Nutritional Immunology and Inflammation
Exosomes are natural nanoparticles that facilitate cell-to-cell communication by transferring RNAs from donor to recipient cells. Previous studies focused on microRNA cargos and disregarded mRNAs. Exosomes and RNA cargos may be absorbed from milk. RNAs elicit antiviral responses by activating interferon type I (IFN) and NF-kappa B (NF-kB) signaling (Fig. 1A).
To assess whether 1) bovine milk exosomes (BME) contain microbial mRNAs, 2) microbial mRNAs are bioavailable in humans and mice, and 3) microbial mRNAs activate antiviral responses in reporter cells and mice.
The mRNA content in BME was assessed by RNA-sequencing. mRNA bioavailability was assessed by RNA-sequencing analysis of human plasma before and 4 h after consumption of 1 L milk and by administering BME transfected with synthetic, fluorophore (IRDye)-labeled Pseudomonas fluorescens capB mRNA to Balb/c mice by oral gavage. (capB mRNA was abundantly expressed in all BME samples sequenced.) Antiviral responses were assessed using liposomes loaded with capB mRNA and cultured with IFN and NF-kB J774 reporter macrophages, and by survival curves of mice fed BME-free or BME-sufficient diets and challenged with influenza A virus. Unpaired t-test was used for statistical analyses; P < 0.05 was considered significant.
About 50% of the mRNA-sequencing reads in BME were non-bovine; after contig assembly, up to two thirds of the non-bovine reads mapped to microbial mRNAs (Fig. 1B). IRDye-capB mRNA accumulated in murine intestinal mucosa and liver (and perhaps brain) 6 h and 24 h, respectively, after oral gavage (Fig. 2). Microbial mRNAs, including 55 and 36 mRNAs from P. fluorescens and E. coli were detected in human plasma after but not before a milk meal (Tables 1, 2). capB mRNA-loaded liposomes activated NF-kB and IFN signaling in reporter macrophages (Fig. 3A). C57BL/6 mice fed a BME-depleted diet died within 11 days of influenza A challenge whereas controls fed a BME-sufficient diet survived (Fig. 3B).
This is the first report to suggest a novel interaction between microbiome and hosts: microbial mRNAs in BME are bioavailable, activate antiviral IFN and NF-kB pathways and increase resistance to influenza A.
Funding Sources :
NIFA, NIH, Gates Foundation, PureTech Health, USDA Hatch & Multistate. J.Z. is a consultant for PureTech.