Molecular imaging with macrophage CRIg-targeting nanobodies for early and preclinical diagnosis in a mouse model of rheumatoid arthritis.

UNLABELLED An accurate and noninvasive tracer able to detect molecular events underlying the development of rheumatoid arthritis (RA) would be useful for RA diagnosis and drug efficacy assessment. A complement receptor of the Ig superfamily (CRIg) is expressed on synovial macrophages of RA patients, making it an interesting target for molecular imaging of RA. We aim to develop a radiotracer for the visualization of CRIg in a mouse model for RA using radiolabeled single-domain variable antibody VHH fragments (Nanobodies). METHODS Quantitative polymerase chain reaction was used to locate CRIg expression in mice with collagen-induced arthritis (CIA). A Nanobody, NbV4m119, was generated to specifically target CRIg. Flow cytometry, phosphorimaging, and confocal microscopy were used to confirm NbVm119 binding to CRIg-positive cells. SPECT (SPECT/CT) was used to image arthritic lesions in the inflamed paws of 29 mice using (99m)Tc-NbV4m119 Nanobody. RESULTS CRIg is constitutively expressed in the liver and was found to be upregulated in synovial tissues of CIA mice. SPECT/CT imaging revealed that (99m)Tc-NbV4m119 specifically targeted CRIg-positive liver macrophages in naïve wild-type but not in CRIg(-/-) (CRIg knockout) mice. In CIA mice, (99m)Tc-NbV4m119 accumulation in arthritic lesions increased according to the severity of the inflammation. In the knees of mice with CIA, (99m)Tc-NbV4m119 was found to accumulate even before the onset of macroscopic clinical symptoms. CONCLUSION SPECT/CT imaging with (99m)Tc-NbV4m119 visualizes joint inflammation in CIA. Furthermore, imaging could predict which mice will develop clinical symptoms during CIA. Consequently, imaging of joint inflammation with CRIg-specific Nanobodies offers perspectives for clinical applications in RA patients.