Mechanisms of type 1 Lewis antigen synthesis and variation in Helicobacter pylori

• Main Author: Pohl, Mary Ann
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2011
• Subjects: Microbiology
• ISBN: 9781124808642; 1124808647

Helicobacter pylori is a ubiquitous Gram-negative, microaerophilic organism, colonizing about half of the world's human population. Although a large portion of the population harbors this bacterium, only a small minority develop disease, including peptic ulceration and gastric cancer. H. pylori has developed mechanisms to mediate persistence in the host. Factors hypothesized to contribute to survival and adaptation include the unique characteristics of the lipopolysaccharide (LPS) layer of the outer membrane of H. pylori. H. pylori LPS is much less immunoreactive than LPS of other enteric bacteria, and within the LPS are fucosylated oliogsaccharides called Lewis (Le) antigens. H. pylori Le antigens are similar to Le antigens present on human tissues, including the gastric epithelium; importantly, both humans and H. pylori are polymorphic for Le phenotypes. Analysis of H. pylori strains from around the world revealed an array of Le phenotypes, with specific Le genotypes and phenotypes associated with the geographic origin of the strains. Molecular mimicry of Le antigens by H. pylori led to the hypothesis that H. pylori can change their Le phenotypes to match that of their host, as a mechanism for niche adaptation and immune evasion. We tested this hypothesis by challenging wild type and Leb-transgenic mice with H. pylori strain HP1. There were no changes in Le phenotypes in H. pylori recovered from wild type mice over 8 months, but isolates from the Leb-transgenic mice were Leb+, suggesting that H. pylori can change its Le expression in response to host-driven selection. Next, I characterized the genetic differences in the Leb+ isolates from the Leb-transgenic mice, and determined that phase variation occurred in β-(1,3)galT, which allowed for expression of Leb. Mutagenesis and complementation assays confirmed that this allele is essential for type I antigen synthesis. Further analyses of the β-(1,3)galT locus revealed that β-(1,3)galT can undergo intragenomic recombination with an upstream homolog, jhp0562, within individual H. pylori strains. Complementation of a β-(1,3)galT null mutant revealed that these chimeric alleles can restore β-(1,3)galT function. Mutagenesis and complementation showed both jhp0562 and galT are essential for synthesis of all Le antigens in strains harboring this gene.