Preexisting immunity to poliovirus does not impair the efficacy of recombinant poliovirus vaccine vectors

Abstract

Recombinant viruses are attractive candidates for the development of novel vaccines. A number of viruses have been engineered as vaccine vectors to express antigens from other pathogens or tumors. Inoculation of susceptible animals with this type of recombinant virus results in the induction of both humoral and cellular immune responses directed against the foreign antigens. A general problem to this approach is that existing immunity to the vector can diminish or completely abolish the efficacy of the viral vector. In this study, we investigated whether poliovirus recombinants are capable of inducing effective immunity to the foreign antigen in previously vaccinated animals. Antipoliovirus immunity was induced in susceptible mice by intraperitoneal immunization with live poliovirus. Immunized mice developed antibodies directed against capsid proteins that effectively neutralized poliovirus in vitro and protected animals from a lethal challenge with a high dose of pathogenic poliovirus. To test whether preexisting immunity reduces the efficacy of vaccination with recombinant poliovirus, immunized mice were inoculated with a recombinant poliovirus expressing the C-terminal half of chicken ovalbumin (Polio-Ova). Animals developed ovalbumin-specific antibodies and cytotoxic T lymphocytes (CTL). While the antibody titers observed in preimmune and naive mice were similar, the overall CTL response appeared to be reduced in preimmune mice. Importantly, vaccination with Polio-Ova was able to effectively protect preimmune mice against lethal challenge with a tumor expressing the antigen. Thus, preexisting immunity to poliovirus does not compromise seriously the efficacy of replication-competent poliovirus vaccine vectors. These results contrast with those observed for other viral vaccine vectors and suggest that preexisting immunity does not equally affect the vaccine potential of individual viral vectors.

FIG. 1

Schematic representation of the experimental design. OVA, C-terminal half of chicken ovalbumin.

FIG. 2

Antipoliovirus serum IgG. Sera from all mice used in this study were analyzed, but only results for 30 mice are shown. Numbers identify individual mice. Mice 1 to 10 were immunized by a single intraperitonal injection of 10⁶ PFU of Mahoney, mice 16 to 25 received 10⁶ PFU of Polio-Sp27 twice by the same route, and naive controls 11 to 15 and 26 to 30 were injected with PBS. (A) Sera from Mahoney-immunized mice were diluted 1:100 and analyzed by Western blot for antibodies against poliovirus capsid proteins. (B) Poliovirus neutralizing antibodies from sera of the same animals were determined by plaque reduction assay. (C) Poliovirus neutralizing antibodies from sera of mice immunized with Polio-Sp27 were determined as in panel B. Neutralizing activity is expressed as fold reduction compared to naive controls.

FIG. 3

Ova-specific antibodies and CTL. (A) Naive (triangles) and Polio-Sp27-immunized (diamonds) mice were inoculated three times with 10⁶ PFU of Polio-Ova. Control animals were inoculated with PBS (circles). Five weeks after the first inoculation, mice were bled, and sera were analyzed for ovalbumin-specific antibodies by ELISA. Antibody titers are expressed as the reciprocal of the lowest serum dilution that gave an absorbance of 0.1 unit above the background. Each symbol represents an antibody titer for an individual mouse. (B) CTL activity measured by a ⁵¹Cr release assay. Naive mice (triangles) and mice vaccinated with Polio-Sp27 (diamonds) were inoculated with Polio-Ova as described in the text. Control mice were injected with PBS only (circles). Three weeks after the immunization, spleens from all mice were harvested and cultured with irradiated EG-7 (EL-4 cells transfected with ovalbumin). After 5 days, cells were collected and tested in ⁵¹Cr release assays on EG-7 targets (solid symbols) or on EL-4 cells as controls (open symbols). Splenocytes restimulated with EL-4 cells did not kill either target (data not shown). Results are means for five mice per group. Each spleen was tested individually. The level of CTL activity is significantly higher (dilutions 1:25 to 1:100) in the groups that were immunized with Polio-Ova (with or without prevaccination with poliovirus) than in control naive mice (P < 0.05).