BackgroundThe immunomodulatory capacity of breast milk may partially be mediated by microRNAs (miRNA), small RNA molecules that regulate gene expression on a post-transcriptional level and are hypothesized to be involved in modulation of immunological pathways. Here, we evaluate the expression of immune-related miRNAs in breast milk after pre- and postnatal supplementation with Limosilactobacillus reuteri and omega-3 (omega-3) polyunsaturated fatty acids (PUFAs), and the association to infant regulatory T cell (Treg) frequencies. MethodsOne-hundred and twenty women included in a double-blind, randomized, placebo-controlled allergy intervention trial received L. reuteri and/or omega-3 PUFAs daily from gestational week 20. Using Taqman qPCR, 24 miRNAs were analyzed from breast milk obtained at birth (colostrum) and after 3 months (mature milk) of lactation. The proportion of activated and resting Treg cells were analyzed in infant blood using flow cytometry at 6, 12, and 24 months. ResultsRelative expression changed significantly over the lactation period for most of the miRNAs; however, the expression was not significantly influenced by any of the supplements. Colostrum miR-181a-3p correlated with resting Treg cell frequencies at 6 months. Colostrum miR-148a-3p and let-7d-3p correlated with the frequencies of activated Treg cells at 24 months, as did mature milk miR-181a-3p and miR-181c-3p. ConclusionMaternal supplementation with L. reuteri and omega-3 PUFAs did not significantly affect the relative miRNA expression in breast milk. Interestingly, some of the miRNAs correlate with Treg subpopulations in the breastfed children, supporting the hypothesis that breast milk miRNAs could be important in infant immune regulation. Trial registrationClinicalTrials.gov-ID: NCT01542970.

Human milk is rich in extracellular vesicles (EV) that may contribute to shaping neonatal immunity. Here, we evaluated whether freezing, and the addition of sodium citrate (SC), affect the characteristics of human milk EVs and their miRNAs. Freezing may compromise the milk EV population and their miRNA profile by creating artificial vesicles due to cell lysis. Furthermore, SC can be added to clear the EV fraction of micelles, that is, protein aggregates that co-isolate with milk EVs, and may affect certain downstream analyses. To investigate potential differences between milk EV and their miRNA cargo when isolated from fresh and frozen samples, mature milk samples were collected from 10 women and subjected to four different treatments: fresh and frozen; freshSC and frozenSC. Ultracentrifugation was used for EV isolation, and subsequently characterized by Nanoparticle tracking analysis, flow cytometry, Western blot and electron microscopy. While freezing without SC has no impact on the evaluated EV parameters, freezing with SC significantly altered particle mean size as measured by NTA and protein levels as studied by MACSPlex flow cytometry. Importantly, neither freezing nor SC had an impact on the EV miRNA cargo, measured by qPCR. These findings also suggest that EV isolates from frozen samples, in comparison to freshly isolated ones, can produce valid results concerning morphology, size, surface markers and the EV miRNA profile.

Breast milk is an essential source of nutrition and hydration for the infant. In addition, this highly complex fluid is rich in extracellular vesicles (EVs). Here, we have applied a microfluidic technology, lipid-based protein immobilization (LPI) and liquid chromatography with tandem mass spectrometry (LC-MS/MS) to characterize the proteome of human milk EVs. Mature milk from six mothers was subjected to EV isolation by ultracentrifugation followed by size exclusion chromatography. Three of the samples were carefully characterized; suggesting a subset enriched by small EVs. The EVs were digested by trypsin in an LPI flow cell and in-solution digestion, giving rise to two fractions of peptides originating from the surface proteome (LPI fraction) or the complete proteome (in-solution digestion). LC-MS/MS recovered peptides corresponding to 582 proteins in the LPI fraction and 938 proteins in the in-solution digested samples; 400 of these proteins were uniquely found in the in-solution digested samples and were hence denoted "cargo proteome". GeneOntology overrepresentation analysis gave rise to distinctly different functional predictions of the EV surfaceome and the cargo proteome. The surfaceome tends to be overrepresented in functions and components of relevance for the immune system, while the cargo proteome primarily seems to be associated with EV biogenesis.