mRNA-LNP vaccine providing antigen and co-stimulation in the tumor microenvironment enhances CAR T cell function (CART-Vac).

Despite advances in multimodal therapies, outcomes for pediatric patients with relapsed or refractory cancers remain poor. Chimeric antigen receptor (CAR) T cell therapy has demonstrated limited efficacy in solid tumors due to the immunosuppressive tumor microenvironment (TME), which promotes T cell exhaustion and restricts CAR T cell expansion. This study evaluated a combinatorial approach to enhance CAR T function by reprogramming the TME to overexpress target antigens (TAs) and co-stimulatory molecules (CSMs) through a lipid nanoparticle-based CAR-T vaccination (CART-Vac). As proof of concept, rhabdomyosarcoma cells (Rh30) engineered to overexpress TAs and CSMs (Rh30-TACS) were examined. EPHB4-directed CAR T cells demonstrated enhanced cytotoxicity, proliferation, and cytokine secretion in vitro, and superior tumor control with increased T cell infiltration in Rh30-TACS tumors in vivo compared with Rh30 tumors. To induce TA and CSM expression in the TME, CART-Vac was designed to deliver mRNAs encoding truncated EPHB4, CD80, and CD137L. CART-Vac effectively mediated transient expression, significantly enhancing CAR T expansion and antitumor activity in both models. These findings suggest that CART-Vac can modulate the TME, offering a promising strategy to improve the therapeutic efficacy of CAR T cells in solid tumors.