Background : The gold standard for diagnosing lupus nephritis is the renal biopsy. While it has several significant benefits, this procedure is not without risk (i.e. bleeding) and only interrogates a minute portion of the renal parenchyma. Recently, near-infrared fluorescent (NIRF) probes have been utilized to image protein activity noninvasively in animals and humans. We hypothesized that specific inflammatory processes observed in nephritis can be noninvasively detected and monitored using NIRF-based optical imaging approaches. Using a probe that becomes fluorescent in the presence of the protease cathepsin B (CatB), we tested the ability of using NIR optical imaging to assess renal inflammation as a noninvasive marker for early-stage glomerulonephritis (GN).
Methods : Experimental GN was induced in 129 mice by nephrotoxic serum (NTS) delivered intravenously. Proteinuria was quantified using albumin ELISA and chromogenic creatinine assay. NIR optical imaging of anesthetized mice was performed following intravenous administration of a cleavable sensor for CatB and fluorescence intensity of kidney regions quantified using fluorescence molecular tomography (FMT)-3000 instrument at days 1 to 10 post-NTS administration.
Results : In NTS-treated mice, a strong signal from the CatB-activatable probe was observed as early as day 1, which associated with the onset of proteinuria (Figure 1). This signal could be detected for at least 10 days. In contrast, control mice were devoid of any CatB signal. To assess the specificity of CatB signal to GN, we examined CD2AP KO mice that develop nephrotic syndrome in the absence of inflammation. CD2AP KO had no CatB signal despite ongoing nephrosis.
Conclusions : Induction of GN by NTS was specifically detected noninvasively using a CatB-activatable probe and NIR optical imaging. These data establish the proof-of-principle that novel noninvasive tomographic approaches may represent a translatable approach to establishing early stages of GN. We believe that this approach can be expanded to other experimental imaging approaches, such as photoacoustics, as a novel method for detecting lupus nephritis in humans.