Antigen cross-presentation potentiating cancer vaccine adjuvant for T cell expansion and synergy with anti-PD-1.

Therapeutic cancer vaccines (TCVs) remain limited in their capacity to elicit robust CD8⁺ cytolytic T lymphocyte responses. An effective cancer vaccine adjuvant should promote expansion of antigen-specific T cells through cross-presentation by type 1 conventional dendritic cells (cDC1s). For anti-PD-1 immune checkpoint inhibitor therapy, being frequently combined with cancer vaccines, requires an expanded pool of precursor-exhausted CD8⁺ T (Tpex) cells. Here, we report Flt3L-FlaB (FB), a hybrid adjuvant that integrates FMS-like tyrosine kinase 3 ligand (Flt3L) with the TLR5 agonist flagellin B (FlaB). FB significantly expanded and activated cDC1s, accompanied by increased CD8⁺ T cells with stem-like memory (Tscm) and Tpex phenotypes in tumors and draining lymph nodes. FB-adjuvanted TCVs, combined with anti-PD-1 therapy, achieved potentiated tumor suppression and provided durable protection against metastasis and high-dose tumor rechallenge. These results establish FB as a potent TCV adjuvant with strong translational potential, particularly the combination with anti-PD-1 immune checkpoint inhibitors.