Synergistic DNA + Adenovirus Prime-Boost Immunization in Metastatic Colorectal Cancer.

摘要

Heterologous combinations of DNA prime and viral boost immunizations elicit immune responses against different types of tumors and infectious diseases. Robust cellular immune responses mediating antitumor efficacy are produced following various DNA + virus prime-boost immunization regimens. However, mechanisms underlying this synergy remain incompletely defined. Here, we utilized the colorectal cancer (CRC) associated antigen guanylyl cyclase C (GUCY2C) to explore the synergy of DNA-Ad5 (adenovirus type 5) prime-boost immunizations. GUCY2C is selectively expressed on the apical surfaces of intestinal epithelial cells and CRC cells through all stages of tumorigenesis. The compartmentalized expression of GUCY2C, normally in the mucosa but extending systemically in metastases, makes GUCY2C a target for CRC immunotherapy. In previous studies, Ad5 expressing GUCY2C (Ad5-GUCY2C) was a safe and effective vaccine in mice. However, it cannot be employed in homologous prime-boost immunization regimens due to Ad5 vector-specific immunity. Therefore, heterologous prime-boost strategies may be required to maximize antitumor efficacy targeting GUCY2C in CRC. In that context, DNA vaccines not only elicit both cellular and humoral immunity and but also are advantageous in heterologous prime-boost strategies due to their lack of vector-specific immunity. However, low transfection rates and poor immunogenicity restrict the utility of DNA vaccine strategies. Here, plasmids expressing CCL20 and CCL21 as chemokine adjuvants to recruit dendritic cells and naive T cells were used to increase DNA vaccine immunogenicity. Further, electroporation (EP) of GUCY2C-expressing plasmids increased transfection rates, optimizing GUCY2C immune responses. Heterologous prime-boost regimens of DNA vaccine and Ad5 vaccine not only elicited robust primary and memory immune responses, but also produced curative antitumor responses, compared to each immunization alone and other combination regimens. Interestingly, DNA priming followed by Ad5 boosting (DNA-Ad5) produced only a modest increase (<2-fold) in the magnitude of GUCY2C-specific CD8+ T cell responses without significant changes in polyfunctionality, compared to other immunizations. However, DNA-Ad5 dramatically improved target sensitivity of GUCY2C-specific CD8+ T cells by increasing T cell receptor (TCR) functional avidity (∼20-fold). Thus, enhanced CD8+ T cell functional avidity, rather than CD8+ T cell quantity or polyfunctionality, mediated antitumor efficacy of DNA-Ad5 in CRC. The synergistic GUCY2C DNA-Ad5 prime-boost strategy inducing robust CD8+ T cell responses and anti-CRC activity may be translated to patients at risk for metastatic CRC.