Immunomodulatory intervention: IL-17A and regulatory T cells in a model of retinopathy of prematurity

Abstract

Retinopathy of prematurity (ROP) is characterised by tissue ischaemia which leads to aberrant vascularisation of the developing retina. This disorder is a leading cause of vision loss and blindness in premature infants. ROP is initiated upon exposure of the infant to high levels of supplemental oxygen to provide for normal cardiovascular development. This results in the degeneration of the peripheral retinal microvasculature. When infants are returned to room air conditions, the reduced vascular network is insufficient in supplying oxygen, resulting in relative hypoxia in the retina. Hypoxia is a trigger for the stimulation of inflammatory and angiogenic factors including vascular endothelial growth factor (VEGF), initiating vascular leakage and pathological neovascularization. Unfortunately preventative treatments are unavailable and hence there is considerable interest in defining the cellular and biochemical mechanisms in ROP in order to identity potential treatment targets. Inflammation has emerged as a key contributor to the progression of ROP. The pro- or anti-inflammatory activation state of the resident retinal immunocompetent cells, microglia has been recognised as a determinant in the development of retinal vasculopathy. However, the factors regulating microglia activation are not well understood. The inflammatory mediator interleukin (IL)-17A, the signature cytokine of TH17 cells, can stimulate VEGF-induced neovascularization. However, it is largely unknown whether non-lymphoid cells can secrete IL-17A in the retina and if this is a contributing factor to the development of ROP. The findings of this thesis established microglia as a source of IL-17A under hypoxic conditions. Muller and ganglion cells however didn’t produce IL-17A, but expressed IL-17 receptors and demonstrated the responsiveness to IL-17A through increased production of VEGF and inflammatory factors. Deficiency in IL-17A via direct neutralisation and upstream inhibition attenuated vascular injury and inflammation in a mouse model of ROP, oxygen-induced retinopathy (OIR). Also reduced were key signifiers of OIR including vascular leakage and microglia infiltration as well as macroglial Muller cell and ganglion cell damage. Regulatory T cells (Tregs) expressing the transcription factor, Forkhead box 3 (Foxp3), play a central role in repressing inflammation and immune-mediated pathology. However, the contribution of Tregs to the development of ROP inflammation and vasculopathy is unknown. It is reasoned that Foxp3+ T regulatory cells (Tregs) penetrate the ischemic retina, immunosuppress microglia and reduce proliferative vasculopathy. This thesis presents novel data utilising OIR and Foxp3+ reporter mice, with Treg numbers in lymphoid organs, retina and blood altered during the course of disease and increased following Treg expansion with an IL-2/anti-IL-2 mAb complex. This data was confirmed using a clinically relevant treatment, the adoptive transfer of Tregs. Both approaches reduced retinal vaso-obliteration, neovascularization, vascular leakage and key retinal pro-angiogenic factors; VEGF and placental growth factor. Tregs physically contacted microglia, which were deactivated as demonstrated by FACS and confocal microscopy. These findings were corroborated in primary cultured microglia activated by hypoxia, where Tregs inhibited microglia activation through cell-to-cell contact and hindered their capacity to produce injurious cytokines. Collectively, this thesis demonstrated that boosting Tregs and IL-17A/RORγ ablation are previously unrecognized avenues for research into new therapeutic interventions for ROP.