Immunocompetent cells in the anterior segment

Immunoregulatory mechanisms of the eye

Immunity to protect the eye from invading pathogens is an absolute requirement for the preservation of vision. However, immune responses carry their own threat of tissue damage, due chiefly to the participation of non-specific inflammation. Because of its delicate microanatomy, the visual axis is vulnerable to distortion (and resulting blindness) from relatively trivial amounts of intraocular inflammation. Therefore, regulation of expression of immunity in the eye is critical to preservation of vision. Regulation of ocular immunity is one expression of the general phenomenon of regional immunity in which local tissue factors mold immune responses to local purposes. Ocular immune privilege is an extreme example of regional immunity. Immune privilege is an active, rather than a passive, process in which regulatory molecules and cells of the eye modulate both the induction and the expression of immunity to eye-derived antigens. Immune privilege is achieved primarily through unique features of the eye, ranging from special microanatomic factors (blood:eye barrier, absence of lymphatic draining), to soluble factors secreted by ocular cells into the ocular microenvironment, to regulatory molecules constitutively expressed on the surfaces of ocular cells. In general, the most important consequence of regulation of ocular immune responses is the virtual elimination of immunogenic inflammation from the eye. While this enables the eye to receive immune protection without the threat of blinding inflammation, it also renders the eye vulnerable to those pathogens whose elimination requires the participation of inflammatory molecules and cells.

The distribution of immune cells in the uveal tract of the normal eye

Inflammatory and immune-mediated diseases of the eye are not purely the consequence of infiltrating inflammatory cells but may be initiated or propagated by immune cells which are resident or trafficking through the normal eye. The uveal tract in particular is the major site of many such cells, including resident tissue macrophages, dendritic cells and mast cells. This review considers the distribution and location of these and other cells in the iris, ciliary body and choroid in the normal eye. The uveal tract contains rich networks of both resident macrophages and MHC class II+ dendritic cells. The latter appear strategically located to act as sentinels for capturing and sampling blood-borne and intraocular antigens. Large numbers of mast cells are present in the choroid of most species but are virtually absent from the anterior uvea in many laboratory animals; however, the human iris does contain mast cells. Small numbers of what are presumed to be trafficking lymphocytes are present in the uveal tract of normal eyes. There is little data available on the presence or absence of eosinophils. The role of these various cell types in immune homeostasis and ocular inflammation is briefly considered.