Fig. 2. Reactive oxygen species and virulence factors produced by persistent Helicobacter pylori infections are implicated in gastric cancer.

H. pylori infection can result in prolonged inflammatory response and DNA damage due to reactive oxygen species (ROS) production. The virulence factors VacA and CagA from H. pylori contribute to ROS production by increasing immune modulation and inflammation^38,39. a) VacA binds to cell-surface components and activates inflammatory responses once internalized, forming anion-selective membrane channels¹⁹⁷. Chloride ions (Cl⁻) pass through these channels increasing intralumenal chloride levels. To counteract this influx, vacuolar ATPase activity is upregulated, increasing proton (H⁺) pumping and reducing compartmental pH. Weak bases, like ammonia (NH₃), diffuse into these compartments and are trapped following protonation, leading to osmotic swelling and cell vacuolation⁴¹. b) CagA is delivered inside gastric epithelial cells by a type IV secretion system, where it is phosphorylated by host cell kinases, and interacts with host cell proteins to alter signaling pathways. Phosphorylated CagA binds and activates the oncoprotein SRC-homology 2 (SHP2), which induces the extracellular signal-regulated kinase (ERK) via RAS-dependent and -independent mechanisms⁴³. Additionally, CagA activates NF-ĸB, upregulating the expression of proinflammatory cytokines interleukin (IL)-8 and tumour necrosis factor (TNF)-α in gastric epithelial cells⁴⁴. Long-term delivery of CagA disrupts tight-epithelial barrier function, alters cell morphology, and prevents normal tight junction formation⁴⁵. c) In infected macrophages, ROS are produced by spermine oxidase. After being transported into the macrophage, ʟ-arginine is converted to ʟ-ornithine by arginase, generating urea as a byproduct⁵¹. ʟ-ornithine undergoes polyamine conversion to putrescine by a decarboxylase, metabolism by spermidine synthase to spermidine, and subsequent conversion to spermine by spermine synthase. Spermine oxidase (SMOX) reverts spermine back to spermidine, generating hydrogen peroxide (H₂O₂). Sperminal and spermindial could induce double-strand DNA breaks^{53–55,198,199}. d) In polymorphonuclear neutrophils (PMN), ROS are produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase⁴⁹. Neutrophils engulf pathogens into phagosomes and use NADPH-dependent ROS within the phagosome to eliminate pathogens. During phagocytosis, the catalytic subunit of NADPH oxidase is activated and receives an electron from cytoplasmic NADPH, which is donated to oxygen either inside or outside the phagosome, forming superoxide (O₂⁻) that is converted to membrane-permeable H₂O₂. Hydrogen peroxide forms hypochlorous acid (HOCl), which can lead to toxic monochloramine (NH₂Cl) production⁴⁹. NH₂Cl is the result of the reaction between HOCl and ammonium, which is available through urea-to-ammonia conversion mediated by the urease activity of H. pylori⁴⁹. Additionally, hydrogen peroxide can react non-enzymatically with iron (Fe²⁺) or copper (Cu⁺) ions to generate hydroxyl radicals. e) H. pylori persists in the gastric environment and evades immune responses, driving long-term inflammation and carcinogenesis. Part a is adapted from ref.³⁹, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Part d is adapted from ref.⁴⁹, Springer Nature Limited.