Dendritic cells are early responders to retinal injury

Abstract

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.

Introduction

Several types of bone marrow (BM)-derived CD45⁺ cells participate in immunity and inflammation in CNS, including perivascular cells (PVC), microglia (MG), and dendritic cells (DC) (Dick, 1999, Gregerson et al., 2004, Hickey & Kimura, 1988, Xu et al., 2007a). MG have been the focus of studies to understand the local response to neural injury. In addition to their well-known ability to scavenge dead or dying neurons (Streit et al., 2004), they have been reported to promote survival of injured neurons (Sanders and Jones, 2006).

There is significant controversy as to whether DC exist in quiescent retina and function as antigen presenting cells (APC). Part of the difficulty is that normal retina has very few DC, and there are few markers for elucidation of their phenotype, function, and origin. Evidence for DC in the inflamed retina has been found using the experimental autoimmune uveoretinitis (EAU) model for retinal autoimmune disease in which CD11c⁺ cells were recruited to retina by the inflammation during pathogenesis of the disease (Jiang et al., 1999). Of several markers associated with murine DC, CD11c is most frequently used. However, antibodies to murine CD11c are difficult to use in immunohistochemistry. Using flow cytometry and antibodies 33D1 (Brasel et al., 2000) and CD205, a small number of MHC class II⁺ putative DC were identified in retina (Gregerson & Yang, 2003, Xu et al., 2007a). The relationship between MG and DC is uncertain, and it is not entirely clear whether retinal DC or other cells with APC ability are derived from MG, recruited from the circulation (Gregerson and Kawashima, 2004), or derived from local progenitor cells.

To visualize and study the responses of retinal DC, we took advantage of a transgenic mouse line (CD11c-DTR) where CD11c⁺ DC express both the diphtheria toxin receptor (DTR) and green fluorescent protein (GFP) under the CD11c promoter (Jung et al., 2002). This mouse model allows identification of DC by their expression of GFP and provides a method for depletion of the DC by treating the mice with diphtheria toxin (DTx). The response of retinal DC to neural injury was examined by analysis for changes in the number and localization of DC following two distinct retinal injuries. First, DC changes occurring after a unilateral optic nerve crush (ONC) were examined bilaterally in the retina. The axonal damage induced by ONC leads to retinal ganglion cell (RGC) death and triggers a retinal MG response (Bodeutsch et al., 1999, Panagis et al., 2005, Sautter & Sabel, 1993, Yoles & Schwartz, 1998). Second, the effect of light-induced retinal damage on DC was examined. Constant light is a well-known model of injury to the retinal photoreceptor cells (LaVail et al., 1987). Both injury models resulted in increased numbers of retinal GFP⁺ DC in areas of the retina associated with the injured cells. Examination of the retinas whose RGC were injured by the ONC revealed that far more GFP⁺ DC than GFP⁻ MG were tightly associated with the damaged axons. A smaller, but significant GFP⁺ DC response was also found in the retina of the unmanipulated contralateral eye.