GENETIC IMMUNIZATION IN THE HORSE: THE POTENTIAL FOR ENHANCED IMMUNE RESPONSES WITH DEACYLATED POLYETHYLENEIMINE (PEI) AND IMMUNOSTIMULATORY CYTOKINES AS VACCINE ADJUVANTS

DNA vaccines in larger animals, such as horses, are generally less effective and elicit significantly weaker immune responses, than in small animal model systems. To provide optimal protection against pathogenic microorganisms, the induction of both humoral and cellular immune responses from DNA vaccination may be necessary. One limitation to DNA immunization in the horse is the difficulty in generating high levels of antigen-specific antibody and CTL responses. Previous work in the laboratory has demonstrated that expression constructs containing native sequences encoding the surface unit (SU) envelope glycoprotein (pCiSU) of the Equine Infectious Anemia Virus (EIAV) are ineffective at stimulating immune responses in the horse. This was attributed to an unusual codon-usage bias of the EIAV genome that significantly limits the expression of SU sequences. Optimizing the codon usage of pCiSU (pSYNSU) in DNA vaccines stimulated low-titer immune responses in inoculated ponies. Another plausible explanation for the reduced effectiveness of these DNA vaccines may be transfection deficiency and low level expression elicited by plasmid vectors in the horse. These studies investigated if the addition of a cationic polymer, deacylated polyethyleneimine (PEI), and/or codon optimized molecular immune-stimulatory cytokines could augment the relatively weak immunogenicity of pSYNSU in DNA vaccination of horses/ponies. Administration of DNA in formulation with PEI resulted in the robust production of very long-lived humoral (15 months after vaccination) responses and induced cell-mediated IFN-y responses five days after immunization.. Additionally, the co-expression of a family of IL-15 cytokines expanded the repertoire of T cell recognition to SU-specific peptides, in terms of lymphoproliferation. DNA vaccination incorporating one IL-15 family member, IL-15 (SSLSS) significantly enhanced serum antibody levels of IgGA and IFN-y mRNA expression levels. These responses were distinctly different from results seen with vaccinates that received „naked‟ pSYNSU DNA vaccines. It is evident from these vaccine studies that PEI can enhance DNA vaccine-elicited antibody and CTL-associated responses in the horse and IL-15 (SSLSS) can dramatically augment these responses. These results demonstrate an important role for PEI in promoting the longevity of immune responses to genetic immunization, which has not been reported previously in any large animal model