Host-directed therapy in foals reduces the severity of clinical pneumonia against <i>Rhodococcus equi</i> intrabronchial challenge

Abstract Pneumonia caused by Rhodococcus equi, an intracellular bacterium that infects macrophages, is an important cause of disease and death in immunocompromised hosts, especially foals. No vaccine is available, and control is based on antibiotic treatment of subclinical cases, increasing the pressure for emergence of antibiotic resistance and indicating novel control approaches are needed. TLR agonists have immunomodulatory function. Our first objective was to evaluate the ex vivo effects of treatment of neonatal foal alveolar macrophages (AMs) with a combination of TLR2 and TLR9 agonists (PUL-042). Using bronchoalveolar lavage (BAL), we collected AMs from 48 foals and determined the killing of intracellular R. equi using the ratio of CFUs at 48 hours (T48) to post-phagocytosis (T0) (i.e, T48:T0). Concentration of pro- and anti-inflammatory cytokines were measured in AM culture supernatant, BAL fluid, and serum. Compared to control, PUL-042 decreased TNF-a in BAL fluid but increased IL-6, IFN-g, and TNF-a in AMs after ex vivo infection with R. equi, suggesting that nebulization induced an immune response upon infection that is favorable for killing intracellular pathogens. We also evaluated the in vivo the efficacy of PUL-042 in reducing the severity of clinical pneumonia in foals after intrabronchial challenge with R. equi. Infected foals nebulized with PUL-042 had a shorter duration of clinical signs of pneumonia and of presence of lung abscesses when compared to foals that did not receive nebulization, consistent with our ex vivo findings. Our results show potential for enhancing neonatal immune responses against respiratory pathogens and reducing severity of clinical pneumonia following intrabronchial challenge with R. equi.