Development and Evaluation of Compact Semi-Synthetic Promoters for Enhanced Antigen Expression in Adenoviral-Vectored Vaccines

Background/Objectives: The large size of commonly used regulatory elements such as the cytomegalovirus (CMV) immediate-early promoter imposes a significant burden on the already restricted payload capacity of first-generation adenoviral vectors, potentially hindering the development of multi-antigen vaccine candidates. To address this limitation, we have engineered a panel of novel, small, semi-synthetic promoters designed to leverage the changes in transcriptomic milieu following adenoviral vector entry. Methods: Eight synthetic enhancer modules (SE1–SE8) were designed in silico, each composed of transcription factor binding sites (TFBSs) previously found in host genes that are upregulated during early adenoviral infection. These synthetic enhancers were coupled with a minimal CMV core promoter to generate a panel of compact semi-synthetic promoters (cSE1–cSE8), and their activity was evaluated in the context of ChAdOx1 viral vectors expressing GFP or a modified Plasmodium falciparum circumsporozoite (CSN) antigen. Promoter performance was characterised in vitro via flow cytometry, RT-qPCR, and Western blotting, and in vivo by quantifying antigen-specific T-cell (IFN-γ ELISpot) and IgG antibody (ELISA) responses in BALB/c mice. Results: In vitro characterisation revealed a wide range of promoter activity across the panel, with cSE3 and cSE5 driving transgene expression levels comparable to the benchmark CMV promoters despite their markedly reduced genomic footprint. In vivo, ChAdOx1 vectors incorporating cSE3 and cSE5 elicited potent antigen-specific T-cell and IgG responses that were comparable to those induced by the larger CMV control promoters. Conclusions: We have successfully developed semi-synthetic promoters that match the potency of the much larger, frequently used CMV promoters whilst simultaneously reducing genomic footprint. These novel regulatory elements will facilitate the design of next-generation vaccines, particularly those requiring large antigens or multi-antigen cassettes.