Pathogenic H elicobacter pylori strains translocate DNA and activate TLR9 via the cancer-associated cag type IV secretion system

Abstract

Helicobacter pylori is the strongest identified risk factor for gastric cancer, the third most common cause of cancer-related death worldwide. An H. pylori constituent that augments cancer risk is the strain-specific cag pathogenicity island, which encodes a type IV secretion system (T4SS) that translocates a pro-inflammatory and oncogenic protein, CagA, into epithelial cells. However, the majority of persons colonized with CagA+ H. pylori strains do not develop cancer, suggesting that other microbial effectors also play a role in carcinogenesis. Toll-like receptor 9 (TLR9) is an endosome bound, innate immune receptor that detects and responds to hypo-methylated CpG DNA motifs that are most commonly found in microbial genomes. High expression tlr9 polymorphisms have been linked to the development of premalignant lesions in the stomach. We now demonstrate that levels of H. pylori-mediated TLR9 activation and expression are directly related to gastric cancer risk in human populations. Mechanistically, we show for the first time that the H. pylori cancer-associated cag T4SS is required for TLR9 activation and that H. pylori DNA is actively translocated by the cag T4SS to engage this host receptor. Activation of TLR9 occurs through a contact-dependent mechanism between pathogen and host, and involves transfer of microbial DNA that is both protected as well as exposed during transport. These results indicate that TLR9 activation via the cag island may modify the risk for malignancy within the context of H. pylori infection and provide an important framework for future studies investigating the microbial-epithelial interface in gastric carcinogenesis.

Human and Helicobacter pylori Interactions Determine the Outcome of Gastric Diseases

“…Studies have highlighted the critical role of the T4SS in this process (Fischer et al 2001; Belogolova et al 2013; Varga et al 2016) (Fig. 2.1).…”

“…2.1). Although several bacterial components, such as peptidoglycan (Viala et al 2004) or DNA (Varga et al 2016), can be translocated into host cells through the T4SS and induce an innate immune response, the role of CagA has been evidenced by the following discoveries: i) H. pylori cagA -positive strains induce more CXCL8 than cagA -negative strains (Crabtree et al 1994; Salih et al 2014); ii) cagA mutant strains have reduced ability to stimulate CXCL8 (Brandt et al 2005; Gobert et al 2013); iii) ectopic expression of CagA in gastric epithelial cells stimulates CXCL8 expression (Kim et al 2006); and iv) inhibition of CagA phosphorylation decreases CXCL8 mRNA expression (Gobert et al 2013). Nevertheless, the effect of CagA on CXCL8 induction has not been observed in some studies (Sharma et al 1995), and the various results could be explained by differences in the type of host cells, in the H. pylori strains that have been used, and in the time of infection.…”

Genetic Manipulation of Helicobacter pylori Virulence Function by Host Carcinogenic Phenotypes

“…One specific cancer-linked H. pylori locus is the cag pathogenicity island ( cag PAI), which encodes a type IV secretion system (T4SS) that translocates the oncoprotein CagA, peptidoglycan, and DNA into host cells (4-6). A structural component of the cag T4SS is CagY, which is required for NF-κB-driven pro-inflammatory cytokine secretion.…”

DNA Transfer and Toll-like Receptor Modulation by Helicobacter pylori

“…DNA transfer events between bacterial species or into the extracellular milieu are very common; however, transkingdom DNA transfer events are rare. To date, there are only a few bacterial species capable of translocating DNA into a eukaryotic host including: Agrobacterium tumefaciens , E. coli (Waters 2001) (Fernandez-Gonzalez et al 2011), Bartonella henselae (Schroder et al 2011), and most recently H. pylori (Varga et al 2016). …”

“…The H. pylori cag T4SS retains significant homology to the archetypal T4SS of A. tumefaciens and has been shown to translocate the effector molecules CagA and peptidoglycan into host cells and has recently been shown to translocate DNA as well (Odenbreit et al 2000; Viala et al 2004; Varga et al 2016). …”