In vivo imaging of experimental arthritis with near‐infrared fluorescence

Objective To visualize early experimental arthritis with near‐infrared fluorescence (NIRF) imaging in a murine model of antigen‐induced arthritis (AIA). Methods The target of NIRF was the F4/80 antigen present on the surface of macrophages infiltrating the inflamed synovial membrane. Imaging was performed using anti‐F4/80 monoclonal antibodies (mAb) labeled with Cy5.5 fluorochrome. On day 7 of AIA, 6 mice received an intravenous (IV) injection of labeled mAb; control AIA mice (n = 6) received an IV injection of Cy5.5‐labeled isotype control antibody. NIRF imaging was performed before injection (baseline) and until 72 hours thereafter. Histologic evaluation of arthritis severity and immunohistochemical assessment of F4/80 antigen density were also performed on day 7. Results NIRF imaging showed an accumulation of fluorochrome probes in the inflamed knee joints and, to a lesser extent, in the contralateral (nonarthritic) knee joints. The signal induced by mAb F4/80 was clearly higher than that generated by the isotype control. Accumulation of fluorochrome probes in the joints was confirmed histologically by confocal laser scanning microscopy. Conclusion The use of fluorochromes allows imaging of arthritis in the near‐infrared range. Accumulation in the contralateral, nonarthritic knee joints can be explained by the presence of sentinel macrophages in normal synovium or by a mild contralateral response due to systemic activation or neurogenic mechanisms.