Figure 1

Endothelial cells are conditional innate immune cells. In their quiescent state, endothelial cells express MHC I (Major histocompatibility class I) molecules and PPRs (pattern-recognition receptors) which detect PAMPs (pathogen-associated molecular patterns). In the presence of inflammatory stimuli and risk factors in the bloodstream, endothelial cells transform from an anti-inflammatory and anti-coagulatory state to a pro-inflammatory and pro-coagulatory state. Endothelial cells can detect inflammatory stimuli and risk factors via PRRs. In response to these stimuli, endothelial cells express MHC II molecules which present endothelial antigens to immune cells. Moreover, endothelial cells can upregulate the expression of surface adhesion molecules that induce the adhesion of immune cells, such as leukocytes, to the endothelium and facilitate transmigration to underlying tissues. In addition, endothelial cells enhance the secretion of pro-inflammatory cytokines and chemokines which can modulate the activities of immune cells.