Localization and characterization of immunocompetent cells in the human retina

Recent studies have shown that experimental uveitis can be induced by the appropriate administration of various retinal antigens. Little is known about the in-situ interactions between immune cells in the retina as a prerequisite for understanding the mechanisms involving the presentation of antigens by local antigen-presenting cells (APC) to invading T cells. The study described here was therefore designed to investigate the presence of immunocompetent cells with a focus on the characterization of retinal APCs.

Retinal wholemounts, cytospins, and ocular sections were prepared from eyebank eyes obtained within 24 hours postmortem. Immunohistochemistry (single staining and double staining) was performed on the retinal wholemounts, cytospins, and the ocular sections using monoclonal antibodies specific for HLA-DR (MHC class II), CD45 (leukocytes), CD68 (macrophages), CD22 (B cells), CD3 (T cells), and CD1a (dendritic cells).

CD68-positive macrophages were observed in one layer of the retina, whereas HLA-DR(+) and CD45(+) cells were seen in two distinct planes: one mainly at the level of the inner nuclear layer to outer plexiform layer (deep layer) and the other mainly at the nerve fiber and ganglion cell layer (shallow layer). There was a significant difference between the different parts of the retina with regard to the density of these cells. Cell density decreased when going from the peripheral to the posterior areas of the retina. The positive cells in the deep layer were frequently associated with blood vessels, whereas the cells in the shallow layer were distributed evenly throughout the retina. Most positive cells displayed a dendritiform appearance, whereas few cells showed a pleiomorphic morphology. Very few CD1a-positive cells were noted in the retina. Neither T cells (CD3) nor B cells (CD22) could be detected in the normal human retina. Double staining showed that the majority (83. 7%) of the CD45(+) cell population was HLA-DR-positive, whereas approximately half (56.8%) the CD68(+) cell population was HLA-DR-positive.

This study shows that the human retina contains a number of different microglia populations, some of which express HLA-DR and could thus be involved in antigen presentation. Marked differences in cell density can be observed within the retina, with the most abundant presence seen in the peripheral retina. The normal retina contains few professional antigen-presenting cells (CD1(+)).