Proton pump inhibitors protect mice from acute systemic inflammation and induce long-term cross-tolerance

Incidence of sepsis is increasing, representing a tremendous burden for health-care systems. Death in acute sepsis is attributed to hyperinflammatory responses, but the underlying mechanisms are still unclear. We report here that proton pump inhibitors (PPIs), which block gastric acid secretion, selectively inhibited tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) secretion by Toll-like receptor (TLR)-activated human monocytes in vitro, in the absence of toxic effects. Remarkably, the oversecretion of IL-1β that represents a hallmark of monocytes from patients affected by cryopyrin-associated periodic syndrome is also blocked. Based on these propaedeutic experiments, we tested the effects of high doses of PPIs in vivo in the mouse model of endotoxic shock. Our data show that a single administration of PPI protected mice from death (60% survival versus 5% of untreated mice) and decreased TNF-α and IL-1β systemic production. PPIs were efficacious even when administered after lipopolysaccharide (LPS) injection. PPI-treated mice that survived developed a long-term cross-tolerance, becoming resistant to LPS- and zymosan-induced sepsis. In vitro, their macrophages displayed impaired TNF-α and IL-1β to different TLR ligands. PPIs also prevented sodium thioglycollate-induced peritoneal inflammation, indicating their efficacy also in a non-infectious setting independent of TLR stimulation. Lack of toxicity and therapeutic effectiveness make PPIs promising new drugs against sepsis and other severe inflammatory conditions.Systemic inflammatory response is a critical clinical response to insults of either infectious or non-infectious origin.¹ Severe sepsis and septic shock are more serious clinical forms with a poor outcome.² The incidence of sepsis is continuously increasing;^{1, 2, 3, 4} the mortality rate ranges between 30 and 50% in severe sepsis and septic shock, and patients who survive have a higher risk of mortality compared with the normal population for months and even years.⁵ Although treatment of the underlying infection and circulatory support decrease mortality, sepsis remains a leading cause of death in critically ill patients, and efficacious therapy is missing.⁶Traditionally, the physiopathology of sepsis is attributed to a hyperinflammatory response, the ‘cytokine storm'', that can directly lead to death or favor the insurgence of an immunosuppressive phase during which multiple organ dysfunction occurs.¹ We have recently reproduced in vitro on primary monocytes the cytokine storm: the simultaneous activation of multiple Toll-like receptors (TLRs) results in oxidative stress responsible for a marked enhancement of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) secretion.⁷Proinflammatory cytokines are indeed increased in sepsis. TNF-α⁸ is the first cytokine detected in the serum of septic patients followed by IL-1β.⁹ High-mobility group box chromosomal protein-1 appears well after TNF-α and IL-1β, supporting its role in mortality due to late sepsis.¹⁰The ‘cytokine storm'' theory is presently debated, mainly because clinical trials with cytokine antagonists were unsuccessful.¹¹ Timing of administration and non-optimal association of cytokine-neutralizing agents may be responsible for the failure of clinical trials; however, other factors besides cytokine hypersecretion, including genetic polymorphism in genes for coagulation and fibrinolysis and the insurgence of oxidative stress,² may participate in the genesis of sepsis. A common trait in septic patients is acidosis, which is more pronounced in non-survivors.¹² Acidosis is mainly due to a shift from oxidative phosphorylation to glycolysis, with accumulation of lactate and decrease of pH,¹³ and occurs upstream of the pathologic events (release of toxic substances, vasoconstriction, endothelial damage) that lead to cell and patient death.¹² Acidosis is also a feature of inflammatory microenvironments:^{14, 15} upon activation, inflammatory cell metabolism shifts toward aerobic glycolysis,¹⁶ with consequent decrease of extracellular pH. Extracellular acidosis is proinflammatory: it induces inflammatory genes^{14, 15} and increases processing and secretion of IL-1β¹⁷ in a NOD-like receptor (NLR) family, pyrin domain containing 3 (NLRP3) inflammasome-dependent^{18, 19} or -independent²⁰ manner.Proton pump inhibitors (PPIs) are a family of prodrugs that, activated by low pH,²¹ are highly efficacious in reducing acidic secretion by gastric cells and therefore largely used in the treatment of peptic ulcers and reflux esophagitis.²² PPIs are also effective against tumors, which are similarly characterized by low pH.^{23, 24} Furthermore, PPIs have been found to exert anti-inflammatory effects unrelated to the inhibition of gastric acid production, although the underlying mechanisms remain to be elucidated.²⁵Here we show that, in vitro, PPIs inhibit the production of proinflammatory cytokines by monocytes stimulated with TLR agonists; in vivo, in a murine model of lethal endotoxic shock,^{8, 26} PPIs protect against lipopolysaccharide (LPS)-induced mortality. Interestingly, mice cured by PPIs develop cross-tolerance: not only are they resistant to a second challenge with LPS but also respond better to zymosan injection.