Abstract 4095: Preclinical characterization of switchable allogeneic chimeric antigen receptor T cells to support first in human clinical study in CD123-positive hematologic and lymphatic malignancies

Approximately 80% of AML patients express CD123 on their leukemic blasts and CD123 is frequently expressed on other hematologic and lymphatic malignancies. Chimeric antigen receptor (CAR) T-cell therapy has demonstrated significant efficacy in B cell malignancies. Breakthrough of conventional CAR-T technology in AML has been hampered by expression of suitable target antigens on normal hematopoietic progenitor cells posing a risk for continued aplasia. Thus, innovative approaches putting the power of CAR-T technology under the control of reliable and fast-acting on/off switches to avoid and/or abrogate acute and long-term side effects are required. We have recently reported clinical proof-of-concept for autologous switchable CAR-T cell therapy in AML (Wermke et al. Blood 2021). However, manufacturing of autologous products is very costly and rrAML patients are often in urgent need for a product and multiple prior lines of treatment may be detrimental for product quality. While current clinical allogeneic approaches successfully prevent graft-versus-host disease, observed persistence has been limited due to rejection of the grafted T cells by host immune cells. Here, we report preclinical development of a donor-derived allogeneic switchable CAR-T therapy for CD123-positive hematologic and lymphatic malignancies (AVC-201). The reverse universal chimeric antigen receptor platform (RevCAR) is a 2-component CAR-T platform. The first component is a universal CAR-T cell. Its binding domain is a short, non-immunogenic peptide that by itself does not recognize any human cell surface antigen but is specifically bound by an scFv included in the second component, a soluble adaptor called targeting module (TM). To target CD123, a TM with a short half-life was selected (R-TM123), enabling a rapid switch-off of the RevCAR system by TM withdrawal to avoid acute and long-term toxicity usually associated with continuous activation of CAR-T cells. The allogeneic cells are generated by three edits with CRISPR-Cas9, which is meant to fully overcome graft-versus-host disease as well as graft rejection by host T and NK cells. Therefore, we expect a persistent T cell product that can be re-expanded with additional TM cycles. A summary of preclinical characterization of AVC-201 will be presented at the meeting including in vitro and in vivo pharmacology and toxicology experiments. Activation of Allo-RevCAR-T cells is strictly dependent on the presence of CD123-positive target cells and R-TM123. Allo-RevCAR-T cells redirected by R-TM123 efficiently lyse CD123-positive AML cells in vitro and in vivo, and in vitro EC50 values are in the low picomolar range of R-TM123 concentrations. In conclusion, preclinical data support the clinical exploration of AVC-201 in a first in human study.

Citation Format: Armin Ehninger, Johannes Spehr, Simon Loff, Anika Langer, Julia Riewaldt, Julia Reinhardt, Jan-Erik Meyer, Josephine Dietrich, Gabriel Jurado, Reynald Lescarbeau, Marc Cartellieri. Preclinical characterization of switchable allogeneic chimeric antigen receptor T cells to support first in human clinical study in CD123-positive hematologic and lymphatic malignancies. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4095.

Chimeric antigen receptor T-cell therapy in acute myeloid leukemia

PURPOSE OF REVIEW

Treatment outcome of relapsed or refractory AML patients remains dismal and new treatment options are needed. Adoptive cell therapy using CAR-T cells is a potentially interesting approach in this.

RECENT FINDINGS

Several potentially interesting AML targets are being investigated with CAR-T therapy with over 60 clinical trials listed on clinicaltrials.gov. The first clinical data are only just emerging with mixed results, once more proving that further research is needed.

SUMMARY

Adoptive cell therapy using chimeric antigen receptor T cells is being investigated in AML through many clinical trials. So far, no AML-specific antigen has been identified, requiring additional strategies to mitigate on-target off-tumor toxicity and to increase efficacy. Focus point is to acquire control over the CAR T cells once administered. Strategies to do so include biodegradable CARs, inducible CARs, suicide-switch containing CARs and two-component modular CARs. Limited and mixed results are available, confirming the risk of lasting toxicity for nonswitchable CARs. Initial results of modular CARs suggest toxicity can be mitigated whilst maintaining CAR activity by the use of modular CAR concepts that allows for 'ON' and 'OFF' switching.

Evaluation of switch-mediated costimulation in trans on universal CAR-T cells (UniCAR) targeting CD123-positive AML

Chimeric antigen receptor T cells (CAR-T) targeting CD19 have achieved significant success in patients with B cell malignancies. To date, implementation of CAR-T in other indications remains challenging due to the lack of truly tumor-specific antigens as well as control of CAR-T activity in patients. CD123 is highly expressed in acute myeloid leukemia (AML) blasts including leukemia-initiating cells making it an attractive immunotherapeutic target. However, CD123 expression in normal hematopoietic progenitor cells and endothelia bears the risk of severe toxicities and may limit CAR-T applications lacking fine-tuned control mechanisms. Therefore, we recently developed a rapidly switchable universal CAR-T platform (UniCAR), in which CAR-T activity depends on the presence of a soluble adapter called targeting module (TM), and confirmed clinical proof-of-concept for targeting CD123 in AML with improved safety. As costimulation via 4–1BB ligand (4–1BBL) can enhance CAR-T expansion, persistence, and effector functions, a novel CD123-specific TM variant (TM123-4-1BBL) comprising trimeric single-chain 4–1BBL was developed for transient costimulation of UniCAR-T cells (UniCAR-T) at the leukemic site in trans. TM123-4-1BBL-directed UniCAR-T efficiently eradicated CD123-positive AML cells in vitro and in a CDX in vivo model. Moreover, additional costimulation via TM123-4-1BBL enabled enhanced expansion and persistence with a modulated UniCAR-T phenotype. In addition, the increased hydrodynamic volume of TM123-4-1BBL prolonged terminal plasma half-life and ensured a high total drug exposure in vivo. In conclusion, expanding the soluble adapter optionality for CD123-directed UniCAR-T maintains the platforms high anti-leukemic efficacy and immediate control mechanism for a flexible, safe, and individualized CAR-T therapy of AML patients.

Abstract 1506: Expansion kinetics and cytokine profiles of UniCAR-T-CD123, a rapidly switchable two-component CAR-T therapy, in patients with relapsed/refractory AML

Background: Conventional CAR-T cells targeting CD123 in rrAML have achieved objective responses, but led to long-lasting myelosuppression due to expression of CD123 on progenitor cells. UniCAR-T-CD123 is a rapidly switchable two-component CAR-T therapy. An inert universal CAR-T cell (UniCAR-T) is combined with a CD123-specific soluble targeting module with a short half-life (TM123). By administering or withholding the continuous infusion of TM123, the UniCAR-T-cell can be rapidly switched on and off. Within the ongoing Phase IA study in rrAML, we investigated the expansion kinetics of UniCAR-T cells during TM123 administration in peripheral blood and bone marrow as well as cytokine profiles of treated patients.

Methods: Prior to administration of autologous UniCAR-T cells, patients received a standard Flu/Cy lymphodepletion regimen at day -5 to -3. TM123 was administered as continuous infusion over 24 days starting at day 0. At day 1 a single dose of UniCAR-T cells was given. Dosing started with 1 x 108 UniCAR-T cells and 0.5 mg TM123 per day in patient 1. Patient 2 received the same TM123 dose and a UniCAR-T dose of 2.5 x 108 cells. Patient 3 received the same cell dose as patient 2 but a higher TM123 dose (1 mg/day). Pharmacokinetic of UniCAR-T and TM123 was determined from peripheral blood and bone marrow by droplet digital PCR and TM123-specific ELISA, respectively. Cytokine levels were measured by microfluidic immunoassay.

Results: All 3 patients treated so far achieved an objective response, with one showing a PR and two a CRi. Treatment proved to be tolerable, no DLTs were observed to date and adverse events were mild. Grade 1 CRS (fever) was observed in 2 patients but subsided within 48 h after use of antipyretics. Myelosuppression was observed starting after lymphodepletion, which immediately recovered after TM123 withdrawal on day 24 in all patients, providing evidence for the rapid off-switch of UniCAR-T cells post TM123 administration. UniCAR-T cells expanded in all patients in peripheral blood and bone marrow comparable to data reported for conventional CD123 CAR-T products, and were so far detectable for up to 6 months after administration. Expansion kinetics were TM123-dependent. Patients showed periods of transient increase of IL-6, IFN-γ and TNF-α levels preceding peak expansion and decreasing after termination of TM123 administration. One patient showed additional coinciding momentary elevation of GM-CSF and IL-2.

Conclusions: The initial clinical and translational results of UniCAR-T-CD123 represent, to our understanding, a first time evidence for a well-tolerated and effective rapidly switchable CAR-T product. Even though the number of patients treated so far is limited, the data obtained provide clinical proof-of-concept for the opportunity to abrogate side effects by withdrawal of TM123. Enrollment into the Phase IA study is ongoing.

Citation Format: Armin Ehninger, Julia Riewaldt, Cordula Gründer, Kristin Franke, Maria Schreiber, Martin Wermke, Sabrina Kraus, Carla Kreissig, Jan Koedam, Michael Pehl, Gerhard Ehninger, Marc Cartellieri. Expansion kinetics and cytokine profiles of UniCAR-T-CD123, a rapidly switchable two-component CAR-T therapy, in patients with relapsed/refractory AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1506.

Abstract 2176: Using a PSMA-specific low-molecular-weight compound for prostate cancer treatment with rapidly switchable universal CAR-T cells: Overcoming the challenges of cellular immunotherapies in solid tumors

CAR-T cell therapy holds great promise for treating a wide range of malignancies. Nevertheless, the CAR-T approach faces multiple challenges, including lack of suitable targets, insufficient tumor penetration and a microenvironment hostile to CAR-T cells. Here we provide pre-clinical evidence for using a low-molecular-weight, chemically synthesized compound to re-target CAR-T cells against solid tumors in conjunction with the CD28 co-stimulatory domain to address several of these challenges. PSMA is a validated target for treatment of advanced and metastatic prostate cancer (PCa), but also broadly expressed on tumor neo-vasculature beyond PCa. PSMA is also known to be expressed on a number of normal tissues, albeit to a much lower extent, which necessitates additional safety mechanisms for CAR-T therapy. Therefore, a rapidly switchable universal CAR-T platform (UniCAR) was developed. In this system, antigen-specificity is provided by soluble adaptors termed targeting modules (TM), which redirect T-cells engineered to express a universal CAR in an antigen-specific manner against tumors. The TM explored in the present study incorporates a clinically well-characterized radiotracer peptide motif binding to the enzymatic groove of PSMA. The small-sized TM has favorable pharmacokinetic and pharmacodynamics properties (short half-life of < 30 min; rapid internalization in < 1h), which allows a fast silencing and excellent controllability of UniCAR-T reactivity (< 4 h). Furthermore, the small molecule TM efficiently penetrates and rapidly accumulates within solid tumors as clinically demonstrated by related radiotracers. Results from our pre-clinical in vivo model demonstrate potent recruitment of UniCAR-T into tumor tissue by the TM and an efficient anti-tumor response. Of note, it was recently shown that the chosen PCa model suppresses CAR-T response by secreting high levels of transforming growth factor beta (TGF-β), creating an immunosuppressive milieu. We could overcome immunosuppression utilizing CD28 compared to 4-1-BB in the intracellular signaling domain of the UniCAR. Administered CD28/zeta UniCAR efficiently suppressed tumor growth in our in vivo model and could be re-activated by consecutive cycles of TM administration even after several weeks. In contrast to CARs with fixed binding moieties, which continuously signal and thereby induce T- cell exhaustion, the discontinuous activation of UniCAR keeps a balanced mix of T-effector and T-memory cells. In summary, our rapidly switchable universal CAR-T platform in combination with a low-molecular-weight TM allows to target less differentially expressed solid tumor antigens. Furthermore, incorporation of the CD28 costimulatory domain overcomes the immunosuppressive tumor microenvironment while discontinuous activation of UniCAR-T prevents exhaustion.

Citation Format: Marc Cartellieri, Simon Loff, Johannes Spehr, Mridula Swayampakula, Julia Riewaldt, Cordula Gründer, Maria Schreiber, Michael Bachmann, Anja Feldmann, Michael Pehl, Gerhard Ehninger, Armin Ehninger. Using a PSMA-specific low-molecular-weight compound for prostate cancer treatment with rapidly switchable universal CAR-T cells: Overcoming the challenges of cellular immunotherapies in solid tumors [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 2176.

Abstract 2209: Rapidly switchable universal CAR-T cells with improved safety profile allow for active targeting of PD-L1 expressing solid tumors

Despite remarkable therapeutic success using chimeric antigen receptor modified T-cells (CAR-T) in hematologic malignancies, targeting solid tumors still remains challenging, given their heterogeneity and immunosuppressive milieu, including the expression of inhibitory immune checkpoints such as PD-1/PD-L1. Preclinical models demonstrate that combining CAR-T therapy with checkpoint inhibitors targeting the PD-1/PD-L1 axis could be a promising strategy to enhance anti-cancer activity of CAR-T in solid tumors and on-going phase I trials are clinically evaluating this strategy. However, primary and acquired resistance to checkpoint blockade is frequently observed limiting its broad applicability and efficacy. In addition, lack of specificity results in frequent occurrence of immune-related adverse events affecting several organs. Alternatively, expression of PD-L1 by solid tumor cells could be an attractive target for CAR-T therapy, especially in order to prevent or reverse tumor re-growth and relapse after successful initial tumor cell lysis. The development of a rapidly switchable universal CAR-T platform (UniCAR) with enhanced safety features allowed us to explore the possibility to directly attack PD-L1 expressing solid tumor cells. An inherent key feature of the UniCAR-T platform is a rapid and reversible turn off mechanism (less than 4 hours) determined by the short pharmacokinetic half-life of soluble targeting modules (TMs) and fast internalization of cell-bound TMs. TMs provide the antigen-specificity for UniCAR gene-modified T-cells (UniCAR-T) and consist of an antigen-binding moiety, e.g. an scFv, linked to a small peptide motif recognized by the UniCAR. For redirecting UniCAR-T against the PD-1/PD-L1 axis, a PD-L1 specific TM (TM-PD-L1) was developed and its functionality confirmed in binding assays to human and murine PD-L1 using surface plasmon resonance as well as in cell-based cytotoxicity assays. Half maximal killing efficacy (EC50) mediated by TM-PD-L1 redirected UniCAR-T was in the low picomolar range. As PD-L1 is also expressed on healthy tissue, toxicity studies were performed in a humanized mouse model with human UniCAR-T. Humanized mice treated with TM-PD-L1 twice a day for ten consecutive days did not reveal any signs of toxicity. In a prostate cancer (PCa) xenograft model efficient suppression of tumor growth could be demonstrated by administration of TM-PD-L1. Our data are the first to demonstrate safety and feasibility of active targeting of PD-L1 expressed by solid tumors with CAR-T and that this approach is capable of inducing a significant anti-tumor response.

Citation Format: Josephine Dietrich, Simon Loff, Johannes Spehr, Julia Riewaldt, Cordula Gründer, Maria Schreiber, Michael Bachmann, Gerhard Ehninger, Michael Pehl, Marc Cartellieri, Armin Ehninger. Rapidly switchable universal CAR-T cells with improved safety profile allow for active targeting of PD-L1 expressing solid tumors [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 2209.

Rapidly Switchable Universal CAR-T Cells for Treatment of CD123-Positive Leukemia

Chimeric antigen receptor T cells (CAR-T) targeting CD19 or B cell maturation antigen (BCMA) are highly effective against B cell malignancies. However, application of CAR-T to less differentially expressed targets remains a challenge due to lack of tumor-specific antigens and CAR-T controllability. CD123, a highly promising leukemia target, is expressed not only by leukemic and leukemia-initiating cells, but also by myeloid, hematopoietic progenitor, and certain endothelial cells. Thus, CAR-T lacking fine-tuned control mechanisms pose a high toxicity risk. To extend the CAR-T target landscape and widen the therapeutic window, we adapted our rapidly switchable universal CAR-T platform (UniCAR) to target CD123. UniCAR-T efficiently eradicated CD123⁺ leukemia in vitro and in vivo. Activation, cytolytic response, and cytokine release were strictly dependent on the presence of the CD123-specific targeting module (TM123) with comparable efficacy to CD123-specific CAR-T in vitro. We further demonstrated a pre-clinical proof of concept for the safety-switch mechanism using a hematotoxicity mouse model wherein TM123-redirected UniCAR-T showed reversible toxicity toward hematopoietic cells compared to CD123 CAR-T. In conclusion, UniCAR-T maintain full anti-leukemic efficacy, while ensuring rapid controllability to improve safety and versatility of CD123-directed immunotherapy. The safety and efficacy of UniCAR-T in combination with TM123 will now be assessed in a phase I clinical trial (ClinicalTrials.gov: NCT04230265).

Anti-CAR-engineered T cells for epitope-based elimination of autologous CAR T cells

Although CAR T-cell therapy has demonstrated tremendous clinical efficacy especially in hematological malignancies, severe treatment-associated toxicities still compromise the widespread application of this innovative technology. Therefore, developing novel approaches to abrogate CAR T-cell-mediated side effects is of great relevance. Several promising strategies pursue the selective antibody-based depletion of adoptively transferred T cells via elimination markers. However, given the limited half-life and tissue penetration, dependence on the patients’ immune system and on-target/off-side effects of proposed monoclonal antibodies, we sought to exploit αCAR-engineered T cells to efficiently eliminate CAR T cells. For comprehensive and specific recognition, a small peptide epitope (E-tag) was incorporated into the extracellular spacer region of CAR constructs. We provide first proof-of-concept for targeting this epitope by αE-tag CAR T cells, allowing an effective killing of autologous E-tagged CAR T cells both in vitro and in vivo whilst sparing cells lacking the E-tag. In addition to CAR T-cell cytotoxicity, the αE-tag-specific T cells can be empowered with cancer-fighting ability in case of relapse, hence, have versatile utility. Our proposed methodology can most probably be implemented in CAR T-cell therapies regardless of the targeted tumor antigen aiding in improving overall safety and survival control of highly potent gene-modified cells.

Tonic Signaling and Its Effects on Lymphopoiesis of CAR-Armed Hematopoietic Stem and Progenitor Cells

Long-term survival of adoptively transferred chimeric Ag receptor (CAR) T cells is often limited. Transplantation of hematopoietic stem cells (HSCs) transduced to express CARs could help to overcome this problem as CAR-armed HSCs can continuously deliver CAR⁺ multicell lineages (e.g., T cells, NK cells). In dependence on the CAR construct, a variable extent of tonic signaling in CAR T cells was reported; thus, effects of CAR-mediated tonic signaling on the hematopoiesis of CAR-armed HSCs is unclear. To assess the effects of tonic signaling, two CAR constructs were established and analyzed 1) a signaling CAR inducing a solid Ag-independent tonic signaling termed CAR-28/ζ and 2) a nonstimulating control CAR construct lacking intracellular signaling domains termed CAR-Stop. Bone marrow cells from immunocompetent mice were isolated, purified for HSC-containing Lin^-cKit⁺ cells or the Lin^-cKit⁺ Sca-1⁺ subpopulation (Lin^-Sca-1⁺cKit⁺), and transduced with both CAR constructs. Subsequently, modified bone marrow cells were transferred into irradiated mice, in which they successfully engrafted and differentiated into hematopoietic progenitors. HSCs expressing the CAR-Stop sustained normal hematopoiesis. In contrast, expression of the CAR-28/ζ led to elimination of mature CAR⁺ T and B cells, suggesting that the CAR-mediated tonic signaling mimics autorecognition via the newly recombined immune receptors in the developing lymphocytes.

Retargeting of T lymphocytes to PSCA- or PSMA positive prostate cancer cells using the novel modular chimeric antigen receptor platform technology "UniCAR"

New treatment options especially of solid tumors including for metastasized prostate cancer (PCa) are urgently needed. Recent treatments of leukemias with chimeric antigen receptors (CARs) underline their impressive therapeutic potential. However CARs currently applied in the clinics cannot be repeatedly turned on and off potentially leading to severe life threatening side effects. To overcome these problems, we recently described a modular CAR technology termed UniCAR: UniCAR T cells are inert but can be turned on by application of one or multiple target modules (TMs). Here we present preclinical data summarizing the retargeting of UniCAR T cells to PCa cells using TMs directed to prostate stem cell- (PSCA) or/and prostate specific membrane antigen (PSMA). In the presence of the respective TM(s), we see a highly efficient target-specific and target-dependent activation of UniCAR T cells, secretion of pro-inflammatory cytokines, and PCa cell lysis both in vitro and experimental mice.

Engrafting human regulatory T cells with a flexible modular chimeric antigen receptor technology

As regulatory T cells (Tregs) play a fundamental role in immune homeostasis their adoptive transfer emerged as a promising treatment strategy for inflammation-related diseases. Preclinical animal models underline the superiority of antigen-specific Tregs compared to polyclonal cells. Here, we applied a modular chimeric antigen receptor (CAR) technology called UniCAR for generation of antigen-specific human Tregs. In contrast to conventional CARs, UniCAR-endowed Tregs are indirectly linked to their target cells via a separate targeting module (TM). Thus, transduced Tregs can be applied universally as their antigen-specificity is easily adjusted by TM exchange. Activation of UniCAR-engrafted Tregs occurred in strict dependence on the TM, facilitating a precise control over Treg activity. In order to augment efficacy and safety, different intracellular signaling domains were tested. Both 4-1BB (CD137) and CD28 costimulation induced strong suppressive function of genetically modified Tregs. However, in light of safety issues, UniCARs comprising a CD137-CD3ζ signaling domain emerged as constructs of choice for a clinical application of redirected Tregs. In that regard, Tregs isolated from patients suffering from autoimmune or inflammatory diseases were, for the first time, successfully engineered with UniCAR 137/ζ and efficiently suppressed patient-derived effector cells. Overall, the UniCAR platform represents a promising approach to improve Treg-based immunotherapies for tolerance induction.

Retargeting of UniCAR T cells with an in vivo synthesized target module directed against CD19 positive tumor cells

Recent treatments of leukemias with T cells expressing chimeric antigen receptors (CARs) underline their impressive therapeutic potential but also their risk of severe side effects including cytokine release storms and tumor lysis syndrome. In case of cross-reactivities, CAR T cells may also attack healthy tissues. To overcome these limitations, we previously established a switchable CAR platform technology termed UniCAR. UniCARs are not directed against typical tumor-associated antigens (TAAs) but instead against a unique peptide epitope: Fusion of this peptide epitope to a recombinant antibody domain results in a target module (TM). TMs can cross-link UniCAR T cells with tumor cells and thereby lead to their destruction. So far, we constructed TMs with a short half-life. The fast turnover of such a TM allows to rapidly interrupt the treatment in case severe side effects occur. After elimination of most of the tumor cells, however, longer lasting TMs which have not to be applied via continous infusion would be more convenient for the patient. Here we describe and characterize a TM for retargeting UniCAR T cells to CD19 positive tumor cells. Moreover, we show that the TM can efficiently be produced in vivo from producer cells housed in a sponge-like biomimetic cryogel and, thereby, serving as an in vivo TM factory for an extended retargeting of UniCAR T cells to CD19 positive leukemic cells.

Characterization of a switchable chimeric antigen receptor platform in a pre-clinical solid tumor model

The universal modular chimeric antigen receptor (UniCAR) platform redirects CAR-T cells using a separated, soluble targeting module with a short half-life. This segregation allows precise controllability and flexibility. Herein we show that the UniCAR platform can be used to efficiently target solid cancers in vitro and in vivo using a pre-clinical prostate cancer model which overexpresses prostate stem cell antigen (PSCA). Short-term administration of the targeting module to tumor bearing immunocompromised mice engrafted with human UniCAR-T cells significantly delayed tumor growth and prolonged survival of recipient mice both in a low and high tumor burden model. In addition, we analyzed phenotypic and functional changes of cancer cells and UniCAR-T cells in association with the administration of the targeting module to reveal potential immunoevasive mechanisms. Most notably, UniCAR-T cell activation induced upregulation of immune-inhibitory molecules such as programmed death ligands. In conclusion, this work illustrates that the UniCAR platform mediates potent anti-tumor activity in a relevant in vitro and in vivo solid tumor model.

A novel nanobody-based target module for retargeting of T lymphocytes to EGFR-expressing cancer cells via the modular UniCAR platform

Recent treatments of leukemias with chimeric antigen receptor (CAR) expressing T cells underline their impressive therapeutic potential. However, once adoptively transferred into patients, there is little scope left to shut them down after elimination of tumor cells or in case adverse side effects occur. This becomes of special relevance if they are directed against commonly expressed tumor associated antigens (TAAs) such as receptors of the ErbB family. To overcome this limitation, we recently established a modular CAR platform technology termed UniCAR. UniCARs are not directed against TAAs but instead against a unique peptide epitope on engineered recombinant targeting modules (TMs), which guide them to the target. In the absence of a TM UniCAR T cells are inactive. Thus an interruption of any UniCAR activity requires an elimination of unbound TM and the TM complexed with UniCAR T cells. Elimination of the latter one requires a disassembly of the UniCAR-TM complexes. Here, we describe a first nanobody (nb)-based TM directed against EGFR. The novel TM efficiently retargets UniCAR T cells to EGFR positive tumors and mediates highly efficient target-specific and target-dependent tumor cell lysis both in vitro and in vivo. After radiolabeling of the novel TM with ⁶⁴Cu and ⁶⁸Ga, we analyzed its biodistribution and clearance as well as the stability of the UniCAR-TM complexes. As expected unbound TM is rapidly eliminated while the elimination of the TM complexed with UniCAR T cells is delayed. Nonetheless, we show that UniCAR-TM complexes dissociate in vitro and in vivo in a concentration-dependent manner in line with the concept of a repeated stop and go retargeting of tumor cells via the UniCAR technology.

Abstract B099: The UniCAR system: Inducible CAR T cells for precise reactivity and high efficacy against hematopoietic malignancies

During the past years promising clinical outcomes have been reported using immunotherapy for treatment of solid tumors and malignancies of the hematopoietic system. Since T cells act as key players in immune surveillance, an encouraging approach is to genetically engineer T cells with lenti- or retroviral vectors, in order to express artificial chimeric antigen receptors (CARs), directing them against cancer cells and leading to a distinct cytotoxic response. CARs are synthetic fusion proteins consisting of an antigen-binding moiety, commonly a tumor-associated antigen (TAA)-specific single-chain fragment variable (scFv), combined with activating signaling domains like the CD3-zeta chain. Signaling can be enhanced by adding co-stimulatory domains, such as the intracellular regions of CD28 or CD137. Although, CAR T cells showed promising results in patients with B cell malignancies, a major drawback of conventional CARs, bearing a tumor-antigen-binding moiety, is the uncontrollable continuous T cell activity. Hence, CAR T cells can achieve a sustained anti-tumoral response. However, reactivity against any healthy tissues expressing the target antigen might cause severe adverse effects.

A chance to gain safety on CAR T cell approaches by retaining efficacy is offered through a novel universal binary CAR system (UniCAR). The system is based on a second generation CAR with an extracellular scFv binding domain, which is directed against a short non-immunogenic peptide motif instead of a cell surface antigen. Thus, engineered T cells are inert per se, remaining in a “sleeping mode” after re-infusion. Small protein links (targeting modules, TMs), consisting e.g. of TAA-specific scFvs harboring the small peptide epitope recognized by the UniCAR are used to redirect UniCAR expressing T cells in an antigen-specific manner. Primary results demonstrate that UniCAR driven T cell activation and cytotoxic effects against tumor cells are strictly dependent on the presence of the antigen-specific TMs. UniCAR T cells show high anti-tumor efficacy even at picomolar TM concentrations and low effector to target cell (e:t) ratios in vitro. After those proof-of-concept studies we disclose positive evidence of high efficacy UniCAR T cells with reactivity against CD123-positive acute myeloid leukemia (AML) and CD19-bearing acute lymphoblastic leukemia (ALL) cells. UniCAR T cell anti-tumor efficacy proofed to be superior when compared to conventional CAR T cells at low e:t ratios adjusted to the situation usually found in patients. Furthermore, UniCAR T cells pre-decorated with CD123-specific TMs in vitro are capable of prohibiting tumor engraftment of CD123-positive AML in vivo and persist in immunodeficient NSG mice. Ex vivo isolated UniCAR T cells retained cytotoxic potential against MOLM-13 cells for at least four month post transplantation fortifying the therapeutic potential in vitro and in vivo.

The tremendous benefit of the novel UniCAR platform is the precise control of CAR T cell activity by regulating TM administration and dosage, preventing off side-toxicities arising from conventional CAR approaches. Moreover, the modular UniCAR system also enables combinatorial targeting strategies against several TAAs to increase therapy performance and circumvent the occurrence of tumor resistance mechanisms due to selectable compositions of target structures. Leukemia-related target antigens like CD19, CD20, CD22, CD33 or CD123 are conceivable therapy settings, offering advantages compared to monotherapeutic CAR-based therapies. Taken together, the modular composition of the UniCAR platform maintains the high anti-tumor potential of CAR-engrafted T cells, while introducing tight control mechanisms and unparalleled target flexibility. These features will allow a more sophisticated application of CAR technology and a reduction of adverse events in the clinical setting.

Citation Format: Simon Loff, Malte v. Bonin, Josephine Dietrich, Jan-Erik Meyer, Anja Feldmann, Claudia Arndt, Johannes Spehr, Cordula Gründer, Gerhard Ehninger, Michael P. Bachmann, Armin Ehninger, Marc Cartellieri. The UniCAR system: Inducible CAR T cells for precise reactivity and high efficacy against hematopoietic malignancies [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B099.

Abstract 2313: Improved killing of tumor cells by a novel flexible antibody-based modular T cell retargeting system

In recent years, bispecific antibodies (bsabs) and chimeric antigen receptors (CARs) emerged as promising candidates for an antigen-specific cancer immunotherapy. Bsabs and CARs redirect T cells in an antigen-specific manner towards tumor cells and unleash their enormous cytotoxic potential to attack the tumor. Nevertheless, several challenges have to be solved to pave the way for a more widespread application of both strategies. Both approaches are monotherapies bearing the inherent risk for development of antigen-loss tumor variants under treatment. In addition, due to their single antigen specificity each given bsab or CAR is only curative for a restricted number of tumor indications and long-lasting and demanding research is necessary to bring a new bsab- or CAR-based drug to patients in need. Moreover, current CAR T cell approaches do not allow for any control of the magnitude and duration of T cell reactivity, thus bearing the risk for overshooting anti-tumor reactions leading to severe side effects and ongoing destruction of healthy tissue carrying the target antigen after tumor clearance. To overcome these limitations, we recently introduced a novel flexible antibody-based modular platform (UniTARG) that can be rapidly and easily adapted for redirection of T cells to any TAA in both a bsAb- or CAR-setting. The UniTARG technology consists of a universal effector arm and individual targeting modules (TMs). The effector arm is either a universal CAR (UniCAR), which has specificity for a short peptide motif of 10 amino acids derived from a human nuclear protein, or a bsAb (UniMAB) with specificity for human CD3 on T cells and the respective peptide epitope. The antigen-specificity of the system is provided by TMs comprising a binding domain e.g., a tumor-antigen specific scFv, fused to the nuclear antigen motif recognized by the UniCAR/UniMAB binding domain. Here we provide first in vitro and in vivo prove of concept for these new approaches. Redirection of T cells armed with the universal effector modules was effective at picomolar concentrations of targeting modules directed against various antigens. The modular composition of both platforms prompted us to explore, if T cell retargeting against several antigens simultaneously might be feasible. Using either two single-specific TMs or single bi-specific TMs significantly enhanced specific lysis of tumor cells in vitro and durable responses in vivo were observed. Furthermore, our results proof that TMs against new targets can be developed in a couple of weeks and all TMs tested so far engaged effector module armed T cells with similar potencies. Taken together, the UniTARG platform technology represents a promising tool in the field of both bsAbs and CARs with the advantage of simultaneous or consecutive dual or even multispecific T cell retargeting. Furthermore the platform approach allows a rapid development of new therapeutic options against additional targets.

Citation Format: Armin Ehninger, Marc Cartellieri, Anja Feldmann, Claudia Arndt, Stefanie Koristka, Simon Loff, Malte von Bonin, Gerhard Ehninger, Michael Bachmann. Improved killing of tumor cells by a novel flexible antibody-based modular T cell retargeting system. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2313.

Multimodal Somatostatin Receptor Theranostics Using [(64)Cu]Cu-/[(177)Lu]Lu-DOTA-(Tyr(3))octreotate and AN-238 in a Mouse Pheochromocytoma Model

Pheochromocytomas and extra-adrenal paragangliomas (PHEO/PGLs) are rare catecholamine-producing chromaffin cell tumors. For metastatic disease, no effective therapy is available. Overexpression of somatostatin type 2 receptors (SSTR2) in PHEO/PGLs promotes interest in applying therapies using somatostatin analogs linked to radionuclides and/or cytotoxic compounds, such as [¹⁷⁷Lu]Lu-DOTA-(Tyr³)octreotate (DOTATATE) and AN-238. Systematic evaluation of such therapies for the treatment of PHEO/PGLs requires sophisticated animal models. In this study, the mouse pheochromocytoma (MPC)-mCherry allograft model showed high tumor densities of murine SSTR2 (mSSTR2) and high tumor uptake of [⁶⁴Cu]Cu-DOTATATE. Using tumor sections, we assessed mSSTR2-specific binding of DOTATATE, AN-238, and somatostatin-14. Therapeutic studies showed substantial reduction of tumor growth and tumor-related renal monoamine excretion in tumor-bearing mice after treatment with [¹⁷⁷Lu]Lu-DOTATATE compared to AN-238 and doxorubicin. Analyses did not show agonist-dependent receptor downregulation after single mSSTR2-targeting therapies. This study demonstrates that the MPC-mCherry model is a uniquely powerful tool for the preclinical evaluation of SSTR2-targeting theranostic applications in vivo. Our findings highlight the therapeutic potential of somatostatin analogs, especially of [¹⁷⁷Lu]Lu-DOTATATE, for the treatment of metastatic PHEO/PGLs. Repeated treatment cycles, fractionated combinations of SSTR2-targeting radionuclide and cytotoxic therapies, and other adjuvant compounds addressing additional mechanisms may further enhance therapeutic outcome.

Bispecific antibody releasing-mesenchymal stromal cell machinery for retargeting T cells towards acute myeloid leukemia blasts

Bispecific antibodies (bsAbs) engaging T cells are emerging as a promising immunotherapeutic tool for the treatment of hematologic malignancies. Because their low molecular mass, bsAbs have short half-lives. To achieve clinical responses, they have to be infused into patients continously, for a long period of time. As a valid alternative we examined the use of mesenchymal stromal cells (MSCs) as autonomous cellular machines for the constant production of a recently described, fully humanized anti-CD33-anti-CD3 bsAb, which is capable of redirecting human T cells against CD33-expressing leukemic cells. The immortalized human MSC line SCP-1 was genetically modified into expressing bsAb at sufficient amounts to redirect T cells efficiently against CD33 presenting target cells, both in vitro and in an immunodeficient mouse model. Moreover, T cells of patients suffering from acute myeloid leukemia (AML) in blast crisis eliminated autologous leukemic cells in the presence of the bsAb secreting MSCs over time. The immune response against AML cells could be enhanced further by providing T cells an additional co-stimulus via the CD137-CD137 ligand axis through CD137L expression on MSCs. This study demonstrates that MSCs have the potential to be used as cellular production machines for bsAb-based tumor immunotherapy in the future.

DAP12-based activating chimeric antigen receptor for NK cell tumor immunotherapy

NK cells are emerging as new effectors for immunotherapy of cancer. In particular, the genetic engraftment of chimeric Ag receptors (CARs) in NK cells is a promising strategy to redirect NK cells to otherwise NK cell-resistant tumor cells. On the basis of DNAX-activation protein 12 (DAP12), a signaling adaptor molecule involved in signal transduction of activating NK cell receptors, we generated a new type of CAR targeting the prostate stem cell Ag (PSCA). We demonstrate in this article that this CAR, designated anti-PSCA-DAP12, consisting of DAP12 fused to the anti-PSCA single-chain Ab fragment scFv(AM1) confers improved cytotoxicity to the NK cell line YTS against PSCA-positive tumor cells when compared with a CAR containing the CD3ζ signaling chain. Further analyses revealed phosphorylation of the DAP12-associated ZAP-70 kinase and IFN-γ release of CAR-engineered cells after contact with PSCA-positive target cells. YTS cells modified with DAP12 alone or with a CAR bearing a phosphorylation-defective ITAM were not activated. Notably, infused YTS cells armed with anti-PSCA-DAP12 caused delayed tumor xenograft growth and resulted in complete tumor eradication in a significant fraction of treated mice. The feasibility of the DAP12-based CAR was further tested in human primary NK cells and confers specific cytotoxicity against KIR/HLA-matched PSCA-positive tumor cells, which was further enhanced by KIR-HLA mismatches. We conclude that NK cells engineered with DAP12-based CARs are a promising tool for adoptive tumor immunotherapy.

In vivo fluorescence imaging and urinary monoamines as surrogate biomarkers of disease progression in a mouse model of pheochromocytoma

Pheochromocytoma (PHEO) is a rare but potentially lethal neuroendocrine tumor arising from catecholamine-producing chromaffin cells. Especially for metastatic PHEO, the availability of animal models is essential for developing novel therapies. For evaluating therapeutic outcome in rodent PHEO models, reliable quantification of multiple organ lesions depends on dedicated small-animal in vivo imaging, which is still challenging and only available at specialized research facilities. Here, we investigated whether whole-body fluorescence imaging and monitoring of urinary free monoamines provide suitable parameters for measuring tumor progression in a murine allograft model of PHEO. We generated an mCherry-expressing mouse PHEO cell line by lentiviral gene transfer. These cells were injected subcutaneously into nude mice to perform whole-body fluorescence imaging of tumor development. Urinary free monoamines were measured by liquid chromatography with tandem mass spectrometry. Tumor fluorescence intensity and urinary outputs of monoamines showed tumor growth-dependent increases (P < .001) over the 30 days of monitoring post-tumor engraftment. Concomitantly, systolic blood pressure was increased significantly during tumor growth. Tumor volume correlated significantly (P < .001) and strongly with tumor fluorescence intensity (rs = 0.946), and urinary outputs of dopamine (rs = 0.952), methoxytyramine (rs = 0.947), norepinephrine (rs = 0.756), and normetanephrine (rs = 0.949). Dopamine and methoxytyramine outputs allowed for detection of lesions at diameters below 2.3 mm. Our results demonstrate that mouse pheochromocytoma (MPC)-mCherry cell tumors are functionally similar to human PHEO. Both tumor fluorescence intensity and urinary outputs of free monoamines provide precise parameters of tumor progression in this sc mouse model of PHEO. This animal model will allow for testing new treatment strategies for chromaffin cell tumors.

Dual role of B7 costimulation in obesity-related nonalcoholic steatohepatitis and metabolic dysregulation

The low-grade inflammatory state present in obesity contributes to obesity-related metabolic dysregulation, including nonalcoholic steatohepatitis (NASH) and insulin resistance. Intercellular interactions between immune cells or between immune cells and hepatic parenchymal cells contribute to the exacerbation of liver inflammation and steatosis in obesity. The costimulatory molecules, B7.1 and B7.2, are important regulators of cell-cell interactions in several immune processes; however, the role of B7 costimulation in obesity-related liver inflammation is unknown. Here, diet-induced obesity (DIO) studies in mice with genetic inactivation of both B7.1 and B7.2 (double knockout; DKO) revealed aggravated obesity-related metabolic dysregulation, reduced insulin signalling in the liver and adipose tissue (AT), glucose intolerance, and enhanced progression to steatohepatitis resulting from B7.1/B7.2 double deficiency. The metabolic phenotype of B7.1/B7.2 double deficiency upon DIO was accompanied by increased hepatic and AT inflammation, associated with largely reduced numbers of regulatory T cells (Tregs) in these organs. In order to assess the role of B7 costimulation in DIO in a non-Treg-lacking environment, we performed antibody (Ab)-mediated inhibition of B7 molecules in wild-type mice in DIO. Antibody-blockade of both B7.1 and B7.2 improved the metabolic phenotype of DIO mice, which was linked to amelioration of hepatic steatosis and reduced inflammation in liver and AT.

CONCLUSION

Our study demonstrates a dual role of B7 costimulation in the course of obesity-related sequelae, particularly NASH. The genetic inactivation of B7.1/B7.2 deteriorates obesity-related liver steatosis and metabolic dysregulation, likely a result of the intrinsic absence of Tregs in these mice, rendering DKO mice a novel murine model of NASH. In contrast, inhibition of B7 costimulation under conditions where Tregs are present may provide a novel therapeutic approach for obesity-related metabolic dysregulation and, especially, NASH.

Redirection of CD4+ and CD8+ T lymphocytes via a novel antibody-based modular targeting system triggers efficient killing of PSCA+ prostate tumor cells

BACKGROUND

There is still a need for new therapeutic options against prostate cancer. Conventional single-chain bispecific antibodies (bsAbs), that directly cross-link T cells and tumor cells, hold great potential for efficient tumor treatment. However, rapid development of novel bsAbs is hampered by laborious optimization to improve their efficacy and reduce potential side effects. To accelerate the development of a novel antibody tool for the redirection of T cells to different tumor-associated antigens, we recently introduced a modular targeting system.

METHODS

We here describe a novel modular system for treatment of prostate cancer by retargeting of T cells to the prostate stem cell antigen (PSCA). Functionality of the novel PSCA-specific modular system was investigated in vitro by T cell activation and chromium release assays as well as in immunodeficient mice.

RESULTS

Similar to a conventional bsAb CD3-PSCA, the novel PSCA-specific modular system induces activation of both CD4+ and CD8+ T cells leading to secretion of pro-inflammatory cytokines and highly efficient target-specific tumor cell lysis. The novel TM was ready-to-use from the time point of construction and functional at low E:T ratios and picomolar concentrations without further optimization. In addition, the PSCA-specific modular system delays outgrowth of s.c. tumors in mice comparable to bsAb CD3-PSCA.

CONCLUSIONS

We have developed a novel PSCA-specific modular system which triggers an efficient T cell-mediated killing of PSCA+ tumor cells in vitro and in vivo. The new Ab-based targeting strategy can functionally replace conventional bsAbs and allows a flexible redirection of T cells to different tumor-associated antigens.

Simultaneous targeting of prostate stem cell antigen and prostate-specific membrane antigen improves the killing of prostate cancer cells using a novel modular T cell-retargeting system

BACKGROUND

Recently, we described a novel modular platform technology in which T cell-recruitment and tumor-targeting domains of conventional bispecific antibodies are split to independent components, a universal effector module (EM) and replaceable monospecific/monovalent target modules (TMs) that form highly efficient T cell-retargeting complexes. Theoretically, our unique strategy should allow us to simultaneously retarget T cells to different tumor antigens by combining the EM with two or more different monovalent/monospecific TMs or even with bivalent/bispecific TMs, thereby overcoming limitations of a monospecific treatment such as the selection of target-negative tumor escape variants.

METHODS

In order to advance our recently introduced prostate stem cell antigen (PSCA)-specific modular system for a dual-targeting of prostate cancer cells, two additional TMs were constructed: a monovalent/monospecific TM directed against the prostate-specific membrane antigen (PSMA) and a bivalent/bispecific TM (bsTM) with specificity for PSMA and PSCA. The functionality of the novel dual-targeting strategies was analyzed by performing T cell activation and chromium release assays.

RESULTS

Similar to the PSCA-specific modular system, the novel PSMA-specific modular system mediates an efficient target-dependent and -specific tumor cell lysis at low E:T ratios and picomolar Ab concentrations. Moreover, by combination of the EM with either the bispecific TM directed to PSMA and PSCA or both monospecifc TMs directed to either PSCA or PSMA, dual-specific targeting complexes were formed which allowed us to kill potential escape variants expressing only one or the other target antigen.

CONCLUSIONS

Overall, the novel modular system represents a promising tool for multiple tumor targeting.

Characterization of a novel single-chain bispecific antibody for retargeting of T cells to tumor cells via the TCR co-receptor CD8

There is currently growing interest in retargeting of effector T cells to tumor cells via bispecific antibodies (bsAbs). Usually, bsAbs are directed on the one hand to the CD3 complex of T cells and on the other hand to a molecule expressed on the surface of the target cell. A bsAb-mediated cross-linkage via CD3 leads to an activation of CD8+ T cells and consequently to killing of the target cells. In parallel, CD4+ T cells including TH₁, TH₂, TH₁₇ cells and even regulatory T cells (Tregs) will be activated as well. Cytokines produced by CD4+ T cells can contribute to severe side effects e. g. life-threatening cytokine storms and, thinking of the immunosupressive function of Tregs, can even be counterproductive. Therefore, we asked whether or not it is feasible to limit retargeting to CD8+ T cells e. g. via targeting of the co-receptor CD8 instead of CD3. In order to test for proof of concept, a novel bsAb with specificity for CD8 and a tumor-associated surface antigen was constructed. Interestingly, we found that pre-activated (but not freshly isolated) CD8+ T cells can be retargeted via CD8-engaging bsAbs leading to an efficient lysis of target cells.

A novel ex vivo isolation and expansion procedure for chimeric antigen receptor engrafted human T cells

Genetically engineered T lymphocytes are a promising option for cancer therapy. Prior to adoptive transfer they have to be expanded in vitro to reach therapeutically sufficient numbers. So far, no universal method exists for selective in vitro expansion of engineered T lymphocytes. In order to overcome this problem and for proof of concept we incorporated a novel unique peptide sequence of ten amino acids as epitope (E-Tag) into the binding domains of two novel chimeric antigen receptors (ECARs) directed against either prostate stem cell antigen (PSCA) for the treatment of prostate cancer (PCa) or CD33 for the treatment of acute myeloide leukemia (AML). The epitope tag then was utilized for expanding ECAR engrafted T cells by triggering the modified T cells via a monoclonal antibody directed against the E-Tag (Emab). Moreover, the E-Tag served as an efficient selection epitope for immunomagnetic isolation of modified T cells to high purity. ECAR engrafted T cells were fully functional and mediated profound anti-tumor effects in the respective models of PCa or AML both in vitro and in vivo. The method can be integrated straightforward into clinical protocols to improve therapeutic efficiency of tumor treatment with CAR modified T lymphocytes.

TCR/CD3 activation and co-stimulation combined in one T cell retargeting system improve anti-tumor immunity

We have recently described a novel modular targeting platform for T cell recruitment that not only efficiently replaces but also is superior to conventional T cell-engaging bispecific antibodies as it allows for the flexible targeting of several antigens and the delivery of co-stimulatory ligands to malignant lesions, thereby enhancing the antitumor potential of redirected T cells.

Retargeting of regulatory T cells to surface-inducible autoantigen La/SS-B

The nuclear autoantigen La can be detected on the surface of dying cells. Here we present an assay which enables us to show that La protein is not limited to the surface of dying cells but will be released upon stress-induced cell death. As released La protein tightly binds to the surface of neighboring intact cells we asked the question whether or not La protein could serve as a stress-inducible target e.g. for redirecting of regulatory T cells (Tregs) into damaged tissues to downregulate an immune response. In order to provide first proof of concept we developed a novel fully humanized single-chain bispecific antibody (bsAb) which on the one hand is directed to the La antigen and on the other hand to the CD3 complex of T cells. A cross-linkage of Tregs with La-decorated target cells mediated by this bsAb resulted indeed in the activation of the Tregs in a target-dependent manner. Moreover, such bsAb activated Tregs displayed a potent suppressive capacity and negatively influenced proliferation, expansion and cytokine production of autologous CD4(+) and CD8(+) Teff cells.

Novel humanized and highly efficient bispecific antibodies mediate killing of prostate stem cell antigen-expressing tumor cells by CD8+ and CD4+ T cells

Prostate cancer is the most common noncutaneous malignancy in men. The prostate stem cell Ag (PSCA) is a promising target for immunotherapy of advanced disease. Based on a novel mAb directed to PSCA, we established and compared a series of murine and humanized anti-CD3-anti-PSCA single-chain bispecific Abs. Their capability to redirect T cells for killing of tumor cells was analyzed. During these studies, we identified a novel bispecific humanized Ab that efficiently retargets T cells to tumor cells in a strictly Ag-dependent manner and at femtomolar concentrations. T cell activation, cytokine release, and lysis of target cells depend on a cross-linkage of redirected T cells with tumor cells, whereas binding of the anti-CD3 domain alone does not lead to an activation or cytokine release. Interestingly, both CD8+ and CD4+ T cells are activated in parallel and can efficiently mediate the lysis of tumor cells. However, the onset of killing via CD4+ T cells is delayed. Furthermore, redirecting T cells via the novel humanized bispecific Abs results in a delay of tumor growth in xenografted nude mice.

Silver staining techniques of polyacrylamide gels

Although the main application for polyacrylamide gels is the separation and subsequent blotting of proteins for immunodetection, there are tasks that need staining of proteins in the polyacrylamide gel. Several different staining techniques exist for protein staining in SDS gels that differ in their sensitivity, their expenditure of time, and other aspects. Still, silver staining is the most sensitive and reliable staining technique. Because this technique was developed in the 1970s, a huge number of variations exist. Therefore, we will provide herein three methods, which are robust and easy to perform.

Unexpected recombinations in single chain bispecific anti-CD3-anti-CD33 antibodies can be avoided by a novel linker module

CD33 is an attractive immunotarget on the surface of tumor cells from patients with acute myeloid leukemia (AML). In a first attempt for immunotargeting of AML blasts we constructed two bispecific antibodies in the single chain bispecific diabody (scBsDb) format by fusing the variable domains of monoclonal antibodies directed against CD3 and CD33. Unfortunately, protein expression of both scBsDbs resulted in varying mixtures of fragmented and full length proteins. As the non-functional fragments competed with the functional full length antibodies we tried to understand the reason for the fragmentation. We found that the anti-CD3 and anti-CD33 antibody genes show striking sequence homologies: during B cell development the same V(h) J558 heavy and V(l) kk4 light chain genes were selected. Moreover, the closely related D genes DSP2 (9 and 11) were combined with the same JH4 gene. And finally, during VJ recombination of the light chain the same JK5 element was selected. These homologies between the two monoclonal antibodies were the reason for recombinations in the cell lines generated for expression of the scBsDbs. Finally, we solved this problem by (i) rearranging the order of the heavy and light chains of the anti-CD3 and anti-CD33 domains, and (ii) a replacement of one of the commonly used glycine serine linkers with a novel linker domain. The resulting bispecific antibody in a single chain bispecific tandem format (scBsTaFv) was stable and capable of redirecting T cells to CD33-positive tumor cells including AML blasts of patients.

RNA interference targeting survivin exerts antitumoral effects in vitro and in established glioma xenografts in vivo

Malignant glioma represents the most common primary adult brain tumor in Western industrialized countries. Despite aggressive treatment modalities, the median survival duration for patients with glioblastoma multiforme (GBM), the highest grade malignant glioma, has not improved significantly over past decades. One promising approach to deal with GBM is the inactivation of proteins essential for survival or progression of glioma cells by means of RNA interference (RNAi) techniques. A likely candidate for an RNAi therapy of gliomas is the inhibitor of apoptosis protein survivin. Survivin is involved in 2 main cellular processes-cell division and inhibition of apoptosis. We show here that stable RNAi of survivin induced polyploidy, apoptosis, and impaired proliferation of human U343-MG, U373-MG, H4, and U87-MG cells and of primary glioblastoma cells. Proteome profiler arrays using U373-MG cells identified a novel set of differentially expressed genes upon RNAi-mediated survivin knockdown. In particular, the death receptor TRAIL R2/DR5 was strongly upregulated in survivin-depleted glioma cells, inducing an enhanced cytotoxic response of allogeneic human NK cells. Moreover, an experimental in vivo therapy using polyethylenimine (PEI)/siRNA complexes for survivin knockdown efficiently blocked tumor growth of established subcutaneous U373-MG tumors and enhanced survival of NMRI(nu/nu) mice orthopically transplanted with U87-MG cells. We conclude that survivin is functionally relevant in gliomas and that PEI-mediated exogenous delivery of siRNA targeting survivin is a promising strategy for glioblastoma therapy.

Retargeting of T cells to prostate stem cell antigen expressing tumor cells: comparison of different antibody formats

BACKGROUND

Prostate cancer (PCa) is the most common malignant disease in men. Novel treatment options are needed for patients after development of metastatic, hormone-refractory disease or for those who have failed a local treatment. The prostate stem cell antigen (PSCA) is expressed in >80% of primary PCa samples and bone metastases. Its expression is increased both in androgen-dependent and independent prostate tumors, particularly in carcinomas of high stages and Gleason scores. Therefore, PSCA is an attractive target for immunotherapy of PCa by retargeting of T cells to tumor cells.

METHODS

A series of different bispecific antibody formats for retargeting of T cells to tumor cells were described but, only very limited data obtained by side by side comparison of the different antibody formats are available. We established two novel bispecific antibodies in different formats. The functionality of both constructs was analyzed by FACS and chromium release assays. In parallel, the release of pro-inflammatory cytokines was determined by ELISA.

RESULTS AND CONCLUSIONS

Irrespective of the underlying antibody format, both novel bispecific antibodies cause an efficient killing of PSCA-positive tumor cells by pre- and non-pre-activated T cells. Killing and release of pro-inflammatory cytokines requires an antigen specific cross-linkage of the T cells with the target cells.

Degree of modification of Ro60 by the lipid peroxidation by-product 4-hydroxy-2-nonenal may differentially induce Sjögren syndrome or systemic lupus erythematosus in BALB/c mice

Our previous work showed that immunization of rabbits with 4-hydroxy-2-nonenal-modified Ro60 (HNE-Ro60) accelerates autoimmunity. We extended this model into mice, hypothesizing that the severity of autoimmunity would be dependent on the degree of HNE modification of Ro60. Five groups of BALB/c mice (10/group) were used. Group I was immunized with Ro60. Groups II to IV were immunized with Ro60 modified with 0.4 mM (low), 2 mM (medium), and 10 mM (high) HNE, respectively. Group V controls received Freund's adjuvant. A rapid abrogation of tolerance to Ro60/La antigens occurred in mice immunized with HNE-modified Ro60, especially in the low and medium HNE-Ro60 groups. Lymphocytic infiltration and significantly high decrement in salivary flow (37%) compared to controls was observed only in the high HNE-Ro60 group, suggesting induction of a Sjögren syndrome-like condition in this group. Anti-dsDNA occurred only in mice immunized with medium HNE-Ro60. This group did not have a significant decrement in salivary flow, suggesting induction of a systemic lupus erythematosus-like manifestation in this group. Significantly high antibodies to Ro60 were found in saliva of mice in the low and medium HNE-Ro60 and the Ro60 groups, as well as anti-HNE Ro60 in the low and medium HNE-Ro60 groups. Understanding the mechanism of this differential induction may help discriminate between these two autoimmune diseases.

A novel modular antigen delivery system for immuno targeting of human 6-sulfo LacNAc-positive blood dendritic cells (SlanDCs)

BACKGROUND

Previously, we identified a major myeloid-derived proinflammatory subpopulation of human blood dendritic cells which we termed slanDCs (e.g. Schäkel et al. (2006) Immunity 24, 767-777). The slan epitope is an O-linked sugar modification (6-sulfo LacNAc, slan) of P-selectin glycoprotein ligand-1 (PSGL-1). As slanDCs can induce neoantigen-specific CD4+ T cells and tumor-reactive CD8+ cytotoxic T cells, they appear as promising targets for an in vivo delivery of antigens for vaccination. However, tools for delivery of antigens to slanDCs were not available until now. Moreover, it is unknown whether or not antigens delivered via the slan epitope can be taken up, properly processed and presented by slanDCs to T cells.

METHODOLOGY/PRINCIPAL FINDINGS

Single chain fragment variables were prepared from presently available decavalent monoclonal anti-slan IgM antibodies but failed to bind to slanDCs. Therefore, a novel multivalent anti-slanDC scaffold was developed which consists of two components: (i) a single chain bispecific recombinant diabody (scBsDb) that is directed on the one hand to the slan epitope and on the other hand to a novel peptide epitope tag, and (ii) modular (antigen-containing) linker peptides that are flanked at both their termini with at least one peptide epitope tag. Delivery of a Tetanus Toxin-derived antigen to slanDCs via such a scBsDb/antigen scaffold allowed us to recall autologous Tetanus-specific memory T cells.

CONCLUSIONS/SIGNIFICANCE

In summary our data show that (i) the slan epitope can be used for delivery of antigens to this class of human-specific DCs, and (ii) antigens bound to the slan epitope can be taken up by slanDCs, processed and presented to T cells. Consequently, our novel modular scaffold system may be useful for the development of human vaccines.

Targeting of tumor cells expressing the prostate stem cell antigen (PSCA) using genetically engineered T-cells

BACKGROUND

Curative therapeutic options for minimal residual disease or advanced tumor stages in prostate cancer (PCa) are still missing. Adoptive transfer of cytotoxic T-cells that have been polyclonally rendered tumor-specific by genetic engineering appears to be a promising immunotherapeutic strategy. Among the numerous prostate tissue/tumor antigens identified during the last years, the "prostate stem cell antigen" (PSCA) is an attractive immunotherapeutic target. It is broadly expressed on the surface of primary PCa cells as well as on PCa metastases.

METHODS

To generate a chimeric T-cell receptor (TCR) recognizing PSCA, a monoclonal anti-PSCA antibody was raised and a single-chain fragment (scFv) was prepared. The resulting anti-PSCA scFv 7F5 was fused to the beta2 constant region derived from the beta-chain of a TCR and to the CD3zeta-signaling domain.

RESULTS

The chimeric alpha-PSCA-beta2/CD3zeta-TCR, expressed in Jurkat cells, was phosphorylated in the ITAMs of the CD3-zeta chain upon cross-linking by insolublized PSCA. When transduced into a mouse cytotoxic T-cell line, the chimeric receptor specifically activated cytotoxicity against PSCA-positive tumor cells.

CONCLUSIONS

We developed a functional chimeric TCR against PSCA for treatment of PCa. The chimeric alpha-PSCA-beta2/CD3zeta-TCR might now be used for arming human cytotoxic T-cells for further studies towards a clinical treatment of PCa.

N-terminal Gag domain required for foamy virus particle assembly and export

Among the Retroviridae, foamy viruses (FVs) exhibit an unusual way of particle assembly and a highly specific incorporation of envelope protein into progeny virions. We have analyzed deletions and point mutants of the prototypic FV gag gene for capsid assembly and egress, envelope protein incorporation, infectivity, and ultrastructure. Deletions introduced at the 3' end of gag revealed the first 297 amino acids (aa) to be sufficient for specific Env incorporation and export of particulate material. Deletions introduced at the 5' end showed the region between aa 6 and 200 to be dispensable for virus capsid assembly but required for the incorporation of Env and particle egress. Point mutations were introduced in the 5' region of gag to target residues conserved among FVs from different species. Alanine substitutions of residues in a region between aa 40 and 60 resulted in severe alterations in particle morphology. Furthermore, at position 50, this region harbors the conserved arginine that is presumably at the center of a signal sequence directing FV Gag proteins to a cytoplasmic assembly site.

Determination of the relative amounts of Gag and Pol proteins in foamy virus particles

We determined the relative ratios of Gag and Pol molecules in highly purified virions of spumaretroviruses or foamy viruses (FVs) using monoclonal antibodies and bacterially expressed reference proteins. We found that the cleaved p68^Gag moiety dominates in infectious FVs. Furthermore, approximate mean ratios in FV are 16:1 (pr71^Gag plus p68^Gag:p85^RT),12:1 (p68^Gag:p85^RT), and 10:1 (pr71^Gag plus p68^Gag:p40^IN). Thus, the results indicate that FVs have found a way to incorporate approximately as much Pol protein into their capsids as orthoretroviruses, despite a completely different Pol expression strategy.

Characterization of prototype foamy virus gag late assembly domain motifs and their role in particle egress and infectivity

Foamy viruses (FV) are unusual among retroviruses since they require both Gag and Env structural proteins for particle egress. Recently significant progress has been made towards the mechanistic understanding of the viral release process, in particular that of retroviruses, and the viral domains and cellular pathways involved. However little is currently known about domains of FV structural proteins and cellular proteins engaged in this process. By mutational analysis of sequence motifs in prototype FV (PFV) Gag, bearing homology to known late assembly (L) domains, a PSAP motif with L domain function that was functionally interchangeable by heterologous L domains was identified. In contrast the inactivation of a PPPI motif had no significant influence on PFV particle release, although mutant viral particles displayed reduced infectivity. Similarly mutation of an evolutionary conserved YXXL motif revealed no classical L-domain function but resulted in release of noninfectious viruslike particles. Biochemical and electron microscopy analysis demonstrated that these mutant particles incorporated all viral structural proteins but contained aberrantly capsid structures, suggesting a role in capsid assembly for this PFV Gag sequence motif. In line with the mutational analysis, overexpression of dominant negative (DN) mutants and wild-type TSG101 but not the DN mutant of AIP-1/ALIX reduced PFV particle release and infectivity. Furthermore, DN mutants of Vps4A, Vps4B, and CHMP3 inhibited PFV egress and infectivity. Taken together these results demonstrate that PFV, like other viruses, requires components of the vacuolar protein sorting (VPS) machinery for egress and enters the VPS pathway through interaction with TSG101.