Features of the Immunologic Microenvironment
The immunologic microenvironments of the retina, RPE, choriocapillaris, and choroid have not been well characterized. The retinal circulation establishes an inner blood–ocular barrier at the level of tight junctions between adjacent endothelial cells, while tight junctions between the cells of the RPE provide an outer barrier between the choroid and the retina. The vessels of the choriocapillaris are highly permeable to macromolecules and allow transudation of most plasma macromolecules into the extravascular spaces of the choroid and choriocapillaris. Well-developed lymphatic channels are absent, although both the retina and the choroid have abundant potential APCs. In the retina, resident microglia (bone marrow–derived cells related to dendritic cells) are interspersed within all layers and can undergo physical changes and migration in response to various stimuli. The choriocapillaris and choroid contain an abundance of certain potential APCs, especially macrophages and dendritic cells.
The RPE can be induced to express major histocompatibility complex (MHC) class II molecules, suggesting that the RPE may interact with T lymphocytes in some circumstances. The presence of T lymphocytes or B lymphocytes within the normal posterior segment has not been carefully studied, but effector cells appear to be absent from the normal retina. Similar to macrophages, RPE cells also express toll-like receptors (TLRs). These special pattern recognition receptors are critical in recognition of pathogens and in the initiation of innate and adaptive immune responses, forming an initial line of defense against invading microorganisms. A moderate density of mast cells is also present in the choroid, especially around the arterioles, but lymphocytes are present only in very low number. Eosinophils and neutrophils appear to be absent. Under various clinical or experimental conditions, however, a large number of T lymphocytes, B lymphocytes, macrophages, and neutrophils can infiltrate the choroid, choriocapillaris, and retina. The RPE and various cell types within the retina and the choroid (eg, pericytes) can synthesize many different cytokines (eg, transforming growth factor-β) that may alter the subsequent immune response. Local immune processing does not seem to occur. (See also BCSC Section 12, Retina and Vitreous.)
Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.