Kidney injury molecule–1 is a phosphatidylserine receptor that confers a phagocytic phenotype on epithelial cells
Takaharu Ichimura, Edwin J.P.v. Asseldonk, Benjamin D. Humphreys, Lakshman Gunaratnam, Jeremy S. Duffield, Joseph V. Bonventre
Takaharu Ichimura, Edwin J.P.v. Asseldonk, Benjamin D. Humphreys, Lakshman Gunaratnam, Jeremy S. Duffield, Joseph V. Bonventre
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Research Article Nephrology

Kidney injury molecule–1 is a phosphatidylserine receptor that confers a phagocytic phenotype on epithelial cells

Abstract

Following injury, the clearance of apoptotic and necrotic cells is necessary for mitigation and resolution of inflammation and tissue repair. In addition to macrophages, which are traditionally assigned to this task, neighboring epithelial cells in the affected tissue are postulated to contribute to this process. Kidney injury molecule–1 (KIM-1 or TIM-1) is an immunoglobulin superfamily cell-surface protein not expressed by cells of the myeloid lineage but highly upregulated on the surface of injured kidney epithelial cells. Here we demonstrate that injured kidney epithelial cells assumed attributes of endogenous phagocytes. Confocal images confirm internalization of apoptotic bodies within KIM-1–expressing epithelial cells after injury in rat kidney tubules in vivo. KIM-1 was directly responsible for phagocytosis in cultured primary rat tubule epithelial cells and also porcine and canine epithelial cell lines. KIM-1 was able to specifically recognize apoptotic cell surface-specific epitopes phosphatidylserine, and oxidized lipoproteins, expressed by apoptotic tubular epithelial cells. Thus, KIM-1 is the first nonmyeloid phosphatidylserine receptor identified to our knowledge that transforms epithelial cells into semiprofessional phagocytes.

Authors

Takaharu Ichimura, Edwin J.P.v. Asseldonk, Benjamin D. Humphreys, Lakshman Gunaratnam, Jeremy S. Duffield, Joseph V. Bonventre

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Figure 8

KIM-1–expressing epithelial cells bind and internalize ox-LDL, and the KIM-1 ectodomain binds specifically to ox-LDL.

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KIM-1–expressing epithelial cells bind and internalize ox-LDL, and the K...
(A) Graph showing internalization of DiI-labeled ox-LDL or native LDL by KIM1-PK1 cells or pcDNA-PK1 cells over 1 hour at 37°C, as quantified by spectrofluorometry of lysed cells. (B) Photomicrographs of KIM1-PK1 cells and pcDNA-PK1 cells showing internalized ox-LDL or native LDL in intracellular vesicles. Original magnification, ×40. (C) Graph showing the effect of a 40-fold excess of unlabeled ox-LDL on internalization of DiI-labeled ox-LDL by KIM1-PK1 cells or pcDNA-PK1 cells incubated at 37°C for 1 hour. Uptake of fluorescent lipoprotein was quantified by spectrofluorometry of lysed cells. (D) Graph showing the effect of doxycycline on KIM1–tet-off MDCK cells’ capacity to internalize DiI-labeled ox-LDL and DiI-labeled native LDL. In the presence of doxycycline (DOX+), KIM-1 expression is suppressed. In these conditions, there is little uptake of labeled lipoprotein after 1 hour. In the absence of doxycycline (DOX-), KIM-1 expression is not suppressed, and there is marked uptake of both lipoproteins. (E) In vitro binding curves for purified KIM1-Fc, human IgG1, or c-Ret–Fc proteins to ox-LDL coated ELISA plates. KIM1-Fc binding was detected by anti-human IgG – HRP-conjugated antibody followed by a colorimetric assay. Note KIM1-Fc strongly binds to ox-LDL but not uncoated plastic, whereas control proteins human IgG and c-Ret–Fc do not bind ox-LDL or plastic. (F) Graph showing the effect of pretreatment of KIM1-PK1 cells with either 40 or 50 μg/ml of ox-LDL on phagocytosis of fluorescently labeled apoptotic LLC-PK1 cells as assessed by flow cytometry. (*P = 0.0009 [40 μg/ml ox-LDL]; **P = 0.0003 [50 μg/ml ox-LDL] compared with no ox-LDL pretreatment. Error bars indicate SD).