Alteration of gut microbiota has been associated with development of immune mediated diseases, such as allergy. In part, this could be due to the influence of microbes in shaping the immune response. In paper I, we investigated the association of early-life gut colonization with bacteria, and numbers of IL-4, IL-10 and IFN-γ producing cells at two years of age in response to PBMC stimulation with phytohemagglutinin (PHA) in vitro. Early Staphylococcus (S) aureus colonization was directly proportional to increased numbers of IL-4 and IL-10 secreting cells, while early co-colonization with lactobacilli and S. aureus associated with a decrease in IL-4, IL-10 and IFN-γ secreting cells compared to S. aureus alone. This was also confirmed in in vitro stimulations of PBMC with Lactobacillus and/or S. aureus strains, where S. aureus-induced IFN-γ production by Th cells was down regulated by co-stimulation with Lactobacillus. In paper II, we investigated the effects of UV-killed and/or culture supernatant (sn) of Lactobacillus strains and S. aureus strains on IEC and immune cell responses. IEC exposed to S. aureus-sn produced CXCL-1/GRO-α and CXCL-8/IL-8, while UV-killed bacteria had no effect. Further, PBMC from healthy donors exposed to Lactobacillus-sn and S. aureus-sn were able to produce a plethora of cytokines, but only S. aureus induced the T-cell associated cytokines: IL-2, IL-17, IFN-γ and TNF-α; which were down regulated by co-stimulation with any of the different Lactobacillus strains. Intracellular staining verified S. aureus-induced IFN-γ and IL-17 production by Th cells, and increased CTLA-4 expression and IL-10 production by T reg cells.

In conclusion, we show that colonization with gut microbiota at early age modulates the cytokine response in infancy. In addition, bacterial species influence cytokine response in a species-specific manner and we demonstrate that lactobacilli modulate S. aureus-induced immune response away from an inflammatory phenotype.

The early-life microbiota is important for postnatal immune maturation and implied in immune mediated diseases. The aim of this work was to study specific species of bacteria in the gut microbiota and relate them to immune function and allergic disease during childhood.

In paper I we investigated gut bacteria in feces from infants included in a prospective allergy cohort. We found that children with non-allergic parents were more likely to be colonized with a group of lactobacilli. Further, lactobacilli colonization was more prevalent in children remaining non-allergic, regardless of allergic heredity. In paper II we related the infant gut bacteria to immune function at two years of age. Infant Staphylococcus (S.) aureus colonization associated with increased immune responsiveness, whereas co-colonization with S. aureus and lactobacilli associated with reduced responses. In paper III we investigated T regulatory (Treg) cell phenotype and cytokine production during childhood, and related S. aureus and lactobacilli colonization to Treg phenotype at the age of two. The Treg population matured with age, regarding phenotype and cytokine production. Furthermore, infant S. aureus colonization associated with Treg phenotype at the age of two. In paper IV we investigated the in vitro peripheral blood mononuclear cells responses to soluble factors produced by lactobacilli and S. aureus. Both T- and natural killer cells responded with cytokine production, degranulation and proliferation after S. aureus and simultaneous culture with lactobacilli could dampen the S. aureus-induced responses.

Taken together this thesis shows that the gut microbiota is altered in children who develop allergies, and that early life bacteria associate with immune function. Our in vitro findings support that lactobacilli modulate immune maturation and responses, and that early lactobacilli-colonization may be important for a properly regulated maturation of the immune system.