Transgenic Expression of IL15 in CD123-Specific BiTE-Secreting Engager T-Cells Results in Improved Anti-AML Activity
Authors: Mu-Mosley Hong , Ostermann Lauren B , Muftuoglu Muharrem , Schober Wendy , Patel Nalini B , Vaidya Abishek , Bonifant Challice L. , Gottschalk Stephen , Velasquez Mireya Paulina , Andreeff Michael 
Published: November 2019

Background: CD123 is frequently expressed on hematologic malignancies including 96-98% of AML. CD123 has been a potential immunotherapeutic target in AML due to its association with leukemic stem cells that play an essential role in disease progression and relapse. Our previous study using T-cells secreting CD123/CD3-bispecific T-cell engagers (BiTEs) (CD123-ENG T-cells) showed a promising approach anti-AML activity, however T-cell persistence was limited. Interleukin-15 (IL15) has emerged as a candidate immunomodulator as it enhances T-cell expansion and persistence, and induces long-lasting memory T-cells. To improve the efficacy and persistence of CD123-ENG T-cells we developed IL15 expressing CD123-ENG T-cells. Here, we report on the characterization and efficacy of IL15-secreting CD123-ENG T cells in vitro and in vivo models of adult AML.
Methods/Results: A cDNA encoding IL15 was cloned into retroviral vectors encoding CD123-ENG or CD19-ENG (CD123-ENG.IL15; CD19-ENG.IL15). ENG T-cells were generated from human peripheral blood mononuclear cells (PBMCs) from normal donors or T-cells from AML patients by retroviral transduction and in vitro expansion. Non-transduced (NT) T-cells and T-cells expressing CD123 (CD123-ENG T-cells) served as controls. IL15 production of CD19-ENG.IL15 and CD123-ENG.IL15 T cells was confirmed by ELISA (144-159 pg/ml vs 38 and 46 pg/ml of NT and CD123-ENG T cells, p<0.01, n=6). Both CD123-ENG and CD123-ENG.IL15 T-cells recognized CD123+ AML cells as judged by IL2 and interferon γ (IFNγ) production (p<0.01, n=5). In contrast, NT and CD19-ENG.IL15 T-cells did not, confirming specificity. In addition, CD123-ENG.IL15 and CD123-ENG T-cells induced killing of only CD123-positive target cells as well as of primary adult patients' AML blasts in luciferase- or 7AAD-based cytotoxicity assays (p<0.001, n=10). CD123-ENG.IL15 T-cells showed greater cytolytic activity than CD123-ENG T-cells as determined by luciferase activity (p=0.0002, n=3). In a Molm13 AML xenograft model, CD123-ENG.IL15 and CD123-ENG T-cells exhibited potent anti-leukemic activity as judged by bioluminescence imaging. Moreover, CD123-ENG.IL15 T cells had enhanced anti-leukemic activity and greater persistence in BMs, spleens, and livers in comparison to CD123-ENG T cells, resulting in improved anti-AML activity (Figure 1, p<0.01 vs CD123-ENG T-cell group, n=12 per group) and extended survival (Figure 2, p=0.0097 vs CD123-ENG T-cell group). Finally, AML PDX models and ENG T-cells were generated from AML blasts and T-cells from 3 patients with active AML. Infusion of autologous ENG T-cells (1.5x106 cells/mouse, n=7) in AML PDX#6697688 mouse model revealed significant reduction of leukemia burden in the CD123-ENG.IL15 or CD123-ENG T-cells mouse groups but not in the mouse group with NT or CD19-ENG.IL15 T-cells or PBS (p=0.004, n=6-8). We are currently monitoring survival of these PDX models.
Conclusion: We here demonstrated that transgenic expression of IL15 in CD123-ENG T-cells results in improved expansion and persistence, and anti-AML activity. These results warrant further exploration of IL15-modified CD123-targeted T-cells as immunotherapy for AML.

Disclosures
Bonifant: Patents filed in the field of engineered cellular therapies: Patents & Royalties: Patents filed in the field of engineered cellular therapies. Gottschalk:EMD Serono: Honoraria; Inmatics: Membership on an entity's Board of Directors or advisory committees; ASSISI fundation of Memphis: Research Funding; TESSA Therapeutics: Other: Research Collaboration; ViraCyte: Consultancy; NHLBI: Research Funding; America Lebanese Syrian Associated Charities: Research Funding; California Institute for Regenerative Medicine: Research Funding; Patents and patent applications in the fields of T-cell & Gene therapy for cancer: Patents & Royalties; MBIO: Other: St. Jude Children's Research Hospital has an existing exclusive license and ongoing partnership with Mustang Bio for the further clinical development and commercialization of this XSCID gene therapy; Sanofi: Honoraria; Tidal: Membership on an entity's Board of Directors or advisory committees; Merck: Consultancy. Velasquez:St. Jude: Patents & Royalties: Patent Applications in the Fields of Cell and Gene Therapy ; Rally! Foundation: Membership on an entity's Board of Directors or advisory committees. Andreeff:Senti Bio: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; AstaZeneca: Consultancy; Amgen: Consultancy; Eutropics: Equity Ownership; Aptose: Equity Ownership; Reata: Equity Ownership; 6 Dimensions Capital: Consultancy; Celgene: Consultancy; Jazz Pharmaceuticals: Consultancy; Daiichi Sankyo, Inc.: Consultancy, Patents & Royalties: Patents licensed, royalty bearing, Research Funding; Cancer UK: Membership on an entity's Board of Directors or advisory committees; NCI-CTEP: Membership on an entity's Board of Directors or advisory committees; German Research Council: Membership on an entity's Board of Directors or advisory committees; Leukemia Lymphoma Society: Membership on an entity's Board of Directors or advisory committees; NCI-RDCRN (Rare Disease Cliln Network): Membership on an entity's Board of Directors or advisory committees; CLL Foundation: Membership on an entity's Board of Directors or advisory committees; BiolineRx: Membership on an entity's Board of Directors or advisory committees; NIH/NCI: Research Funding; Center for Drug Research & Development: Membership on an entity's Board of Directors or advisory committees; CPRIT: Research Funding; Oncoceutics: Equity Ownership; Oncolyze: Equity Ownership; Breast Cancer Research Foundation: Research Funding.