PROJECT SUMMARY / ABSTRACT ARDS results from a severely dysregulated immune response that leads to lung vascular injury and protein-rich edema. Excessive activation of neutrophils (PMNs) is a primary cause of the lung damage. In experimental sepsis and septic patients, some PMNs are intensely activated and sequestered in lungs while others pass through the lung microvasculature unimpeded and function as essential host-defense cells. Previous findings suggest that PMNs might exist as various subsets and in different stages of activation. The concept of heterogeneity of PMNs raises the possibility that a subset of activated PMNs may contribute to a maladaptive inflammatory response and be responsible for lung injury. We found that a subset of PMNs specifically internalized 100 nm albumin nanoparticles (ANPs). As our Supporting Data show this population of PMNs increased significantly in experimental sepsis in mice and they were shown to be essential for the development of inflammatory lung injury. We also conjugated drugs to ANP for their precise delivery into these PMNs. These results raise several fundamental questions: What is the nature of this PMN population? Is there a related population in humans? What is their function and what is their origin? What is the mechanism of ANP internalization? Do these cells mediate lung injury and can ANP deliver drugs into this PMN population to reverse the course of the disease? We will address these PMNs by characterizing the function of CD11bhighCD16+CD45highANPhigh PMN subset as opposed to control CD11bhighCD16+CD45highANPlow PMNs in mediating inflammatory lung injury (Aim 1). Here we will test the hypothesis that ANPhigh PMNs are functionally distinct from ANPlow PMNs and that the former are crucial in mediating lung injury. Next, we will determine differential β2 integrin signaling in the distinct PMN sub-populations and their role in mediating lung injury (Aim 2). Here we will test the hypothesis that β2 integrins and downstream signaling pathway are hyper-activated in ANPhigh PMNs compared to ANPlow PMNs and differential PMN signaling is required inflammatory lung injury. Finally, we will define the origin, fate, and phenotypic heterogeneity of PMNs mediating lung injury (Aim 3). Here, RNA-Seq profiling has thus far revealed distinct chemokine receptors as markers of ANPhigh PMNs in lungs, and we will use this information to isolate this subset to further characterize them and assess their functional role in mediating lung injury. We will also define the time- dependent transcriptomic profiles and networks of ANPhigh vs. ANPlow PMNs during inflammatory activation to assess their differential properties. Thus, together studies will not only define a population toxic injury-promoting population of PMN but also hopefully identify new therapeutic targets to reverse the course of inflammatory lung injury.