PNAG-specific equine IgG₁ mediates significantly greater opsonization and killing of Prescottella equi (formerly...

Prescottella equi (formerly Rhodococcus equi) is a facultative intracellular bacterial pathogen that causes severe pneumonia in foals 1-6 months of age, whereas adult horses are highly resistant to infection. We have shown that vaccinating pregnant mares against the conserved surface polysaccharide capsule, β-1 → 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 IgG₁ might be more important than IgG_4/7 for 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 IgG₁ or IgG_4/7 enriched from either PNAG hyperimmune plasma (HIP) or standard plasma. Subclasses IgG₁ and IgG_4/7 from 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 IgG₁ deposited significantly (P < 0.05) more C1 onto PNAG than did IgG_4/7 from PNAG HIP or subclasses IgG₁ and IgG_4/7 from standard plasma. In addition, IgG₁ from PNAG HIP mediated significantly (P < 0.05) greater OPK than IgG_4/7 from PNAG HIP or IgG₁ and IgG_4/7 from standard plasma. Our findings indicate that anti-PNAG IgG₁ is 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.