Roles of reactive oxygen species in monocyte activation induced by photochemical reactions during photodynamic therapy.

This study attempts to investigate the mechanism of the vascular shut down (VSD) effect during photodynamic therapy (PDT) with zinc coproporphyrin III tetrasodium salt as a photosensitizer. PDT is a treatment based on photochemical reactions and the resultant cytotoxic reactive oxygen species (ROS). Platelet thrombus formation leading to stasis observed in vivo during PDT is called the VSD effect. Leukocytes play an important role in the VSD effect in vivo, but the mechanism how activated monocytes generate ROS is not known in detail. To evaluate ROS generation by activated monocytes is especially important to clarify leukocyte-endothelium interactions in the VSD mechanism. The dichlorofluorescein fluorescence intensity of monocytes with four types of free radical scavenger was investigated by confocal laser scanning microscopy. The fluorescence intensity of monocytes that had been incubated with superoxide dismutase and incubated and added with L-histidine was decreased by about 20 and 30%, respectively. The result affirms the predominant role of singlet oxygen and superoxide anion radicals in monocyte activation in the VSD effect during PDT.