Selective binding of platelet factor 4 to regions of active angiogenesis in vivo

In a previous study we suggested that recombinant human platelet factor 4 (rhPF4) preferentially binds in vivo to regions of active angiogenesis/endothelial cell migration. To test this hypothesis, binding of fluorescently labeled rhPF4 to newly formed vessels was compared with that of the normal skin vasculature, using syngeneic Langerhans islets as inducers of angiogenesis. Islets were implanted in the dorsal skinfold chamber of the hamster, and the binding of rhPF4 was studied using intravital fluorescence microscopy. Intra-arterially injected rhPF4 labeled, with high intensity, the endothelium along newly formed vessels of the islets (1,632 +/- 617 microns labeled vessel length per islet), and only on rare occasions (1 +/- 2 sites per cm2 skinfold) were short (62 +/- 48 microns) intense-labeled sites found in the normal vasculature of the skinfold. Heparin could displace most of the label if injected within 10 min after the rhPF4 injection, but not 30 min after. In conclusion, rhPF4-preferentially binds to regions of active angiogenesis in vivo. On binding, rhPF4 is internalized as judged from a decreasing heparin sensitivity with time after rhPF4 injection. The infrequent rhPF4-labeling sites in the normal skin vasculature most likely represent regions of newly formed cells/migration, i.e., normal endothelial turnover, supporting our previous findings demonstrating that the occurrence of such regions is rare in the normal microvasculature. Furthermore, despite the previously demonstrated short half-life in plasma, systemically injected rhPF4 will target regions of angiogenesis with high affinity, thereby facilitating the antiangiogenic effect of PF4.