Background We have developed a positron emission tomography (PET)-based imaging probe for detecting inflammation in the kidneys. After immune-complexes deposit in the glomeruli of patients with lupus nephritis (LN), circulating C3 is cleaved and covalently fixed to tissue surfaces. Kidney biopsies are routinely immunostained for deposited C3 fragments as a marker of immunologic activity. Although generally safe, biopsies are invasive procedures, and they are also subject to sampling error. Our new method is based on non-invasive detection of C3d deposits in an animal model of LN and can be translated for use in patients. Furthermore, it will not be limited by the small size of the biopsy or require an invasive procedure.

Methods We developed a murine monoclonal antibody to tissue bound C3d (mAb 3d29) that targets tissue-bound C3d when injected in vivo. We stained biopsy tissue from patients with active LN to confirm that 3d29 detects tissue C3d in human disease. We also analyzed biopsy reports from 76 patients with LN to assess whether C3 deposits correlate with disease activity.

We next radiolabeled mAb 3d29 with ¹²⁴I and injected 16-week-old MRL/lpr mice (a model of lupus-like disease) or wild-type mice intravenously with 20 µg of protein (120 µCi). The mice underwent whole body PET imaging using a Siemens Inveon microPET/CT scanner at 4, 24, 48, 72, and 96 hours post injection. After the mice were sacrificed, organs were weighed and counted in a gamma counter along with a known amount of the radioactive injectate to determine the biodistribution of the antibody.

Results By immunostaining, mAb 3d29 detects glomerular C3d in kidney samples from MRL/lpr mice and humans with LN. In patients with LN, the abundance of glomerular C3 fragments (0-3+) was highest in those patients with proliferative disease and with high activity scores.

¹²⁴I-PET with mAb 3d29 revealed rapid tracer uptake in the kidneys of MRL/lpr mice in the first hours after injection compared to controls, with retention up to 96 hours (figure 1). At the end of the study (144 hours), there was still a trend towards greater antibody in kidneys of MRL/lpr mice than controls (4.93±1.42% vs 1.77±0.17 percentage of injected radioactivity per gram of tissue, P=0.07)

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Abstract 508 Figure 1

C3d-PET of MRL/lpr and wild-type mice. 16-week-old animals were injected with 124I labeled anti-C3d antibody and PET imaging was performed at various timepoints. The kidneys of the MRL/lpr mice revealed higher uptake and retention of 124I-labeled antibody. *P < 0.05. “K” indicates kidney, “BL” indicates bladder.

Conclusion Glomerular C3 deposition is an important marker of disease activity in LN. C3d-imaging can be used to non-invasively detect inflammation in the kidneys of patients with LN.