Background Glomerulonephritis (GN) represents a major cause of morbidity and mortality in many conditions such as systemic lupus erythematosus. The standard for diagnosing GN is through renal biopsy, but this is not performed uniformly across many centers. Furthermore, it is well established that early treatment of GN improves complication and survival outcomes (1,2). While proteinuria and/or hematuria is typically the first clinical sign of GN, immunologic changes in the kidney such as macrophage infiltration is observed well before. Thus, there is an unmet need to identify a noninvasive approach for recognizing GN, especially the preclinical stages. Recent advances in deep tissue imaging using probes detected by near-infrared (NIR) wavelengths have enabled the noninvasive probing of biologic activity. Activated macrophages that infiltrate the kidney early in GN express the papain-like cysteine protease cathepsin B (3). Thus, renal macrophage activation can be assessed using an NIR probe that becomes fluorescent upon cleavage by cathepsin B.
Objectives We tested whether NIR optical imaging can assess renal macrophage activation as a noninvasive marker for early-stage GN.
Methods GN was induced in 129 mice by nephrotoxic serum (NTS) delivered intravenously. Proteinuria was quantitated using albumin ELISA and chromogenic creatinine assay. Presence of renal macrophages was confirmed using FACS. NIR optical imaging of anesthetized mice was performed following intravenous administration of a cleavable sensor for cathepsin B and fluorescence intensity of kidney regions quantified using the Perkin Elmer FMT4000 fluorescence molecular tomography device.
Results In mice with uninflamed kidneys, we confirmed the paucity of renal macrophages. Accordingly, there was minimal renal fluorescence signal as determined by fluorescent molecular imaging of cathepsin B activity. 3 days post-NTS administration, we observed a massive influx of macrophages into the kidney by flow cytometry, which correlated with a significant increase in renal fluorescence intensity signal in NTS mice compared to control mice. This was followed by the onset of proteinuria on days 5–7 in NTS mice.
Conclusions Induction of GN by NTS caused significant macrophage infiltration, which could be detected noninvasively by a cathepsin B-activatable probe and NIR optical imaging. Importantly, detection of activated macrophages preceded the onset of proteinuria by several days. These data establish the proof-of-principle that NIR optical imaging may represent a translatable approach to detecting the preclinical stages of GN.
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Acknowledgement A.H.J.K. would like to acknowledge the Rheumatology Research Foudation for funding through an Investigator Award.
Disclosure of Interest S. Braehler: None declared, M. Cheung: None declared, D. Huang: None declared, W. Akers: None declared, A. Kim Grant/research support from: Rheumatology Research Foundation, Kypha Inc.