DNA vaccination with map1 gene followed by protein boost augments protection against challenge with Cowdria ruminantium, the agent of heartwater

Abstract

A DNA vaccine encoding the immunodominant MAP1 protein of Cowdria ruminantium (Crystal Springs (CS) strain) was shown to partially protect DBA/2 mice against homologous lethal challenge. To enhance the protective capacity of this DNA vaccine, the effects of length of interval between vaccinations and of prime-boost regimes were investigated. Increasing the interval between vaccinations from 2 to 12 weeks did not result in better protection ( P=0.900). However, boosting DNA vaccine-primed mice with recombinant MAP1 protein significantly augmented protection on homologous challenge in various trials from 13–27 to 53–67% ( P<0.050). The augmented protection by the prime-boost regimen correlated with augmented T H1 type immune responses that were induced by the DNA vaccine. These responses were characterized by production of IFN-γ, IL-2 and anti-MAP1 antibodies of predominantly IgG2a isotype, and were critical for protection against C. ruminantium infection. Cytokine analyses were done at 48 h after in vitro stimulation of splenocytes with C. ruminantium or control antigens. In contrast, splenocytes of DNA vector control mice produced no cytokines and these mice were fully susceptible to challenge. In addition, DBA/2 mice immunized with the recombinant MAP1 protein without DNA vaccine priming produced non-protective T H2 type immune responses which were characterized by production of IL-4, IL-5, IL-10 and IgG1 anti-MAP1 antibodies. A second DNA vaccine containing map1 gene from the Mbizi strain of C. ruminantium also delivered by a prime-boost regime, conferred less protection against heterologous challenge. Hence, in developing DNA vaccines against heartwater that contain map1 gene, a prime-boost regimen should be adopted and gene sequence heterogeneity of field isolates should also be considered.