The secretomes of bovine mammary epithelial cell subpopulations differentially modulate macrophage function

Bovine mammosphere-derived epithelial cell (MDEC) cultures are heterogeneous and enriched for stem and progenitor cells. We previously reported that the bovine MDEC secretome, comprised of all bioactive factors secreted by the cells, displays regenerative properties, exerts antimicrobial effects, and modulates neutrophil activity, positioning it as a promising non-antibiotic biologic therapy for infectious diseases important to the dairy industry, like mastitis. Mastitis is defined as inflammation of the udder, and it is typically caused by bacterial infection. The effect of the MDEC secretome on macrophages, a first line of defense against bacterial infections in the udder, is unknown and could impact the utility of the secretome as a therapy for mastitis. To address this, we isolated bovine monocytes from peripheral blood and maintained them as an unpolarized (M0) population or polarized them into M1 or M2 phenotypes. Macrophages cultured with the secretome of bovine MDECs were assessed for their ability to phagocytose labeled bacterial particles and accumulate reactive oxygen species (ROS). We used single-cell RNA sequencing (scRNA-seq) and fluorescence-activated cell sorting (FACS) to isolate a subpopulation of MDECs that exert enhanced effects on macrophages. We found that the secretome of MDECs that do not express cluster of differentiation (CD) 73, a cell surface enzyme used as a marker for mesenchymal stromal cells, most strongly increased macrophage phagocytosis and ROS accumulation. These findings will help optimize the generation of the bovine MDEC secretome as a suitable treatment option for mastitis.