Re: Endothelial activation and dysfunction in the pathogenesis of influenza A virus infection
Influenza and Platelet?Endothelial Interactions
In addition to leak,
there is evidence to suggest that endothelial dysfunction following influenza virus infections may manifest as altered thrombogenicity. Healthy endothelial monolayers are anti-thrombogenic; during the H1N1 pandemic in 2009 (H1N1pdm09), there were multiple reports of flu-associated thrombosis. A retrospective review of 119 hospitalized patients found that 7 patients (5.9%) had thrombotic vascular events that were diagnosed or occurred during hospitalization.40 Three of these patients had arterial thrombosis and four had venous thrombosis. A study in Michigan looked at 10 patients with H1N1pdm09 influenza who were admitted to the intensive care unit with ARDS.41 Five had pulmonary emboli, while two others showed evidence of hypercoagulation. Whether these complications are directly attributable to the virus or simply reflect the overall severity of illness remains unclear.
Similarly, epidemiological evidence supporting a link between influenza and cardiovascular disease has been reported for decades.42-44 A temporal relationship between influenza infections and the incidence of cardiovascular disease has been reported by a number of groups42-47 and several studies have found the influenza vaccine to be associated with a reduction in stroke, transient ischemic attack, and hospitalization due to cardiac disease.48-50
The link between influenza infection and cardiovascular disease has also been reported in animals. In one study, apoE−/− mice (an accepted murine model of atherosclerosis) were infected with influenza A/Hong Kong/68 (H3N2) and vascular histology of the aorta was compared with apoE−/− uninfected mice as well as infected wild-type mice. Infected apoE−/− mice showed increased subendothelial cellular infiltration in atherosclerotic plaques compared with uninfected apoE−/− mice. Infected wild-type mice showed no evidence of cellular infiltration of the vascular intima.51 This group found clustered platelets on the plaques of the majority of infected animals, but none in uninfected animals. Thus, influenza may worsen existing vascular disease resulting in increased endothelial damage and platelet adhesion.
While a causal link between influenza and thrombotic disease has not yet been definitely established,52 a variety of plausible mechanisms have been proposed that highlight the relationship between the influenza virus, platelet activation, and endothelial dysfunction. Elevated levels of circulating cytokines associated with influenza infection53 can induce endothelial activation leading to upregulation of cell surface adhesion molecules that favor platelet adhesion. In HUVECs, TNF and IL-1β have been shown to induce the expression of type 1 plasminogen activator inhibitor (PAI-1), which promotes platelet binding, while inhibiting the expression of tissue-type plasminogen activator (tPa) and thrombomodulin, which are anticoagulant.54 Similarly, infusion of IL-1 into rabbits caused a time-dependent increase in tissue factor (TF) expression on aortic endothelial cells55; TF is known to be a key player in the coagulation process.56 Endothelial cells also increase production of platelet activating factor (PAF) upon stimulation with TNF and IL-1α.57 Cytokines can also induce endothelial cell retraction, exposing the pro-atherogenic extracellular matrix.
Influenza-induced lung injury itself may be an important factor promoting thrombosis. Patients with severe influenza require supplemental oxygen because of profound hypoxemia.1,2 Hypoxia has been shown to induce a pro-inflammatory state in endothelium causing the increased release of IL-1, IL-6, PAF, ICAM-1, p-selectin, and VWF,58 all of which are associated with platelet activation and adhesion.
There is also evidence that the influenza virus per se may directly affect the endothelium resulting in platelet adhesion. Influenza H3N2 virus has been shown to infect endothelial cells in vitro and to trigger endothelial cell apoptosis,32 which is known to enhance platelet adhesion.59 Endothelial cell death would cause exposure of the extracellular matrix to circulating blood, favoring platelet binding.60 Cultured HUVEC monolayers infected with influenza have been shown to reduce clotting times by 55% after 3 h of infection and by 66% after 24 h of infection, compared with uninfected monolayers.61 This was attributed to an increase in TF expression, but at least some of the effect may have been mediated through cytokines and/or the induction of apoptosis as neither was measured in this study.
In addition to affecting the endothelium, the influenza virus may have a direct effect on platelets. An H3N2 virus added directly to platelets was found to induce clumping of both human and rabbit platelets.62 In this study, both live and dead virus were adsorbed by platelets and the adsorption period was linked to clumping. Infusion of influenza into rabbits induced rapid thrombocytopenia. Platelet activation by influenza has also been documented in humans. In one prospective study comparing patients with severe influenza (H1N1) and patients with severe bacterial pneumonia (all being treated for ARDS) to healthy controls, patients with influenza showed the greatest degree of baseline platelet activation as evidenced by increased formation of platelet-monocyte aggregates and increased binding of the PAC-1 antibody, which binds to the active conformation of αIIbβ3 integrin on platelets.63 Platelets could also promote endothelial damage during influenza infection through their interaction with neutrophils. As mentioned earlier, excessive neutrophils have been associated with worse lung pathology in mice infected with H1N1 influenza, attributed to the formation of neutrophil extracellular traps.26 Intriguingly, it has been reported that addition of activated platelets to neutrophils in vitro is sufficient to induce NET formation.64 In a mouse model of transfusion-related lung injury, inhibition of platelets either with aspirin (acetylsalicylic acid) or a glycoprotein IIb/IIIa inhibitor reduced both NET formation and lung injury.64 In a study of acid-induced lung injury in mice, platelet depletion reduced lung neutrophil infiltration, improved histological changes, and increased survival.65 Depletion of platelets was also found to reduce neutrophil influx and pathological changes associated with LPS-induced lung injury.66 Remarkably, however, little is known about platelet?endothelial interactions and their contribution to acute lung injury during severe influenza. This is the subject of ongoing work in our laboratory.
In summary, influenza virus, platelets, and endothelial cells may interact in a variety of ways to induce lung endothelial dysfunction. These include induction of pro-coagulant pathways, activation of platelets and the endothelium, and enhanced interaction between platelets and neutrophils leading to the breakdown of the lung endothelial barrier (Fig. 1B).
http://www.landesbioscience.com/journals/virulence/article/25779/
