Analysis of anamnestic immune responses in adult horses and priming in neonates induced by a DNA vaccine expressing the vapA gene of Rhodococcus equi.

Rhodococcus equi remains one of the most important pathogens of early life in horses, yet conventional vaccines to prevent rhodococcal pneumonia have not been successful. DNA vaccination offers an alternative to conventional vaccines with specific advantages for immunization of neonates. We developed a DNA vaccine expressing the vapA gene (pVR1055vapA) that induced an anamnestic response characterized by virulence associated protein A (VapA)-specific IgG antibodies in sera and bronchoalveolar lavage fluid (BALF) as well as VapA-specific proliferation of pulmonary lymphocytes when tested in adult ponies. In contrast, none of the adults receiving the control plasmid responded. To determine if pVR1055vapA induced VapA-specific responses in the foal, the targeted age group for vaccination against R. equi, 10 naïve foals were randomly assigned at birth to two groups of five. At 8-15 days of age (day 1), foals were vaccinated by intranasal and intradermal (i.d.) routes with either pVR1055vapA or the negative control pVR1055vapA_rev. All foals were DNA boosted at day 14 and protein boosted at day 30 with either recombinant VapA or recombinant CAT (control group). Prior to the protein boost, neither group developed VapA-specific immune responses. However, at day 45, two of the VR1055vapA-vaccinated foals had increased titers of VapA-specific IgGb, IgM and IgGa in the sera, and IgG in the BALF. The induction of the opsonizing isotypes IgGa and IgGb has been previously shown to be associated with protection against R. equi. No VapA-specific immune responses were detected in the control group. This study indicates that the DNA vaccine effectively stimulates anamnestic systemic and pulmonary immune responses in adult horses. The results in foals suggest that the DNA vaccine also primed a subset of immunized neonates. These data support further development and modification to produce a DNA vaccine to more effectively prime neonatal foals.