PROJECT SUMMARY/ABSTRACT – updated for supplement Pediatric arterial ischemic stroke afflicts ≈2,000 U.S. children every year, permanently disabling most, yet is poorly understood. The Vascular effects of Infection in Pediatric Stroke (VIPS I) study established an international network of 37 sites that enrolled 355 children with stroke and 354 controls. We discovered: (a) minor clinical infections act as a stroke trigger, while routine childhood vaccinations are protective; (b) almost half of children with stroke have an acute herpesvirus infection (the cause of chicken pox, cold sores, and other common illnesses); and (c) children with stroke have a high risk of recurrent stroke, particularly if they have an arteriopathy. A VIPS pilot study suggests that other common childhood pathogens may also play a role, possibly in combination with herpesviruses. Infection is compelling as a treatable stroke risk factor, with available therapies for both pathogens and downstream inflammatory effects. However, VIPS I findings present a paradox: infection is common, while childhood stroke is uncommon. VIPS II is exploring two potential explanations: (1) the “infection hypothesis”: unusual pathogen strains, or combinations of pathogens, lead to stroke; and (2) the “host response hypothesis”: an unusual inflammatory response to infection leads to stroke. The specific aims are to (1) identify known and novel pathogens in children with stroke, and determine whether different pathogens are seen with arteriopathic versus other stroke types; (2) determine whether children with arteriopathic stroke have a different inflammatory response compared to those with other stroke types; and (3) use data from Aims 1 and 2 to explore different mechanistic pathways from infection, to inflammation, to arteriopathic stroke, and other stroke types. In February 2022, VIPS II completed enrollment of 205 new cases of childhood stroke and 147 controls. For Aim 1, we are using next generation sequencing (NGS) of nucleic acids to detect pathogens in blood samples and throat swabs collected in VIPS II. For Aim 2, we are measuring levels of inflammatory markers in blood samples from cases and controls. We have already studied the samples from VIPS I cases. Using these results, we have refined our list of markers and will now proceed with testing the VIPS II samples. The goal of VIPS II is to gain the knowledge needed to protect children with stroke from additional brain injury. Its results will guide the selection of currently available therapies—such as antimicrobials and anti- inflammatory medications—for a pediatric clinical trial aimed at preventing recurrent stroke.