89Zr anti-CD44 immuno-PET monitors  CD44 expression on splenic myeloid cells and HT29 colon cancer cells

CD44 is a cell-surface glycoprotein involved in cell–cell interaction, adhesion, and migration. CD44 is found on colon cancer cells and on immune cells. Previous studies of 89 Zr PET imaging of CD44 have relied on an anti-human antibody (Ab), which can influence biodistribution in murine models. In this study, we used an Ab that cross-reacts with both human and mouse origin CD44 of all isoforms to unveil the type of leukocyte responsible for high splenic anti-CD44 uptake and investigate how its regulation can influence tumor immuno-PET. The Ab was site-specifically labeled with 89 Zr-deferoxamine on cysteine residues. 89 Zr-anti-CD44 demonstrated high-specific binding to HT29 human colon cancer cells and monocytic cells that showed CD44 expression. When 89 Zr-anti-CD44 was administered to Balb/C nude mice, there was remarkably high splenic uptake but low SNU-C5 tumor uptake (1.2 ± 0.7%ID/g). Among cells isolated from Balb/C mouse spleen, there was greater CD44 expression on CD11b positive myeloid cells than lymphocytes. In cultured monocytic and macrophage cells, LPS stimulation upregulated CD44 expression and increased 89 Zr-anti-CD44 binding. Similarly, normal Balb/C mice that underwent lipopolysaccharide (LPS) stimulation showed a significant upregulation of CD44 expression on splenic myeloid cells. Furthermore, LPS treatment stimulated a 2.44-fold increase of 89 Zr-anti-CD44 accumulation in the spleen, which was attributable to splenic myeloid cells. Finally, in Balb/C nude mice bearing HT29 tumors, we injected 89 Zr-anti-CD44 with greater Ab doses to reduce binding to splenic cells. The results showed lower spleen uptake and improved tumor uptake (2.9 ± 1.3%ID/g) with a total of 300 μg of Ab dose, and further reduction of spleen uptake and greater tumor uptake (5.7 ± 0.0%ID/g) with 700 μg Ab dose. Thus, using an 89 Zr labeled Ab that cross-reacts with both human and mouse CD44, we demonstrate that CD44 immuno-PET has the capacity to monitor CD44 regulation on splenic myeloid cells and may also be useful for imaging colon tumors.