Dendritic cells are early responders to retinal injury

The presence and activity of dendritic cells (DC) in retina is controversial, as these cells are difficult to identify in retina due to limited markers and sparse numbers. Transgenic mice that express green fluorescent protein (GFP) on the CD11c promoter to label DC allowed the visualization and quantification of retinal DC. Two retina injury models, the optic nerve crush (ONC) and light injury, were used to study their injury response. Many GFP⁺ DC were tightly associated with retinal ganglion cell nerve fibers following ONC, while very few microglia (GFP^-CD11b⁺ cells) were found in close contact. The GFP⁺ cells were greatly elevated in the outer plexiform layer following photic injury. All of the GFP⁺ DC were CD11b⁺, suggesting a myeloid origin. In addition, the GFP⁺ DC upregulated expression of MHC class II after injury, while the GFP^-CD11b⁺ microglia did not. This study shows that DC were found in the retina and that they rapidly responded to neural injuries. We propose that they are a previously overlooked population, distinct from microglia, and may be important in the injury response.