Peripheral T-cell tolerance toward human carcinoembryonic self-antigen (CEA) was broken in CEA-transgenic C57BL/6J mice by an oral CEA-based DNA vaccine. This vaccine, delivered by the live, attenuated AroA− strain of Salmonella typhimurium (SL7207), induced tumor-protective immunity mediated by MHC class I-restricted CD8+ T cells. Activation of these T cells was indicated by increased secretion of proinflammatory cytokines IFN-γ, interleukin (IL)-12 and granulocyte/macrophage-colony stimulating factor, as well as specific tumor rejection and growth suppression in vaccinated CEA-transgenic mice after a lethal challenge with murine MC38 colon carcinoma cells. These tumor cells were double transfected with CEA and the human epithelial cell adhesion molecule (Ep-CAM)/KSA and consequently served as a docking site for a recombinant antibody-IL2 fusion protein (KS1/4-IL2) recognizing KSA. Importantly, the efficacy of the tumor-protective immune response was markedly increased by boosts with this antibody-IL2 fusion protein, resulting in more effective tumor rejection coupled with increased expression of costimulatory molecules B7.2/B7.2 and intercellular adhesion molecule 1 (ICAM-1) on dendritic cells and intensified release of proin-flammatory cytokines IFN-γ, IL-12, and granulocyte/macrophage-colony stimulating factor from T cells of successfully vaccinated CEA-transgenic C57BL/6J mice. Increased T-cell activation mediated by boosts with KS1/4-IL2 fusion protein after tumor cell challenge was further indicated by expanded expression of T-cell activation markers CD25, CD28, CD69, and LFA-1. The application of such CEA-based DNA vaccines and its further improved versions may ultimately prove useful in combination therapies directed against human carcinomas expressing CEA self-antigens.
This work was supported by NIH Grants CA42508 and CA83856 (to R. A. R.); Merck KGaA, Darmstadt, Germany (to R. A. R.) and NIH Grant CA70320 (to F. J. P.); and NIH Cancer Core Grant to the Vanderbilt-Ingram Cancer Center (to F. J. P.). J. M. R. is a Fellow of the Deutsche Forschungsgemeinschaft, and A. G. N. is a Fellow of the Deutsche Krebshilfe. This is The Scripps Research Institute's manuscript number 13392-IMM.