The NKG2A–HLA-E Axis As a Novel Checkpoint in the Tumor Microenvironment Linda Borst, Sjoerd H

Published OnlineFirst May 14, 2020; DOI: 10.1158/1078-0432.CCR-19-2095

CLINICAL CANCER RESEARCH | REVIEW

The NKG2A–HLA-E Axis as a Novel Checkpoint in the Tumor Microenvironment Linda Borst, Sjoerd H. van der Burg, and Thorbald van Hall

ABSTRACT ◥ The success of checkpoint blockade therapy revolutionized binds a limited peptide repertoire and its expression is commonly cancer treatment. However, we need to increase the fraction of detected in human cancer. NKG2A blockade as a standalone responding patients and overcome acquired resistance to these therapy appears poorly effective in mouse tumor models, how- therapies. Recently, the inhibitory receptor NKG2A received ever, in the presence of activated T cells, for example, induced by attention as a new kid on the block of immune checkpoints. PD-1/PD-L1 blockade or cancer vaccines, exerts strongly This receptor is selectively expressed on cytotoxic lymphocytes, enhanced efﬁcacy. Clinical trials demonstrated safety of the þ including natural killer cells and CD8 T cells, and NKG2A T humanized NKG2A-blocking antibody, monalizumab, and ﬁrst cells are preferentially residing in tissues, like the tumor micro- results of phase II trials demonstrate encouraging durable environment. Its ligand, histocompatibility leucocyte antigen E response rates. Further development of this axis is clearly (HLA-E), is a conserved nonclassical HLA class I molecule that warranted.

Introduction The Similarities and Differences within The unprecedented clinical impact of immune checkpoint block- the NKG2A Family ade therapy induced a new era of cancer treatment (1). After Activation of T cells is the result of T-cell receptor (TCR) ligation introduction in clinical practice of antibodies to CTLA-4 and the with antigen–HLA complexes in combination with costimulatory PD-1/PD-L1 axis, additional checkpoints like TIM-3, TIGIT, and signals, like CD28 and CD27, and cytokines. To avoid unrestricted VISTA and stimulation of activating receptors like CD27, CD40, immune responses that might result in pathology toward host and 4-1BB are evaluated for their antitumor-inducing immunity. tissues, T cells are also equipped with multiple inhibitory receptors, Recently, the immune inhibitory receptor NKG2A draw the atten- which are expressed in a programmed way (11). As revealed, CTLA- – tion as the new kid on the block of checkpoints (2 5). NKG2A 4 and PD-1 inhibitory receptors are also considered as markers of forms heterodimers with the CD94 chain (6) and recognizes the recent T-cell activation. The spaciotemporal dynamics are different nonclassical HLA class I molecule HLA-E. These recent studies for each inhibitory receptor, resulting in distinct proﬁles of immune demonstrated high expression of NKG2A on natural killer (NK) pathology revealed in knockout mice. The CTLA-4–knockout cells and cytotoxic CD8 T cells in the tumor microenvironment as a mouse displays spontaneous and overt lymphoproliferation, where- result of PD-1 blockade therapy as well as after immune activation as PD-1 and NKG2A knockouts show much milder phenotypes by cancer vaccines (7, 8). First clinical trials in gynecologic cancers (12–15). The cytoplasmic tail of the NKG2A receptor contains two and head and neck carcinoma with the NKG2A-blocking antibody immunoreceptor tyrosine-based inhibition motifs (ITIM) capable monalizumab shows safety and promising clinical responses in of recruiting both SHP-1 (16) and SHP-2 phosphatases, but not the refractory disease, including regressions of target lesions (8, 9). inositol phosphatase SHIP (Fig. 1;refs.17–19). Both ITIMs Interestingly, protein levels of histocompatibility leucocyte antigen are required to mediate the maximal inhibitory signal, but the E (HLA-E), which is the sole ligand of the CD94/NKG2A recep- membrane-distal ITIM is of primary importance rather than the tor (10), are frequently upregulated in many cancers, suggesting membrane-proximal ITIM (20). The partner CD94 has only seven that this axis functions as an acquired resistance mechanism after cytoplasmic amino acids, thus lacks ITIMs and has no role in immune activation in the tumor microenvironment. Here, we downstream signaling. Interestingly, CD94 can also form a hetero- provide an overarching view on the current understanding of dimer with a very close family member of NKG2A (encoded by the NKG2A and HLA-E in cancer and touch upon gaps in our Klrc1 gene), which is named NKG2C (encoded by the Klrc2 gene). knowledge. NKG2C is an activating receptor transducing a stimulating signal via association with DAP12, bearing an immunoreceptor tyrosine- based activation motif (21). The NKG2C protein carries some Department of Medical Oncology, Oncode Institute, Leiden University Medical amino acid differences compared with NKG2A, resulting in a 6-fold Center, Leiden, the Netherlands. lower afﬁnity for the shared ligand HLA-E (22, 23). In NK cells, the Corresponding Authors: Thorbald van Hall, Leiden University Medical Center, two family members are usually not expressed together in the same Albinusdreef 2, Leiden 2333 ZA, the Netherlands. Phone: 3107-1526-6945; Fax: cell and a switch to NKG2C marks more mature NK cells, also 31 71 5264036; E-mail: [email protected]; and Linda Borst, E-mail: “ ” [email protected] referred to as adaptive NK cells (24, 25). Another activating family member, NKG2D (encoded by the Klrk1 gene) is more distantly Clin Cancer Res 2020;26:1–8 related and does not partner with CD94, but rather forms homo- doi: 10.1158/1078-0432.CCR-19-2095 dimers, and is engaged by stress-induced self-proteins, for example, 2020 American Association for Cancer Research. MICA and ULBP members (26).

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Expression and Function of NKG2A in Box 1: The basics of HLA-E Cytotoxic Lymphocytes In contrast to classical MHC class I molecules, HLA-E is Approximately half of peripheral NK cells display the NKG2A high virtually nonpolymorphic with only two functional alleles present receptor and these cells are mostly present in the CD56 fraction, in the human population: the HLA-E01:01 and the HLA-E01:03 which contains the more immature cells. Intratumoral NK cells have variants. These two alleles only differ in a single amino acid at somewhat higher frequencies of NKG2A (7). Interestingly, NKG2A position 107, being arginine (01:01) or glycine (01:03). Position 107 expression on CD8 T cells seems to be highly regulated, as peripheral is located outside the peptide-binding groove of the heavy cells hardly express the receptor, but a substantial fraction of intra- chain (53). The crystal structure of the peptide-binding groove, tumoral T cells, especially those in immune reactive milieu, display single alanine substitutions, and peptide elution, have demonstrat- NKG2A (7, 27). These CD8 T cells often display a late effector memory ed that HLA-E has an optimal structure to bind peptides with two phenotype and lack expression of typical central memory markers primary anchor residues at positions 2 and 9, and secondary anchor CCR7, CD27, and CD28 or late effector marker KLRG1. Interestingly, residues at position 7 and possibly 3 (54–59). Surprisingly, a very we and others found NKG2A expression on CD8 T cells harboring a limited repertoire of peptides was found under homeostatic con- tissue-resident signature, marked by specific integrins like CD103 – ditions and the most prominent are the signal peptides of classical (7, 28 30). Although induction of NKG2A is initiated by TCR MHC class I molecules. The majority of HLA class I alleles share a b triggering, the presence of tissue cytokines like IL15 and TGF might consensus sequence in their signal peptides with the amino acid – b enhance its expression (31 33). TGF is often overtly present in the motif of VMAPRTLLL (Table 1). Positions 7 and 8 vary between tumor microenvironment (34). However, the functional relationship leucine and valine, without affecting binding affinity. Some HLA b between the tissue residence program, NKG2A, and TGF is unclear at alleles, however, contain a substitution at residues p2, 3, or 6, which the moment. impair binding (23). In addition, the immunotolerance molecule Aside from its expression regulation, the NKG2A downstream HLA-G, which is expressed in immune-privileged sites and fre- effects are also not completely unraveled. The inhibitory signals quently in cancers, encodes a unique leader peptide that binds with induced by NKG2A receptor engagement results in decreased capacity fi – high af nity to HLA-E (60, 61). Interestingly, the leader peptide of NK cells and CD8 T cells to lyse target cells (33, 35 39). NKG2A motif is extremely conserved among mammalian species, implying triggering inhibits cytotoxic effector functions on NK cells by disrupt- an important role in immune defense (62) and is even copied by the ing the actin network at the immunologic synapse of activating human cytomegalovirus gpUL40 protein to sustain HLA-E surface receptor NKG2D (40). For CD8 T cells such mode of action is still display on infected cells while shutting down classical HLA class I to be elucidated, although NKG2D or the immunologic synapse of the – fi antigen presentation of host cells (63 65). Importantly, the CD94/ TCR could be lucid candidates (41). In this context, the recent ndings NKG2A receptor exhibits clear preference for peptide-containing on the operational mechanism of PD-1, which was suggested to HLA-E, especially for this consensus leader peptide (56). Under dephosphorylate T-cell costimulatory receptor CD28, illustrate our stress, HLA-E can present a peptide of multidrug resistance– limited understanding at molecular level of these important check- associated protein 7 (ALALVRMLI; ref. 66) and under high cell points (42, 43). CTLA-4 executes its inhibitory effects by competing densities the leader peptide QMRPVSRVL of hSP60 stabilizes with CD28 for the same ligands on antigen presenting cells, thereby HLA-E (67). In cancers, classical HLA class I molecules are limiting T-cell costimulation (44). Such molecular details are lacking frequently lost to prevent T-cell–mediated recognition (68, 69), for NKG2A, let alone our insight on direct downstream target genes but surprisingly, expression of the HLA-E molecule is often affected by NKG2A triggering. enhanced (70–78). Finally, HLA-E surface display is determined by expression levels of classical alleles through the availability HLA-E as a Molecule of Immune of their leader peptide and the binding affinity of this peptide to Tolerance HLA-E, as peptides with a threonine at position 2 poorly bind (see also Table 1; refs. 79, 80). The ligand of the heterodimeric receptor CD94/NKG2A is the human HLA-E and its mouse ortholog Qa-1b. These nonclassical MHC class Ib molecules are surprisingly conserved in the population and present signal peptides of classical MHC class I molecules (Box 1; ref. 45). Interestingly, whereas most tissues express low basal cell surface levels, HLA-E is highly expressed in immune- HLA-E Expression in Cancer privileged sites of the body, including trophoblast cells of the Whereas classical HLA alleles are frequently lost in human cancer to placenta and ductal epithelial cells in the testis and epididymis, prevent T-cell recognition (68, 69), we and others reported high levels suggesting that HLA-E has a role to counter potential attack by CD8 of HLA-E in several cancer types, including gynecologic cancers (up to T cells and NK cells against the partly HLA-mismatched fetus and 90% of tumor samples; refs. 70–72) and up to 50% in breast cancer, haploid reproductive cells (Fig. 2;ref.46).Interestingly,CD94/ non–small cell lung carcinoma (NSCLC), liver, pancreas, kidney, NKG2Aisratherpeptide-specific and preferentially interacts with melanoma, prostate, head and neck, stomach, rectal, and colorectal HLA-E when canonical peptides are bound, but not when these are cancer (73–78). Figure 2 displays some examples of tissue sections of absent (22, 47). These canonical peptides are derived from the human cancers and their healthy counterparts. The surface expression leaders of HLA class I molecules, including the immune-tolerant of HLA-E is correlated with functional antigen-processing compo- molecule HLA-G (48). In addition, the expression of HLA-E or its nents and infiltration of CD8 T cells, however, expression can also be mouse ortholog Qa-1b provides protection of tissue integrity during observed in tumors with downregulated classical HLA class I expres- viral clearance and limits excessive activation and apoptosis of CD8 sion. In a cohort of patients diagnosed with high-grade serous ovarian T cells (15, 49–52). carcinoma, HLA-E expression correlated with a significantly worse

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Table 1. Sequences and origins of HLA-E–binding peptides.

HLA-derived leader peptides HLA locus Allele

Substitution at residues P7 and P8 MAVMAPRTLLLLLSGALALTQTWAa HLA-A A01, A03, A11, A29, A30, A31, A32, A33, A36, A74 HLA-C C02, C15 VMAPRTLVL HLA-A A02, A23, A24, A25, A26, A34, A43, A66, A68, A69 VMAPRTVLL HLA-B B07, B08, B14, B38, B39, B42, B48, B67, B73, B81 VMAPRTLIL HLA-C C01, C03, C04, C05, C06, C08, C12, C14, C16 VMAPRTVFL HLA-G G01 Substitution at residues P2, P3, and P6 VMPPRTLLL HLA-A A80 VTAPRTVLL HLA-B B15, B35, B40, B41, B45, B46, B49, B50, B52, B53, B57, B58 VTAPRTLLL HLA-B B13, B18, B27, B37, B44, B47, B54, B55, B56, B59, B82, B83 VMAPRALLL HLA-C C07, C18 VMAPQALLL HLA-C C17 MVDGTLLLb HLA-E E01 MAPRSLLLb HLA-F F01

Note: IPD-IMGT/HLA Database, http://www.ebi.ac.uk/ipd/imgt/hla/. aConsensus HLA class I-derived signal sequence. bLeader peptides from HLA-E and HLA-F are shorter and are therefore unable to serve as HLA-E binders.

survival (81). A poor relapse-free survival rate was also observed in cervical, and ovarian carcinomas. However, this predictive value is breast cancer, but only when classical class I expression was lost (73). abrogated when tumor display high HLA-E levels, suggesting that CD8 T-cell inﬁltration and retained expression of classical HLA class I HLA-E mediates resistance against CD8 T-cell attack (70, 71, 74). In strongly associate with a better prognosis for patients with NSCLC, primary colorectal cancers, patients with high HLA-E expression

Tumor cell (Polymorphic) (Monomorphic) Classical Nonclassical HLA class I HLA class I molecule molecule HLA-E

HLA class I epitope Signal peptide β α

TCR CD8 NKG2A CD94 CD94 NKG2C (KLRC1) (KLRD1) (KLRD1) (KLRC2) CD3

ITIM DAP12 ITAM ITAM P SHP1 P P ITIM P P P

CD8 T lymphocyte Cytotoxic CD8 T and NK lymphocyte

Figure 1. NKG2 family and their ligands. The polymorphic classical HLA class I molecules present antigenic peptide epitopes to the TCR in complex with CD3 and coreceptor CD8 at the surface of CD8 T cells. In contrast, HLA-E is a monomorphic nonclassical HLA class I molecule that presents a limited set of conserved signal peptides. These peptides are derived from leader sequences of classical HLA class I molecules. NKG2A and NKG2C form heterodimer receptors with CD94 and both target the same p/ HLA-E complex, but ligation induces an inhibitory signal for NKG2A and an activation signal for NKG2C. The NKG2 family of mouse and man also comprises the activating family member NKG2E (not shown here), but its function remains elusive. The more distantly related activating homodimer NKG2D receptor binds MICA/B and ULBP1-6, “empty” molecules that fold like HLA class I heavy chains but do not contain b2-microglobulin (not shown here). The NKG2 family is expressed by both cytotoxic CD8 T cells and NK lymphocytes. Adapted from an image created with BioRender.com.

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Healthy Cancer also signify the negative effect of HLA-E in these cohorts as observed in others. Finally, it should be noted that HLA-E–directed antibodies fail to distinguish between peptide- ﬁlled molecules and empty conformers (see Box 2), complicating the Colon interpretation of these tissue slide results, as especially HLA-E ﬁlled with leader peptides constitute ligands for the CD94/NKG2A receptor (56).

Kidney Box 2: Antibodies against HLA-E Generation of specific antibodies to HLA-E was challenging due Liver to its close homology with classical HLA class I molecules. Three antibodies stand out as most specific: (i) clone 3D12 is frequently used in flow cytometry and was generated against soluble HLA-E with the HLA-A2 leader peptide (83) and is capable to stain the Lung native conformation of the molecule and also binds to b2m-free HLA-E conformations (84). (ii) Clone MEM-E/02 recognizes a linear epitope on HLA-E and is mostly used in formalin-fixed, paraffin-embedded tissue slide staining (84, 85). (iii) Clone TFL- Cervix 033 binds HLA-E in a highly specific manner and is reactive to the a1 and a2 helices of HLA-E, which are the same sequences recognized by the CD94/NKG2A inhibitory receptors (86). A Healthy debate on the exact specificity and potential cross-reactivity with classical HLA molecules of these antibodies is ongoing (83–85, 87, 88), and stirred by recent findings that HLA-E and Epididymis its mouse ortholog Qa-1b poorly associate with b2m and accu- mulates as an b2m-free and peptide-empty form at the cell surface (84, 89–91). As currently available antibodies fail to distinguish between peptide-loaded or open HLA-E conformers and the biological relevance of open conformations remains to be deter- Placenta mined, novel antibodies to peptide-containing forms of HLA-E and preferably even peptide-specific antibodies are urgently required to promote the development of this field. Figure 2. HLA-E expression in healthy and cancer tissue. IHC staining of healthy and cancer tissues for HLA-E, as deposited in the Human Protein Atlas (www. proteinatlas.org). Tissue slides were stained either with MEM-E/02 antibody Induction of HLA-E or rabbit polyclonal HPA031454 (85, 86). Of note, HLA-E antibodies also binds free heavy chains. The fact that HLA-E is generally upregulated in cancers compared with healthy tissue touches upon responsible gene transcription mechanisms. IFNg is proposed as an important cytokine and is locally showed a significantly decreased disease-free survival for Dukes C produced by tumor-infiltrating T cells and NK cells. At the transcrip- patients (76). In another retrospective cohort of 234 colorectal patients, tion level, this is substantiated through binding of a STAT1-containing the expression of HLA-E, b2m, CD94, CD8, and NKp46 was examined complex to distinct IFNg-responsive region (IRR) present upstream of by IHC on tissue microarray. HLA-E/b2m was overexpressed in the HLA-E gene (92). In addition, the upstream UIRR region mediates microsatellite instable tumors and to a lesser extend in microsatellite a 3- to 8-fold increase in HLA-E transcription in response to IFNg stable ones (45% vs. 19%, respectively) and corresponded with worse upon binding of GATA-1 (93). In vitro experiments indeed showed survival (78). Unfortunately, NKG2A expression cannot be quantified that IFNg significantly increases the surface expression of HLA-E and in paraffin-embedded tissues because of lack of specific antibody. In an the shedding of soluble HLA-E by melanoma cells, in a metallopro- independent study of colon cancers, the researchers concluded that the teinase-dependent fashion (94). This also corresponds to studies total absence of HLA class I, including HLA-E and HLA-G, related to a showing enhanced HLA-E expression on tumors upon increased better overall and disease-free survival (77). This was attributed to NK- cytotoxic lymphocyte infiltration (71) and enhanced Qa-1b expression cell–mediated killing in the portal vein of MHC class I–negative cancer in mice in response to immune therapy leading to increased T-cell cells, which give rise to liver metastases. infiltration (7). Nevertheless, there are also tumors and tumor cell lines It should be stated that there are also several studies of patient that constitutively upregulate HLA-E molecules, soluble and at the cell cohorts in which the researchers did not observe a relevant corre- surface, irrespective of IFNg treatment (94). Importantly, HLA-E lation with HLA-E expression (75, 82). However, as previously surface expression is posttranslationally regulated by availability of demonstrated, the role of HLA-E expression seems to be dependent the conserved leader peptides, the peptide transporter TAP, and on immune contexture, like lymphocyte infiltration and presence of proteolytic enzymes. Moreover, HLA-E can present alternative classical HLA class I molecules. Stratification on these factors might peptides aside from the HLA class I leaders and can be recognized

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by ab TCRs (95, 96). Clearly, novel research tools are required to of other forms of immunotherapy, like PD-L1 blockade or cancer distinguish HLA-E complexed with the monomorphic leader peptides vaccines (7, 8, 101). Cancer vaccines induced strong tumor-directed from those complexed with alternative peptides, which might actually T-cell responses and infiltration of CD8 T cells in the tumors, be present in tumors (58). leading to local IFNg release and increase of the mouse HLA-E þ ortholog (7). In addition, frequencies of NKG2A CD8 T cells were increased in tumor-infiltrating lymphocyte (TIL). The association – þ Targeting the NKG2A HLA-E Axis of immune reactivity and NKG2A T-cell frequency was corrob- Similarities between the PD1–PD-L1 axis and the NKG2A– orated in human TIL of oropharyngeal carcinomas (7). Together, HLA-E axis are obvious, as they both involve lymphocytes and these observations designated a role for NKG2A as an acquired represent inhibitory immune receptors, and their ligands are resistance mechanism (102). This concept is further substantiated expressed on cancer cells and inducible by the proinflammatory by preclinical in vitro data demonstrating a requirement of immune cytokine IFNg. Of note, HLA-E transcripts in The Cancer activation signals to reveal beneficial effects of NKG2A block- Genome Atlas (TCGA) database exceed those of PD-L1, suggest- ade (8, 27, 38). Design of clinical trials should therefore be based ing a high and general overexpression (Fig. 3). These two pairs on combination therapy leading to inflammatory responses in reflect feedback signals to dampen overt T-cell–mediated tissue patients with cancer. Induced expression of HLA-E or enhanced þ damage at affected lesion sites, which might result in resistance and frequencies of NKG2A immune cells in the tumor might serve as immune escape of cancer (97, 98). In contrast to PD-1, NKG2A is predictive biomarkers. selectively expressed on lymphocytes with cytolytic function, including NK cells, NKT cells, and a subset of CD8 T cells. Thereby, the NKG2A–HLA-E axis is suggested to predominantly act at the NKG2A in Clinical Trials terminal tumor–attack stage and not to be involved in priming or Monalizumab (IPH2201, Innate Pharma/AstraZeneca) is a human- regulation of immune responses. Interruption of this axis by ized IgG4 antibody blocking the interaction of human NKG2A with blocking antibodies can be envisaged at both sides. However, HLA-E and showed a therapeutic effect in immunodeficient mice NKG2A is the preferred target instead of HLA-E, because blockade harboring human leukemia (103). The NKG2A-blocking antibody of HLA-E would also prevent interaction with the activating monalizumab was furthermore administered in a dose ranging phase II NKG2C receptor. A NKG2C-expressing subset of NK cells, also clinical trial in patients with gynecologic malignancies (9). Adminis- called “adaptive NK cells,” displays an altered receptor profile and is tration of 10 mg/kg i.v. every 2 weeks was well-tolerated and even associated with chronic viruses, like cytomegalovirus (CMV; short-term disease stabilizations were observed (9). In another recent refs. 99, 100). Its role in immunity to CMV remains elusive, but clinical trial, monalizumab was combined with the EGFR-blocking anti-HLA-E antibodies would also block activation of such NK cells antibody cetuximab in previously treated squamous cell carcinoma of through NKG2C. the head and neck. This combination showed a promising 31% Disappointingly, mouse cancer models demonstrated limited objective response rate (8). Cetuximab is able to activate NK cells via success of NKG2A blockade therapy when provided as a standalone Fcg receptors, leading to antibody-mediated cellular cytotoxicity. blocking antibody, however, it greatly improved antitumor efficacy Addition of monalizumab enhanced NK-cell effector functions (8). Currently, several trials in which monalizumab is tested are enrolling patients. Different combinations with anti-EGFR, anti-PD-L1, tyrosine kinase inhibitors, and chemotherapy are tested in several 2,500 cancer indications, including patients with resectable NSCLC HLA-E (NCT03794544), patients with PD-1 therapy-resistant NSCLC 2,000 PD-L1 (NCT03833440), and patients with advanced nonresectable stage III NSCLC (NCT03822351), as well as patients with advanced squamous cell carcinoma of the head and neck (NCT02643550), refractory 1,500 chronic lymphocytic leukemia (NCT02557516), or other hematologic malignancies after stem cell transplantation (NCT02921685), and also FPKM 1,000 a basket trial with advanced solid malignancies (NCT02671435). This latter trial reported a manageable safety profile and durable partial 500 responses in patients with microsatellite stable (MSS) colorectal carcinoma treated with combinations of FOLFOX chemotherapy and blocking antibodies to VEGF, PD-L1, and NKG2A (104). On the basis 0 of these promising results, a larger phase II trial will start in the near Liver Lung Renal Breast future for patients with high-risk MSS colorectal carcinoma who Cervical Ovarian Colorectal received radical surgery, testing different adjuvant therapies based on Head and neck standard-of-care chemotherapy FOLFOX in combination with durvalumab (anti-PD-L1) and monalizumab (anti-NKG2A; Columbia 2; NCT04145193). Figure 3. HLA-E and PD-L1 transcription levels in various cancer types. RNA-sequencing data for HLA-E and PD-L1 is reported as median FPKM (number fragments per Conclusions kilobase of exon per million reads), generated by TCGA. Normal distribution across the dataset is visualized with box plots, shown as median and 25th and The current challenge of checkpoint blockade therapy is to 75th percentiles. Points are outliers when they are above the 97.5 or below the increase the fraction of responding patients and, second, to over- 2.5 percentile. come acquired resistance. NKG2Ablockadedisplaysunique

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characteristics compared with other immune checkpoints, as this Disclosure of Potential Conflicts of Interest target is selectively expressed within tumor lesions on cytotoxic S.H. van der Burg reports receiving other commercial research support from lymphocytes and is not involved in priming or regulation of Innate Pharma. T. van Hall reports receiving commercial research grants from þ fl immunity against cancer. NKG2A T cells are mostly residing in Innate Pharma. No potential con icts of interest were disclosed by the other author. the tumor microenvironment where they are effector cells attacking the transformed cells. Moreover, we and others showed increased Acknowledgments frequencies of these cytotoxic lymphocytes upon administration of This study was financially supported by the Dutch Cancer Society (2014-7146 therapeutic cancer vaccines. These observations plea for combina- to T. van Hall and S.H. van der Burg) and by a research grant from Innate Pharma tion therapies of the NKG2A-blocking antibody monalizumab with (to T. van Hall and S.H. van der Burg). such vaccines. In the meanwhile, fundamental investigation on NKG2A and its ligand HLA-E should provide mode-of-action of Received January 27, 2020; revised March 31, 2020; accepted May 12, 2020; this novel immune checkpoint. published first May 14, 2020.

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OF8 Clin Cancer Res; 26(21) November 1, 2020 CLINICAL CANCER RESEARCH

The NKG2A−HLA-E Axis as a Novel Checkpoint in the Tumor Microenvironment

Linda Borst, Sjoerd H. van der Burg and Thorbald van Hall

Clin Cancer Res Published OnlineFirst May 14, 2020.

Updated version Access the most recent version of this article at: doi:10.1158/1078-0432.CCR-19-2095

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