Vascular cell adhesion molecule-1-targeted detection of endothelial activation in human microvasculature

The hallmark of endothelial activation, an early and critical step in many alloimmune and inflammatory responses, is the transcriptional induction and expression of endothelial adhesion molecules (eg, vascular cell adhesion molecule-1 [VCAM-1]). We assessed the feasibility of VCAM-1-targeted in vivo detection of endothelial activation using I-125-labeled-F(ab')2 fragments of E1/6, a monoclonal antibody against human but not murine VCAM-1. The Kd and Bmax, determined by saturation binding in tumor necrosis factor (TNF)-activated human endothelial cells (ECs), were 3.2 +/- 0.6 nmol/L and 5600 +/- 300 binding sites per EC, respectively. Biodistribution and in vivo binding characteristics of I-125-E1/6 F(ab')2 were assessed in a novel chimeric human/mouse model, in which human skin (as a source of human microvasculature) is grafted onto SCID/beige mice. I-125-E1/6 F(ab')2 localized to TNF-activated human skin grafts as detected by autoradiography and gamma well-counting. Relative uptakes (uptake in human skin graft/uptake in the surrounding mouse skin) were, respectively, 2.6 +/- 0.8 (n = 14) and 1.6 +/- 0.3 (n = 12) for E1/6 and MOPC-21, an isotype-matched control antibody (P < .01). The preferential uptake in human skin graft was not due to differences in tissue vascularity assessed by Tc-99m-labeled murine red blood cells. In conclusion, the chimeric human/mouse model is a novel experimental tool for in vivo evaluation of human endothelial cell-specific radiopharmaceuticals. Although I-125-E1/6 F(ab')2 localized to human skin grafts, the limited number of VCAM-1 molecules/endothelial cell adversely affects its suitability as a target for in vivo imaging of endothelial activation.