Abstract
Messenger RNA vaccines have shown strong prophylactic efficacy against viral infections. Here we show that antigen-encoding small circular RNAs (circRNAs) loaded in lipid nanoparticles elicit potent and durable T cell responses for robust tumour immunotherapy after subcutaneous injection in mice, particularly when combined with immune checkpoint inhibition. The small circRNA vaccines are highly stable and show low levels of activation of protein kinase R as well as low cytotoxicity, enabling long-lasting antigen translation (longer than 1 week in cells). Relative to large protein-encoding unmodified or modified mRNAs and circRNAs, small circRNA vaccines elicited up to 10-fold antigen-specific T cells in mice and accounted for 30-75% of the total peripheral CD8+ T cells over 6 months. Small circRNA vaccines encoding tumour-associated antigens, neoantigens and oncoviral or viral antigens elicited substantial CD8+ and CD4+ T cell responses in young adult mice and in immunosenescent aged mice. Combined with immune checkpoint inhibition, monovalent and multivalent circRNA vaccines reduced tumour-induced immunosuppression and inhibited poorly immunogenic mouse tumours, including melanoma resistant to immune checkpoint blockade.