S. aureus Endophthalmitis Induces Increased Retinal Damage in B-Crystallin Deficient Mice

Abstract

Purpose:: Bacterial toxins are known to induce retinal destruction during endophthalmitis. However, it is not known whether apoptosis contributes to infection pathology. We are investigating the role of αB-crystallin in protecting retinal tissue during Staphylococcus aureus endophthalmitis. αB-crystallin is an anti-apoptotic small heat shock protein constitutively expressed in the retina. We reported previously that αB-crystallin is upregulated in the retina during the successful clearance of an intravitreal injection of 500 CFU S. aureus in C57BL/6J mice. In contrast, active caspase-3 is expressed in the retina when αB-crystallin is cleaved by S. aureus proteases following an injection of 5000 CFU that destroys the eye with complete loss of retinal ERG function. We hypothesize that αB-crystallin protects the retina from apoptosis and that αB-crystallin-deficient mice will have increased retinal damage during endophthalmitis.

Methods:: C57BL/6J, and 129S6/SvEv αB-crystallin knock-out and wild-type mice received intravitreal injections of 500 or 5000 CFU S. aureus (RN6390). Clinical examinations and ERGs were performed at 24, 48, 72 and 96 hours post injection. Eyes were analyzed histologically for apoptosis via TUNEL staining.

Results:: Only low levels of apoptosis were observed in the inner nuclear layer of the retina in C57BL/6J mice that received 500 CFU S. aureus. In contrast, a significant increase in apoptosis was observed in all retinal layers of mice receiving 5000 CFU S. aureus. 129S6/SvEv αB-crystallin KO mice injected with 500 CFU cleared the infection but displayed: (i) decreased ERG retinal function, (ii) increased retinal destruction as determined by histology, and (iii) higher levels of apoptosis in the ganglion cell, inner nuclear and outer nuclear retinal layers as compared to wild-type controls at 48 hours.

Conclusions:: Bacterial-induced apoptosis of the retina is an important component in the pathology of endophthalmitis. Upregulation of αB-crystallin protects against retinal apoptosis and damage during infection. Together with our previous report that S. aureus proteases cleave αB-crystallin, these data imply that either blocking the cleavage of αB-crystallin or increasing expression during infection will decrease retinal apoptosis and preserve retinal function.

Keywords: endophthalmitis • apoptosis/cell death • crystallins