PROJECT SUMMARYThe COVID-19 pandemic has dramatically highlighted the serious unmet needs of acute respiratory distresssyndrome (ARDS) including the lack of key genetic insights into ARDS susceptibility and health disparities andthe absence of effective FDA-approved pharmacologic interventions that address ARDS mortality. ARDSphenotype heterogeneity the complexity of dysregulated inflammation and the absence of predictive biomarkershave all contributed to failed ARDS therapeutic clinical trials. We previously identified a missense genetic variant(Met62Ile) located in the selectin P ligand gene (SELPLG) which encodes for P-selectin glycoprotein ligand 1(PSGL1) that was associated with increased susceptibility to ARDS in Blacks. PSGL1/P-selectin interactionsare a highly biologically-plausible target for ARDS therapies due to the critical role of this inflammatory pathwayin polymorphonuclear (PMN) leukocyte trafficking platelet aggregation and thrombosis. In published studiesSELPLG expression was significantly increased with ventilator (VILI)- and lipopolysaccharide (LPS)-inducedlung injury that was significantly attenuated by either SELPLG knock down or PSGL1 inhibition. We have recentlyshown that plasma PSGL1 and P-selectin levels are significantly elevated in sepsis ARDS and COVID-19pneumonia patients compared to controls. We and others have identified SELPLG and SELP variants associatedwith ARDS risk and mortality and SELPLG and SELP variants that are associated with elevated plasma PSGL1and P-selectin. These preliminary data suggest that the combination of SELPLG/SELP variants with elevatedplasma PSGL1/P-selectin levels may be integrated into a genetics-based biomarker risk score (GBRS) forARDS. We speculate that a PSGL1 /P-Selectin pathway-based GBRS may define an at risk subgroup of whoare excellent candidates for inclusion in a clinical trial targeting PSGL1 /P-Selectin interactions (endotype).Focused recruitment of this at risk ARDS subgroup would represent a novel approach to ARDS clinical trialdesign. Specific Aim (SA) #1 will define the genetic regulation of plasma PSGL-1 and P-selectin levels usingdata and samples from the NHLBI ARDSnet ALVEOLI study (539 patients). Innovative use of MendelianRandomization and mediation analyses will allow us to generate the genetics-based biomarker risk score forARDS initially focusing on 10 SELPLG/22 SELP SNPs identified that potentially predict plasma PSGL1 and P-selectin levels respectively. SA #2 will leverage biospecimens from the NIAID Immunophenotyping Assessmentin a COVID-19 Cohort (IMPACC) study (>1000 patients) to validate GBRS utility in predicting risk of and mortalityfrom COVID-19-associated ARDS. SA #3 will assess the therapeutic efficacy of targeting PSGL-1/P-selectininteractions in well-established small and large animal preclinical ARDS/VILI models utilizing the FDA IND-approved P-selectin inhibitor recombinant tandem PSGL1 immunoglobulin fusion molecule (TSGL-Ig).Successful completion of this highly translational R01 grant will generate a novel point of care pharmacogeneticenrichment tool to be leveraged in innovatively designed human ARDS clinical trials of TSGL-Ig.