The cag pathogenicity island (PAI)-encoded type IV secretion system (T4SS) of the bacterial pathogen Helicobacter pylori triggers a massive inflammatory response during infection of the gastric epithelia by a mechanism not fully understood. Previous studies showed that the H. pylori protein CagA and the cell wall component peptidoglycan, when delivered into the host cell by the T4SS, activated transcriptional factor NF-kB leading to induction of the potent proinflammatory chemokine interleukin-8 (IL-8). However, deletion of cagA and knockdown of the intracellular peptidoglycan-sensing nucleotide-binding oligomerisation domain 1 (NOD1) receptor fails to completely reduce T4SS-dependent IL-8. Thus it has long been speculated that the T4SS apparatus itself might contribute to IL-8 induction but direct proof was lacking. Here we provide multiple lines of novel evidence that a non-effector T4SS surface component, CagL, contributes directly and predominantly to the induction of IL-8.
Inhibition of α5β1 integrin or mutation of the arginine-glycine-aspartate (RGD) motif in CagL significantly attenuated IL-8 induction. CagL-directed induction of IL-8 involved activation of integrin α5β1, Src kinase, Ras GTPase, the mitogen-activated protein kinase (MAPK) pathway and NF-κB. Remarkably, exogenous CagL dramatically increased the ability of the H. pylori mutant P12ΔcagL, but not the P12ΔcagPAI mutant, to activate NF-κB and IL-8 secretion, suggesting that CagL functions synergistically with the rest of the T4SS in triggering proinflammatory responses. This occurred independently of CagA translocation and NOD1-dependent signaling.
These findings reveal a previously uncharacterised innate immune mechanism whereby the T4SS apparatus of a persistent human pathogen is recognised by the host adhesion receptor integrin β1, which activates the MAPK pathway to mount a potent proinflammatory response.