PNAG-specific equine IgGmediates significantly greater opsonization and killing of Prescottella equi (formerly Rhodococcus equi) than does IgG.

Abstract

Prescottella equi (formerly Rhodococcus equi) is a facultative intracellular bacterial pathogen that causes severe pneumonia in foals 1-6&#x202f;months of age, whereas adult horses are highly resistant to infection. We have shown that vaccinating pregnant mares against the conserved surface polysaccharide capsule, &#x3b2;-1&#x202f;&#x2192;&#x202f;6-linked poly-N-acetyl glucosamine (PNAG), elicits opsonic killing antibody that transfers via colostrum to foals and protects them against experimental infection with virulent. R. equi. We hypothesized that equine IgGmight be more important than IgGfor mediating protection against R. equi infection in foals. To test this hypothesis, we compared complement component 1 (C1) deposition and polymorphonuclear cell-mediated opsonophagocytic killing (OPK) mediated by IgGor IgGenriched from either PNAG hyperimmune plasma (HIP) or standard plasma. Subclasses IgGand IgGfrom PNAG HIP and standard plasma were precipitated onto a diethylaminoethyl ion exchange column, then further enriched using a protein G Sepharose column. We determined C1 deposition by enzyme-linked immunosorbent assay (ELISA) and estimated OPK by quantitative microbiologic culture. Anti-PNAG IgGdeposited significantly (P&#x202f;<&#x202f;0.05) more C1 onto PNAG than did IgGfrom PNAG HIP or subclasses IgGand IgGfrom standard plasma. In addition, IgGfrom PNAG HIP mediated significantly (P&#x202f;<&#x202f;0.05) greater OPK than IgGfrom PNAG HIP or IgGand IgGfrom standard plasma. Our findings indicate that anti-PNAG IgGis a correlate of protection against R. equi in foals, which has important implications for understanding the immunopathogenesis of R. equi pneumonia, and as a tool for assessing vaccine efficacy and effectiveness when challenge is not feasible.