Background We have developed imaging probes for detecting C3d deposits in the kidneys and other tissues. After immune-complexes deposit in the glomeruli of patients with lupus nephritis (LN) and other forms of glomerulonephritis, 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, however, biopsies are invasive procedures. They are also subject to sampling error. Our new method is based on non-invasive detection of C3d deposits in animal models of inflammatory disease 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.

Materials and Methods We developed several murine monoclonal antibodies that target tissue-bound C3d when injected in vivo. We stained biopsy tissue from patients with active LN to confirm that these antibodies 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 have labeled anti-C3d antibodies for molecular imaging using several different types of reporters. We have radiolabeled antibody with ¹²⁴I and 89Zr, with FITC for fluorescence imaging, and with a bioluminescence resonance energy transfer (BRET) reporter called ‘Antares’.

Results By immunostaining, our antibodies detect glomerular C3d in kidney samples from mice and humans with various forms of glomerulonephritis. 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. Using Antares-labeled anti-C3d, we were able to detect C3d deposits in the kidneys of live mice with glomerulonephritis, and we could non-invasively distinguish the mice from controls (figure 1).

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Abstract 1004 Figure 1

C3d-imaging of mice with glomerulonephritis. A) Mice with glomerulonephritis (factor H knock-out, or Cfh-/-), C57BL/6 controls, and C3 knockout mice (C3-/-) were injected with an anti-C3d antibody labeled with a bioluminescence resonance energy transfer (BRET) reporter. The kidneys of the Cfh-/-mice revealed significantly higher signal than kidneys in control mice. B) 3D reconstruction of the images demonstrates signal limited to the kidneys of experimental mice

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