Heat shock protein hyperexpression on chorioretinal layers after transpupillary thermotherapy

To assess a biological effect induced by temperature elevation during transpupillary thermotherapy (TTT).

Six pigmented rabbits were anesthetized, and TTT was performed on the right eye using an 810-nm diode laser installed on a slit lamp (spot size, 1.3 mm; duration, 60 seconds; power, 92-150 mW). A series of laser pulses were aimed at the posterior pole of the retina. The left eyes were used as the control. Twenty-four hours after laser irradiation, a histologic study was performed on the chorioretinal layers. Tissue samples were fixed in formalin and embedded in paraffin. A monoclonal antibody was used to detect heat shock protein (Hsp)70 immunoreactivity, followed by a biotinylated goat anti-mouse antibody, revealed by the avidin-biotin complex and the 3-amino-9-ethyl-carbazole (AEC) chromogen. Retinal structures were further identified by hematoxylin erythrosin saffron (HES) coloration.

The photocoagulation threshold was found to be at the 150-mW laser power. Under this threshold, Hsp70 immunostaining was the strongest at the 127-mW power, with staining of some choroidal cells, including capillary endothelial cells. No Hsp70 immunoreactivity was observed on the retina. At the 107-mW power, Hsp70 reactivity was observed only in occasional choroidal cells. At the 98-mW power, only mild, diffuse Hsp70 immunoreactivity was observed in the choroid. At the 92-mW power, as in nonirradiated eyes, no Hsp70 immunoreactivity was detected.

Subthreshold transpupillary 810-nm laser irradiation induces choroidal Hsp hyperexpression. This confirms that choroidal Hsp hyperexpression can be induced during TTT, as has been recently hypothesized by several investigators.