Micrornas in bovine milk exosomes are bioavailable in humans but do not elicit a robust pro-inflammatory cytokine response

Background: Bovine milk exosomes are studied for their roles as bioactive food compounds and as vehicles for drug delivery. Both lines of investigation converge on immune function, e.g., immune regulation by absorption of microRNAs encapsulated in milk exosomes across species boundaries, and the possibility of exosomes and their cargos triggering an immune response if used in drug delivery. This study assessed the bioavailability of immune-related microRNAs from bovine milk and changes in plasma cytokine concentrations after milk consumption in humans, and the secretion of cytokines by human peripheral blood mononuclear cells (PBMCs) cultured with milk exosomes transfected with immune-relevant microRNAs. Results: Human plasma samples were collected before and at timed intervals after a milk meal and analyzed for concentrations of six immune-relevant microRNAs and nine cytokines. The peak plasma concentrations of miR-15b-5p, miR21-5p, miR-106b-5p, and miR-223-3p were 60 ± 9.80% to 162 ± 31.80% higher after milk consumption (C_t values 23 ± 1.2 to 26 ± 1.1 cycles) compared to baseline values (P < 0.05). Plasma concentrations of TNF-alpha were not significantly different before versus after milk consumption; eight other cytokines were below detection limit. PBMCs were collected before and six hours after milk consumption and cultured with or without concanavalin A (ConA). TNF-alpha, IL-1β,IL-6andIL-10were detectable in culture media, but concentrations did not depend on milk consumption prior to PBMC isolation (P > 0.05). When PBMC cultures from fasted subjects were supplemented with milk exosomes that had been transfected with immune-relevant microRNAs, the concentrations of IL-1β, IL-6, IL-10 and TNF-alpha were 29 ± 12% to 220 ± 33% higher than controls cultured with non-transfected exosomes (P < 0.05), but cytokine concentrations were not different compared with control exosomes transfected with scrambled microRNA (P > 0.05). Conclusions: MicroRNAs in bovine milk exosomes are bioavailable. Milk exosomes do not elicit an increase of plasma cytokines following oral administration. Trial registration: ISRCTN registry ID: 16329971. Retrospectively registered on February 7th, 2019.