Anti-inflammatory pathways were also modulated during the repeat phase. LXR/RXR signaling was doubly inhibited, directly through inhibition of anti-inflammatory pathway LXR/RXR activation
39,40 and indirectly by activation of the LPS/IL-1–mediated inhibition of RXR function pathway. This is the opposite of the LXR/RXR activation profile observed during vessel regression in this model,
25 indicating an upregulation of inflammation to reverse the regression phenotype during the repeat phase of neovascularization. Activation of these and other inflammatory pathways may partially be attributed to upstream regulators, such as
Il1β,
Vegfa, and
Il6 that were more strongly activated during repeat neovascularization. Interestingly, Th1, Th2, and interferon signaling pathways were additionally activated, suggesting a possible role of the adaptive immune response in priming the tissue for faster and more aggressive repeat neovascularization. In line with this, CD8
+ T cells were shown to be important for corneal neovascularization in HSV-1 infected mice.
41 Notably, stimulated T helper cells are a source of VEGF,
42,43 while adaptive immune activation has been linked to the pathogenesis of nAMD.
44 The involvement of other vessel types, such as lymphatics during revascularization, is open for discussion, given that it has been shown that lymphangiogenesis occurs alongside hemangiogenesis, for example during inflammation in penetrating keratoplasty.
45 Using our rat model of suture-induced corneal angiogenesis however, we observed previously that lymphangiogenesis occurs typically at 14 days after induction of angiogenesis,
46 and lymphatics were therefore not observed during initial angiogenic stimulation in the present study. Interestingly, however, recurrent inflammation is thought to accelerate the development of functional lymphatic vessels,
47 in conjunction with enhanced inflammation as observed here during revascularization. In addition, macrophages, cells we have previously shown to dominate a regressing corneal vascular bed,
21 are known to support lymphangiogenesis in many ways, including by transdifferentiation into lymphatic endothelial cells.
48 A recent study showed that targeting lymphatics promotes graft survival via a reduced activation of the immune system,
49 suggesting lymphatics as mediators of immunological memory, that may impact rapid corneal revascularization.