In vitro evaluation of complement deposition and opsonophagocytic killing of Rhodococcus equi mediated by poly-N-acetyl glucosamine hyperimmune plasma compared to commercial plasma products.

Background: The bacterium Rhodococcus equi can cause severe pneumonia in foals. The absence of a licensed vaccine and limited effectiveness of commercial R. equi hyperimmune plasma (RE-HIP) create a great need for improved prevention of this disease.
Hypothesis: Plasma hyperimmune to the capsular polysaccharide poly-N-acetyl glucosamine (PNAG) would be significantly more effective than RE-HIP at mediating complement deposition and opsonophagocytic killing (OPK) of R. equi.
Animals: Venipuncture was performed on 9 Quarter Horses.
Methods: The ability of the following plasma sources to mediate complement component 1 (C1) deposition onto either PNAG or R. equi was determined by ELISA: (1) PNAG hyperimmune plasma (PNAG-HIP), (2) RE-HIP, and (3) standard non-hyperimmune commercial plasma (SP). For OPK, each plasma type was combined with R. equi, equine complement, and neutrophils isolated from horses (n = 9); after 4 hours, the number of R. equi in each well was determined by quantitative culture. Data were analyzed using linear mixed-effects regression with significance set at P < .05.
Results: The PNAG-HIP and RE-HIP were able to deposit significantly (P < .05) more complement onto their respective targets than the other plasmas. The mean proportional survival of R. equi opsonized with PNAG-HIP was significantly (P < .05) less (14.7%) than that for SP (51.1%) or RE-HIP (42.2%).
Conclusions and Clinical Importance: Plasma hyperimmune to PNAG is superior to RE-HIP for opsonizing and killing R. equi in vitro. Comparison of these 2 plasmas in field trials is warranted because of the reported incomplete effectiveness of RE-HIP.
(© 2019 The Authors. Journal of Veterinary Internal Medicine published by Wiley Periodicals, Inc. on behalf of the American College of Veterinary Internal Medicine.)