Activation of microglia in the inherited retinal dystrophy of rd mice

Objective: To study the relationship between activation of microglia and photoreceptor apoptosis in the rd mice. Methods: Photoreceptor apoptosis in the rd mice at postnatal day 8, 10, 12, 14, 16 and 18 was detected by TUNEL assay. CD11b antibody was used to label retinal microglia. Results: TUNEL assay demonstrated that positive cells began to emerge in the outer nuclear layer(ONL) of retina at 10 postnatal days and reached a peak at 16 postnatal days in the rd mice. Immunohistochemical labeling of retinal microglia showed that CD11b-labled cells were restricted to the inner retinal layers in the normal retina. However, CD11b-labled cells began to increase and migrate to the inner portion of the ONL in the rd retina at 10 postnatal days and infiltrated the entire ONL and subretinal space at 14 postnatal days. By 18 postnatal days, only a few CD11b-labled cells scattered among the remaining photoreceptor cells. These results suggested that photoreceptor apoptosis started at 10 postnatal days and got a peak at 16 postnatal days, and activition of microglia was observed at 10 postnatal days and reached a top level at 14 postnatal days. A close temporal and spatial relationship between migration of microglial cells toward the photoreceptor cell layer and progression of photoreceptor degeneration was noted. Conclusion: Photoreceptor cell death in the rd retina occur through apoptosis. Activation of microglia may play an important role in the retinal dystrophy of rd mice.