Final Report Summary - HYGIENEANDIMMUNITY (Mechanisms of hygiene-mediated immune dysregulation and impact on the susceptibility to allergic and autoimmune diseases) Mucosal sites are colonized with a diverse consortium of commensal bacteria that play a key role in health and disease. Although the density and complexity of the microbiota are highest in the lower intestine, all mucosal surfaces, including the skin, the oral cavity and the lung, harbor a resident microbiota. Host-microbial interactions throughout life are critical for homeostasis and regulation of the immune system. The increased susceptibility to autoimmune or allergic diseases that has occurred over the past several decades often correlates with changes in the composition and diversity of the intestinal microbiota. We have been studying how microbial exposure shapes the developing innate and adaptive immune system and sets the baseline for a regulated immune system. Microbial colonization of mucosal surfaces is initiated during birth as soon as the newborn leaves the sterile uterine environment, and microbial exposure after birth is thought to be one of the main stimulus to postnatal immune development. We have found that in the absence of any intestinal bacteria, germ-free mice have a dysregulated immune system with high levels of IgE, the antibody that mediates allergic reactivity. We found that colonization with a complex mixture of bacteria was required to inhibit immune dysregulation and, importantly, colonization in early life was required critical for protection. Therefore, we found that there is a critical window after birth whereby the neonatal immune system is particularly sensitive to microbial signals. Once this critical window has passed, these same microbial signals will no longer provide the protective signals that they could in early life. These data are important because animals that were colonized with only a limited diversity of microbes early in life were more susceptible to IgE-mediated allergic reactions later in life. We have also characterized the IgE that develops in the absence of microbial colonization in order to understand whether it is a result of an enhanced sensitivity and over-reactivity to food or self-proteins. These studies should provide insight into the growing epidemic of allergic and autoimmune diseases. Although live bacteria are confined to the mucosa, studies have shown that products from the intestinal microbiota can easily reach systemic sites in the body. We therefore questioned whether the developing immune system could be influenced by the microbiota of the mother prior to birth. We found that the maternal microbiota can influence neonatal immune development already starting in utero. We could show that antibodies from the mother transfer microbial products across the placenta and into the maternal milk and these products help to shape the developing innate immune system so that it is prepared to face the challenge of colonization with its own microbiota at birth. Innate and adaptive immunity are clearly heavily influenced by microbial colonization and we have further investigated how the development and function of key innate immune cells in the intestine. Although this work is ongoing, we are defining the key molecular and cellular pathways by which our microbial partners shape and define our immune system during health and this affects our susceptibility to diseases.