Direct and indirect anti-inflammatory effects of tulathromycin in bovine macrophages: inhibition of CXCL-8 secretion, induction of apoptosis, and promotion of efferocytosis.

Recent evidence indicates that immunomodulation by antibiotics may enhance their clinical efficacy. Specifically, drug-induced leukocyte apoptosis and macrophage efferocytosis have been shown to promote the resolution of inflammation in a variety of disease settings. Tulathromycin is a new macrolide antibiotic for the treatment of bovine respiratory disease. The direct antimicrobial effects of the drug alone do not fully justify its superior clinical efficacy, and we hypothesize that tulathromycin may have immunomodulating properties. We recently reported that tulathromycin promotes apoptosis and inhibits proinflammatory NF-κB signaling in bovine neutrophils. In this study, we investigated the direct and indirect anti-inflammatory effects of tulathromycin in bovine macrophages. The findings indicate that bovine monocyte-derived macrophages and alveolar macrophages readily phagocytose tulathromycin-induced apoptotic neutrophils both in vitro and in the airways of Mannheimia haemolytica-infected calves. Moreover, tulathromycin promotes delayed, concentration-dependent apoptosis, but not necrosis, in bovine macrophages in vitro. Activation of caspase-3 and detection of mono- and oligonucleosomes in bovine monocyte-derived macrophages treated with tulathromycin was observed 12 h posttreatment; pretreatment with a pan-caspase inhibitor (ZVAD) blocked the proapoptotic effects of the drug. Lastly, tulathromycin inhibited the secretion of proinflammatory CXCL-8 in lipopolysaccharide (LPS)-stimulated bovine macrophages; this effect was independent of caspase activation or programmed cell death. Taken together, these immunomodulating effects observed in bovine macrophages help further elucidate the mechanisms through which tulathromycin confers anti-inflammatory and proresolution benefits. Furthermore, these findings offer novel insights on how antibiotics may offer anti-inflammatory benefits by modulating macrophage-mediated events that play a key role in inflammation.