Engineering Naturally Occurring CD7 Negative Cells for the Immunotherapy of CD7 Positive Leukemia
Authors: Freiwan Abdullah , Vaidya Abishek , Zebley Caitlin , Fan Yiping , Langfitt Deanna , Youngblood Benjamin , Mamonkin Maksim , Gottschalk Stephen , Velasquez Mireya Paulina 
Published: February 2020

Background:
CD7 has emerged as a promising target for the adoptive immunotherapy with T-cells expressing chimeric antigen receptors (CAR T-cells) of CD7+ T-cell acute lymphoblastic leukemia (T-ALL) and acute myeloid leukemia (AML). However, expressing CD7 CARs in T-cells results in fratricide due to high expression of CD7 in most T-cells. While investigators have developed strategies to overcome this limitation by additional genetic modifications of CD7 CAR T-cells, the goal of this project was to explore the feasibility of selecting and genetically modifying naturally occurring CD7 negative (CD7-) T cells for the adoptive immunotherapy of CD7+ leukemia.
Methods:
CD7- T-cells were isolated from PBMCs using a 2-step magnetic bead depletion/selection procedure (CD7 depletion followed by selection of CD3+ T cells from the CD7- fraction). Non-selected T-cells (bulk T-cells), CD7+ and CD7- T cells were activated and transduced with a retroviral vector encoding a second-generation CD7 CAR with a CD28 costimulatory endodomain, and expanded with IL7 and IL15. The effector function of CD7- T-cells expressing CD7 CARs (CD7 CARCD7- T cells) was assessed in vitro as well as in xenograft models.
Results:
To assess the feasibility of our approach, we first determined the frequency of CD7- T-cells in PBMCs. On average, 4.7 % of T cells were CD7- (range: 2% - 12.3%; N=22), and we successfully selected these cells from bulk PBMCs with a combined CD7 depletion/CD3 selection procedure. We genetically modified CD7-, CD7+ and bulk T cellsto express CD7 CARs (CD7 CARCD7-, CD7 CARCD7+, CD7 CARBulk). Transduction efficiencies ranged from 31% to 75% (± 5%) for each T-cell population. Post transduction, CD7 CARCD7- T-cells did not undergo fratricide and had similar expansion kinetics (N=6, p=ns) in comparison to non-transduced (NT) T-cell cultures (NT CD7-, NT CD7+, NT bulk). In contrast, CD7 CARCD7+or CD7 CARBulk T-cells underwent fratricide and did not expand (N=6, p&lt;0.0001). CD7- T-cells (NT and CD7 CARCD7-) had a predominantly CD4+ effector memory phenotype at day 7 and 14 of culture. To assess the effector function of CD7 CARCD7- T-cells, we co-cultured them with CD7+ T-ALL cell lines (CCRF, MOLT3). CD7 CARCD7- T-cells recognized CD7+ targets in contrast to CD7- targets (BV173, Daudi) as evidenced by significant (N=6, p&lt;0.0001) IFN-γ and IL-2 production. Control CAR T-cells (CD19 CARCD7-) did not recognize CD7+ target cells, confirming specificity. CD7 CARCD7- T-cells also had potent cytolytic activity against CD7+ targets in cytotoxicity assays. To assess in vivo the anti-tumor activity of CD7 CARCD7- T-cells, we used a NSG mouse xenograft model with CCRF cells, genetically modified to express firefly luciferase (CCRF.ffluc) to allow for serial bioluminescence imaging. A single infusion of CD7 CARCD7- T-cells had potent anti-leukemia activity as judged by serial imaging resulting in a significant survival (p&lt;0.003) advantage in comparison to control mice.
Conclusion:
We have successfully generated CD7 CARCD7- T-cells from peripheral blood CD7- T-cells. CD7 CARCD7- T-cells had a predominantly CD4+ effector memory phenotype, and potent anti-leukemia activity in vitro and in vivo. Thus, naturally occurring CD7- T cells may present a promising T-cell source for the cellular immunotherapy of CD7+ leukemia.

Disclosures
Langfitt: 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. Youngblood: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. Gottschalk:NHLBI: Research Funding; America Lebanese Syrian Associated Charities: Research Funding; ASSISI fundation of Memphis: Research Funding; California Institute for Regenerative Medicine: Research Funding; ViraCyte: Consultancy; 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; Patents and patent applications in the fields of T-cell & Gene therapy for cancer: Patents & Royalties; TESSA Therapeutics: Other: Research Collaboration; Tidal: Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria; EMD Serono: Honoraria; Merck: Consultancy; Inmatics: Membership on an entity's Board of Directors or advisory committees. 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.