CAR T-cells targeting CD117 effectively eliminate mast cells in preclinical models of advanced systemic mastocytosis

Leukemia ARTICLE www.nature.com/leu OPEN CAR T-cells targeting CD117 effectively eliminate mast cells in preclinical models of advanced systemic mastocytosis Anne Kaiser1,2,3, Veronika Lysenko 1, Renier Myburgh1, Laura Volta1, Christian Pellegrino 1, Alexandre P. A. Theocharides Deborah Christen2,3, Jens Panse 2,3, Marco M. Bühler 4,5,6, Michel Arock 7, Joseph Butterfield8, Marcelo A. S. de Toledo ✉ Peter Valent 9,10, Martin Zenke 2,3, Tim H. Brümmendorf2,3 and Markus G. Manz 1,4 1,4 , , 2,3 © The Author(s) 2026 1234567890();,: Systemic mastocytosis (SM) is characterized by uncontrolled expansion of neoplastic mast cells (MCs) and their accumulation in various tissues and organs, ranging from indolent variants to more advanced forms (advSM). Although several MC- and SMexpressed cell surface antigens have been identified, no immune therapy has been developed for advSM so far. The receptor tyrosine kinase KIT (CD117) is highly expressed on MCs, exceeding the levels of expression on hematopoietic stem and progenitor cells (HSPC). Therefore, targeting CD117 in advSM could be of therapeutic value. In this study, we assessed the therapeutic potential of anti-CD117 chimeric antigen receptor (CAR) T-cells to target neoplastic MCs in SM. In vitro, anti-CD117-CAR T-cells efficiently lysed several SM-related human MC cell lines, MCs differentiated from SM patient-derived induced pluripotent stem (iPS) cells, and neoplastic bone marrow cells obtained from SM patients. Furthermore, in immunocompromised mice engrafted with an advSM-like MC cell line, repetitive applications of anti-CD117-CAR T-cells were able to inhibit MC expansion. These data may pave the way for the development of anti-CD117-CAR T-cell therapies in advSM. Leukemia; https://doi.org/10.1038/s41375-026-02968-5 INTRODUCTION In up to 90% of all patients with systemic mastocytosis (SM), the neoplastic cells harbor the KIT D816V mutation. This mutation is located in the intracellular domain of the tyrosine kinase receptor KIT (CD117), leading to constitutive activation of its kinase activity and thereby to ligand-independent differentiation and survival of neoplastic mast cells (MCs) [1–3]. Despite recent advancements in the treatment of SM, such as tyrosine kinase inhibitors (TKIs) like midostaurin [4] and the more specific KIT D816V-targeting TKI avapritinib [5–8], advanced forms of SM (advSM: aggressive SM, ASM, SM with an associated hematological neoplasm, SM-AHN, and MC leukemia, MCL [2, 9, 10]) still pose substantial therapeutic challenges. While avapritinib improved the overall response rate to 75% in advSM, including several patients in complete remission, not all patients respond and several relapse after an initial response to avapritinib [5]. In the pre-TKI era, median overall survival ranged from 3.5 years for patients with ASM to less than six months for MCL [11, 12]. Currently, the only available curative approach, especially for TKI refractory patients, is conditioning polychemotherapy followed by allogeneic hematopoietic stem cell transplantation (allo-HSCT) [13–17]. This procedure is recommended for young and fit patients as early as possible during the 1 disease course to achieve optimal MC debulking and remission [13–19]. However, allo-HSCT is associated with substantial transplant-related morbidity and mortality as well as a considerable relapse rate, with three-year overall survival of only 43% for patients with ASM and 17% for patients with MCL [16]. These limitations highlight the urgent need for more effective therapeutic strategies. In pursuit of improved treatment options for advanced and resistant neoplasms in clinical hematology, novel drugs and novel immunotherapeutic strategies, particularly chimeric antigen receptor (CAR) T-cell therapies, have been developed and have shown therapeutic efficacy across various hematological malignancies [20, 21]. The clinical success, which led to the approval of CAR T-cell therapy for B-cell malignancies e.g., [22, 23], paved the way for exploring this approach also for targets expressed on myeloid malignancies [24, 25]. Notably, our group has demonstrated the efficacy of anti-CD117-CAR T-cells and anti-CD117 bispecific T-cell engagers in CD117+ human acute myeloid leukemia (AML) cells both in vitro and in vivo models [26–28]. Given that MCs express CD117 in excess compared to other myeloid cells, we here tested the efficacy of human anti-CD117CAR T-cells against various SM cell lines and primary SM cells using preclinical in vitro and in vivo model systems. Department of Medical Oncology and Hematology, University Hospital Zurich and University of Zurich, Zurich, Switzerland. 2Department of Hematology, Oncology, Hemostaseology, and Stem Cell Transplantation, Faculty of Medicine, Uniklinik Aachen, RWTH Aachen University, Aachen, Germany. 3Center for Integrated Oncology Aachen Bonn Cologne Düsseldorf (CIO ABCD), Aachen, Germany. 4Comprehensive Cancer Center Zurich (CCCZ), Zurich, Switzerland. 5Department of Pathology and Molecular Pathology, University Hospital Zurich, Zurich, Switzerland. 6The LOOP Zurich, Zurich, Switzerland. 7CEREMAST, Department of Hematological Biology, Pitié-Salpêtrière Hospital, Paris Sorbonne University, Paris, France. 8Divisions of Allergy, Asthma and Immunology, Mayo Clinic, Rochester, Minnesota, USA. 9Department of Internal Medicine I, Division of Hematology and Hemostaseology, Medical University of Vienna, Vienna, Austria. 10Ludwig Boltzmann Institute for Hematology and Oncology, Medical University of Vienna, Vienna, Austria. ✉email: Received: 23 July 2025 Revised: 20 March 2026 Accepted: 14 April 2026 A. Kaiser et al. 2 MATERIAL AND METHODS In vitro culture of cell lines and patient-derived cells Cell lines were established and cultured as previously described [29–32], details are provided in the Supplementary Material and Methods. Isogenic iPS cells with and without KIT D816V mutation (Supplementary Fig. 1A) and MC differentiation were previously reported [33, 34]. All patient samples were obtained during routine sampling with informed written consent, in accordance with the Declaration of Helsinki, and were stored at RWTH Aachen University centralized Biomaterial Bank (RWTH cBMB; Project number 41-2020, application number 274, ethical approval number 206/09). CAR T-cell generation Second-generation anti-CD117-CAR T-cells (clone 79D, 4-1BB costimulatory domain) were generated via lentiviral transduction as previously described [26, 27]. A detailed description is provided in Supplementary Material and Methods. Lentiviral particles were produced by transfection of HEK293T cells [27]. The RQR8 gene sequence was kindly provided by Dr. Martin Pule (University College London, UK). In vitro studies on CAR T-cell mediated MC lysis Human MCs (cell lines, iPS cell-derived, bone marrow-derived) were cultured in the appropriate medium (Supplementary Material and Methods). CAR T-cells and control (ctrl.) T-cells were thawed and (...truncated)