The function of injury-specific macrophages in acute kidney injury

Acute kidney injury (AKI) is defined as an abrupt, and often reversible, decline in kidney function that is characterized by a decrease of urine output, an increase in creatinine, or both. The ischemia reperfusion injury (IRI) model most effectively recapitulates the pathophysiological conditions observed following AKI. Mechanistically, post-ischemic tubular epithelial cells (TECs) undergo necrosis and secrete damage associated molecular patterns (DAMPs), initiating proinflammatory responses by recruiting monocytes, neutrophils, dendritic cells and other immune cells. Prolonged inflammation results in further damage to tubular cells, and eventually CKD. Data from our lab and others has identified macrophages as a key contributor to the injury and repair process after IRI; however, a detailed description of macrophage phenotype at a cell-by-cell level after IRI has not been performed and represents a significant gap in knowledge. To address this gap in knowledge, we generated a comprehensive single-cell RNA (scRNA) sequencing dataset of macrophages at early, intermediate, and late stages post IRI. Using this dataset, we identified two injury-specific macrophage clusters: one predominant during the early inflammatory phases (injury-specific macrophage 1 or ISM1), and the other emerging during the reparative phase of IRI (injury-specific macrophage 2 or ISM2). A detailed analysis revealed that ISM2 highly expressed Triggering receptor expressed on myeloid cells 2 (Trem2), a phagocytic receptor involved in anti-inflammatory signaling. Temporal analysis revealed that Trem2 expression begins 2 days post injury and persists until day 28. CellChat analysis predicted that ISM2 promote tubular repair after injury by producing reparative ligands including IGF1 that bind to cognate receptors on injured TECs. Igf1 has previously been associated with improved kidney function after injury by activating cell survival and cell cycle in an ERK/MAPK and Akt/Rb dependent manner. Based on these data, I hypothesize that the TREM2 receptor is required for transition of macrophages from a pro-inflammatory to pro-reparative state, and that ISM2 derived Igf1 promotes tubular epithelial cell repair after injury. In this application, I will test these hypotheses through a mixture of in vivo and in vitro techniques. There are currently no FDA approved therapies to treat AKI, or to prevent the AKI to CKD transition. Treatments for patients developing CKD are palliative in nature and limited to dialysis and transplant, both of which pose a significant health care burden. Thus, there is an urgent need to develop new therapies to treat AKI and CKD. An understanding at the molecular level of the AKI-to-CKD transition will aid in the development of effective treatments. Further, the comprehensive training plan will provide me with the skills necessary to become an independent investigator study the function of macrophages after AKI.