Periodic Reporting for period 4 - PERIF (Perivascular cells at the crossroads of inflammation, regeneration and fibrosis) Reporting period: 2020-05-01 to 2021-10-31 Summary of the context and overall objectives of the project The survival of organisms requires the ability to repair tissues upon injury, as well as, after birth, to fight foreign invaders that may have contaminated the wound. This last function is mediated by a complex host response involving innate immune cells, blood vessels and inflammatory mediators that collectively intend to neutralize the harmful agent and eliminate damaged/necrotic tissue. Initially beneficial, this massive inflammatory response comes with a cost, and adult injured tissues usually heal with a scar, which is an area of fibrous tissue that transiently replaces normal tissue. This is in contrast to the complete tissue repair with no signs of scarring observed during fetal development until late stages of organogenesis, resembling regeneration. In chronic settings, scarring can become excessive in a process called fibrosis, to the point of preventing functional recovery of the injured organ and be life threatening. Nearly half of all deaths in industrialized world are due to diseases involving inappropriate, often chronic, inflammatory and fibrotic responses, including chronic lung, kidney and liver diseases, scleroderma, inflammatory bowel diseases, muscular dystrophies, cardiovascular diseases, and desmoplastic tumors. However, our current knowledge of the biological processes regulating fibrosis is partial, which has hindered therapeutic advances in the field. Recent data from our team and others drew new attention on a discrete population of mesenchymal cells that wraps around vessels, variously called mural cells, perivascular cells or pericytes, as a major source for profibrotic stromal cells generating scar tissue. Previously known for their vascular protective functions, increasing evidence suggests new and unexpected roles for these cells also in inflammation, repair/regeneration, and cancer. These new findings raise a number of challenging questions relative to their functional diversity, and mechanisms of activation/ regulation in disease.The overall objectives of this project are to improve our understanding of the repair process, and identify new cellular partners in the perivascular niche and molecular pathways that lead to dysregulated repair and fibrosis. In the long term, this project may pave the way for the development of novel (co)-therapeutic strategies in inflammatory diseases, regenerative medicine and cancer. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In this project, we are using a multidisciplinary approach based on new experimental models of lineage tracing and inducible ablation of specific mesenchymal cells in vivo. We will then confirm our results by analyzing samples of patients suffering from fibrotic diseases or cancer that undergo surgery for medical reasons. So far, we have successfully set up a number of different experimental models that allow us to visualize and characterize subsets of perivascular mesenchymal cells developing in inflammatory diseases and cancer. The next step is to isolate these cells from the damaged organs and perform transcriptomic analysis to obtain their molecular signature. By looking at modifications of gene expression, we expect to better understand their function and how these cells are dysregulated in disease. At this point of the project, we are investigating specific genes that mediate the crosstalk of such cells with immune cells, the vasculature and tissue stem cells. These results will allow us to better decipher the coordination of these different cell types in vivo, which, altogether, will tip the balance toward healing or chronic repair/fibrosis. Ultimately, we aim at identifying new targets for chronic diseases. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) This research project addresses current challenges in the field of tissue repair and chronic diseases, with emerging medical applications in fibrosis, regenerative medicine and cancer. Chronic fibrotic diseases are a huge burden for our societies, and still represent a major cause of mortality, nevertheless efficient treatments are rare. In this project, we are addressing the role of resident perivascular mesenchymal cells as key sensors of tissue injury and upstream organizers of inflammation. It is known since a long time that resident mesenchymal/stromal cells that form the structure of our organs and vessels are activated upon injury and foster chronic diseases. However, the underlying mechanisms remain unclear, which has prevented development of new therapeutic approaches. One of the major roadblock is the paucity of specific markers and tools to study these cells. In this project, we generated different genetic tools and experimental systems that allow us to study the function of specific subsets of mesenchymal cells in disease. Using these tools, we expect to shed new light on the fundamental process of tissue repair, which is still not fully understood. In the long term, we plan to test whether and how we can target specific subsets of “pathology-associated” mesenchymal cells to ameliorate current therapies in fibrotic diseases and cancer.