Role of Microglia in the Retina in Retinal Diseases

小胶质细胞是视网膜的常驻免疫细胞，在人类疾病和小鼠模型中，小胶质细胞参与了光感受器退化的进程。临床前研究表明，调节小胶质细胞激活(例如，使用米诺环素)可以改善变性。然而，小胶质细胞和光感受器退化的细胞机制还不完全清楚。 RD10小鼠品系的眼睛在杆状变性的早期(出生后(P)18)、中期(P25)和晚期(P60)被采集，固定，包埋在琼脂糖中，并切成100微米厚的振镜切片。切片用DAPI和抗Iba1、视紫红质和视锥视蛋白的一抗染色，并用TUNEL法标记凋亡细胞。在细胞染色后，用共聚焦显微镜对视网膜下中周区域进行成像。 在出生后18天的RD10小鼠，外核层(ONL)厚度正常，仅含有少量孤立的TUNEL+细胞。IBA1+小胶质细胞仅见于视网膜内侧，呈分枝状。在P25，大量小胶质细胞迁移到ONL，此时ONL明显变薄，多个核呈TUNEL阳性。位于ONL的小胶质细胞具有大的、去角化的、阿米巴样的形态，可观察到吞噬多个视紫红质阳性杆状感光细胞。有趣的是，大多数被吞噬的视杆细胞TUNEL染色为阴性；相反，ONL中的大多数TUNEL+核不被小胶质细胞吞噬。小胶质细胞的吞噬作用也使视锥视蛋白标记的视锥感受器幸免于难。在P60，ONL被还原为单层锥体光感受器，含有小胶质细胞，已恢复为分支的非阿米巴形态，没有明显的持续锥体吞噬作用的证据。 在视杆细胞退行性变过程中，在视网膜变性过程中迁移到ONL的阿米巴样小胶质细胞似乎能够特异性地形成嗜多染红细胞视杆。小胶质细胞吞噬作用可能构成一种不依赖于细胞凋亡的机制，从而导致整个光感受器细胞死亡。抑制小胶质细胞的吞噬功能可能会延缓视网膜退行性变的全程，延长视网膜退行性变中感光细胞的存活时间。

Microglia, the resident immune cell of the retina, have been implicated in the progress of photoreceptor degeneration in human disease and in mouse models. Preclinical studies modulating microglial activation (e.g. with minocycline) have been shown to ameliorate degeneration. However, the cellular mechanisms underlying microglia and photoreceptor degeneration are not fully understood. Eyes from the rd10 mouse strain were harvested during early- (postnatal (P) 18), mid- (P25), and late- (P60) phases of rod degeneration, fixed, embedded in agarose, and sectioned into 100 micrometer-thick vibratome sections. Sections were stained with DAPI and primary antibodies to Iba1, rhodopsin, and cone opsin, and apoptotic cells were marked using a TUNEL assay. The inferior mid-peripheral retinal region was imaged using confocal microscopy following cell staining. In the rd10 mouse at P18, the outer nuclear layer (ONL) was of normal thickness and contained only a few isolated TUNEL+ cells. Iba1+ microglia were found only in the inner retina and demonstrated ramified morphologies. At P25, numerous microglia migrated into the ONL, which at this point demonstrated significant thinning with multiple nuclei showing TUNEL positivity. Microglia located in the ONL possessed large, deramified, amoeboid morphologies, and were observed to phagocytose multiple rhodopsin-positive rod photoreceptors. Interestingly, the majority of these phagocytosed rods were negative for TUNEL staining; conversely most TUNEL+ nuclei in the ONL were not phagocytosed by microglia. Microglial phagocytosis also spared cone-opsin labeled cone photoreceptors. At P60, the ONL was reduced to a single layer of cone photoreceptors and contained microglia that had reverted back to a ramified, non-amoeboid morphology, without evidence of significant ongoing cone phagocytosis. Amoeboid microglia migrating into the ONL during retinal degeneration appear capable of specifically phaogocytosing rods during the period of rod degeneration. Microglial phagocytosis may constitute an apoptosis-independent mechanism that contributes to overall photoreceptor cell death. Inhibition of microglial phagocytosis may delay overall rod degeneration and prolong the survival of photoreceptors in retinal degenerations.