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Introduction/Hypothesis: The sterile inflammatory response involves activation of the immune system in the absence of infection following trauma, ischemia, or toxins. Matrix metalloproteinase-9 (MMP-9) is a neuro-inflammatory zinc metalloproteinase implicated in secondary injury after acute ischemic stroke. In order to determine whether MMP-9 was associated with a sterile inflammatory response, we examined the association between circulating MMP-9 and extracellular ATP [a marker of a damage-associated molecular pattern (DAMP)] and acute neutrophil count.Methods: We measured plasma MMP-9 and ATP levels in 338 patients who presented to two institutions with acute ischemic stroke and were enrolled in the Specialized Program of Translational Research in Acute Stroke (SPOTRIAS) study. Plasma samples and admission leukocyte subsets were collected at 7.0 ± 3.0 hours and 47.7 ± 7.3 hours from stroke onset. ATP was measured using a liquid chromatography-tandem mass spectrometry metabolomics assay. MMP-9 levels were measured by ELISA using a commercially available assay.Results: 338 acute stroke patients were available for analysis. MMP-9 was associated with plasma ATP on admission (Pearson r = 0.24, P<0.0001) and at 48 hours (Pearson r = 0.21, P = 0.0028). MMP-9 levels were also associated with total leukocyte count on admission (Pearson r=0.32, P<0.0001) and at 48 hours (Pearson r=0.39, P<0.0001). Absolute neutrophil counts were the only leukocyte subsets to be associated with MMP-9 on both admission (Pearson r=0.31, P<0.001) and at 48 hours (Pearson r=0.34, P<0.0001).Conclusions: MMP-9 is associated with markers of the sterile inflammatory response, including circulating plasma ATP levels and absolute neutrophil count. In vitro studies have demonstrated that an initial rise in extracellular ATP is associated with increased neutrophil-mediated MMP-9 release. These findings suggest that ATP may be functioning as a DAMP in response to acute ischemia, and is associated with MMP-9 acutely in patients. This study marks the first clinical cohort to begin to characterize determinants of the sterile inflammatory response in acute ischemic stroke.