Microbial colonization shifts the Vbeta repertoire of intestinal T cells and polysaccharide expression in the microbiota

The goal of my dissertation research has been to examine molecular aspects of the relationship between host and intestinal microbiota. In order to do so I have utilized the germ-free (GF) mouse model of colonization and the model symbiont, Bacteroides fragilis.;From the perspective of the host, I determined the effects of colonization on the T cell receptor Vbeta repertoire of CD4+ T cells in mucosal and systemic lymphoid organs. GF mice were colonized with a complex microbiota at birth or at the time of weaning or were monocolonized with B. fragilis. Although CD4+ T cell responses to the microbiota were largely independently of Vbeta usage, the presence of a microbiota did induce subtle but significant shifts in the Vbeta repertoire in all three groups of colonized mice. Relative to GF mice, acquisition of a complex flora at birth had a greater effect on the CD4+ Vbeta repertoire than colonization upon weaning. However, in both groups of complex colonized mice a majority of the Vbeta shifts were observed in the Peyer's patches. In contrast, in monocolonized mice, the Vbeta repertoire of the spleen was most affected by colonization. Overall the results suggest that microbiota-associated antigens can generate T cell responses capable of shifting the Vbeta repertoire of CD4+ T cell populations in mucosal and systemic lymphoid organs.;From the perspective of B. fragilis, I studied preferences in capsular polysaccharide (CPS) promoter orientation during intestinal colonization. B. fragilis produces eight CPS, seven of which are regulated by the reversible inversion of the DNA containing the promoter of the biosynthesis locus. B. fragilis chromosomal DNA was isolated from the intestinal contents and fecal samples from monocolonized and complex colonized mice. CPS promoter orientations were determined using a PCR digestion method. Though there are similarities in CPS promoter orientations between B. fragilis populations in monocolonized and complex colonized mice, there are also distinct differences that may significantly alter the surface diversity of the population, suggesting that environmental factors can influence CPS expression.