CD123 and CD33 Co-Targeting By Balanced Signaling on CAR-CIK Cells Reduces Potential Off-Target Toxicity While Preserving the Anti-Leukemic Activity of Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) still represents an unmet clinical need for adult and pediatric high-risk patients. Adoptive cell therapy by chimeric antigen receptor (CAR) T cells demonstrated a high therapeutic potential in B-acute lymphoblastic leukemia, but translation in AML is limited by the absence of an ideal tumor-specific antigen. CD123 (known as IL-3 receptor alfa) and CD33 satisfy several features of ideal target antigens as they are expressed in almost all AML patients (mainly overexpressed in NPM1 and FLT-3 mutated AML), and conserved at disease relapse . Despite that, CD33 expression on hemopoietic stem/progenitor cells (HSPC) induced prolonged myelotoxicity after anti-CD33 CAR-T cell therapy and CD123 expression on endothelium was responsible for severe capillary leak syndrome after anti-CD123 CAR therapy. With the aim to improve selectivity for CD123+/CD33+ AML cells while minimizing toxicity against healthy cells, we probe a dual targeting model by Cytokine Induced Killer (CIK) cells co-expressing a first generation low affinity anti-CD123 IL-3 zetakine (IL3z.CAR) and an anti-CD33 as costimulatory receptor without activation signaling domains (CD33.CCR). This trans-signaling strategy could allow: 1) low toxicity profile against CD123+ endothelial cells and HSPC, due to a reduced cell activation given by the suboptimal first-generation CAR signal; 2) no or low myelotoxicity against CD33+ HSPC cells, due to absence of CIK cell activation upon the sole costimulatory signal engagement; 3) full CAR-CIK activation only against double expressing CD123 + / CD33 + leukemic cells.

Fresh and frozen peripheral blood mononuclear cells were transduced with retroviral vectors during the CIK cell differentiation process . The functional activity of CAR-CIK cells was assessed in vitro by means of short- and long- term cytotoxicity and cytokine production assays upon challenge with different CD123/CD33 positive AML cell lines (THP-1 and KG-1) or with a CD123 positive human endothelial cell line (TIME). To assess the myelotoxicity against HSPC, CD34+ cells were sorted after 24h co-culture with CAR-CIK cells and mixed with methylcellulose-based semisolid medium to evaluate Colony Forming Units (CFU) after 14 days through an automated imaging and counting system for hematopoietic colonies (Stemvision). The in vivo anti-leukemic activity was evaluated by monitoring luciferase-expressing KG1 AML cell line growth over time in NSG mice treated with 3 infusions of all different CAR-CIK conditions.

Dual CAR-CIK cells (IL-3z.CAR/CD33.CCR) display a potent and specific in vitro anti-leukemic efficacy against all the AML cell lines tested, compared to single targeting IL-3z.CAR and non-transduced CIK cells. To further minimize toxicities against healthy cells exploiting differences in CD123 antigen density between leukemic and healthy cells, we generated a low affinity dual CAR decreasing the IL3z.CAR binding affinity to CD123, by site-directed mutagenesis. Low affinity dual CAR-CIK cells show irrelevant cytotoxicity against the TIME endothelial cell line, comparable to non-transduced CIK cells, while preserving the efficacy against THP1 and KG1 AML cell lines. As a confirm of the reduced reactivity against endothelial cells, low affinity dual CAR-CIK cells produce the same levels of IL-2 and IFNγ either alone or in the presence of TIME cell line. Low affinity dual CAR-CIK cells also preserve the in vitro CFU-E, CFU-GM and CFU-GEMM colony forming capacity as compared to wild type (wt) dual CAR-CIK and wt IL-3z.CAR-CIK cells, highlighting the possibility to find a therapeutic window to minimize toxicity on healthy cells. The anti-leukemic activity of low affinity dual CAR-CIK cells has been confirmed also in vivo, with a significant suppression of leukemic growth against KG1 AML cell line. Mice treated with 3 infusions of low affinity dual CAR-CIK cells exhibited a significant better anti-leukemic profile, when compared to IL-3-single targeting CAR-CIK cells in terms of tumor growth and overall survival.

These preclinical data demonstrate a powerful antitumor efficacy mediated by low affinity dual targeting IL-3z.CAR/CD33.CCR CIK cells against AML targets without any relevant in vitro toxicity on HSPC and endothelial cells, offering a proof-of-concept strategy to increase selectivity for CD123+/CD33+ AML cells whilst reducing the risk of "on-target off-tumor toxicity".