Lung epithelial cell-derived C3 in acute lung injury

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PROJECT SUMMARY/ABSTRACT The goal of this R01 proposal is to investigate how complement component C3 derived from the lung epithelium can improve host resilience during acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). C3 is a central component of the complement system and one of the most abundant circulatory proteins. It is an early responder to injury and operates by killing pathogens and clearing debris. However, we have shown that it takes time for circulating C3 to reach the alveolar space in ALI, thereby increasing the dependence on C3 present in the lung. We have also shown that not only is lung epithelial cell-derived C3 secreted, but also its intracellular stores are central to the protection against ALI and cell death. These findings are a shift from the conventional thinking that liver-derived C3 is the main operational form of complement in the lung and increases the emphasis on epithelial cells as a source. Additionally, we have shown that many immune cell types—including macrophages—derive C3 stores via uptake from the extracellular space, and this uptake improves their effector response. Hence, this proposal focuses on an emerging role for lung epithelial cell-derived C3 in promoting the survival and function of epithelial cells and macrophages, each one vital in ALI. A major hurdle for investigating tissue-specific roles of complement has been the limited availability of models and assays. We have developed new transgenic mouse models and functional assays that distinguish the roles of liver- and lung-derived complement and show that epithelial cell-derived C3 is central to protection in ALI. We have supplemented these models and assays with data from ex vivo human models to show key roles for C3 in the cellular response to injury. These results support our central hypothesis that lung epithelial cell-derived C3 modulates the bronchoalveolar epithelial and immune cell niche to reduce ALI. This proposal will test our hypothesis by achieving two Specific Aims. Aim 1 compares targeted liver derived C3-deficient mice with lung epithelial cell- derived C3-deficient mice to assess if the cytoprotection offered by lung epithelial cell-derived C3 is cell-type intrinsic or also regional. It will also address how to optimally augment C3 to protect the injured lung. Aim 2 uses a combination of conditional knockout mice and deceased human donor lungs to assess how lung epithelial cell- derived C3 influences the pulmonary myeloid cell compartment, especially alveolar macrophage survival and function. It also addresses how to modulate C3 uptake in these cells to optimize their function. The proposal brings together a principal investigator with expertise in complement biology and acute lung injury, and co-investigators with expertise in gene delivery, CRISPR screens and bioinformatics to determine the immunobiological role of lung epithelial cell-derived C3 in facilitating host defense at the site of injury and promoting tissue resilience. The proposed work is important because understanding how the early host immune response modulates tissue damage is essential for designing and implementing new therapies for ALI. The knowledge will form the basis of locally delivered, host-focused therapies for ARDS, thus aligning with a priority research area for the NHLBI.