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.