Intact Gram-NegativeHelicobacter pylori,Helicobacter felis, andHelicobacter hepaticusBacteria Activate Innate Immunity via Toll-Like Receptor 2 but Not Toll-Like Receptor 4

Abstract

Molecular and genetic studies have demonstrated that members of the Toll-like receptor (TLR) family are critical innate immune receptors. TLRs are recognition receptors for a diverse group of microbial ligands including bacteria, fungi, and viruses. This study demonstrates that distinct TLRs are responsible for the recognition ofHelicobacterlipopolysaccharide (LPS) versus intactHelicobacterbacteria. We show that the cytokine-inducing activity ofHelicobacterLPS was mediated by TLR4; i.e., TLR4-deficient macrophages were unresponsive toHelicobacter pyloriLPS. Surprisingly, the cytokine response to wholeHelicobacterbacteria (H. pylori,H. hepaticus, andH. felis) was mediated not by TLR4 but rather by TLR2. Studies of HEK293 transfectants revealed that expression of human TLR2 was sufficient to confer responsiveness to intactHelicobacterbacteria, but TLR4 transfection was not sufficient. Our studies further suggest thatcagpathogenicity island genes may modulate the TLR2 agonist activity ofH. pyloriascagA⁺bacteria were more active on a per-cell basis compared tocagAmutant bacteria for interleukin-8 (IL-8) cytokine secretion. Consistent with the transfection studies, analysis of knockout mice demonstrated that TLR2 was required for the cytokine response to intactHelicobacterbacteria. Macrophages from both wild-type and TLR4-deficient mice produced a robust cytokine secretion response (IL-6 and MCP-1) when stimulated with intactHelicobacterbacteria. In contrast, macrophages from TLR2-deficient mice were profoundly unresponsive to intactHelicobacterstimulation, failing to secrete cytokines even at high (100:1) bacterium-to-macrophage ratios. Our studies suggest that TLR2 may be the dominant innate immune receptor for recognition of gastrointestinalHelicobacterspecies.