Abstract 1630: FLT3 chimeric antigen receptor T cell therapy induces B to T cell lineage switch in infant acute lymphoblastic leukemia

Though childhood acute lymphoblastic leukemia (ALL) is highly treatable, there remain subsets of pediatric ALL with very poor prognoses. Infant ALL, found in children under the age of 1, is difficult to treat due to the scarcity of cases impeding the ability of even the largest pediatric oncology centers from gaining experience in treating the disease, the more aggressive initial clinical presentation, as well as the inability for these young patients to tolerate toxicities associated with chemotherapeutic regimens and procedures. Despite being only 5% of total ALL cases, 80% of infant ALL cases are marked by mixed lineage leukemia (MLL/KMT2A) rearrangements. In KMT2A rearranged (KMT2Ar) ALL, FLT3 is the most differentially expressed gene that distinguishes KMT2Ar ALL from non-KMT2Ar ALL making FLT3 an attractive target for infant ALL. CD19 and CD22 targeting chimeric antigen receptor (CAR) T cell therapy has demonstrated outstanding responses in phase 1 clinical trials for relapsed/refractory B ALL, leading to tremendous interest in testing other CAR targets. Here we explore FLT3 CAR as a potential treatment for B ALL and the unexpected finding that FLT3 CAR T cells induce lineage switch of an infant ALL from a B to T cell phenotype. We generated a FLT3-targeting CAR consisting of a FLT3 binding domain derived from a well-characterized anti-human FLT3 antibody coupled to 4-1BB costimulatory and CD3-zeta activation domains. In vitro studies confirmed that human T cells expressing the FLT3 CAR produced interferon-gamma and interleukin-2 after co-culture with KMT2Ar B ALL SEM and infant B ALL KOPN8. FLT3 CAR T cells eliminated ALL in vivo in NOD-SCID-IL2Rγc-/- (NSG) mice engrafted with high FLT3 expressing SEM. KOPN8, which expresses lower levels of FLT3 when treated with FLT3 CAR T cells showed an initial clearance of leukemia in NSG mice, however relapsed with ALL that no longer expressed FLT3 or B cell marker CD19. Interestingly, this loss of FLT3 and CD19 happened concurrently with gain of T cell markers (CD3+ and either CD4+ or CD8+). The durability of this T cell phenotype was transient because when T lineage switched KOPN8 was cultured ex vivo without immune pressure, the KOPN8 cells reverted to the parental B ALL phenotype (FLT3+, CD19+, CD3 neg, CD4 neg, CD8 neg) suggesting that the ability to lineage switch is not a selection of a clone that genetically does not express B cell markers while expressing T cell markers, but rather a potential epigenetic mechanism driving the cell lineage change. Contrary to reports from CD19 CAR treated KMT2Ar B ALL that switched to a myeloid phenotype, these cells did not upregulate myeloid markers (CD33, CD11b). Taken together these data imply that lineage switch driven by CAR T cell immune pressure may cause different types of lineage switch based on the target of the CAR. Furthermore, using CAR T cell immunotherapy we may be able to interrogate the biology of leukemia. Citation Format: Christopher D. Chien, Lila Yang, Sang M. Nguyen, Christopher T. Sauter, Kazusa Ishii, Feng Shen, Sarah K. Tasian, Terry J. Fry. FLT3 chimeric antigen receptor T cell therapy induces B to T cell lineage switch in infant acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1630.