A reengineered common chain cytokine augments CD8+ T cell–dependent immunotherapy

Cytokine therapy is limited by undesirable off-target side effects as well as terminal differentiation and exhaustion of chronically stimulated T cells. Here, we describe the signaling properties of a potentially unique cytokine by design, where T cell surface binding and signaling are separated between 2 different families of receptors. This fusion protein cytokine, called OMCPmutIL-2, bound with high affinity to the cytotoxic lymphocyte-defining immunoreceptor NKG2D but signaled through the common γ chain cytokine receptor. In addition to precise activation of cytotoxic T cells due to redirected binding, OMCPmutIL-2 resulted in superior activation of both human and murine CD8⁺ T cells by improving their survival and memory cell generation and decreasing exhaustion. This functional improvement was the direct result of altered signal transduction based on the reorganization of surface membrane lipid rafts that led to Janus kinase-3–mediated phosphorylation of the T cell receptor rather than STAT/AKT signaling intermediates. This potentially novel signaling pathway increased CD8⁺ T cell response to low-affinity antigens, activated nuclear factor of activated T cells transcription factors, and promoted mitochondrial biogenesis. OMCPmutIL-2 thus outperformed other common γ chain cytokines as a catalyst for in vitro CD8⁺ T cell expansion and in vivo CD8⁺ T cell–based immunotherapy.

Chimeric Cytokine Targeting NKG2D and IL-2 Receptor Augments CD8+ T Cell-Metabolic Fitness and Improves Immunotherapy

T cell mediated immunotherapy has gained credible traction for cancer treatment. To date experimental and clinical strategies have relied on antibody-mediated co-stimulatory blockade, or the administration of high doses of naturally occurring or engineered cytokines to increase T cell activation. Cytokine therapy is by limited undesirable off-target side effects as well as terminal differentiation and exhaustion of chronically stimulated T cells. Here we describe functionality of a unique chimeric cytokine by design, called OMCPmutIL-2, binds with high affinity to the cytotoxic lymphocyte-defining immunoreceptor NKG2D but signals through the common γ-chain cytokine receptor, thus eliminating IL-2Ra mediated off-target side effects. In addition to precise activation of cytotoxic T cells due to redirected binding we describe that OMCPmutIL-2 results in superior activation of both human and murine CD8+ T cells by improving their survival, decreasing exhaustion, and improving memory cell generation. This functional improvement is the result altered signal transduction based on the reorganization of surface membrane lipid rafts resulting in JAK-mediated phosphorylation of the T cell receptor (TCR) rather than canonical STAT/AKT signaling intermediates activated by other common γ-chain cytokines. This altered signaling pathway increases CD8+ T cell response to low affinity antigens, activates Nuclear Factor of Activated T Cells (NFAT) transcription factors, and enhances mitochondrial biogenesis with improved T cell survival, cytotoxicity, and memory cell generation. OMCPmutIL-2 thus outperforms other common γ-chain cytokines as both a reagent for T cell expansion and in vivo CD8+ T cell-based immunotherapy.

Supported by grant  I01 BX002299

Dissecting molecular mechanisms underlying T cell co-potentiation when targeting the TCR/CD3 complex with anti-CD3 Fab fragments

A cognate antigen engagement of TCR induces a conformational change in the CD3 complex (CD3Δc) that is required for productive T cell activation. CD3Δc fails when TCRs interact with poorly immunogenic antigens that do not active T cells. Anti-CD3 monovalent Fab fragments bound to the CD3 complex mimic CD3Δc, producing T cell “co-potentiation” when associated with TCRs bound to weak antigens that enhances T cell activation, neither stimulating non-engaged T cells, nor interrupting T cell responses to strong antigens. Previously, we showed T cell co-potentiation can be exploited as a novel immunotherapeutic principle by reducing melanoma burden in B6 mice treated with anti-CD3 Fabs. TCR signaling initiation requires CD3Δc for the exposure of a conserved sequence in the cytosolic domain of CD3ɛ rich in prolines (PRS) that enables the recruitment of adaptor protein Nck. The mutation of cysteines in the CXXC motif at the extracellular domain of CD3ɛ inhibits the conformational change across the plasma membrane that results on PRS accessibility. Here, using two strains of B6 mice that express mutations in either the CXXC or the PRS motifs in CD3ɛ, we study the potential contribution of each motif in T cell co-potentiation. Our results from these studies favor the involvement of the CXXC and PRS motifs in the molecular mechanism supporting the co-potentiation of T cell activation by anti-CD3 Fab fragments.

Supported by MU Start-up funds and NIH grants: NCI, U01 CA244314; NIAID, R01 AI097187

Anti-CD3 Bi-Fab crosslinking of T cells enables their fratricide by activation induced cell death (AICD) and cytotoxicity

We showed in the past anti-CD3 Fabs can form stable bivalent non-covalent dimers we called Bi-Fabs, which appear stimulatory to T cells according to the up-regulation of the early activation marker CD69. Now we show that incubation of naïve T cells with anti-CD3 Bi-Fabs in the presence of proper co-stimulation results in their robust stimulation in comparison to mAb and F(ab’)2 species with same specificity. Nonetheless, anti-CD3 Bi-Fab treatment leads to abortive T cell proliferation with impaired accumulation of divided T cells. After detailed studies to dissect the mechanisms of T cell death enabled by Bi-Fab molecules in comparison with anti-CD3 mAb and F(ab′)2 recognizing the same epitope, we observed that anti-CD3 Bi-Fabs promote T cell fratricidal killing via activation induced cell death (AICD) and CD8 cytolytic activity (CTL) more efficiently than the mAb and F(ab′)2 counterparts. While low resolution structural studies indicate that Bi-Fab and F(ab’)2 adopt similar shapes, molecular dynamic simulation shows that F(ab’)2 is a highly flexible molecule, whereas Bi-Fabs are more rigid and of linear conformation, sampling a smaller range of Fab-Fab angles than F(ab’)2. We hypothesize anti-CD3 Bi-Fab is superior as T cell killer stimulatory modality over mAb and F(ab’)2 of equal specificity due to Bi-Fab’s greater rigidity when crosslinking in trans TCR/CD3 molecules located in neighboring T cells. These findings may be applicable into the design of Ig-based immunotherapeutic drugs targeting the TCR/CD3 complex.

Supported by MU start-up funds

Abstract B78: Co-potentiation of human T cells to identify subdominant tumor neoantigens from melanoma patients responding to immune checkpoint blockade

Malignant melanoma is the most aggressive form of skin cancer, accounting for 10,000 deaths annually. Largely resistant to conventional cytotoxic chemotherapy and radiation, immune checkpoint blockade (ICB) therapies have revolutionized patient care, accounting for partial or complete responses in up to 70% of patients. An important aspect of cancer elimination is activation of the cytotoxic T lymphocyte (CTL) response. Recent data suggest that ICB mediated broadening of the peripheral blood CTL repertoire correlates with good clinical outcomes. Thus, recent studies in cancer immunotherapy have focused on targeting tumor neoantigens (tNeoAg) with cancer vaccines for use in combination with ICB. Computational algorithm-driven predictions of immunogenic tNeoAgs yield vast numbers of potential peptide targets, only a fraction of which may be immunogenic in patients. This likely contributed to the modest success of cancer vaccines targeting predicted, high-avidity tNeoAgs, emphasizing the importance of identifying true tumor rejection antigens, including sub-dominant antigens, outside the scope of predictive models. In addition to promoting T cell-mediated tumor rejection, ICB often comes at the expense of treatment-induced immune-related adverse effects (irAE) that frequently require discontinuation of treatment. Modulation of ICB towards antitumor immunity and away from autoimmunity may dramatically improve the therapeutic index of modern cancer therapy. This appears feasible considering the clinical observation that antitumor efficacy does not correlate with type/severity of irAE, suggesting that the mechanisms of both processes, though likely related (CTL mediated), may not be identical. We aim to identify and separate non-cross-reacting antigenic targets mediating tumor rejection from those mediating irAEs to enable therapeutic interventions that maximize the efficacy of ICB, expanding tumor-specific CTLs with vaccines while minimizing irAEs through desensitization. We have access to samples from responding/nonresponding melanoma patients subjected to ICB with varying degrees of irAEs. We have designed an experimental approach that combines established mass spectrometry and sequencing techniques to identify peptides and matching TCR clones with a novel strategy that targets the TCR-associated CD3 complex to allow inclusion of subdominant antigens in our studies. T-cell “co-potentiation” is achieved when anti-CD3 monovalent Fabs induce a conformational change in the CD3 complex that sustains the T-cell response to weak antigenic stimulation. We have successfully used anti-human CD3 Fabs to co-potentiate in vitro the activation and subsequent response to weak peptide-HLA/TCR interactions of human T cells found in PBMCs isolated from healthy donors. Our preliminary data suggest T-cell co-potentiation may allow identification of ICB-induced CTL clones specific for subdominant tNeoAgs and irAE targets in patients with melanoma undergoing active immunotherapy.

Citation Format: Laura Elsbernd, Alfreda Nelson, Hien Huynh, Michele Hoffmann, Christopher Parks, Wendy Nevala, Shari Sutor, Larry Pease, Adam Schrum, Diana Gil, Svetomir N. Markovic. Co-potentiation of human T cells to identify subdominant tumor neoantigens from melanoma patients responding to immune checkpoint blockade [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B78.

A monomeric fab fragment capable of inducing CD3 conformational change increases T cell receptor reactivity to poorly immunogenic antigens (TUM2P.1020)

TCR interactions with agonist pMHCs induce a conformational change in the tails of the associated CD3 complex (CD3Δc) that contributes to efficient TCR/CD3 triggering and productive T cell responses. In contrast, TCR engagement by antagonistic pMHCs does not induce either CD3Δc or full TCR/CD3 signaling in mature T cells. Here, we have used a monovalent Fab fragment (Mono-Fab) specific for the murine CD3 complex to provide induction of CD3Δc to mature T cells engaging pMHC ligands. This Mono-Fab does not interfere with TCR capacity to bind pMHC and fails to elicit signaling in the absence of TCR/MHC interactions. Using OT-I Tg T cells and a collection of altered peptide ligands derived of the agonist peptide OVA, we show that addition of CD3Δc to OT-I T cells using Mono-Fab enables T cell responses to antagonist ligands that fail to induce CD3Δc on their own. Moreover, using a mouse model of metastatic melanoma, we show that Mono-Fab promotes in vivo T cell dependent anti-melanoma responses that are antigen-specific. This Mono-Fab can also be combined with established therapies targeting signals 2 and 3 involved in T cell activation for a synergistic effect. Our data suggests that in-trans CD3Δc provision decreases the activation threshold of T cells. We propose that targeting the CD3 complex for the induction of CD3Δc could be exploited to achieve efficient TCR/CD3 triggering by poorly immunogenic antigens in the context of vaccination against cancer and infectious diseases.

Reevaluating the biochemical valency of native TCR/CD3 complexes isolated from T cells (IRM7P.708)

The αβ T cell antigen Receptor (TCR) is a multi-protein complex expressed together with CD3 subunits (γδεζ). The conventional model has it as monovalent with a stochiometry of (αβγδε2ζ2); however, some previously published work reported observation of bivalent complexes with two αβ heterodimers. To reevaluate the valency of the TCR, we have subjected T cell digitonin-solubilized complexes to immunoprecipitation detected by flow cytometry (IP-FCM), size exclusion chromatography, and other methods for detecting complex size and subunit composition. Our results indicate that precisely bivalent TCRs are found among surface-expressed complexes independent of cross-linking by strong antigenic ligands. Their detection can be masked in the presence of Coomassie reagent from Blue-native PAGE conditions, or if antibodies used in detection assays are themselves bivalent. Therefore, bivalent TCR/CD3 complexes can be found under the biochemical conditions that have classically defined the minimally expressed TCR/CD3 unit (digitonin solubilization). Further investigation is required to determine the functional relevance of these bivalent TCR complexes, although greater antigenic binding and signaling is predicted over that of monovalent TCRs.

Reconstitution of TCR/CD3 expression in B cell lymphoma to dissect the contribution of CD3 conformational change from downstream signaling events in the process of TCR/CD3 downregulation. (IRM7P.714)

T cell receptor (TCR) stimulation induces conformational change in CD3 (CD3Δc), followed by phosphorylation of ITAMs that leads to receptor downregulation among other T cell responses. To begin to dissect the contribution of CD3Δc from downstream signal transduction mechanisms associated with these events, we have transduced the mouse B cell lymphoma X63.Ag8.653, which lacks expression of the src-family kinase Lck required for proper ITAM phosphorylation, with the OT-I TCR chains Vα2 and Vβ5, together with all CD3 subunits and the coreceptor CD8αβ, generating the cell line X63.OT1.CD8. Stimulation of the OT-I TCR with anti-CD3 antibody or the cognate antigen SIINFEKL-H2Kb induces CD3Δc in this cell line. As expected by the absence of Lck, CD3 ITAM phosphorylation was undetectable in our cell line and no calcium mobilization was found upon antibody stimulation. Unexpectedly, stimulation with either SIINFEKL-H2Kb or anti-CD3 did induce TCR/CD3 downregulation in our cells. These data suggest CD3ε PRS exposure via CD3Δc might be sufficient to transduce some signals that contribute to TCR downregulation in the absence of CD3 ITAM phosphorylation.

An innovative systems perspective of protein-protein interactions in the physiologic T cell antigen receptor signaling network (TECH2P.929)

Signaling networks and the protein interactions that define them are of great interest, but techniques to assess these networks in unmanipulated primary human cells are limited. We have developed and applied a novel technological and analytical platform to measure proteins in shared complexes detected by exposed surface epitopes (PiSCES). Immunoprecipitation reactions are performed in multiplex using microsphere beads to measure 20+ proteins and 210+ distinct pair-wise protein associations that are important for T cell antigen receptor signaling. Jurkat T cells stimulated with superantigen generated an extensive PiSCES signature using this method and provided a dataset from which we optimized an analytical package. The system is currently being utilized to address the hypothesis that T cells isolated from skin lesions of autoimmune alopecia areta (AA) patients will have a unique PiSCES signature compared to those isolated from non-AA donors. To investigate this, we have isolated non-engineered primary cells from AA patient skin lesions and healthy controls, and have begun to identify differences in their PiSCES signatures, either directly after isolation or after stimulation with anti-CD3 and anti-CD28. We conclude that this novel PiSCES approach is capable of generating protein-protein interaction signatures that provide a unique view of TCR signalosome activity solicited from physiologic non-engineered primary human patient samples.

A Versatile Simple Capture Assay for Assessing the Structural Integrity of MHC Multimer Reagents (TECH1P.859)

MHC multimers including tetramers and pentamers provide a powerful way to visualize antigen specific T cell responses in both experimental and clinical immune assays. The multimers can be acquired commercially, from the NIH, or assembled in the laboratory for measuring immune responses to commonly-studied or novel-hypothetical antigens. The multimer reagents can be expensive and time consuming to acquire and in many cases are uncontrolled reagents as robust positive control T cells used to assess the reagents functionally may not be available. This is particularly problematic when using MHC multimers to investigate less well-defined T cell responses, such as those characterized by low T cell frequencies or by specificity to hypothetical antigens relevant to cancer, infection, or autoimmune disease. These problems are magnified when studying irreplaceable clinical trial samples where having confidence that reagents are biochemically intact is critical. To address this problem, we have devised a versatile, flow cytometry-based, capture assay to probe the structural integrity of MHC multimer reagents. In principle, the idea is to immobilize on a bead antibody specific for conformational determinants expressed on a properly folded MHC molecule. The particle becomes labeled and can be visualized and quantified by flow cytometry when a fluorescently tagged tetramer is adsorbed to the particulate upon binding to the antibody.

Multiplex IP-FCM reveals network-scale protein-protein interaction differences between pro-immunogenic and pro-tolerogenic TCR signaling (P1130)

T cell receptor signaling involves interactions of various kinases, phosphatases, ubiquitin ligases and adaptor proteins. However, the network-scale protein-protein interactions (PPIs) that mediate signaling, as well as the key differences between pro-immunogenic (agonist) and pro-tolerogenic (antagonist) signals, remain unclear. We have developed a novel method to examine large networks of signaling PPIs, multiplex immunoprecipitation measured by flow cytometry (mIP-FCM). Using multiple classes of Luminex® microspheres, we created a 96-well-formatted matrix array capable of measuring 484 PPIs in a single overnight assay. The array is focused on the most proximal signaling proteins associated with the TCR/CD3 complex. As a model system, we have used Jurkat cells that express the public LC13 TCR, stimulated by antigen presenting cells loaded with either LC13 agonist (FLRGRAYGL) or antagonist (FLRGRFYGL) peptide. Time-course studies have revealed both quantitative and kinetic differences between the two types of signal. We present our latest results mapping the PPI network that mediates TCR signaling. Our results have significantly impacted our understanding of the difference between pro-immunogenic and pro-tolerogenic TCR signals.

Mechanism of MDSC induced T cell tolerance in cancer (88.26)

Tumor antigen-specific T-cell tolerance is a critical element of tumor escape. It is known that MDSC can induce antigen-specific CD8+ T cell tolerance in an experimental system with adoptive transfer of transgenic T cells. In this study we addressed the critical question whether a similar phenomenon can be observed in a "natural" non-transgenic experimental model. Balb/C mice were subcutaneously inoculated with two different tumor cell lines, a derivative A2L2 cell line, which expresses high levels of HER2/neu and the parental cell line 66.3. HER2/neu specific CD8+T cells were generated by immunization of mice with peptide. MDSC were able to tolerize antigen-specific T cells in vivo if they were collected from A2L2 but not from 66.3 tumor-bearing mice indicating the tumor-specific nature of T cell tolerance induced by MDSC. Next we investigated the type of signaling caused by MDSC in CD8+T cells that could lead to tolerance. MDSC dramatically reduced the ability of CD8+ T cells to respond to the specific peptide. However, study of various signaling molecules using different experimental systems and tests found that MDSC did not induce negative signaling in T cells. This conclusion was confirmed in adoptive transfer experiments with CD8+T cells expressing dual TCR: LCMV glycoprotein peptide gp33-41 and OVA-derived H-2Kb-restricted peptide ova257-264. We found that MDSC loaded with peptide specific for one TCR induced tolerance only against that specific epitope but CD8+ T cells retained the ability to respond to the peptide specific for the other TCR expressed on the same cell.

Junctate is a key element in calcium entry induced by activation of InsP3 receptors and/or calcium store depletion

In many cell types agonist-receptor activation leads to a rapid and transient release of Ca(2+) from intracellular stores via activation of inositol 1,4,5 trisphosphate (InsP(3)) receptors (InsP(3)Rs). Stimulated cells activate store- or receptor-operated calcium channels localized in the plasma membrane, allowing entry of extracellular calcium into the cytoplasm, and thus replenishment of intracellular calcium stores. Calcium entry must be finely regulated in order to prevent an excessive intracellular calcium increase. Junctate, an integral calcium binding protein of endo(sarco)plasmic reticulum membrane, (a) induces and/or stabilizes peripheral couplings between the ER and the plasma membrane, and (b) forms a supramolecular complex with the InsP(3)R and the canonical transient receptor potential protein (TRPC) 3 calcium entry channel. The full-length protein modulates both agonist-induced and store depletion-induced calcium entry, whereas its NH(2) terminus affects receptor-activated calcium entry. RNA interference to deplete cells of endogenous junctate, knocked down both agonist-activated calcium release from intracellular stores and calcium entry via TRPC3. These results demonstrate that junctate is a new protein involved in calcium homeostasis in eukaryotic cells.