Flt3 ligand augments immune responses to soluble PD1-based DNA vaccine via expansion of type 1 conventional DCs.

DNA vaccines are prospective for their efficient manufacturing process, but their immunogenicity is limited as they cannot efficiently induce CD8 T cell responses. A promising approach is to induce cross-presentation by targeting antigens to DCs. Flt3L can expand the number of type 1 conventional DCs and thereby improve cross-presentation. In this study, we first constructed a DNA vaccine expressing soluble PD1 and found that the therapeutic effect of targeting DCs with only the sPD1 vaccine was limited. When combined the vaccine with Flt3L, the anti-tumor effect was significantly enhanced. Considering the complexity of tumors and that a single method may not be able to activate a large number of effective CD8 T cells, we combined different drugs and the vaccine with Flt3L based on the characteristics of different tumors. In 4T1 model, we reduced Tregs through cyclophosphamide. In Panc02 model, we increased activated DCs by using aCD40. Both strategies triggered strong CD8 T cell responses and significantly improved the therapeutic effect. Our study provides important support for the clinical exploration of DC-targeted DNA vaccines in combination with Flt3L.