Regulation of Microbial Trafficking to the Thymus in Early Life

Project Summary Early life is a critical period for the proper development of the immune system. As mammalian hosts have co- evolved with their symbiotic bacteria, signals from the commensal microbiota are crucial for the prevention of allergy, inflammatory disorders, and protection against enteric pathogens. Our lab has recently uncovered that

commensal antigens are trafficked to the thymus by dendritic cells, where they are presented to developing thymocytes. Rather than inducing deletion or Treg development, this results in an expansion of naïve microbiota- specific T cell populations, which are functional and egress to populate the peripheral tissue. This process is

limited to early life, suggesting that it is critical for the establishment of the commensal-reactive T cell repertoire. Analysis of thymic 16S sequencing from mice at weaning reveals a stark enrichment of select taxa, such as members of Escherichia. When considering the vast diversity of the intestinal microbiome, these data imply an

antigenic filter through which only select microbes are trafficked to the thymus. This proposal seeks to understand how the unique environment of the early-life intestine promotes commensal ferrying to the thymus, and the microbial and host factors that curate specific taxa for sampling by sub-epithelial dendritic cells. My central

hypothesis is that this process has evolved to prioritize the generation of naïve T cell clones capable of recognizing antigens associated with epithelial invasion. We posit that the signals promoting commensal antigen trafficking to the thymus are 1) adherence to the intestinal epithelium and 2) opsonization by maternal IgG. As

recent publications have identified commensal-specific IgG as targeting microbes with the ability to translocate, both of these signals indicate the capacity for epithelial invasion – whether defined by the neonate or by maternal cues. Preliminary data indicate that non-adherent commensals are significantly underrepresented in the thymic

tissue. I have also found increased expression of the IgG receptor FcRn in migratory thymic DCs in early life, indicating their enhanced ability to recognize and process IgG-opsonized commensals. In Aim 1, I will quantify the epithelial adherence of a model commensal E. coli throughout early development. I will genetically target

adhesins in this commensal to determine how these factors impact epithelial adherence and thymic trafficking in vivo. I will utilize 16S sequencing to compare the mucosa-associated microbiome with that of the thymus. Aim 2 will focus on the role of maternal IgG opsonization, and will quantify thymic trafficking in B cell- and antibody-

deficient mouse models throughout early life development. I will manipulate maternal IgG titers through transfer of commensal-specific IgG into IgG-deficient dams, and the impacts on thymic trafficking in offspring will be examined. The proposed experiments will reveal crucial elements of how the commensal-reactive T cell

repertoire is curated in early life, and will have important insight into how the microbiome affects the development of the mucosal immune system.