Opsonization but not pretreatment of equine macrophages with hyperimmune plasma nonspecifically enhances phagocytosis and intracellular killing of Rhodococcus equi.
Authors: Harvey Aja B, Bordin Angela I, Rocha Joana N, Bray Jocelyn M, Cohen Noah D
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Hyperimmune Plasma and *Rhodococcus equi* Control in Foals Whilst hyperimmune plasma (HIP) is widely used to prevent *Rhodococcus equi* pneumonia in foals, evidence supporting its efficacy remains inconsistent. Researchers exposed equine alveolar macrophages to *R. equi* under four conditions: bacteria opsonized with hyperimmune plasma, bacteria opsonized with normal plasma, macrophages pretreated with hyperimmune plasma then exposed to nonopsonized bacteria, and control groups, measuring bacterial uptake and survival at 0 and 48 hours. Opsonization of the bacteria themselves—regardless of whether hyperimmune or normal plasma was used—significantly enhanced macrophage phagocytosis and reduced intracellular bacterial survival (both P <0.0001), whereas pretreating macrophages with either plasma type had no effect on these outcomes. These findings suggest that any clinical advantage of hyperimmune plasma over standard plasma in preventing foal pneumonia cannot be explained by enhanced macrophage immune function, implying that protective mechanisms operate through alternative pathways—possibly involving systemic opsonization, complement activation, or direct bacterial neutralisation outside the alveolar space. For practitioners, this indicates that further investigation into HIP efficacy is warranted before assuming superiority over conventional preventative strategies, and highlights the importance of considering multiple immune mechanisms when evaluating disease prevention protocols in neonatal foals.
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Practical Takeaways
- •Hyperimmune plasma's clinical benefit in preventing foal R. equi pneumonia likely does not work through direct enhancement of alveolar macrophage function, suggesting other protective mechanisms may be involved
- •Normal plasma appears equally effective as hyperimmune plasma at opsonizing R. equi in vitro, questioning the additional cost and logistics of hyperimmune plasma use for this indication
- •Any clinical advantage of hyperimmune plasma over standard plasma for R. equi prevention in foals must involve mechanisms beyond macrophage-mediated killing, such as systemic immunity or other immune pathways
Key Findings
- •Opsonization of R. equi with hyperimmune plasma (HIP) increased phagocytosis by alveolar macrophages (P<0.0001) compared to nonopsonized bacteria
- •Opsonization with HIP decreased intracellular survival of R. equi in alveolar macrophages (P<0.0001), with no significant difference between HIP and normal plasma
- •Pretreatment of macrophages with HIP had no effect on subsequent phagocytosis or intracellular replication of nonopsonized R. equi
- •The enhanced bacterial killing effect was dependent on opsonization of the pathogen itself, not on prior macrophage conditioning with hyperimmune plasma