An investigation into the role of pericytes in regulation of vascular morphology and function using murine models of inflammation

Abstract

Leukocyte recruitment to sites of inflammation is a crucial event in host defense against pathogens and tissue injury. Although there is at present much interest in deciphering the mechanisms of leukocyte transendothelial cell migration, little attention has been paid to the subsequent steps, i.e. leukocyte migration through the pericyte layer and the venular basement membrane. In this context, results from this group previously demonstrated that neutrophils preferentially transmigrate through gaps between adjacent pericytes, regions associated with sites of low matrix protein expression within the vascular basement membrane. The aim of this thesis was to extend these findings by investigating the impact of inflammatory mediators on pericyte morphology and vascular basement membrane deposition using both in vitro and in vivo models. Flow cytometry analysis of pericyte-like C3H/10T1/2 cells and primary lung pericytes revealed the expression of key pro-inflammatory molecules on their surface (including cytokine receptors and adhesion molecules) and the regulation of these molecules upon cytokine stimulation. Using the murine cremaster muscle model it was further demonstrated that key neutrophil chemoattractants (i.e. LTB4, KC, C5a and fMLP) induced neutrophil transmigration that was associated with a change in pericyte morphology (as quantified through enlargement of gaps between adjacent pericytes). These changes in pericyte gap size were neutrophil-dependent and mediated by endogenously generated TNF as demonstrated in neutrophil-depleted mice and TNFR-/- mice, respectively. In addition, TNF appeared to mediate post-inflammatory BM deposition in response to LTB4 and was required for chemoattractant-induced vascular permeability. Hence, the results of the present work have demonstrated the ability of pericytes to respond to both cytokines and chemoattractants, suggesting an active role for pericytes in the regulation of inflammatory responses. In addition, findings provide the first evidence for chemoattractant-induced changes in vascular morphology and barrier functions of venular walls in vivo via the release of endogenous TNF as a secondary mediator, effects that may contribute to the pro-inflammatory properties of these stimuli.