Effects of environmental and dietary factors on the development of the immune system in children at genetic risk of type 1 diabetes

Type 1 diabetes (T1D) and allergies are considered autoimmune diseases. The exact mechanisms leading to these diseases are mostly unknown, but genetic and environmental factors are involved. Regulatory T cells (Tregs) have a crucial role in initiating and maintaining tolerance, limiting harmful autoantigen-specific inflammation processes. This doctoral thesis explores the effect of environmental factors on the development of the immune system by studying peripheral immunological responses in healthy infants at genetic risk of T1D in two follow-up studies. The first study investigated the development of peripheral Treg cells in infants from Estonia and Finland, neighboring countries with differing living standards, and the incidence of T1D and allergies. We demonstrated a two-step maturation process in the circulating Treg cell population in the first two years of life. First, a decrease in the proportion of Treg cells followed by an increase in the highly activated Treg cells (TregFOXP3high), with enhanced suppressive activity. The Treg cell maturation process coincided with the microbiota composition development from a Bifidobacteria dominated to one dominated by butyrate-producing species by 36 months. The abundance of Bifidobacteria and the relative abundance of B.longum alone at three months showed an inverse association with atopic sensitization. The switch in the microbiota happened earlier in Estonia and was associated with more highly activated TregFOXP3high cells with FOXP3 and CTLA-4 expression. This earlier maturation of the circulating Treg population is associated with a lower risk of allergic diseases in Estonian children. Additionally, we analyzed the common enteral virus infections in the first year of life. We observed a temporal association of virus infection with the expression of FOXP3 in regulatory T cells. Infants with enterovirus infection during the preceding 30 or 60 days had a significantly decreased FOXP3 expression in TregFOXP3high cells. In addition, we observed upregulation of Th1- and Th17 -related cytokines and a decreased activation of CCL22, associating with activation of proinflammatory pathways and reduced immune regulation. In the second study, we examined whether exposure to bovine insulin is a trigger of autoimmunity by studying the appearance of diabetes autoantibodies. We studied exclusively breastfed infants or infants after exposure to different infant formulas; cow-milk formula (CMF) with bovine insulin, whey-based hydrolyzed formula, or a virtually bovine insulin-free whey-based hydrolyzed FINDIA formula during the first six months of life, whenever breast milk was not available. The follow-up was up to at least three years of age. We showed that weaning to the FINDIA formula reduced the incidence of autoantibody positivity when compared with CMF-formula. This result might reflect a dysfunctional immune system. Bovine insulin peptides from the infant formula may activate autoreactivity against the primary autoantigen, i.e., human insulin, resulting in non-tolerogenic T cells. To conclude, the study shows that environmental factors may influence regulatory T cells and immunotolerance. Also, nutrition may influence the development of the immune system or autoantibodies to T1D in infants genetically at risk.