CD117 As an Immunotherapeutic Target in Advanced Forms of Mastocytosis
Authors: Kaiser Anne , Myburgh Renier , Volta Laura , Pellegrino Christian Edoardo , Manz Markus G 
Published: November 2021

Abstract
Mastocytosis is a malignant disease resulting from oncogenic transformed mast cells. Up to 80% of malignant cells harbor a D816V mutation in the KIT-receptor (CD117), leading to constitutive kinase activation and proliferation and survival of mast cells. Advanced forms of mastocytosis (aggressive systemic mastocytosis: ASM, systemic mastocytosis with associated hematological disease: SM-AHN, mast cell leukemia: MCL) present as a therapeutic challenge. Although the recently approved poly tyrosine kinase inhibitor Midostaurin provides some improvement, the median overall survival ranges from 3.5 years (ASM) to less than six months (MCL). The reduced life expectancy is frequently due to mast cell infiltration resulting in multi organ failure. Additionally, there are patients who do not benefit from the treatment with Midostaurin (overall response 60%) or suffer from side effects, which lead to reduction or termination of therapy. Currently, the only available curative approach is conditioning poly-chemotherapy followed by allogenic stem cell transplantation (allo-HSCT). However, allo-HSCT is associated with substantial side-effects and, also due to high rates of relapse, only leads to an overall survival of 43% for ASM and 17% for MCL after three years. Thus, better therapeutic options are needed.
Recently, we demonstrated that CD117 (KIT-receptor) positive human AML can be efficiently eradicated by anti-CD117 CAR T-cells in vitro and in vivo (Myburgh et al., Leukemia 2020). As mast cells, and also transformed mast cells, highly express CD117, we here tested if anti-CD117 CAR T-cells would equally efficiently eliminate this malignant cell population. We thus co-cultured various established mast cell lines (partly harboring the oncogenic driver mutation KIT D816V) with anti-CD117-CAR T-cells in a 1:1 effector to target ratio in vitro. After 24 hours of co-culturing, the tumor cells were effectively killed, and this was still observed despite increasing the effector to target ratio to 1:4. Also, within 28 days of co-culture, the longest time followed in vitro, tumor cells were controlled and did not outgrow. Increased proliferation of anti-CD117-CAR T-cells in the presence of mast cells was observed and tracked throughout the 28-day experiment.
In conclusion, we demonstrate that the human mast cell lines HMC-1.1 KIT V560G, HMC-1.2 KIT V560G, KIT D816V, ROSA KIT WT, ROSA KIT D816V, LAD2 and MCPV-1 can be efficiently targeted and killed in vitro by allogeneic anti-CD117-CAR T-cells. Given that CD117 is expressed on healthy hematopoietic stem and progenitor cells (HSPCs) on a substantially lower level, there might be a therapeutic window for anti-CD117 immunotherapy in advanced forms of mastocytosis. However, as CAR T-cells are highly efficient, collateral damage on healthy HSPCs will likely need to be compensated by subsequent HSC transplantation. We are currently translating these promising in vitro immunotherapeutic settings into surrogate xenogeneic in vivo models.

Disclosures
Myburgh: University of Zurich: Patents & Royalties: CD117xCD3 TEA. Manz: CDR-Life Inc: Consultancy, Current holder of stock options in a privately-held company; University of Zurich: Patents & Royalties: CD117xCD3 TEA.