Acute kidney injury (AKI) is a major determinant of mortality and morbidity in hospitalized veterans. Understanding the pathophysiology of AKI is essential for the development of therapy. This application proposes to study the role of Tamm-Horsfall protein (THP) in AKI. This protein is expressed exclusively in the kidney by cells of the thick ascending limbs (TAL) of Henle. In the last funding cycle, we uncovered a key role of THP in regulating neutrophil infiltration in AKI. The current proposal expands these findings by building on strong preliminary data showing that THP is important in recovery from AKI by enhancing the healing functions of renal mononuclear phagocytic cells (MPCs) such as macrophages. These studies will form the basis to transition into therapeutic applications. Indeed, the PI’s long term goal is to understand how the role of THP can be modulated to treat veterans who develop AKI. The central hypothesis is that Tamm-Horsfall Protein is essential to promoting recovery from AKI by enhancing the abundance and activity of renal MPCs needed for healing. This hypothesis has been formulated on the basis of strong preliminary and published data. The following specific aims will be used to investigate this hypothesis. Aim 1 will define the role of THP on the homeostasis of renal MPCs after AKI Aim 2 will define the role of THP on the activity of renal MPCs during recovery from AKI This research will involve the use of THP knockout and wild type mice. A tissue specific knockout mouse will also be used to study the effect of THP on MPC signaling. The animal model used for AKI is renal ischemia- reperfusion injury achieved through renal pedicle clamping. Other techniques used in this work include: Immunofluorescence confocal 3D imaging and tissue cytometry, innovative large scale multiplexed imaging, flow cytometry, real time-PCR, western blot, protein purification and pharmacological dosing. This research is innovative, because it will uncover novel regulatory functions for THP that may be relevant not only in kidney injury but also in other forms of acute or chronic renal disease. Future strategies that enhance the role of THP in the kidney will be of important therapeutic significance.