Differential Expressions of Immunoregulatory microRNAs in Breast Milk of Mothers of Preterm Versus Term Infants: A Preliminary Study

<i>Background and Objectives</i>: Human breast milk contains essential nutrients for infant growth, as well as bioactive molecules such as exosomes and miRNAs, which play a key role in the maturation of the infant's immune system. Breast milk from mothers of preterm and term infants shows significant differences in its nutrient contents and bioactive components. This preliminary study aimed to compare the expressions of 13 immunomodulatory microRNAs present in breast milk from the mothers of preterm and term infants. <i>Materials and Methods</i>: Breast milk samples were obtained from 5 breastfeeding mothers of term infants and 5 breastfeeding mothers of preterm infants. Every mother provided morning, noon, and evening milk samples. The total protein, fat, and lactose concentrations were measured. In addition, miRNAs were extracted from the exosomal fraction of each sample. The expression levels of the 13 miRNAs were compared and analyzed at the three time points each day. <i>Results</i>: The preterm infants' milk had higher average fat and lactose levels. There were no differences in the total protein concentrations. The expressions of miRNAs in the preterm infants' milk showed significantly higher variations in miR-17-5p, miR-24, miR-29b, miR-30a-5p, and miR-146a. The other miRNAs did not show variations. Interestingly, the highest miRNA expression was only observed in breast milk from the nighttime. The morning and midday samples showed distinct expression patterns. <i>Conclusions</i>: We identified the immunomodulatory microRNA components and their changes in expression levels at different times of the day, as well as those most strongly expressed in breast milk consumed by preterm infants.