Application of Novel T Cell Immunotherapeutics to Drive Antigen-Specific Activation, Expansion, and Differentiation of CD19 Chimeric Antigen Receptor T Cells (CAR T-cells)

Background: While chimeric antigen receptor T cells (CAR T-cells) induce dramatic remissions of refractory or recurrent B cell malignancies, the durability of these remissions is frequently limited by subsequent reduction in circulating CAR T-cells and/or by diminution of their effector function. We hypothesized that we could overcome this therapeutic limitation and increase the functional activity and longevity of CAR T-cells by selectively deriving them from virus-specific effector memory T cells. We have developed biologics we termed synTacs (artificial immunological synapse for T-cell activation), which selectively activate and expand antigen-specific CD8+ T cells in vitro and in vivo by recapitulating signals delivered at the immunological synapse. The synTacs consist of dimeric Fc domain scaffolds linking CD28- or 4-1BB-specific ligands to HLA-A2 MHC molecules covalently tethered to virus-derived peptides. Treatment of PBMCs from CMV-exposed donors with synTacs presenting a CMV-derived peptide (pp65-NLVPMVATV) induce vigorous and selective ex vivo and in vivo expansion of highly functional CMV-specific CD8+ T cells, with potent antiviral activity. We used these synTacs to selectively generate CAR T-cells from CMV-specific effector memory CD8+ T cells, which could be further expanded by restimulation with the CMV-specific synTacs.

Methods: We treated PBMCs from CMV-exposed donors in media supplemented with either IL-2 or IL-7/12/15 with a synTac containing the CMV-derived pp65 peptide presented by HLA-A2 MHC molecules linked to ligands capable of stimulating CD28- or 4-1BB-dependent costimulatory pathways. PBMCs activated either with anti-CD3/CD28 or the CMV-specific synTacs were transduced with lentivirus expressing an anti-CD19 CAR and a GFP reporter gene. CMV-specific CD8+ T cells were quantified by tetramer staining and CAR T-cells were detected by GFP expression determined by flow cytometric analysis. The functional activity of the CD19 CAR T-cells was determined by a B cell-specific cytotoxic assay.

Results: After 7 days, treatment of PBMCs with CMV-specific synTacs rapidly induced robust activation and >50-fold expansion of CMV-specific CD8+ T cells expressing effector memory markers. Treatment of the PBMCs with CMV-specific synTacs selectively activated CMV-specific T cells and enabled them to be specifically transduced with a CD19-specific CAR lentivirus and converted into CD19 CAR T-cells. These CMV-specific CD19 CAR T-cells displayed potent dose-responsive cytotoxic activity targeting purified primary B cells. Furthermore, these CMV-specific CD19 CAR T-cells could be selectively expanded by in vitro treatment with CMV-specific synTacs.

Conclusions: SynTacs are versatile immunotherapeutics capable of selective in vitro and in vivo activation and expansion of virus-specific CD8+ T cells with potent antiviral cytotoxic activity. After selective lentiviral transduction and conversion into CD19 CAR T-cells, their co-expression of the CMV-specific T cell receptor enabled them to be potently stimulated and activated by in vitro treatment with CMV synTacs. The modular design of synTacs facilitates efficient coupling of other costimulatory ligands – such as OX40 or GITRL – or cytokines, such as IL-2, IL-7, or IL-15, to enable the selective in vivo delivery of defined costimulatory signals or cytokines to the CAR T-cells expressing CMV-specific TCR. This strategy has the potential to boost the in vivo activity of tumor-specific CAR T-cells after infusion and enable more durable and potent treatment of refractory/recurrent B cell malignancies.