Increased immunogenicity of a DNA vaccine encoding pseudorabies virus IE180 protein by co-administration of IL-2 or by delivery in liposomes

Abstract - A recent breakthrough in vaccinology which may be effective in controlling infectious diseases is the development of DNA vaccines. A number of approaches have been investigated to optimize the efficacy of DNA vaccines. For instance, the use of plasmids encoding various cytokines with the DNA vaccines have been demonstrated to enhance and to modulate immune responses. Thus, IL-12, INF- , IL-2 and GM-CSF have been shown to induce Th1 responses, while IL-4 is a cytokine that can increase Th2 type responses. Among these cytokines, IL-2 is unique in that it can activate multiple compartments of the immune system and has been shown to enhance both humoral and cell-mediated immune responses with DNA vaccines for HBV and HSV. Liposomes are under extensive investigation as vehicles for DNA vaccine delivery. Plasmid DNA complexed or entrapped in liposomes can be protected from exposure to nucleases and increase the efficiency of vaccine delivery. Liposomes also have the capacity to target the antigen into antigen presenting cells, which is critical for the induction of immune responses. Recent studies have shown that DNA vaccines delivered in cationic liposomes result in increased humoral and cell-mediated immunity. Moreover, DNA-liposome formulations were applied in mucosal immunization and demonstrated to be an efficient way to induce both mucosal and systemic immunity. Our group has recently demonstrated, with a DNA vaccine, that the immediate early protein (IE180) of pseudorabies virus provides a moderate level of protection in mice. In order to improve its immunogenicity and protective capacity, this IE180 DNA vaccine (pcDNAIE180) was delivered to C3H/HeJ mice either in combination with an IL-2 expressing plasmid or complexed with cationic liposomes (DDAB/Cholesterol). Antibody, delayed-type hypersensitivity (DTH) responses and protection against lethal challenge were evaluated. Co-delivery of the pcDNAIE180 vaccine and IL-2 DNA by gene gun resulted in seroconversion in 5 out of 5 mice after a single administration, whereas two intramuscular (i.m.) injections were required to achieve seroconversion in all mice. Anti-IE180 antibody and virus-specific DTH responses were augmented in mice receiving the DNA vaccine and the IL-2 gene compared to those receiving the pcDNAIE180 vaccine alone. The proportion of surviving mice (40%) was similar to that obtained in our previous study where mice received the pcDNAIE180 vaccine alone by gene gun. However, the time of death after challenge was significantly delayed for mice which received the IL-2 gene. Liposome-mediated vaccine delivery also resulted in high rates of seroconversion. Intravenous (i.v.) injection of the liposome/DNA complex was more efficient than i.m. administration. Thus, all vaccinated mice seroconverted after either two i.v. or three i.m. injections of the liposome/DNA complex. Significant DTH responses were detected in mice which received i.v., but not i.m. injections of the liposome/DNA complex. The protection was 40% or 25% for mice receiving i.v. or i.m. injections of the liposome/DNA complex, respectively. In contrast, a lack of response and no protection against challenge were observed after repeated i.m. or i.v. injections of the pcDNAIE180 vaccine alone. These data support that both co-administration of the pcDNAIE180 vaccine with the IL-2 gene or delivery of the pcDNAIE180 vaccine in liposomes are effective approaches that significantly increase the immunogenicity of this vaccine.