Toward a poliovirus-based simian immunodeficiency virus vaccine: correlation between genetic stability and immunogenicity.

Recombinant polioviruses expressing foreign antigens may provide a convenient vaccine vector to engender mucosal immunity. Replication-competent chimeric viruses can be constructed by fusing foreign antigenic sequences to several positions within the poliovirus polyprotein. Artificial cleavage sites ensure appropriate proteolytic processing of the recombinant polyprotein, yielding mature and functional viral proteins. To study the effect of the position of insertion, two different recombinant polioviruses were examined. A small amino-terminus insertion delayed virus maturation and produced a thermosensitive particle. In contrast, insertion at the junction between the P1 and P2 regions yielded a chimeric poliovirus that replicated like the wild type. Eight different chimeras were constructed by inserting simian immunodeficiency virus (SIV) sequences at the P1/P2 junction. All recombinant viruses replicated with near-wild-type efficiency in tissue culture cells and expressed high levels of the SIV antigens. One of the inserted fragments corresponding to gp41 envelope protein was N-glycosylated but was not secreted. Inserted sequences were only partially retained after few rounds of replication in HeLa cells. This problem could be remedied to some extent by altering the sequences flanking the insertion point. Reducing the homology of the direct repeats by 37% decrease the propensity of the recombinant viruses to delete the insert. To determine the immunogenic potential of the recombinants, mice susceptible to poliovirus infection were inoculated intraperitoneally. The antibody titers elicited against Gag p17 depended on the viral doses and the number of inoculations. In addition, recombinants which display higher genetic stability were more effective in inducing an immune response against the SIV antigens, and inoculation with a mix of recombinants carrying different SIV antigens (a cocktail of recombinants) elicited humoral responses against each of the individual SIV sequences.