Persistent inhibition of oxygen-induced retinal neovascularization by anthrax lethal toxin.

Purpose: To evaluate the role of mitogen-activated protein kinase kinase (MKK) signaling in a mouse model of oxygen-induced retinopathy (OIR) that mimics retinopathy of prematurity (ROP).

Methods: Postnatal day 7 mice were exposed to elevated oxygen for 5 days to induce retinopathy. Anthrax lethal toxin (LeTx), an MKK inhibitor, was injected into the vitreous after restoration to normoxia, and its effects on vascular growth were analyzed by whole mount immunofluorescence and confocal microscopy. Pericyte coverage was determined by PDGFR-β and α-SMA staining. Macrophage presence was determined by F4/80 staining. Vitreal cytokine secretion was measured by ELISA and multianalyte profiling.

Results: Intravitreal injection of LeTx over a restricted time interval after return to normoxic conditions blocked the progression of OIR. This block was independent of vascular endothelial growth factor (VEGF) release and did not alter the release of cytokines and growth factors associated with OIR. VEGFR2 expression and activation were similarly unaffected. LeTx had no statistically significant effect on macrophage recruitment. LeTx sensitivity correlated with vessel maturity, extent of hypoxia, and growth of the deep vascular plexus network.

Conclusions: Correlation among pericyte coverage, deep vascular plexus growth, and hypoxia after LeTx treatment indicate immature vessels in a hypoxic environment are preferentially sensitive to LeTx-mediated MKK inhibition. The persistence of VEGF without concomitant induction of neovascular growth or revascularization of vaso-obliterated zones suggests MKK inhibition causes an inability of the cells that are present, or a failure to recruit cells able, to respond to proangiogenic stimuli. These results indicate the inhibition of MKK signaling presents a novel strategy for the inhibition of vascular retinopathies such as OIR and ROP.