Secretome of bovine mammosphere-derived epithelial cells activates the local immune response in an experimental Staphylococcus aureus mastitis model in dairy cows.

Staphylococcus aureus often causes chronic mastitis in dairy cows that does not respond well to antimicrobial therapy. Secreted molecules from stem cells, collected as conditioned medium (CM), exert antimicrobial, immunomodulatory, and tissue-reparative effects, which could be of therapeutic benefit. We tested the effects of CM from bovine mammosphere-derived epithelial cells (MDECs), primary cell cultures enriched for stem and progenitor cells, on the immune response in quarters of healthy cows and cows experimentally challenged with S. aureus. Healthy cows (n = 3) were treated with an intramammary infusion of MDEC-CM in two quarters for 3 days or left untreated (NT). Cows (n = 11) experimentally challenged with S. aureus in all four quarters were treated in all quarters by intramammary infusion with MDEC-CM (n = 4), a conventional antimicrobial (ABX) (n = 3) for 3 days, or left NT (n = 4). Treatment with MDEC-CM increased influx of leukocytes into udder tissue and milk, as well as IL-10 concentrations in milk in unchallenged and S. aureus-challenged quarters. In S. aureus-challenged quarters, ABX reduced bacterial counts in milk on day 5, with 82% of quarters free of S. aureus, while MDEC-CM treated and NT quarters were 31% and 29% S. aureus free, respectively. Transcriptomic profiles of MDEC-CM-treated udder tissue showed a significantly higher number of differentially expressed genes compared to ABX and NT, but differences of selected genes could not be confirmed with qPCR or RNAScope by in situ hybridization. Our results showed that MDEC-CM induced local immune changes in the udder, but more work is needed to better understand this effect.