P1356: cd84-targeted cart-cell therapy: a novel strategy for the treatment of haematological malignancies

Topic: 24. Gene therapy, cellular immunotherapy and vaccination - Biology & Translational Research Background: Chimeric antigen receptor (CAR) T-cell therapies have achieved outstanding results in relapsed or refractory (R/R) B-cell malignancies, including multiple myeloma. However, none has been yet approved for other haematological malignancies. CD84 (SLAMF5) is a SLAM family immunoreceptor that is expressed in several immune cells and it has been reported to be overexpressed in chronic lymphocytic leukaemia (CLL). Aims: Validating CD84 as a target for haematological malignancies, in concrete, for acute myeloid leukaemia (AML) and T-cell acute lymphoblastic leukaemia (T-ALL). Engineer a first-in-class CD84-targeted CAR T-cell therapy (CART84) for the treatment of multiple haematological malignancies. Methods: CD84 expression was studied in immune cells (among them, in hematopoietic stem/progenitor cells (HSPC)) and in malignant cells by flow cytometry; and in human tissues by immunohistochemical staining. Second generation CART84 were designed with different murine or fully human anti-CD84 scFv and a 4-1BB as a costimulatory domain. CART84 in vitro efficacy was determined by cytotoxicity, cytokine production and proliferation assays. Potential myelotoxicity of CART84 was studied by cytotoxicity assays towards CD34+HSPC. CART84 in vivo efficacy was studied in immunocompromised NSG mice using AML and T-ALL cell lines or patient derived-xenografts (PDX). Results: We found that CD84 is highly expressed on the surface of primary patient malignant cells from CLL, AML and T-ALL, whereas its expression was low on CD34+ HSPC. Immunohistochemistry analysis of healthy human tissues revealed that CD84 is not expressed in lung, liver, kidney, myocardium, skin, or brain (cerebral cortex and cerebellum), although expression was detected in tissue macrophages. This expression pattern is similar to that of CD123, the target of several CAR T cells in clinical development. CART84 successfully expanded in vitro and exerted high cytotoxicity towards cell lines from different haematological malignancies including aggressive B-cell lymphoma (Ramos), AML (MOLM-13) and T-ALL (MOLT-4) (Figure 1.A). CART84 CD84.02, CD84.03, and CD84.05 showed specific proliferation and cytokine secretion when co-cultured with CD84-positive cells in vitro. The cytotoxic effect of CART84 against CD34+ HSPC was much lower than that against leukemic cells. Despite low cytotoxicity towards HSPC, we decided to focus on AML and T-ALL, since these patients can be rescued with an allogeneic hematopoietic stem cell transplant (allo-HSCT). In an immunodeficient NSG mouse model, CART84 CD84.02 and CD84.03 controlled both MOLM-13 (AML) and MOLT-4 (T-ALL) tumour progression and increased mice survival in comparison with the control group (Figure 1.B). Similarly, positive results were obtained in AML and T-ALL PDX models. Summary/Conclusion: CD84 is a promising CART target for the treatment of several haematological malignancies. CART84 displayed high cytotoxicity towards AML and T-ALL cells both in vitro and in vivo. Our data support the therapeutic use of CART84 for haematological malignancies. However, due to its potential myelotoxicity, we encourage it as a bridge therapy to allo-HSCT for patients with R/R AML and T-ALL.Keywords: CAR-T, T cell acute lymphoblastic leukemia, Acute myeloid leukemia