Combined blockade of VEGFR-3 and VLA-1 markedly promotes high-risk corneal transplant survival.

Transplantation remains the last hope to restore the functions of a tissue or an organ to patients whose other treatments have failed or who are experiencing medical emergencies. This hope, however, is greatly jeopardized by immune-mediated rejection, which is the primary reason for transplant failure.1–5 Among all solid organ or tissue transplantations, corneal transplantation is the most common and successful form; it enjoys a 2-year survival rate of 90% in patients with uninflamed and avascular (low-risk) graft beds. The rejection rate, however, dramatically increases and reaches as high as 90% when the grafting is performed on inflamed and highly vascularized (high-risk) corneas and the immune privilege of this site is compromised.1–3,5–7 To date, there is still little effective management of this high rejection situation. Unfortunately, many patients who are blind as a result of corneal diseases fall in this category after a traumatic, inflammatory, infectious, or chemical insult. It is, therefore, a field with an urgent demand for new therapeutic protocols. Corneal transplantation also provides an ideal model for the study of allogenic transplantation as it relates to vessel formation and regulation. This is largely because as the forefront tissue of the visual pathway, the normal adult cornea is both transparent and avascular. It is, therefore, both easy and straightforward to spot and assess conditions of the grafts and newly formed vessels in this tissue.2,8 Both lymphatic and blood vessels are involved in the immune reflex arc of transplantation, which mainly consists of the following components: the afferent pathway of lymphatic vessels through which antigens and antigen-presenting cells migrate to the draining lymph nodes, the lymph nodes where T cell priming occurs, and the efferent pathway of blood vessels through which the primed T cells are homed to the targeted grafts. In high-risk host corneas, cellular trafficking afforded by both lymphatic and blood vessel channels are greatly enhanced, which accelerates transplant rejection.1–3,5,9 Compared with blood vessels that have been studied extensively in the past, lymphatic research has been neglected for centuries but has experienced exponential growth in recent years. This is largely because of the advancement of modern technologies and the discoveries of several lymphatic endothelial-specific molecules, including vascular endothelial growth factor receptor-3 (VEGFR-3), lymphatic vessel endothelial hyaluronic acid receptor-1 (LYVE-1), and Prox-1. The lymphatic network penetrates most tissues in the body, and its dysfunction has been found in a broad spectrum of disorders, such as cancer metastasis, inflammatory and immune diseases, tissue and organ (heart and kidney) transplant rejection, obesity, hypertension, and lymphedema.2,10–15 There are few effective treatments for lymphatic diseases, which defines another field with a great need for new therapeutic strategies. Previous data from us and other researchers have demonstrated that VEGFR-3 mediates corneal lymphangiogenesis (LG; the development of new lymphatic vessels); its blockade suppresses donor-derived cell trafficking to draining lymph nodes and promotes transplant survival in normal or low-risk corneas.16–18 More recently, we also demonstrated that very late antigen-1 (VLA-1; also known as integrin α1β1) mediates corneal inflammatory LG in vivo and lymphatic endothelial cell functions in vitro.19 Its inhibition reduces macrophage, leukocyte, and T cell infiltrations of low-risk corneal grafts, which also tend to survive better.20 Although these preliminary data on VEGFR-3 and VLA-1 obtained from normal or low-risk transplantation studies are promising, it remains unknown whether it is possible to interfere with both pathways to promote high-risk transplant survival. Answers to this question are critical because although low-risk transplantation provides a relatively simple model with which to study transplantation immunity, investigation of high-risk transplantation is of more clinical importance. Indeed, most patients who do not respond to current treatment regimens of transplant rejection fall in the high-risk category. Because VEGFR-3 and VLA-1 exhibit distinctive yet overlapping functions involved in the afferent and efferent arms of the immune reflex arc, it is plausible to hypothesize that combined blockade of both pathways may promote high-risk transplant survival, which is investigated in the present study. In this article, we present the first evidence showing that a combined blockade of VEGFR-3 and VLA-1 markedly promotes 90% survival of high-risk transplants compared with 20% of the control condition. Moreover, this strategy selectively suppresses LG in high-risk host beds, along grafting borders, and in donor buttons. A strong correlation between high-risk transplant rejection and severe lymphatic invasion across the donor-graft border is also revealed. These novel findings not only confirm a critical role of the lymphatic pathway in mediating high-risk transplant rejection, they also provide a new and potentially powerful strategy to combat high-risk transplant rejection and possibly other immune- or lymphatic-related diseases in the body.