Cell microvesicles during experimental endotoxemia

Circulating Myeloid Cell-derived Extracellular Vesicles as Mediators of Indirect Acute Lung Injury

Blood-borne myeloid cells, neutrophils and monocytes, play a central role in the development of indirect acute lung injury (ALI) during sepsis and noninfectious systemic inflammatory response syndrome. By contrast, the contribution of circulating myeloid cell-derived extracellular vesicles (EVs) to ALI is unknown, despite acute increases in their numbers during sepsis and systemic inflammatory response syndrome. Here, we investigated the direct role of circulating myeloid-EVs in ALI using a mouse isolated perfused lung system and a human cell coculture model of pulmonary vascular inflammation consisting of lung microvascular endothelial cells and peripheral blood mononuclear cells. Total and immunoaffinity-isolated myeloid (CD11b⁺) and platelet (CD41⁺) EVs were prepared from the plasma of intravenous LPS-injected endotoxemic donor mice and transferred directly into recipient lungs. Two-hour perfusion of lungs with unfractionated EVs from a single donor induced pulmonary edema formation and increased perfusate concentrations of RAGE (receptor for advanced glycation end products), consistent with lung injury. These responses were abolished in the lungs of monocyte-depleted mice. The isolated myeloid- but not platelet-EVs produced a similar injury response and the acute intravascular release of proinflammatory cytokines and endothelial injury markers. In the in vitro human coculture model, human myeloid- (CD11b⁺) but not platelet- (CD61⁺) EVs isolated from LPS-stimulated whole blood induced acute proinflammatory cytokine production and endothelial activation. These findings implicate circulating myeloid-EVs as acute mediators of pulmonary vascular inflammation and edema, suggesting an alternative therapeutic target for attenuation of indirect ALI.

The LPS-induced increase in circulating microparticles is not affected by vitamin C in humans

OBJECTIVE

Microparticles (MP) are considered to promote coagulation. This study aimed to characterize the time course of MP levels and the effect of high-dose vitamin C on MP formation during inflammation in an in vivo Escherichia coli endotoxin (LPS) model.

METHODS

Microparticle formation was studied in 14 male subjects in a cross-over trial who received either intravenous vitamin C at 320 mg/kg body weight (BW) or 480 mg/kg BW or saline solution in a random order on alternate trial days 3 h after intravenous exposure to LPS (2 ng/kg BW). Venous blood samples were taken before, 3 and 6 h after LPS. D-dimer, leucocyte count, C-reactive protein, plasma vitamin C and body temperature were assessed as inflammatory parameters. MP were detected using flow cytometric analysis and expressed in 10³ MP/mL plasma.

RESULTS

Microparticles levels were decreased from baseline 848 units [range 431-1705] by 21% to 671 units [253-1586] at 3 h and increased by 32% to 1119 units [288-4443] at 6 h after LPS. This pattern was not influenced by administration of vitamin C, with a change from 730 units [399-1396] at baseline by an increase to 832 units [215-2168] at 3 h to 1055 units [350-4858] at 6 h. MP subpopulations followed similar dynamics. Alterations in inflammatory parameters were independent from vitamin C administration during endotoxemia.

CONCLUSION

Microparticles are increased in acute systemic inflammation with inconsistent changes in MP subgroups in healthy subjects. Systemic vitamin C administration does not mitigate MP formation and D-dimer levels during acute systemic inflammation, suggesting that MP-induced coagulation activity is not affected by vitamin C.

Microparticles as immune regulators in infectious disease - an opinion

Despite their clear relationship to immunology, few existing studies have examined the potential role of microparticles (MP) in infectious disease. MP have a different size range from exosomes and apoptotic bodies, with which they are often grouped and arise by different mechanisms in association with inflammatory cytokine action or stress on the source cell. Infection with pathogens usually leads to the expression of a range of inflammatory cytokines and chemokines, as well as significant stress in both infected and uninfected cells. It is thus reasonable to infer that infection-associated inflammation also leads to MP production. MP are produced by most of the major cell types in the immune system, and appear to be involved at both innate and adaptive levels, potentially serving different functions in each. Thus, they do not appear to have a universal function; instead their functions are source- or stimulus-dependent, although likely to be primarily either pro- or anti-inflammatory. We argue that in infectious diseases, MP may be able to deliver antigen, derived from the biological cargo acquired from their cells of origin, to antigen-presenting cells. Another potential benefit of MP would be to transfer and/or disseminate phenotype and function to target cells. However, MP may also potentially be manipulated, particularly by intracellular pathogens, for survival advantage.