Homologous and Heterologous Immunization with a PIV5-Based Modified OspA Vaccine Confers Equivalent Protection...

Vaccines targeting outer surface protein A (OspA) of Borrelia burgdorferi protect against Lyme disease by inducing antibodies in the host that neutralize spirochetes in the Ixodes scapularis tick midgut during engorgement before transmission occurs. We evaluated whether heterologous vaccination enhances protection compared to homologous delivery of the immunogen. C3H-HeN mice were immunized with a parainfluenza virus 5 vector (PIV5) containing a modified OspA protein (OspA_BPBPk) using three prime-boost immunization regimens: homologous PIV5 intranasal/intranasal (IN/IN), homologous rOspA_BPBPk (protein) subcutaneous/subcutaneous (SC/SC), or heterologous intranasal PIV5-A_BPBPk/subcutaneous rOspA_BPBPk (IN/SC). Immunized mice were then challenged with nymphal I. scapularis ticks infected with 19 strains of B. burgdorferi three months post-prime vaccination. Three weeks after the last day of tick challenge, blood and tissues were collected from euthanized mice. All OspA-containing regimens elicited strong systemic IgG antibody responses that exceeded established protective thresholds. Vaccination markedly reduced B. burgdorferi loads in engorged nymphal ticks. Homologous IN/IN and SC/SC regimens produced the lowest geometric mean flaB burdens in nymphs (2.6 × 10³ and 1.8 × 10³ copies, respectively), corresponding to ~1.8-2.0 log₁₀ reductions relative to controls; the heterologous IN/SC regimen produced a more modest reduction (~1.7 log₁₀; p = 0.0071 vs IN/IN, p = 0.0003 vs SC/SC). Across all vaccinated groups, no systemic infection with B. burgdorferi was observed as evidenced by absence of motile spirochetes in cultures from tissues, although one mouse (1/9, 11%) in the heterologous IN/SC regimen, had evidence of increased pepVF seroconversion and low-level flaB DNA in culture. Thus, homologous regimens yielded more consistent protective immunity with absent of signs of B. burgdorferi dissemination, suggesting that high systemic anti-OspA_BPBPk IgG antibody titers, rather than alternate immunization routes, were associated with the most consistent protection outcomes. PIV5-A_BPBPk is a promising vaccine candidate for development of next-generation homologous or heterologous human Lyme disease vaccines.