Immune privilege of the eye results from the need to keep vasculature from the central light path where it would impair vision. Therefore, organs and tissues of the eye like the cornea, have developed alternative mechanisms of immune surveillance to protect them and in response to injury or tissue pathogenesis. In the CNS, novel immune surveillance adaptations have been discovered for protection and repair that demonstrate a state of immune quiescence. In the cornea, which can tolerate foreign antigens, there are immune cells that surveille it in the peripheral regions of the cornea, the aqueous humor, and the tears. The lens has remained an enigma in terms of immune surveillance and protection. We now show that, like other tissues, the lens has developed mechanisms of immune surveillance to protect it throughout a lifetime and to respond to stresses and injury while maintaining its transparency. Since dysregulation of immune surveillance is tightly linked to development of fibrosis, it is possible that the immune cells that associate with the lens may be an unexplored cause of cataract and posterior capsule opacification (PCO). We have discovered resident immune cells (?2 integrin/CD45+) in the lens, that are activated upon mock cataract surgery and have essential functions in regenerative repair of the wound area. These cells are susceptible to being diverted from this role, and induced to acquire a myofibroblast phenotype, the cell type that underlies fibrotic PCO. The resident immune cells first appear in the lens during development, delivered by the tunica vasculosa, a vasculature that surrounds the developing lens; however, this vasculature degenerates after birth. It was assumed that there are no active sources of immune cells that could surveille and protect the adult lens during homeostasis and in response to stress, injury, or pathogenesis. However, we discovered that in response to lens dysgenesis (N-cad?lens mice) or corneal debridement wounding, an extrinsic immune surveillance mechanism is activated resulting in the recruitment of immune cells to the lens. These immune cells move between the ciliary body and the lens across the ciliary zonules that connect them, in the absence of a vasculature. Over time, these immune cells can acquire a myofibroblast phenotype and lead to lens opacity. Studies of eyes from these N-cad?lens mice also revealed that there is an immune response to lens dysgenesis in the central cornea, vitreous, and retina. We will build on these findings in three specific aims expected to elucidate how extrinsic immune cells are able to travel to the lens, how the immune system is activated to protect the lens in response to corneal injury and to protect the cornea in response to lens dysgenesis, and how immune cells that surveille the lens in response to its damage/pathogenesis, or that of another tissue, may lead to cataract and PCO.
As the lens is a transparent tissue, immune cells were not thought to play a role in maintaining, protecting, or repairing it. Our previous studies have revealed that the lens contains resident immune cells that are normally in a resting or quiescent state, but which are activated in response to lens and corneal injury. We now build on these findings with studies aimed at investigating the mechanisms of lens immune surveillance, the coordination of lens and corneal immune responses, and how immune cells may promote fibrosis of the lens in cataract and PCO.
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