Early-life consumption of unboiled cow’s milk has been characterized in several studies of human infants to be a protective factor for the development of atopy [
74‐
83]. Indeed, farm milk exposure has been associated with increased numbers of CD4
+CD25
+FoxP3
+ Tregs, lower atopic sensitization and asthma in 4.5-year-old children [
84].
The heat-sensitive atopy-preventive factor in fresh unboiled cow’s milk has not yet been identified. Nevertheless, human and bovine milk contain substantial amounts of exosomal microRNAs, which have been postulated to be involved in postnatal immune regulation [
85‐
90]. Milk microRNAs are transported by membranous microvesicles, called exosomes that play a pivotal role for horizontal microRNA transfer [
91]. Raposo et al. [
92] provided first evidence for exosome-mediated immune cell communication. Unidirectional transfer of microRNA-loaded exosomes from T-cells to antigen-presenting cells has recently been confirmed [
93]. For immune cell-cell interactions exosome transport exchanging genetic messages over distances has been appreciated [
94,
95]. Human and bovine milk contain high amounts of exosomal microRNA-155 [
86,
87,
96,
97]. Admyre et al. [
88] showed that incubation of human peripheral blood mononuclear cells with isolated human milk exosomes increased the number of CD4
+CD25
+FoxP3
+ Tregs in a dose-dependent manner. Substantial evidence underlines that the ancient immune-regulatory microRNA-155 is required for the development of Tregs [
98]. Notably, microRNA-155-deficient mice have reduced numbers of Tregs both in the thymus and in the periphery [
98]. FoxP3 binds to the promoter of
bic, the gene encoding microRNA-155 [
99‐
101]. T-cell receptor (TCR) and Notch signalling upregulate the IL-2R α-chain (CD25), rendering thymocytes receptive to subsequent cytokine signals that foster their development into fully functional FoxP3
+ Tregs [
102‐
104]. IL-2 is capable of transducing signals in CD4
+FoxP3
+ Tregs as determined by phosphorylation of signal transducer and activator of transcription 5 (STAT5) [
104]. Deletion of microRNA-155 results in limited IL-2/STAT5 signalling, which reduced Treg numbers [
105]. Remarkably, microRNA-155 enhances FoxP3 expression by targeting suppressor of cytokine signalling 1 (SOCS1), an important negative regulator of IL-2R/STAT5 signalling [
105].
Boiling of farm milk abolishes the atopy-preventive effect of cow’s milk [
74‐
83]. Boiling of cow’s milk degrades milk-derived bioactive microRNAs [
90]. It has been shown that exosome membrane integrity is essential for the uptake of milk microRNAs into cultured cells [
96]. The boiling process may disrupt the exosome lipid bilayer thus exposing the microRNA cargo to rapid RNase-mediated degradation. These observations support the recent concept that milk’s exosomal microRNAs may be involved in the maturation of Tregs and provide a potential signalling network of fresh milk that controls adequate maturation of Tregs preventing allergic immune deviations [
106]. Indeed, exosomes have been detected in the intestine [
107], and in the human and murine thymus [
108,
109], where they induce Tregs [
108]. It is thus conceivable that during the postnatal period, a time with higher intestinal permeability, immune-regulating exosomal microRNAs may pass the intestinal permeability barrier and traffic to the thymus or peripheral lympoid organs to promote Treg maturation [
110]. Whereas raw unprocessed cow’s milk contains the highest amounts of bioactive microRNAs of all known body fluids, pasteurized milk contains much lower levels and milk powder commonly used for infant formula production only exhibits trace amounts of RNAs [
86,
87]. The absence of bioactive microRNA-155 in infant formula may lead to inappropriate intestinal or thymic Treg maturation providing a further argument for the allergy-promoting effect of formula feeding and for the atopy-preventive effect of raw cow’s milk consumption early in life [
106].