Near-infrared imaging of flare-up arthritis with native fluorochrome Cy5.5 and albumin-bound Cy5.5

The aim of the study was to visualize chronic experimental arthritis with near-infrared fluorescence imaging (NIRF) in a murine experimental arthritis model of rheumatoid arthritis (RA) (flare-up arthritis). The flare-up arthritis model is a modification of the primary antigen-induced arthritis (AIA) model. NIRF was done for two preparations of the fluorochrome Cy5.5, one native and the other albumin conjugated. Histological features of flare-up arthritis were evaluated.

AIA was induced in 16 mice (strain C57/Bl6); flare-up arthritis was induced in a subgroup of eight. On day 7 after induction of flare-up arthritis, four mice received 50 nmol/kg native dye and four mice equimolar concentrations of the dye as albumin-dye conjugate intravenously. NIRF imaging was performed immediately before injection (baseline) and until 72 h thereafter. Arthritis severity was evaluated histologically for primary AIA and flare-up arthritis mice.

NIRF imaging revealed higher fluorochrome uptake in all inflamed knees compared to contralateral ones. The signal intensities induced by native Cy5.5 were higher than those generated by albumin–Cy5.5 conjugate. Histological evaluation of arthritic joints showed similar abnormalities in flare-up arthritis and in primary AIA joints.

Imaging of flare-up arthritis in the near-infrared range was successful for both fluorochrome preparations, but albumin conjugation prior to injection does not improve the uptake of dye in arthritic joints. Flare-up arthritis is a feasible model of chronic relapse of arthritis in human RA.