Abstract: Development of shock in sepsis defines a dramatic deterioration of clinical status and is linked to a significant increase in morbidity and mortality rates. However, the cellular and molecular mechanisms determining the vascular pathology of septic shock remain undefined. Our past work established mast cells (MC) as key effector cells of vascular pathology in different disease contexts. Because MC products are found in the plasma in shock but not during sepsis, this supports that MC activation is a central event leading to septic shock. Employing state-of-the- art technologies such as dynamic photoacoustic imaging of the microvasculature, we provide first evidence that MCs shape key features of shock in sepsis: systemic hypotension, vascular leakage and microvascular perfusion abnormalities. In addition, we elucidate that in sepsis MCs, which are located on the abluminal side of a relatively impermeable endothelium, are stimulated by platelets, which can aggregate in their close proximity following inflammatory triggers. Based on these observations, we hypothesize that specific signaling interactions between platelets, perivascular MCs and the endothelium drive the vascular pathology of septic shock. The objective of our work is first to comprehensively define the mechanisms of MC-mediated vascular pathology in sepsis, second to elucidate the specific mechanism by which platelets trigger MC responses and resultant vascular pathology and lastly, to establish the clinical relevance of our findings in a cohort of septic patients. Together, this project constitutes a key step towards our long-term goal to establish MC responses as a biomarker of sepsis biology and to develop novel therapeutic strategies that may directly target the mechanisms of disease progression in sepsis.