Gut bacteria, regulatory T cells and allergic sensitization in early childhood

The hygiene hypothesis postulates that reduced or altered microbial exposure early in life may lead to impaired immune maturation and, as consequence of this, development of allergic disorders. Thus, we examined if the infantile gut microbiota was related to the postnatal T cell development in vivo and if certain commensal gut bacteria were able to induce regulatory T cells (Tregs) in vitro. We also investigated if the proportion of Tregs was associated with allergic sensitization and allergic disease in the first 3 years of life. We showed that the gut commensal Staphylococcus aureus (S. aureus) could convert neonatal CD4+ T cells into FOXP3+CD25+CD127low Tregs in vitro and that certain culture conditions were required for this conversion. Depletion of pre-existing Tregs before stimulation with S. aureus resulted in activated CD25+CD127low T cells that increased proliferation of CD4+ responder T cells. In contrast, naive CD4+ T cells stimulated in the presence of pre-existing Tregs induced suppressive FOXP3+CD25+CD127low Tregs. Finally, blocking programmed cell death ligand-1 (PD-L1) expressed on antigen presenting cells during stimulation with S. aureus, reduced or completely inhibited the induction of FOXP3+CD25+CD127low T cells. In the prospective FARMFLORA birth-cohort study, we found that children with an early gut microbiota including bifidobacteria and Escherichia coli (E. coli) had mononuclear cells with higher capacity to produce proinflammatory and Th2-related cytokines in response to phytohaemagglutinin (PHA) than children not colonized by these bacteria. In contrast, early colonization by S. aureus and enterococci was inversely related with the PHA-induced cytokine responses. The early bacterial gut colonization pattern was not associated with the proportion of putative FOXP3+CD25high Tregs within the circulating CD4+ T cell population during early childhood. However, high proportions of FOXP3+CD25high cells of the CD4+ T cell population in early infancy were inversely related to the capacity of mononuclear cells to produce cytokines in response to PHA as well as to the proportions of CD45RO+ of CD4+ T cells later in childhood. Moreover, children who were sensitized at 18 and 36 months of age had higher proportions of putative FOXP3+CD25high Tregs at birth and 3 days of life than children who remained non-sensitized. Allergic disease, on the other hand was not associated with the proportion of putative FOXP3+CD25high Tregs. In conclusion, these results indicate that S. aureus has an ability to convert naïve neonatal CD4+ T cells into FOXP3+CD25+CD127low regulatory T cells in vitro, a process which is dependent on the presence of both thymic derived Tregs and of APCs that express PD-L1. However, the early bacterial gut colonization pattern was not related to the proportion of putative FOXP3+CD25high Tregs within the circulating CD4+ T cell population in children during the first 3 years of life. Furthermore, as infants who were sensitized had higher proportion of FOXP3+CD25high within the CD4+ T cell population early in life compared to healthy children, higher proportions of Tregs early in life do not seem to be protective against atopic disorders. Thus, it is possible that high proportions of putative FOXP3+CD25high Tregs within the CD4+ T cell population early in infancy may modulate the effector T cell development in a way that could predispose to allergic sensitization. However, early gut colonization with a gut microbiota including bifidobacteria and E. coli might instead enhance the effector T cell development.