Aarthi Rajesh, Rebecca M. Harman, Gerlinde R. Van de Walle
Neutrophil chemotaxis and phagocytosis are critical for protection against bacteria, but can be compromised by methicillin-resistant Staphylococcus aureus (MRSA). MRSA can also circumvent macrophage surveillance by affecting polarization and reactive oxygen species (ROS) production. We previously demonstrated that the secretome of equine mesenchymal stromal cells (MSCs), comprised of all secreted bioactive factors and collected as conditioned medium (CM), reduces the growth of MRSA both in vitro and in vivo. This study aimed to determine if the equine MSC secretome has additional anti-MRSA properties by studying its effects on equine neutrophil and macrophage functions in vitro. Transwell assays demonstrated that CM from adipose tissue- and bone marrow-, but not peripheral blood-, derived MSCs significantly enhanced neutrophil migration. In addition, CM from all three MSC sources significantly reduced the phagocytic capacity of neutrophils but did not alter ROS production. MSC CM from all three tissue sources promoted macrophage polarization toward both CD86-positive (M1-like) and CD206-positive (M2-like) phenotypes, but did not change phagocytic capacity or ROS production. Further experiments showed that the complete CM, rather than the soluble and extracellular vesicle (EV) subfractions, was responsible for the increased neutrophil chemotaxis. Additionally, the primary effect of EVs on macrophages was cell death, possibly through autophagy, which can be beneficial if tissue damaging inflammation is diminished with decreased numbers of viable macrophages. Collectively, our findings show that the equine MSC secretome modulates various innate immune responses in vitro and may have therapeutic potential for managing dysregulated inflammation associated with bacterial diseases in vivo.