Platelet Toll-Like Receptors: Bridging Inflammation and Immunity

Since the discovery that there is a series of pattern recognition receptors that allow the immune system to detect pathogens, there has been a lot of work to elucidate the signaling molecules that contribute to this detection system. Although the cell types involved would seem to be just as important, our understanding of which cells are critical remains less well explored. Using intravital imaging to visualize the different cell types, we were surprised to find that in almost all conditions that we examined, platelets were rapidly recruited to afflicted tissues. Under some conditions, platelets bound vascular macrophage including Kupffer cells and helped contain bacteria. These were instantaneous responses. At later times, platelets bound neutrophils and induced the production of neutrophil extracellular traps (NETs) that helped to catch bacteria as well as viruses but did induce some local vascular injury. In some scenarios, platelets bound endothelial cells and whether this was to wall off and contain infections or a hijacking of platelets by bacteria to induce vascular dysfunction and poor perfusion remains unclear. Visualizing platelets in sterile injury also revealed important contributions to helping recruit other immune cells that help to heal. The role of the platelet as an effector in infections and inflammation will be discussed.