KIR3DL3 Is an Inhibitory Receptor for HHLA2 that Mediates an Alternative Immunoinhibitory Pathway to PD1

Blockade of the PD1 pathway is a broadly effective cancer therapy, but additional immune-inhibitory pathways contribute to tumor immune evasion. HERV-H LTR-associating 2 (HHLA2; also known as B7H5 and B7H7) is a member of the B7 family of immunoregulatory ligands that mediates costimulatory effects through its interaction with the CD28 family member transmembrane and immunoglobulin domain containing 2 (TMIGD2). However, HHLA2 has also been known to have inhibitory effects on T cells. Here, we report that we have identified killer cell immunoglobulin-like receptor, three immunoglobulin domains and long cytoplasmic tail 3 (KIR3DL3) as an inhibitory receptor for HHLA2 in T cells and natural killer (NK) cells and have generated HHLA2 and KIR3DL3 antibodies that block the immune-inhibitory activity of HHLA2, preserving the costimulatory signal. It is known that HHLA2 is frequently expressed in several tumor types, including clear cell renal cell carcinoma (ccRCC). We found that HHLA2 expression was nonoverlapping with PDL1 expression in ccRCC, suggesting that HHLA2 mediates a mechanism of tumor immune evasion that is independent from PDL1. Blockade of both the PD1 and KIR3DL3 pathways may be a more effective way to reverse tumor immune evasion.See related Spotlight on p. 128.

Abstract 926: Binding of the novel inhibitory receptor KIR-X to its ligand HHLA2 negatively regulates T cell activation

Cancer immunotherapies blocking immuno-inhibitory pathways have become a successful treatment method in the last decade. HHLA2 (human endogenous retrovirus-H long terminal repeat-associating protein 2) is a B7 family member and ligand that is often expressed on cancer cells and antigen presenting cells. HHLA2 has the ability to contribute to both T cell stimulation and inhibition. Previous results have shown that HHLA2 binding to TMIGD2 induces T cell activation, however, the inhibitory receptor was unknown. We identified KIR-X as an additional receptor for HHLA2. In order to test the functional consequence of HHLA2 binding to KIR-X, we set up a KIR-X receptor transfected Jurkat T cell model. Jurkat cells transfected with KIR-X or TMIGD2 and luciferase under the control of an NFAT-response element or IL-2 promoter were stimulated with CHO cells expressing scFv anti-human CD3 +/- HHLA2. The bioluminescence produced by the firefly luciferase reporter reflects NFAT or IL2 transcriptional activation. We confirmed that HHLA2 binding to TMIGD2 stimulates T cells in this Jurkat T cell model as we saw an increase in NFAT mediated transcription of luciferase. The goal of this project was to first test the immunological effect of HHLA2 binding to KIR-X on T cells, and second, test a panel of candidate therapeutic antibodies directed against KIR-X or HHLA2 for their ability to reverse effects observed in Jurkat cells. We found that HHLA2 binding to KIR-X inhibited a T cell antigen receptor/CD28 signal. Next, the antibodies were tested for their capacity to reverse HHLA2 mediated effects on T cell activation. I tested HHLA2 antibodies which block binding to KIR-X, TMIGD2 or both as well as KIR-X antibodies that block binding to HHLA2. I also tested antibodies known to not block binding to ligand. My results show that KIR-X antibodies 2F11 and 1G7 and the HHLA2 antibodies 2G2, 1C8, 6F10, and 2C4 enhanced activation of KIR-X Jurkat cells, consistent with the blockade of a KIR-X mediated inhibitory signal. The HHLA2 antibody 6F10 also reduced TMIGD2 Jurkat T cell activation while the 2C4 antibody did not, consistent with their ability or inability to block TMIGD2 binding, respectively. We found that antibodies that did not block the KIR-X/HHLA2 interaction did not have a functional consequence in the Jurkat T cell luciferase assay. Therefore, these assays showed that there was good correlation between blockade of receptor/ligand interactions and effects on T cell activation and that the binding sites on HHLA2 for KIR-X and TMIGD2 were close but distinguishable. We believe that blocking the KIR-X inhibitory receptor will be valuable for increasing T cell activation in cancer, while blocking TMIGD2 has potential to alleviate autoimmune disease.This work was supported by the Dana-Farber/Harvard Cancer Center Kidney Cancer SPORE P50CA101942, (GJF and KMM), Department of Defense Early Investigator Idea Development Award W81XWH1810500 (KMM), Kidney Cancer Association Advanced Discovery Award (KMM, GJF), and P01AI056299 (GJF).

RSB was supported by NIH R01 CA196996.

Citation Format: Alyssa N. Klee, Kathleen M. Mahoney, Antonio R. Arulanandam, Rupal Bhatt, Baogong Zhu, Gordon J. Freeman. Binding of the novel inhibitory receptor KIR-X to its ligand HHLA2 negatively regulates T cell activation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 926.

Current strategies in the non-clinical safety assessment of biologics: New targets, new molecules, new challenges

Nonclinical safety testing of biopharmaceuticals can present significant challenges to human risk assessment with these innovative and often complex drugs. Emerging topics in this field were discussed recently at the 2016 Annual US BioSafe General Membership meeting. The presentations and subsequent discussions from the main sessions are summarized. The topics covered included: (i) specialty biologics (oncolytic virus, gene therapy, and gene editing-based technologies), (ii) the value of non-human primates (NHPs) for safety assessment, (iii) challenges in the safety assessment of immuno-oncology drugs (T cell-dependent bispecifics, checkpoint inhibitors, and costimulatory agonists), (iv) emerging therapeutic approaches and modalities focused on microbiome, oligonucleotide, messenger ribonucleic acid (mRNA) therapeutics, (v) first in human (FIH) dose selection and the minimum anticipated biological effect level (MABEL), (vi) an update on current regulatory guidelines, International Council for Harmonization (ICH) S1, S3a, S5, S9 and S11 and (vii) breakout sessions that focused on bioanalytical and PK/PD challenges with bispecific antibodies, cytokine release in nonclinical studies, determining adversity and NOAEL for biologics, the value of second species for toxicology assessment and what to do if there is no relevant toxicology species.

The neutralization of type I IFN biologic actions by anti-IFNAR-2 monoclonal antibodies is not entirely due to inhibition of Jak-Stat tyrosine phosphorylation

A panel of monoclonal antibodies (mAb) derived against human interferon-alpha/beta receptor-2 (IFNAR-2) was evaluated for their ability to antagonize the biologic effects of type 1 interferons (IFN-alpha1, IFN-alpha2a, and IFN-beta). Anti-IFNAR-2 mAb 117.7, 35.9, 53.2, and 51.44 neutralized type I IFN-mediated antiviral, antiproliferative, and major histocompatibility complex (MHC) class I upregulation functions. However, only mAb 51.44 neutralized IFN-alpha2a and IFN-beta-mediated natural killer (NK) cell cytotoxicity. In BIAcore and cell binding studies, only mAb 51.44 and 234.28 inhibited IFN-alpha2a and IFN-beta binding to its receptor. The receptor blockade by mAb 51.44 and 234.28 resulted in the inhibition of IFN-alpha2a and IFN-beta-induced tyrosine phosphorylation of Jak1, Tyk2, Stat1/2/3, and IFNAR-1/2 and inhibition of IFN-stimulated gene factor 3 (ISGF3) formation. mAb 117.7, 35.9, and 53.2, although antagonists of IFN's biologic activities, did not block the binding of IFN-alpha/beta to its receptor. The 117.7 mAb, representative of this class of receptor nonblocking mAb, induced hyper-tyrosine phosphorylation of IFNAR-2 in the presence of IFN-alpha/beta but did not inhibit IFN-alpha/beta-induced Jak-Stat tyrosine phosphorylation and ISGF3 complex formation. These results show that the neutralization of type I IFN biologic actions by anti-IFNAR-2 mAb cannot be entirely explained by inhibition of Jak-Stat tyrosine phosphorylation.

Conformation and function of the N-linked glycan in the adhesion domain of human CD2

The adhesion domain of human CD2 bears a single N-linked carbohydrate. The solution structure of a fragment of CD2 containing the covalently bound high-mannose N-glycan [-(N-acetylglucosamine)2-(mannose)5-8] was solved by nuclear magnetic resonance. The stem and two of three branches of the carbohydrate structure are well defined and the mobility of proximal glycan residues is restricted. Mutagenesis of all residues in the vicinity of the glycan suggests that the glycan is not a component of the CD2-CD58 interface; rather, the carbohydrate stabilizes the protein fold by counterbalancing an unfavorable clustering of five positive charges centered about lysine-61 of CD2.

Interaction between human CD2 and CD58 involves the major beta sheet surface of each of their respective adhesion domains

The CD58 binding site on human CD2 was recently shown by nuclear magnetic resonance structural data in conjunction with site-directed mutagenesis to be a highly charged surface area covering approximately 770A2 on the major AGFCC'C" face of the CD2 immunoglobulin-like (Ig-like) NH2-terminal domain. Here we have identified the other binding surface of the CD2-CD58 adhesion pair by mutating charged residues shared among CD2 ligands (human CD58, sheep CD58, and human CD48) that are predicted to be solvent exposed on a molecular model of the Ig-like adhesion domain of human CD58. This site includes beta strand residues along the C strand (E25, K29, and K30), in the middle of the C' strand (E37) and in the G strand (K87). In addition, several residues on the CC' loop (K32, D33, and K34) form this site. Thus, the interaction between CD2 and CD58 involves the major beta sheet surface of each adhesion domain. Possible docking orientations for the CD2-CD58 molecular complex are offered. Strict conservation of human and sheep CD58 residues within the involved C and C' strands and CC' loop suggests that this region is particularly important for stable formation of the CD2-CD58 complex. The analysis of this complex offers molecular insight into the nature of a receptor-ligand pair involving two Ig family members.

The CD58 (LFA-3) binding site is a localized and highly charged surface area on the AGFCC'C" face of the human CD2 adhesion domain

Using site-directed mutagenesis in conjunction with NMR structural data on the adhesion domain of human CD2, we have defined the binding region for CD58. Previous structural studies of rat and human CD2 indicate that this adhesion domain is immunoglobulin-like. Here we report that the CD58 binding site is a well-circumscribed, charged surface area covering approximately 770 A2 on the AGFCC'C" face of the CD2 beta barrel. This site contains beta-strand residues in the carboxyl-terminal half of the F strand (including Lys-82 and Tyr-86), the top of the C strand (Asp-32 and Lys-34), and the C' strand (Gln-46), which are all solvent exposed. In addition, several exposed residues on the FG loop (Gly-90, Lys-91, Asn-92, and Val-93), the CC' loop (Lys-41 and Lys-43), and the C'C" loop (Arg-48 and Lys-51) form this site. In contrast, neither residues on the more peripheral G and C" strands of the same CD2 surface nor residues on B, E, and D strands of the opposite face are involved in CD58 binding. This CD58 binding site is predicted to lie most distal to the T-lymphocyte surface membrane, with ready access to CD58 on the surface of the opposing antigen-presenting cell.

Structure of the glycosylated adhesion domain of human T lymphocyte glycoprotein CD2

BACKGROUND

CD2, a T-cell specific surface glycoprotein, is critically important for mediating adherence of T cells to antigen-presenting cells or target cells. Domain 1 of human CD2 is responsible for cell adhesion, binding to CD58 (LFA-3) expressed on the cell to which the T cell binds. Human CD2 domain 1 requires N-linked carbohydrate to maintain its native conformation and ability to bind CD58. In contrast, rat CD2 does not require N-linked carbohydrate, and binds to a different ligand, CD48.

RESULTS

The three-dimensional structure of the glycosylated form of domain 1 of human CD2 has been determined by NMR spectroscopy. The overall structure resembles the typical beta-barrel of an immunoglobulin variable domain. Nuclear Overhauser enhancement contacts between the protein and the N-linked glycan have been tentatively identified.

CONCLUSION

Based on our results, we propose a model showing how the N-linked glycan might be positioned in the human CD2 domain 1 structure. The model provides an explanation for the observed instability of deglycosylated human CD2, and allows residues that are important for CD58 binding to be differentiated from those affecting conformational stability via interactions with the glycan.

A soluble multimeric recombinant CD2 protein identifies CD48 as a low affinity ligand for human CD2: divergence of CD2 ligands during the evolution of humans and mice

To search for possible ligands of CD2 distinct from CD58 (lymphocyte function-associated antigen 3), we have produced a soluble pentameric CD2-immunoglobulin (Ig) fusion protein (spCD2) linking the 182-amino acid human CD2 extracellular segment with CH2-CH3-CH4 domains of human IgM heavy chain, thus enhancing the micromolar affinity of the CD2 monomer through multimeric interaction. Using quantitative immunofluorescence and standard stringency wash conditions, we observed that the binding of spCD2 to human B lymphoblastoid JY cells and red blood cells is virtually inhibited by anti-CD58 TS2/9 monoclonal antibody, even though these cells express levels of CD48 and CD59 comparable to CD58. Consistent with these results, spCD2 did not show any binding to Chinese hamster ovary (CHO) cells transfected with human CD48 or CD59. However, binding studies on CD48-, CD58-, or CD59-transfected CHO cells with spCD2 under low stringency wash conditions revealed that human CD48 is a low affinity ligand of human CD2 compared with CD58 (Kd approximately 10(-4) vs. approximately 10(-6) M, respectively). The findings are noteworthy given that in the murine system CD48 is the major ligand for CD2. No detectable binding was observed to CD59-transfected CHO cells despite a report suggesting that CD59 may bind to the human CD2 adhesion domain. Importantly, in cell-cell adhesion assays between CD2+ Jurkat T cells and CD48- or CD59-transfected CHO cells, there was no conjugate formation, whereas binding of Jurkat T cells to CD58-transfected CHO cells was readily detected. Collectively, our findings provide evidence for a conservation of the CD2-CD48 interaction in the human species that may be of limited, if any, functional significance. Given the importance of the CD2-CD48 interaction in the murine system and CD2-CD58 interaction in humans, it would appear that there has been a divergence of functional CD2 ligands during the evolution of humans and mice.

T cell receptor complexes containing Fc epsilon RI gamma homodimers in lieu of CD3 zeta and CD3 eta components: a novel isoform expressed on large granular lymphocytes

CD3 zeta and CD3 eta form disulfide-linked homo- or heterodimers important in targeting partially assembled Ti alpha-beta/CD3 gamma delta epsilon T cell receptor (TCR) complexes to the cell surface and transducing stimulatory signals after antigen recognition. Here we identify a new TCR isoform expressed on splenic CD2+, CD3/Ti alpha-beta+, CD4-, CD8-, CD16+, NK1.1+ mouse large granular lymphocytes (LGL), which are devoid of CD3 zeta and CD3 eta proteins. The TCRs of this subset contain homodimers of the gamma subunit of the high affinity receptor for IgE (Fc epsilon RI gamma) in lieu of CD3 zeta and/or CD3 eta proteins. The LGL display natural killer-like activity and are cytotoxic for B cell hybridomas producing anti-CD3 epsilon and anti-CD16 monoclonal antibodies, demonstrating the signaling capacity of both TCR and CD16 in this cell type. These findings provide evidence for an additional level of complexity of TCR signal transduction isoforms in naturally occurring T cell subsets.

The high affinity Fc epsilon receptor gamma subunit (Fc epsilon RI gamma) facilitates T cell receptor expression and antigen/major histocompatibility complex-driven signaling in the absence of CD3 zeta and CD3 eta

The T cell receptor (TCR) is a molecular complex formed by at least seven transmembrane proteins: the antigen/major histocompatibility complex recognition unit (Ti alpha-beta heterodimer) and the invariant CD3 chains (gamma, delta, epsilon, zeta, and eta). In addition to targeting partially assembled Ti alpha-beta CD3 gamma delta epsilon TCR complexes to the cell surface, CD3 zeta appears to be essential for interleukin-2 production after TCR stimulation with antigen/major histocompatibility complex. The gamma chain of the high affinity Fc receptor for IgE (Fc epsilon RI gamma) has significant structural homology to CD3 zeta and the related CD3 eta subunit. To identify the functional significance of sequence homologies between CD3 zeta and Fc epsilon RI gamma in T cells, we have transfected a Fc epsilon RI gamma cDNA into a T cell hybridoma lacking CD3 zeta and CD3 eta proteins. Herein we show that a Fc epsilon RI gamma-gamma homodimer associates with TCR components to up-regulate TCR surface expression. A TCR composed of Ti alpha-beta CD3 gamma delta epsilon Fc epsilon RI gamma-gamma is sufficient to restore the coupling of TCR antigen recognition to the interleukin-2 induction pathway, demonstrating the functional significance of structural homology between the above receptor subunits. These results, in conjunction with the recent finding that CD3 zeta, CD3 eta, and Fc epsilon RI gamma are coexpressed in certain T cells as subunits of an unusual TCR isoform, suggest that Fc epsilon RI gamma is likely to play a role in T cell lineage function.

T cell receptor-independent CD2 signal transduction in FcR+ cells

CD2 subserves both adhesion and signal transduction functions in T cells, thymocytes, and natural killer (NK) cells. In mature T lymphocytes, CD2-mediated signaling function apparently requires surface expression of T cell receptors (TCRs). In contrast, in CD2+ CD3- NK cells and thymocytes, signal transduction through CD2 is TCR independent. To resolve this paradox and characterize TCR-independent triggering mechanisms, we transfected a human CD2 cDNA into a murine mast cell line, C1.MC/57 (Fc epsilon RI+, Fc gamma RII+, Fc gamma RIII+), which is known to produce interleukin 6 (IL-6) as well as release histamine in response to crosslinking of Fc epsilon RI. In the CD2 transfectant, a combination of anti-T11(2) + anti-T11(3) monoclonal antibodies (mAbs) induced a rise in intracellular free calcium [( Ca2+]i), IL-6 production, and histamine release. As expected, no activation was mediated by the same mAbs in C1.MC/57. F(ab)'s fragments of the activatory combination of anti-T11(2) + anti-T11(3) mAbs induced IL-6 in the CD2-transfected mast cells, demonstrating an Fc gamma receptor ectodomain-independent triggering mechanism. In addition, either intact anti-T11(2) or anti-T11(3) IgG alone, which failed to induce [Ca2+]i mobilization in the transfectant, was able to induce IL-6 production. A mAb directed against both Fc gamma RII (previously denoted as Fc gamma RIIb) and Fc gamma RIII (previously denoted as Fc gamma RIIa) inhibits this induction. These results indicate that: (a) Ca2+ mobilization is not essential for IL-6 production; and (b) crosslinking of CD2 and Fc gamma receptors via intact anti-CD2 IgG stimulates IL-6 production. Thus, CD2-mediated IL-6 production occurs by both Fc receptor ectodomain-independent as well as Fc receptor ectodomain-dependent mechanisms in these nonlymphoid cells. Northern blot analysis demonstrates that although the mast cells do not express CD3 zeta or CD3 eta mRNA, they express Fc epsilon RI gamma mRNA. The latter is a known component of Fc gamma RIII as well as Fc epsilon RI, has significant homology to CD3 zeta/eta, and is thought to have a signal transduction function. In these mast cells, CD2 signaling machinery does not require CD3 zeta/eta and may be linked to the Fc epsilon RI gamma subunit. We predict that this subunit or a related structure may confer a TCR-independent signal transduction pathway upon CD2 in CD3- NK cells, thymocytes, and certain B lymphocytes.