(PA-013) Synergistic Anti-tumor Effects of CAR T Cells and a Personalized Tumor Vaccine

Introduction: Chimeric antigen receptor (CAR) T cell therapy has shown great efficacy in hematologic malignances. However, relapse is common due to mechanisms such as antigen loss, CAR T cell dysfunction and limited persistence. We developed a personalized dendritic cell (DC)-based cancer vaccine whereby diverse tumor antigens are presented in the context of autologous MHC. In clinical trials, the vaccine has shown safety and immunogenicity. We hypothesized that a combinatorial immunotherapy strategy integrating CAR T cells with our vaccine will enhance CAR T efficacy overcoming single-antigen targeting limitations and promoting durable tumor-specific immunity.

Methods: To evaluate this approach, we used the A20 murine lymphoma model. Syngeneic BALB/c T cells were transduced with a CD19-targeting CAR. The DC/tumor vaccine was generated by PEG-mediated fusion of A20 cells and BALB/c DC. For in vitro studies, CAR T or T cells were ex vivo primed with vaccine. For in vivo studies, vaccine-primed T cells (Vac T) were isolated from vaccinated mice and ex vivo expanded with CD3/CD28 Dynabeads and 4-1BB antibody. Naïve T cells from matched unvaccinated mice served as controls. Lymphoma was induced by A20 tail vein injection. Vac T or CAR T cells were infused 1 or 7 days later, respectively. Mice were lymphodepleted 24 hours prior to CAR T infusion. Tumor burden was assessed by luciferase activity in CD19⁺ (Firefly) and CD19⁻ (Renilla) A20 cells.

Results: Ex vivo vaccine priming enhanced CAR T activation and persistence, resulting in heightened tumor cytotoxicity. Yet, ex vivo vaccine primed CAR T remained unable to target tumor variants lacking the CAR antigen. The combination of CAR T and ex vivo primed T cells resulted in enhanced in vitro killing of tumor cells, regardless of their expression of the CAR antigen, compared to either monotherapy.
In vivo, the combination of CAR T and Vac T cells demonstrated a synergistic effect, with median survival not reached at 8 months post-tumor inoculation (vs 21, 22, and 46 days in mice treated with naïve T, Vac T, or CAR T cells, respectively). Similarly, in a model of limited CAR T cell efficacy, the combination therapy demonstrated a survival advantage (p≤0.05 vs control) not achieved by either therapy alone. Importantly, in an in vivo heterogenous tumor model composed of antigen-positive and -negative tumor clones, the combination efficiently targeted both tumor populations (median survival 40 days vs 22-34 in CAR T or Vac T alone; p vs naïve T≤0.001; p vs CAR T≤0.05).

Conclusions: The DC/tumor vaccine enhances CAR T persistence and elicits a polyclonal T cell response capable of targeting tumor clones refractory to CAR T killing due to antigen-loss. The combination of CAR T and vaccine may represent a novel strategy to overcome resistance and improve the durability of responses to CAR T therapy. A clinical trial of vaccination and CAR T therapy is planned for patients with advanced myeloma.