P389: BISPECIFIC ANTIBODIES ACTIVATE AUTOLOGOUS LEUKEMIA-REACTIVE T CELLS IN ACUTE MYELOID LEUKEMIA

Abstract

Background: Acute myeloid leukemia (AML) is a highly immunosuppressive disease with a high risk of relapse. This makes the finding of effective immunotherapy-based treatments important, but also difficult. T cell-engaging bispecific antibodies (TCEs) are designed to bind to both T cells and target cells, activate the T cells and thus induce an immune response against the leukemia cells without employing the MHC-T cell receptor (TCR) interaction. Several therapies of this kind are under development for treatment of AML. Surprisingly, both in vitro and in vivo, a subset of these TCE immunocoached T cells (ICTs) can subsequently target autologous AML cells and mediate efficient TCR-MHC-dependent AML cell killing in the absence of the TCE. This points to that a) a portion of the T cells present in an AML are leukemia-reactive but have been immunosuppressed - a suppression that can be reversed by the addition of a TCE, b) that this anti-AML T cell reactivation may play an important part of the mechanism of action of AML-targeted TCEs and c) that by understanding this reactivation mechanism, we will be able to design improved AML immunotherapies in the future. Aims: To understand the mechanisms by which a TCE induces reactivation of leukemia-reactive T cells (LRTs) in AML. Methods: T cells in biobanked AML patient bone marrow samples were activated with either an anti-CD3 x anti-CD123 TCE or a conventional anti-CD3 antibody for 5 days before isolating the ICTs. ICT assay: ICTs were washed to remove the TCE, and isolated ICTs were co-cultured with additional autologous bone marrow cells at different E:T ratios. Killing efficiency and T cell expansion were determined over several days using flow cytometry. Trogocytosis assay: ICTs were co-cultured with stained additional autologous AML cells for 1 h and T cell populations of interest were isolated using FACS. T cell repertoires were analyzed by targeted RNAseq of the TCR. Results: ICTs generated with a TCE kill autologous AML cells efficiently without further addition of the TCE. Adding additional TCE at this stage also does not increase the AML killing efficiency of the ICTs. ICTs generated with an activating anti-CD3 antibody show a similar AML killing efficiency and a similar trogocytotic behavior as TCE ICTs. Most of the TCR clones present in the AML bone marrow pre-stimulation were also represented among the ICTs and trogocytotic ICTs (ICTs that are directly targeting the AML cells after either TCE or anti-CD3 stimulation; Figure 1), suggesting that the bulk of T cells in an AML bone marrow represent heterogeneous immunosuppressed LRTs which can be equally well reactivated by a TCE or an anti-CD3 antibody. Trogocytotic ICTs generated from both types of T cell activation have a similar TCR distribution. ICTs derived from the bone marrow target autologous AML cells more efficiently than ICTs from peripheral blood. Image:Summary/Conclusion: Similarities between TCE ICTs and CD3 ICTs in regards to TCR distributions and killing efficiencies suggest that the TCE reactivates leukemia-reactive T cells via a CD3-dependent mechanism rather than a more complex proximity mechanism. Our data further suggests that T cells in the AML bone marrow are highly heterogeneous and leukemia targeted to a very high degree. In summary, AML bone marrow ICTs are targeting and killing leukemic cells efficiently in a polyclonal fashion and our results highlight that boosting this mechanism can leverage the clinical impact of AML TCEs and be utilized in other novel AML immunotherapy approaches.