Opsonization but not pretreatment of equine macrophages with hyperimmune plasma nonspecifically enhances phagocytosis and intracellular killing of Rhodococcus equi

Abstract

Background

Evidence regarding the efficacy of equine hyperimmune plasma to prevent pneumonia in foals caused by Rhodococcus equi is limited and conflicting.

Hypothesis

Opsonization with R. equi‐specific hyperimmune plasma (HIP) will significantly increase phagocytosis and decrease intracellular replication of R. equi by alveolar macrophages (AMs) compared to normal plasma (NP).

Animals

Fifteen adult Quarter Horses were used to collect bronchoalveolar lavage cells.

Methods

In the first experiment, AMs from 9 horses were pretreated (incubated) with either HIP, NP, or media only (control) and then infected with nonopsonized R. equi. In a second experiment, AMs from 6 horses were infected with R. equi either opsonized with HIP or opsonized with NP. For both experiments, AMs were lysed at 0 and 48 hours and the number of viable R. equi quantified by culture were compared among groups using linear mixed‐effects modeling with significance set at P < .05.

Results

Opsonization with either HIP or NP increased phagocytosis by AMs ( P < .0001) and decreased intracellular survival of organisms in AMs ( P < .0001). Pretreating AMs with either HIP or NP without opsonizing R. equi had no effects on phagocytosis or intracellular replication.

Conclusions and Clinical Importance

Opsonizing R. equi with either NP or HIP decreases intracellular survival of organisms in AMs, but the effect does not appear to be enhanced by using HIP. Mechanisms other than effects on AMs must explain any clinical benefits of using HIP over NP to decrease the incidence of R. equi pneumonia in foals.