Viral escape from NK‐cell‐mediated immunosurveillance: A lesson for cancer immunotherapy?

Natural killer (NK) cells are innate lymphocytes that participate in immune responses against virus‐infected cells and tumors. As a countermeasure, viruses and tumors employ strategies to evade NK‐cell‐mediated immunosurveillance. In this review, we examine immune evasion strategies employed by viruses, focusing on examples from human CMV and severe acute respiratory syndrome coronavirus 2. We explore selected viral evasion mechanisms categorized into three classes: (1) providing ligands for the inhibitory receptor NKG2A, (2) downregulating ligands for the activating receptor NKG2D, and (3) inducing the immunosuppressive cytokine transforming growth factor (TGF)‐β. For each class, we draw parallels between immune evasion by viruses and tumors, reviewing potential opportunities for overcoming evasion in cancer therapy. We suggest that in‐depth investigations of host–pathogen interactions between viruses and NK cells will not only deepen our understanding of viral immune evasion but also shed light on how NK cells counter such evasion attempts. Thus, due to the parallels of immune evasion by viruses and tumors, we propose that insights gained from antiviral NK‐cell responses may serve as valuable lessons that can be leveraged for designing future cancer immunotherapies.


Introduction
Natural killer (NK) cells are part of the innate immune system and participate in the immediate defense of their host.Although NK cells have been identified due to their capacity to kill tumor cells without prior sensitization [1], their contribution to controlling virus infections is well established [2].NK cells respond to diverse threats and distinguish between healthy and aberrant cells by using an array of receptors that allow for integrating multiple distinct signals [3].Activating receptors such as NKG2D detect stressinduced ligands on infected or transformed cells and the lowaffinity Fc receptor III (CD16) elicits activating signals upon bind-ing to antibody-coated target cells.In contrast, inhibitory receptors such as NKG2A allow NK cells to monitor the expression level of human leukocyte antigen (HLA) class I molecules, including the nonclassical HLA-E as an indicator of healthy self.Simultaneous integration of activating and inhibitory signals leads to a tightly controlled balance that determines whether NK cells remain in a resting state or trigger their effector functions.Besides monitoring cellular ligands, NK cells also sense soluble factors in their environment.Pro-inflammatory cytokines such as interleukin (IL)-2, IL-12, IL-15, IL-18, or type I interferons (IFNs) prime NK cells for potentiated functionality and prompt secretion of NK-cell-derived cytokines.Conversely, anti-inflammatory cytokines such as transforming growth factor (TGF)-β suppress NK-cell functions [4].Upon NK-cell activation, their hallmark effector function is the killing of target cells by the release of cytolytic proteins, which directly contributes to eliminating tumors and restraining viral infections.In addition, activated NK cells secrete cytokines and chemokines, including IFN-γ, tumor necrosis factor (TNF)-α, CCmotif chemokine ligand 3, and CC-motif chemokine ligand 4, which alert bystander cells and guide ensuing immune responses [5].As a consequence of NK-cell-mediated immunosurveillance, several viruses have developed advanced mechanisms to escape from NK-cell control [6].Similarly, cancer progression is an indicator that tumor cells have undergone adaptations and subvert NK-cell reactivity in their host [7].
In this review, we explore selected immune evasion strategies employed by viruses, especially examining examples from molecular mechanisms used by human CMV (HCMV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).We illustrate parallels in evasion strategies used by tumors and propose that shared axes, which are targeted by viruses as well as tumors, may serve as valuable lessons for understanding how NK cells overcome immune evasion.Finally, we suggest that in-depth knowledge about antiviral NK-cell responses might have implications for rationally designing future NK-cell-based cancer therapies.

Escape by providing ligands for inhibitory receptors to limit NK-cell reactivity
Maintaining tolerance to self is an essential requirement for immune effector cells.One mode of ensuring tolerance of NK cells is the interaction between self-ligands on healthy cells and inhibitory receptors on NK cells.Consequently, exploiting inhibitory pathways that are designed to maintain tolerance represents a promising strategy for evasion from NK-cell responses.

HCMV providing HLA-E-restricted peptides as ligands for NKG2A
HCMV, which has co-evolved with its human hosts over millennia [6], adeptly exploits a key inhibitory pathway of NK cells by providing peptides from the viral gp UL40 that are presented on the nonclassical HLA-E [8] (Fig. 1).Complexes of HLA-E and UL40 peptides on the surface of infected cells serve as ligands for the inhibitory receptor NKG2A, which instills the recognition of self to NKG2A-expressing NK cells and thereby limits their reactivity [8][9][10][11][12].HCMV UL40 peptides are heterogeneous and display substantial overlap with human sequences [13,14].This molecular mimicry allows the virus to exploit the HLA-E/peptide-NKG2A axis.As NKG2A is expressed on a large proportion of NK cells [15], this strategy efficiently and broadly restrains NK-cell responses.Interestingly, a subpopulation of NK cells expresses the receptor NKG2C, which possibly emerged through a gene duplication of NKG2A [16].The NKG2C receptor also binds to HLA-E/peptide complexes but transmits activating signals [10].NK cells that lack NKG2A are not inhibited by HLA-E/peptide complexes and concomitant expression of NKG2C further equips this subpopulation of NK cells with a countermeasure to over-come UL40-mediated immune evasion in a peptide-dependent manner [14].Expansion of NKG2A − NKG2C + NK cells in peripheral blood is observed during acute HCMV infection in vivo [17,18] and correlates with control of viral load [19].Emergence of NKG2C as an activating receptor with cognate specificity for a viral immune evasion epitope illustrates how the evolutionary arms race between NK cells and HCMV might shape adaptations and counter-adaptations on both sides.

The HLA-E-NKG2A axis in tumor immune evasion
Analogous to the example of HCMV, tumors also employ strategies targeting the HLA-E-NKG2A axis (Fig. 1).For instance, HLA-E expression by acute myeloid leukemia (AML) blasts protects them against cytolytic NK-cell activity [20].This strategy is especially effective shortly after stem cell transplantation when almost all NK cells express NKG2A [20].The inhibitory effect is further enhanced by the exposure of AML blasts to NK-cell-derived IFNγ [20], implying a context-dependent and targeted adaptation to evade NK-cell responses.Many cancer types are enriched for HLA-E, with squamous cell carcinoma of the head and neck displaying elevated expression levels [21].In keeping with immune escape mediated through the HLA-E-NKG2A axis, high abundance of KLRC1 transcripts (encoding NKG2A) is associated with poor clinical outcome of squamous cell carcinoma of the head and neck [22].As the inhibitory effect of NKG2A extends beyond NK cells and is also operational in NKG2A-expressing T cells [21][22][23][24][25], providing HLA-E as a ligand achieves a widespread inhibition of innate and adaptive immune cells and effectively limits their reactivity.

Strategies for overcoming immune evasion through the HLA-E-NKG2A axis
In light of the escape strategies mentioned above, therapeutic interventions aim at overcoming tumor immune evasion by ablating HLA-E-mediated inhibition (Fig. 1, Table 1).Anti-NKG2A antibodies have been used to successfully disrupt the HLA-E-NKG2A axis and restore antitumor functionality [21,22,25].Genetically silencing NKG2A expression is an alternative approach [26,27].Transduction of NK cells with a synthetically designed expression blocker that retains NKG2A intracellularly is an elegant complementary option [28].Recapitulating the HCMVinduced expansion of NKG2A − NKG2C + NK cells to overcome viral immune evasion and translating this naturally occurring process into a tool for therapeutic application is an intriguing approach.Leveraging the inherent response pattern during coculture with HLA-E-expressing feeder cells allows for the selective expansion of NKG2A − NKG2C + NK cells in vitro [29][30][31][32].These NKG2A − NKG2C + NK-cell products combine the absence of inhibition through the lack of NKG2A with robust activation through the presence of NKG2C and thus represent optimal candidates for the treatment of tumors enriched for HLA-E expression.Of  interest, a sensitizing effect of HLA-E in viral infections has also been demonstrated: Not all viral HLA-E-restricted peptides mediate immune evasion and peptides from human immunodeficiency virus (HIV)-1, Epstein-Barr virus (EBV), as well as SARS-CoV-2 have been shown to prevent the binding of NKG2A to HLA-E/viral peptide complexes, thereby rendering infected cells susceptible to NKG2A + NK-cell attack [33][34][35][36].Along those lines, an intriguing future approach to fine-tune NK-cell responses could lie in specifically altering the tumor peptidome with the goal to reduce inhibition through NKG2A, similar to what has been suggested regarding the activation of unconventional T cells [37].Furthermore, potential future therapeutics might seek to carefully balance the inhibitory effect of NKG2A that is important for avoiding autoreactivity with its dampening effect on antitumor responses.The use of induced pluripotent stem cells as a source for in vitro-generated NK cells opens numerous opportunities for the development of multiedited NK-cell products [38].This may allow to refine the rational design of NK cells with desired functional specificities against tumors that attempt to limit NKcell reactivity by the provision of HLA-E, possibly combined with additional tumor-specific activating input and non-HLA-E-binding inhibitory receptors to ensure tolerance to healthy tissues.

Escape by cloaking of stress indicators to subvert NK-cell-mediated immunosurveillance
A fundamental concept of NK-cell regulation is that their effector functions are elicited based on a balance of activating and inhibitory signals.Therefore, restricting the activating input that NK cells receive by reducing the density of ligands for activating receptors can successfully shift the balance away from activation and enable escape from NK-cell-mediated immunosurveillance.
Table 1.Current and potential therapeutic strategies to overcome immune evasion.

Evasion category Therapeutic strategy Intended effect References
Providing ligands for inhibitory receptors (i.e., HLA-E-NKG2A axis) Treatment with anti-NKG2A antibodies Blockade of binding between NKG2A and HLA-E to alleviate inhibition, reinvigorating the patients' own immune cell response [21] Infusion of NK cells with genetic deletion of KLRC1 (encoding NKG2A) Prevention of binding between NKG2A and HLA-E to avoid inhibition of infused cells [26,27] Infusion of NK cells with blocked NKG2A expression Prevention of binding between NKG2A and HLA-E to avoid inhibition of infused cells [28] Infusion of NKG2A − NKG2C + NK cells Selective growth of NKG2A − NKG2C + NK cells in vitro to preclude NKG2A-mediated inhibition and simultaneously enables NKG2C-driven activation upon binding to HLA-E [32] Potential modulation of the tumor HLA-E peptidome Window of opportunity to adjust the pool of peptides presented on HLA-E with the goal to interfere with the binding between HLA-E/peptide complexes and NKG2A Discussed in [37] Potential infusion of multiedited iPSC-derived NK cells Genetic engineering to specifically tailor activity, including prevention of the binding between NKG2A to HLA-E alone or in combination with stimulatory modalities (such as NKG2C or CAR) Discussed in [38] Cloaking of stress indicators (i.e., NKG2D-ligand-NKG2D axis)

Treatment with affinity-matured NK-cell engagers
Optimizing binding potency between NK-cell receptors and engagers to enable efficient engagement despite low ligand expression density [62], discussed in [63] Treatment with bispecific NKG2D/CD16 fusion molecules Utilizing potent activation through CD16 for NKG2D-specific engagement of tumor cells to achieve activation despite low expression of NKG2D-ligands [64] Treatment with multifunctional NK-cell engagers Merging two activating receptors into a combined targeting approach for enhanced activation to reduce dependency on one ligand-receptor pair [65] Treatment with sensitizing agents Restoring and elevating expression or preventing shedding of NKG2D-ligands to recover binding between NKG2D-ligands and NKG2D [60,71] Potential infusion of NK cells with logic-gated activating signals Combinatory engagement of several activating receptors to reduce dependency on one ligand-receptor pair [66,68] Shaping the soluble factor environment (i.e., TGF-β-mediated suppression)

Infusion of cytokine-induced memory-like NK cells
Prestimulation in vitro to maintain activity of infused cells despite suppressive signals in vivo [97] Treatment with IL-15 superagonist Stimulation in vivo to enhance activity and overcome suppression of the patients' own immune cell response [99] Infusion of NK cells with genetic deletion of TGFBR2 Precluding binding of TGF-β to TGF-βR, preventing suppression of infused cells [87] Infusion of NK cells expressing dominant negative TGF-βR Converting signaling downstream of TGF-βR, resulting in stimulation instead of suppression of infused cells [100] (Continued) Discussed in [105] Abbreviations: CAR, chimeric antigen receptor; HLA, human leukocyte antigen; iPSC, induced pluripotent stem cells.

HCMV-and SARS-CoV-2-driven downregulation of NKG2D-ligands
NKG2D is a prototypic activating receptor that is central to NK-cell regulation and orchestrates NK-cell activity by binding to a range of different ligands, which jointly function as danger signals and are upregulated in response to cellular stress, including during viral infection [39,40].Accordingly, viruses have evolved numerous mechanisms to target this particular activating pathway (Fig. 2).HCMV is again one example of numerous viruses that have adopted this pathway for evasion from NK cells, with the HCMV gp UL16 inhibiting the surface expression of several NKG2D-ligands, including ULBP1, ULBP2, and MICB [41][42][43][44][45][46].A systematic virus screen further revealed that the HCMV proteins US18 and US20 act in concert to enforce lysosomal degradation of the NKG2D-ligand MICA [47].Another viral gene product, UL142, also targets MICA and induces its downregulation from the surface of infected cells by retaining it in the Golgi apparatus [48,49].Interestingly, UL142 functions in an allele-specific manner, and the common MICA*008 variant is resistant to UL142-induced downregulation [48], potentially hinting at selective pressure that is exerted by the virus and contributes to host diversity as response to viral escape strategies [46].Despite a much shorter coevolution period with humans as a host compared to HCMV, SARS-CoV-2 also induces the downregulation of NKG2D-ligands on the surface of infected epithelial cells [50,51].
In this scenario, ligand downregulation does not occur due to ligand-specific repressed transcription, intracellular retention, or degradation [51] but rather seems to be part of a globally altered plasma membrane protein landscape driven by low protein abundance upon SARS-CoV-2 infection [50].Compared to HCMV, the exact molecular mechanisms underlying these alterations are currently less well defined, but the SARS-CoV-2 proteins Nsp1 and Nsp14 have been reported to mediate this immune evasion [50,51].Interestingly, the downregulation of NKG2D-ligands is more pronounced at later stages of infection, during which NKcell activity is diverted away from infected target cells and shifted toward uninfected bystanders [51].Impaired NK-cell responses due to the reduced density of NKG2D-ligands can be overcome by the presence of anti-SARS-CoV-2 antibodies that elicit antibodydependent NK-cell activation through CD16 [50].In addition to HCMV and SARS-CoV-2, multiple other viruses target NKG2Dligands for downregulation, including HSV 1 and varicella zoster virus [52] as well as HIV-1 [53], hepatitis B virus [54], and adenoviruses [55].Overall, the observation that several unrelated viruses share NKG2D-ligands as a common target for immune escape underscores the relevance of this axis in NK-cell-mediated immunosurveillance.

The NKG2D-ligand-NKG2D axis in tumor immune evasion
Besides its relevance in restraining viral infections, NKG2D also plays a crucial role in NK-cell-dependent tumor rejection [56,57].Consequently, tumors have developed NKG2D-specific immune evasion strategies that parallel those of viruses (Fig. 2).For instance, the shedding of MICA from tumor cells of epithelial origin reduces its availability at the cell surface, thereby subverting NKG2D-dependent immunosurveillance [58,59].The functionality of the NKG2D-ligand-NKG2D axis is further diminished as tumor-derived soluble MICA binds to NKG2D and induces receptor internalization [58,59].Moreover, the absence of NKG2Dligands has been identified as a hallmark of leukemia stem cells, conferring survival advantages compared to NKG2D-ligandexpressing AML blasts [60].This escape from NKG2D-expressing NK cells is likely one of the factors contributing to the capacity of leukemia stem cells to cause disease relapse [61], further highlighting the importance of this activating pathway.

Strategies for overcoming immune evasion due to loss of NKG2D-ligands
Considering the downregulation of NKG2D-ligands as immune evasion strategy, therapeutic interventions aim at restoring NK-cell-mediated tumor surveillance by overcoming low ligand density on the surface of tumor cells (Fig. 2, Table 1).Biochemical engineering has been successfully used to considerably increase the affinity between NK-cell receptors and their ligands [62].The development of ligand-specific engagers with supraphysiological functional avidity has the potential to reach the required activation threshold of NK cells despite low ligand density [63].Bispecific fusion proteins, which contain extracellular NKG2D domains as antigen-binding component coupled to Fab-fragments of anti-CD16 as cross-linking module, can potentiate NK-cell activation [64].Such reagents maintain the NKG2D-intrinsic specificity for its natural ligands while simultaneously augmenting downstream activating input through the potent activating receptor CD16 and thus may represent another promising therapeutic tool, akin to anti-SARS-CoV-2 antibodies restoring NK-cell activity against infected cells despite low NKG2D-ligand density [50].Future therapeutics may leverage the fact that NK cells express an array of activating receptors, which allows for rationally designing multispecific engagers that target multiple complementary receptors to overcome limited ligand availability [65].Moreover, combining different receptor-ligands axes into one tumor-targeting approach might enable optimal responses by logic-gated activation.Cooperative engagement of several activating receptors in parallel is one such concept that has been proposed for chimeric antigen receptor (CAR) T-cell therapies to reduce the dependency on ligand expression levels [66][67][68].Using CAR NK cells as a baseline platform to combine tumorspecific CAR with therapeutic antibody-mediated triggering of CD16 and additional co-stimulatory receptors will permit exploring distinct permutations of activating signals and thereby generate numerous possibilities of enhancing NK-cell activation despite low ligand expression [69].Finally, the pharmacological inhibition of a transcriptional repressor effectively restored highdensity NKG2D-ligand expression and enabled the elimination of previously resistant leukemia stem cells by NKG2D-expressing NK cells [60].This suggests that sensitizing agents, which promote ligand expression or prevent shedding thereof by multiple mechanisms [70][71][72][73], may prove valuable as combination treatments acting in concert with other therapeutic approaches.

Escape by shaping the soluble factor environment to instruct NK-cell unresponsiveness
Apart from ligands present on the surface of target cells, NK cells are capable of integrating cytokine cues into their effector response.One integral element of the multicomponent soluble factor environment is the immunosuppressive cytokine TGFβ, which regulates a range of immunological processes and, at steady state, plays a crucial role in maintaining homeostasis [74].Exposure of NK cells to TGF-β profoundly impairs their functional capacity via diverse mechanisms [75][76][77][78].Consequently, the induction of TGF-β represents an attractive target for immune escape as its suppressive impact can be exploited to instruct unresponsiveness and dampen NK-cell responses.

SARS-CoV-2 infection-mediated induction of TGF-β
The development of severe COVID-19 following infection with SARS-CoV-2 has been associated with pronounced systemic levels of TGF-β early after infection [79].Moreover, in a followup study, early levels of TGF-β were identified as a predictor of disease severity and fatality [80].Although mild and moderate COVID-19 are characterized by activated NK cells that efficiently control SARS-CoV-2 replication in vitro, TGF-β pervasively suppresses antiviral functions of in severe disease [79] (Fig. 3).Suppression of NK-cell activity manifests as defects in conjugate formation, cytotoxic killing, and cytokine secretion [79].These data exemplify that robust induction of TGF-β early during infection acts as an inappropriate tolerogenic signal and prompts functional editing of NK cells into an unresponsive state.Besides SARS-CoV-2 infection, TGF-β is induced in numerous virus infections, including HCMV [81][82][83], HCV [84], and HIV-1 [85]; albeit its modulatory role in these conditions is not well understood.Overall, it is conceivable that the excessive and/or improperly timed production of TGF-β hampers otherwise appropriate antiviral NKcell functions by enforcing out-of-context tolerance, which in turn facilitates viral replication and may alter disease trajectories.

TGF-β in tumor immune evasion
In a similar manner to viral infections, TGF-β also acts as a crucial soluble factor in cancer, where tumor-derived TGF-β contributes to a soluble factor milieu that suppresses immune responses in the tumor microenvironment [4] (Fig. 3).The suppressive effect of TGF-β on NK cells in cancer results in diverse molecular changes, including the downregulation of activating receptors such as NKG2D [86].Additionally, TGF-β was demonstrated to impair NK-cell cytolytic capacity [75] and to reduce their metabolic activity [77,78], together extensively inhibiting NKcell effector functions.Illustratively, tumor-infiltrating NK cells in glioblastoma multiforme are functionally suppressed in a TGFβ-dependent manner and fail to kill tumor targets [87].Apart from directly inhibiting NK-cell functions, TGF-β is also reported to induce the trans-differentiation of NK cells into noncytotoxic group 1 innate lymphoid cells [88,89].In the context of a solid tumor model, TGF-β-enforced phenotypic alteration in the form of impaired cytotoxicity leads to escape from NK-cell surveillance and diminished tumor control [90].Additionally, an innate lymphocyte population with a phenotype resembling an intermediate between cytotoxic NK cells and noncytotoxic group 1 innate lymphoid cells is described in AML [91], further emphasizing tumorimposed differentiation.Here, TGF-β likely contributes to instructing NK-cell dysfunction both directly by inducing suppression and indirectly by guiding trans-differentiation.

Strategies for overcoming TGF-β-mediated suppression
With regards to TGF-β-mediated immune evasion, therapeutic modulation aims at circumventing suppressive cues and maintaining the functional potential of NK cells (Fig. 3, Table 1).Global inhibition of TGF-β is a challenge due to its pleiotropic functions that likely cause adverse effects [92], thus warranting well-balanced and specifically targeted approaches.As SARS-CoV-2 infection elicits an activated state in NK cells through pro-inflammatory cytokines [93][94][95], onto which suppressive TGF-β signals are superimposed, priming of NK cells with proinflammatory cytokines may be sufficient to counter-balance antiinflammatory signals and maintain functionality.Treatment of NK cells with IL-12, IL-15, and IL-18 imprints potent antitumor functions that are maintained after adoptive transfer in AML [96][97][98] and delivery of an IL-15 superagonist potentiates NK-cell activity in vivo [99].Genetic deletion of TGFBR2 in primary NK cells effectively prevents tumor-induced unresponsiveness [87] and expression of a dominant negative variant of TGFBR2, which is decoupled from suppressive signaling and instead transmits pro-inflammatory signals, represents an elegant strategy to overcome TGF-β-mediated suppression and simultaneously arms NK cells with enhanced functional potential in response to tumor-derived TGF-β [100].Future therapeutic efforts will face the challenge of preserving homeostatic signals of TGF-β while simultaneously averting immune evasion.Arming multicompo-  nent engager molecules with cytokine domains is generating exciting possibilities to integrate proinflammatory cues into cellcell contact-dependent antitumor specificity [101][102][103].Such strategies might also be applicable in settings, where TGF-β only plays a minor role and other suppressive signals dominate the tumor microenvironment.Furthermore, the dichotomy between immunosuppressive environments of cold tumors and immuneresponsive hot tumors is a key difference between antitumor responses and viral infections and poses a central obstacle in cancer therapy [104].Utilizing cancer vaccines to artificially generate proinflammatory environments that mimic those found in acute virus infections may provide the required context for NK cells to overcome suppression and appropriately respond [105].

Concluding remarks
During coevolution with humans over millennia, viruses developed sophisticated strategies to evade NK-cell-mediated immunosurveillance.Similarly, tumors develop numerous immuneevading strategies despite a much shorter time of coevolution within the lifespan of their host.Although the molecular mechanisms of viral and tumor immune evasion strategies are seemingly diverse, they converge toward common axes with shared targets that are critical for antiviral as well as antitumor NK-cell responses.
Viruses are excellent NK-cell biologists, who proficiently exploit their understanding of NK-cell functional regulation to gain a survival advantage and promote their continuance.Hence, meticulously monitoring how NK cells and viruses coevolve and how they continuously develop countermeasures against the opponents' approach provides valuable insights into fundamental NK-cell biology.The interplay between viral escape strategies and compensatory mechanisms that enable NK cells to protect their host despite such evasion attempts might thus serve as a valuable lesson for anticancer NK-cell responses.Leveraging knowledge obtained from antiviral immune responses may allow for rationally designing NK-cell-based cancer immunotherapies, further boosting their considerable clinical promise despite substantial tumor immune evasion [106][107][108][109][110][111].
Acknowledgments: We thank Josefine Dunst, Christopher Tibbitt, and Evren Alici for critical feedback.We are grateful to all members of the Center for Infectious Medicine for inspiring discussions, and we apologize to colleagues whose work could not be included due to space constraints.This work was supported by funding from Region Stockholm

Figure 1 .
Figure 1.Escape by providing ligands for inhibitory receptors to limit NK-cell reactivity.Both virus-infected cells (left) and cancer cells (right) provide human leukocyte antigen (HLA)-E to inhibit NKG2A-expressing NK cells.Human CMV (HCMV) provides a viral peptide that mediates immune evasion by inhibiting NKG2A-expressing NK cells (left top).NKG2C + NKG2A − NK cells, which expand in response HCMV, are a naturally occurring countermeasure that allows for NK-cell attack (left bottom).Anti-NKG2A-blocking antibodies prevent the inhibition of NKG2A + NK cells (right top).Genetically silenced NKG2A expression (KLRC1 −/− or expression blockers) counters inhibition (right middle).In vitro expansion of NKG2C + NKG2A − NK enables robust activation against HLA-E-expressing tumor cells (right bottom).

Figure 2 .
Figure 2. Escape by cloaking of stress indicators to subvert NK-cell-mediated immunosurveillance.Both virus-infected cells (left) and cancer cells (right) downregulate the surface expression of NKG2D-ligands to reduce NK-cell activation.Human CMV (HCMV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) curtail surface expression of NKG2D-ligands, subverting NKG2D-dependent immunosurveillance (left top).Antibodies specific for virus antigens can recover NK-cell activation despite low NKG2D-ligand expression density (left bottom).Fusion proteins of NKG2D and CD16 potentiate NK-cell function despite low NKG2D-ligand density due to potent signaling downstream of CD16 (right top).Multispecific engagers binding to tumor antigens and cooperatively cross-linking multiple activating receptors (ActR) enhance NK cell anticancer functions (right bottom).

Figure 3 .
Figure 3. Escape by shaping the soluble factor environment to instruct NK-cell unresponsiveness.Both virus infection (left) and tumor transformation (right) induce the secretion of TGF-β, functionally altering NK cells and enforcing inappropriate tolerance.TGF-β-mediated signaling through TGFβR suppresses NK-cell functions, including the downregulation of activating receptors such as NKG2D (left top), and can induce functional editing by trans-differentiating NK cells into noncytotoxic group 1 innate lymphoid cells (ILC1)-like cells (left bottom).Stimulatory input through proinflammatory cytokines, such as IL-15, IL-15 superagonist (IL-15-IL15Rα fusion), or multicomponent engager molecules armed with cytokine domains, can maintain NK-cell functionality (right top).Genetic deletion of TGFBR2 counters TGF-β-mediated suppression (right middle).Expression of a dominant negative (DN) TGFβR converts suppressive into stimulatory signals and potentiates NK activity (right bottom).