Host Determinants of Expression of the Helicobacter pylori BabA Adhesin.

Expression of the Helicobacter pylori blood group antigen binding adhesin A (BabA) is more common in strains isolated from patients with peptic ulcer disease or gastric cancer, rather than asymptomatic colonization. Here we used mouse models to examine host determinants that affect H. pylori BabA expression. BabA expression was lost by phase variation as frequently in WT mice as in RAG2-/- mice that do not have functional B or T cells, and in MyD88-/-, TLR2-/- and TLR4-/- mice that are defective in toll like receptor signaling. The presence of other bacteria had no effect on BabA expression as shown by infection of germ free mice. Moreover, loss of BabA expression was not dependent on Leb expression or the capacity of BabA to bind Leb. Surprisingly, gender was the host determinant most associated with loss of BabA expression, which was maintained to a greater extent in male mice and was associated with greater bacterial load. These results suggest the possibility that loss of BabA expression is not driven by adaptive immunity or toll-like receptor signaling, and that BabA may have other, unrecognized functions in addition to serving as an adhesin that binds Leb.

Expression of the BabA adhesin during experimental infection with Helicobacter pylori.

The Helicobacter pylori babA gene encodes an outer membrane protein that mediates binding to fucosylated ABH antigens of the ABO blood group. We recently demonstrated that BabA expression is lost during experimental infection of rhesus macaques with H. pylori J166. We sought to test the generality of this observation by comparison of different H. pylori strains and different animal hosts. Challenge of macaques with H. pylori J99 yielded output strains that lost BabA expression, either by selection and then expansion of a subpopulation of J99 that had a single-base-pair mutation that encoded a stop codon or by gene conversion of babA with a duplicate copy of babB, a paralog of unknown function. Challenge of mice with H. pylori J166, which unlike J99, has 5' CT repeats in babA, resulted in loss of BabA expression due to phase variation. In the gerbil, Leb binding was lost by replacement of the babA gene that encoded Leb binding with a nonbinding allele that differed at six amino acid residues. Complementation experiments confirmed that change in these six amino acids of BabA was sufficient to eliminate binding to Leb and to gastric tissue. These results demonstrate that BabA expression in vivo is highly dynamic, and the findings implicate specific amino acid residues as critical for binding to fucosylated ABH antigens. We hypothesize that modification of BabA expression during H. pylori infection is a mechanism to adapt to changing conditions of inflammation and glycan expression at the epithelial surface.

Inhibition of heat shock protein expression by Helicobacter pylori.

Heat shock proteins (HSPs) are primarily known as molecular chaperones that are induced by cell stress and prevent protein aggregation and facilitate folding. Recent evidence suggests that exposure of cells to microbial pathogens can also induce HSPs, which then modulate both innate and adaptive immune responses. Paradoxically, Helicobacter pylori has been found to decrease expression of HSPs. We sought to investigate this phenomenon further and to examine the role of different H. pylori strains and recognized virulence factors in cell culture and in the mouse model. Co-culture of H. pylori with two gastric carcinoma cell lines reduced expression of HSP70 and, to a lesser extent, HSP60. Down modulation of HSPs was not dependent on the presence of the vacuolating cytotoxin (VacA) or the cag pathogenicity island (cag PAI). C57BL/6 mice infected with a human H. pylori strain also demonstrated reduced expression of HSP70, HSP8, and heat shock factor 1 (HSF-1), a transcriptional activator of HSP70. In contrast, the bacterial pathogen, S. Typhimurium up-regulated HSP expression. Since HSPs are thought to function as danger signals during microbial infection, H. pylori down-regulation of HSPs may be a mechanism of immune evasion that promotes chronic infection.

Limbic dermoid in a New Zealand White rabbit (Oryctolagus cuniculus).

Superficial keratectomy was performed in a New Zealand White rabbit for a suspected limbic dermoid. Histology confirmed the diagnosis. Ocular dermoids have been reported in a variety of laboratory animals. This is the first report of a corneal dermoid in rabbits.