Optimization of a HIV-1 DNA vaccine using a novel DNA vector and molecule adjuvants

Abstract

Circular DNA plasmid is a promising vaccine vector because of its high safety profile, its ease for mass production and its capacity to induce both humoral and cellular immunity. Current DNA vaccines, however, have relatively limited immunogenicity due to the modest transgene expression over a short period of time and lower ability to activate the innate immunity. Previous studies have demonstrated that the large size of the DNA vectors and the inclusion of bacteria backbone sequences are part of reasons behind the low efficacy of DNA vaccines. Minicircle DNA vector is a new form of circular DNA that contains a small expression cassette without the bacterial backbone sequence, making it to be much smaller in size than conventional plasmid vectors. Several studies have reported that minicircle vectors can effectively improve the level and the duration of gene expression in vitro and in vivo. Moreover, the inclusion of molecule adjuvants during DNA vaccination could further enhance their immunogenicity by activating the relevant innate immunity. In this study, I examined extensively the potential use of minicircle DNA as the vaccine vector and novel molecule adjuvants to induce HIV-specific T cell immunity in mouse models. The antigen focused in this study is a recombinant protein including a soluble program death 1 (PD-1) molecule fused to a secreted form of a soluble mosaic HIV-1 Gag p41 antigen, designed in silico based on major circulating HIV-1 sequences in China. It was previously demonstrated that the soluble PD-1 allows effective antigen targeting to dendritic cells for the antigen cross presentation. Two separate DNA vectors, namely the conventional pVAX vector and the minicircle vector, were used to express the recombinant antigen. The novel molecule adjuvants examined in this study were a recombinant activated RIG-I gene expressed from pVAX plasmid and a triphosphorylated stem-loop RNA. The protocol of minicircle production was firstly optimized to allow the generation of a highly pure minicircle product for in vivo use. Following the confirmation of the long-term transgene expression in mice treated with minicircle vectors intramuscularly with electroporation, a comprehensive platform for the evaluation of vaccine-inducing tissue-localized immunity was established and comparable levels of vaccine-inducing HIV-specific T cell responses were detected in the spleens, lungs and genital tracts between MCmax-vector and pVAX-vector HIV-1 vaccines during both acute and memory phases in mice. Furthermore, the PD-1 based HIV-1 vaccines in both minicircle and pVAX vectors successfully elicited the protection in mice against the tumorigenesis of malignant mesothelioma AB1 cells expressing HIV-Gag antigens in lungs. Finally, the application of novel molecule adjuvants effectively enhanced the immunogenicity of DNA-based HIV-1 vaccines. In summary, my results in this study demonstrated that the minicircle vector was not a favourable vaccine vector for inducing strong protective T cell immunity. The generation of minicircle vectors was more complicated than conventional vectors, without any significant enhancement of immunogenicity. In contrast, the use of novel molecule adjuvants to activated the RIG-I innate responses may be a better option to optimize DNA-based HIV vaccines.