Circulating microvesicles as mediators of acute pulmonary vascular inflammation

Abstract

Acute lung injury (ALI) resulting from remote ‘indirect’ causes is a major problem in sepsis and systemic inflammatory response syndrome (SIRS) but the underlying mechanisms are poorly understood. Circulating microvesicles (MVs) have been implicated as long-range mediators of vascular inflammation and their role as biomarkers in sepsis and SIRS has been widely investigated in clinical studies in recent years. However, the in vivo functional roles of MVs in sepsis and ALI have received less attention. Specifically, the role of in vivo MVs in the development of sepsis/SIRS-induced indirect ALI has not been previously evaluated.

We hypothesised that circulating microvesicles (MVs) play a crucial role in propagating inflammation to the lungs, contributing to the development of pulmonary vascular inflammation in indirect ALI. The overall aims of this project were to: 1) evaluate MV uptake by pulmonary vascular cells and the mechanisms involved, 2) characterise the intravascular production of MVs in animal models of sepsis and sterile extrapulmonary organ injury, and 3) identify the contribution of in vivo-derived circulating MVs to the development of indirect ALI.

The major findings of this work were that during sub-clinical endotoxaemia in mice, lung-marginated Ly6Chigh monocytes become a major target for circulating MV uptake via a phosphatidylserine receptor mechanism1. In mouse models of sepsis and extrapulmonary organ injury, neutrophil- and monocyte-derived MVs were the predominant MV subtypes being produced during endotoxaemia, while platelet- and endothelial-derived MVs were predominant during kidney ischaemia reperfusion injury. When MVs obtained from plasmas of endotoxaemic mice were adoptively transferred to isolated perfused lungs (IPLs), they induced significant increases in lung oedema. Depletion of intravascular lung monocytes by treatment with clodronate liposomes resulted in the reversal of the oedema, demonstrating the role of monocytes in MV-induced ALI. To investigate the contribution of different circulating MV subtypes, we immunoaffinity isolated myeloid (CD11b+) and platelet (CD41+) MVs from endotoxaemic mouse plasmas and transferred these to the IPL. We found that myeloid-MVs induced significant lung oedema and potent release of soluble mediators, whereas platelet-MVs produced a statistically significant, but much lower level of oedema and negligible release of soluble mediators.

In summary, these findings indicate an important role of myeloid-derived MVs, particularly those derived from neutrophils and/or monocytes, and their interaction with lung-marginated monocytes in the pathogenesis of pulmonary vascular inflammation in indirect ALI. Further work to elucidate the specific MV molecular effectors mechanism involved will facilitate an enhanced understanding of ALI pathobiology.