Corneal lymphangiogenesis: implications in immunity

The lymphatic system is a network of vessels throughout the body that serves to return lymphatic fluid to the systemic blood circulation and serves as the immune system's highway in the afferent limb of the immune response. This paper reviews the basics of structure, function and development of the lymphatic system. The emphasis is on understanding the role of lymphatics in the cornea as related to angiogenesis and the clinical setting of corneal transplant rejection. The lymphatic system has two main functions: to drain excess extracellular fluid from capillary beds to return to the systemic blood circulation and to capture antigen for presentation to the immune system in the lymphoid compartment (e.g. nodes) connected in parallel to the lymphatic system. The normal cornea is devoid of lymphatic vessels and blood vessels, thus allowing for its unique immune privileged status. Abnormal blood vessel growth into the cornea has been studied for many years, but the study of lymphatic vessels has been hampered until the recent discovery of lymphatic-specific markers. Subsequent studies of vascularized corneas support the presence of lymphatic vessels in the cornea. The growth of new lymphatic (lymphangiogenesis) and blood (hemangiogenesis) vessels in the cornea is closely related to the abrogation of the immune privileged status of the cornea. This review summarizes our current understanding of the parallel development of hemangiogenesis and lymphangiogenesis as related to the mechanisms of immune rejection of corneal transplants.