, NKp46, and inhibitory KIR2DL receptors.

Figure 5

Coculture of NK cells with different IVA PPs induce differential NK cell activation. In vitro experiments coculturing NK cells isolated from PBMCs with pseudotype particles (PPs) of different IVA viruses found differential activation between PPs in terms of CD69, CD107a, and IFN-γ expression (139). The findings presented in the figure correspond to the 500 HAU/mL IVA coculture dose. The PPs modeling the more virulent 1918 H1N1 and H5N1 IVA viruses induced greater activation than the less virulent 2009 H1N1 PPs. This suggests that in IVA infection, disparities in early NK cell activation between influenza viruses may play a role in contributing to differential virulence between them. Further work is required to characterize this effect in vivo with whole virus that is not replication deficient due to IVA infecting and killing NK cells.

Figure 6

Severe SARS-CoV-2 is Associated with a Significant decrease in the number of CD56⁺ CD16⁺ NK Cells in the Blood. The decline in CD56⁺ CD16⁺ NK Cells is due to CD16 shedding catalyzed by ADAM17. This results in an accumulation of CD56⁺ CD16^- NK cells of less cytotoxic potential and in severe cases likely contributes to impaired capability to engage in ADCC. CD16 mediates ADCC by binding to antibodies attached to target cells, ultimately resulting in degranulation.

Figure 7

A visual overview of a few emerging therapeutics seeking to address NK cell dysregulation during viral infection. (A) TGF-β signaling inhibits the mTOR pathway which has been shown to result in impaired NK cell effector functions. The use of TGF-β inhibitors can promote mTOR signaling and in turn NK cell activity. (B) Immune checkpoint inhibitors can be applied to reduce inhibitory signaling that is causing NK cell exhaustion. For example, Monalizumab, an NKG2A inhibitor, can block inhibitory NKG2A contributing to a shift towards NK cell activation and effector functions. (C) Adoptive NK cell therapies entail the administration of exogenous NK cells to supplement the immune response. (D) N-803 is an IL-15 superagonist consisting of a mutant IL-15 molecule associated with an IL-15 receptor fusion protein. Administration of N-803 promotes NK cell proliferation and maturation. (E) An example of an NK cell engager are TRiKE™ constructs which consist of a CD16 engaging molecule linked to an IL-15 molecule that’s linked to an antigen linking molecule. Binding of the TRiKE™ to an NK cell CD16 molecule induces activation while IL-15 supports proliferation and maturation. The antigen linking molecule is designed to target an antigen of interest. (F) The extracellular antigen recognition domain of CAR receptors enables CAR NK cells to specifically target cells producing an antigen of interest. CAR NK cells can be administered to supplement an exhausted NK cell response, with less risk than CAR T-cells.

Figure 8

Schema of the Logic Underpinning the Use of NK Cell Therapeutics in Viral Infection. Many viral infections are associated with changes in the NK cell repertoire in terms of receptor expression and subset prevalence that result in altered NK cell functionality. Specific changes in NK cell repertoire can be targeted for therapeutic modulation to alter functionality in order to combat pathogenesis. For example, chronic HCV is associated with a significant increase in the expression of NKG2A, ultimately contributing to NK cell exhaustion, and a diminished antiviral response. The administration of NKG2A blockade interferes with inhibitory signaling in an effort to help restore effector functions to promote antiviral immunity.