Acute kidney injury (AKI) arises from multiple forms of renal damage, including ischemia, toxin exposure, and urinary tract obstruction. AKI-associated inflammation exacerbates damage and can lead to development of chronic kidney disease (CKD). Kidney injury molecule-1 (KIM-1) is a mucin and immunoglobulin domain-containing transmembrane protein that is highly upregulated in the proximal tubule after kidney injury. While chronic KIM-1 expression promotes renal fibrosis and kidney failure, Li Yang, Craig Brooks, and colleagues at Harvard Medical School reveal that KIM-1 dampens early inflammatory responses and protects the kidney following injury. Mice expressing a mutant form of KIM-1 that lacks the mucin domain (KIM-1Δmucin) were more susceptible to both ischemia-induced and toxin-induced AKI compared to control animals. Loss of the mucin domain disrupted KIM-1-mediated phagocytosis in proximal tubular cells. This reduction in phagocytosis in KIM-1Δmucin mice reduced clearance of apoptotic and necrotic cells and luminal debris after injury, leading to increased immune infiltration and inflammation. Moreover, KIM-1-mediated phagocytosis in proximal tubular cells from WT mice triggered PI3K-dependent downregulation of NFkB activity, resulting in an anti-inflammatory phenotype that was characterized by decreased expression of TLR4 and pro-inflammatory cytokines, and reduced macrophage activation. Taken together, these data establish a protective role for KIM-1 in response to AKI. The accompanying image shows post ischemic kidney tissue from WT mice that was stained for KIM-1 (red), active caspase 3 to label apoptotic cells (green), and DAPI (blue). Differential interference contrast (DIC) microscopy was used to visualize kidney structure and the DIC image was overlaid with the fluorescent images. KIM-1-expressing proximal tubule cells bind caspase 3 positive apoptotic cells.
Kidney injury molecule 1 (KIM-1, also known as TIM-1) is markedly upregulated in the proximal tubule after injury and is maladaptive when chronically expressed. Here, we determined that early in the injury process, however, KIM-1 expression is antiinflammatory due to its mediation of phagocytic processes in tubule cells. Using various models of acute kidney injury (AKI) and mice expressing mutant forms of KIM-1, we demonstrated a mucin domain–dependent protective effect of epithelial KIM-1 expression that involves downregulation of innate immunity. Deletion of the mucin domain markedly impaired KIM-1–mediated phagocytic function, resulting in increased proinflammatory cytokine production, decreased antiinflammatory growth factor secretion by proximal epithelial cells, and a subsequent increase in tissue macrophages. Mice expressing KIM-1Δmucin had greater functional impairment, inflammatory responses, and mortality in response to ischemia- and cisplatin-induced AKI. Compared with primary renal proximal tubule cells isolated from KIM-1Δmucin mice, those from WT mice had reduced proinflammatory cytokine secretion and impaired macrophage activation. The antiinflammatory effect of KIM-1 expression was due to the interaction of KIM-1 with p85 and subsequent PI3K-dependent downmodulation of NF-κB. Hence, KIM-1–mediated epithelial cell phagocytosis of apoptotic cells protects the kidney after acute injury by downregulating innate immunity and inflammation.
Li Yang, Craig R. Brooks, Sheng Xiao, Venkata Sabbisetti, Melissa Y. Yeung, Li-Li Hsiao, Takaharu Ichimura, Vijay Kuchroo, Joseph V. Bonventre