Monocyte and endothelial cell interaction during sepsis

Abstract

Sepsis complicated by coagulopathy is associated with a higher mortality rate. The cell surface expression of tissue factor (TF) may be important in its development. There at least three subsets of monocytes – classical, intermediate and non-classical – that vary in proportion during sepsis. This project used three approaches to investigate how the monocyte subsets and their endothelial interaction could contribute to sepsis-associated coagulopathy: the in vitro stimulation of healthy monocytes with lipopolysaccharide (LPS) and endothelial co-culture; the use of a human model of endotoxaemia; and the collection of blood samples from individuals with sepsis. LPS stimulation of healthy monocytes demonstrated that the monocyte subsets express cell surface TF to different extents. Classical and intermediate monocytes express the highest proportion of TF in response to LPS. An in vitro monocyte-endothelial co-culture model demonstrated that monocytes could promote coagulation through the increased cell surface expression of TF, independent of LPS, and influence the endothelial fibrinolytic response. Blood samples were taken at a range of time-points following an injection of LPS into a healthy volunteer. An increase in cell surface TF occurred within 90 minutes of exposure to LPS and was associated with an increase in markers of coagulation. Individuals varied in their response to LPS, with two groups identified: high and low responders. The response of individuals was consistent between the subsets. Samples from individuals with sepsis expressed a higher level of cell surface TF compared to individuals who were critically ill but did not have sepsis. The surface expression of TF increased further when measured following recovery from sepsis. This work demonstrates that the cell surface expression of TF varies between the monocyte subsets, that individuals may increase monocyte TF expression to different levels, and that individuals with sepsis express higher levels of TF both at the time of sepsis and following recovery.