Immune-Mediated Retinal Vasculitis in Posterior Uveitis and Experimental Models: The Leukotriene (LT)B4-VEGF Axis

Abstract

Retinal vascular diseases have distinct, complex and multifactorial pathogeneses yet share several key pathophysiological aspects including inflammation, vascular permeability and neovascularisation. In non-infectious posterior uveitis (NIU), retinal vasculitis involves vessel leakage leading to retinal enlargement, exudation, and macular oedema. Neovascularisation is not a common feature in NIU, however, detection of the major angiogenic factor-vascular endothelial growth factor A (VEGF-A)-in intraocular fluids in animal models of uveitis may be an indication for a role for this cytokine in a highly inflammatory condition. Suppression of VEGF-A by directly targeting the leukotriene B4 (LTB4) receptor (BLT1) pathway indicates a connection between leukotrienes (LTs), which have prominent roles in initiating and propagating inflammatory responses, and VEGF-A in retinal inflammatory diseases. Further research is needed to understand how LTs interact with intraocular cytokines in retinal inflammatory diseases to guide the development of novel therapeutic approaches targeting both inflammatory mediator pathways.

Keywords: BLT1; EAU; LTB4; VEGF; inflammation; neovascularisation; retinal vasculitis; uveitis.

Figure 1

Vascular endothelial growth factor (VEGF) detection in experimental autoimmune uveitis (EAU). (a). Fundoscopy on B10RIII EAU mice on day 14 and 19 and (b) VEGF levels (pg/mL) detected in vitreoretinal fluids from healthy control and EAU eyes on days 14 and CFA (Complete Freund’s Adjuvant) control and EAU eyes on day 19. Data are representative of at least 4 independent experiments with different numbers of mice (n = 3–6) in each group. ct = control, O = optic disk, V = vessel, iO = inflamed optic disk, iV = inflamed vessel, H = haemorrhage.

Figure 2

The schematic figure showing some of the microenvironmental conditions in the inflamed posterior eye in EAU with infiltrating immune cells and structural damage along with relevant inflammatory pathways. (a) A summary of biosynthesis of leukotriene B4 (LTB4) and cysLTs (LTC4, LTD4 and LTE4) from AA from potential immune cells in inflamed posterior chamber. (b) LTB4 interacts with LTB4 receptor (BLT1), a G Protein Coupled Receptor, on the infiltrating macrophages and also T cells and activates signalling pathways. Th17 cells produce cytokines including interleukin (IL)-17 and VEGF. Producing IL-6 and tumour necrosis factor-alpha (TNFα) by activated macrophages also enhances VEGF expression. The speculated signalling through LTB4-BLT1 pathway which leads to production of VEGF in Th17 and infiltrating macrophages showed by dotted lines. (c) The Hematoxylin and Eosin section from EAU shows histological changes in the posterior chamber of the eye with infiltrating immune cells in vitreous space and structural damages in retinal layers. (d) Fundoscopy of healthy eye, inflamed eye (EAU) with vascular inflammation and sign of neovascularisation during EAU progression has been shown in this figure. O = optic disk, V = vessel, iO = inflamed optic disk, iV = inflamed vessel, nV = new vessel.

Figure 3

Disease suppression and VEGF downregulation. (a) Clinical score of EAU mice (C57Bl/6) on day 15 post-immunisation and pre-treatments. Clinical score of EAU mice at the end point on day 26 post-immunisation. Mice were treated intravitreally with 1–2 µL of L-nomacopan (L-Nom; only targeting LTB4, 20 mg/mL), saline or anti-VEGF (5 mg/kg, Ultra-LEAF™ Purified anti-mouse VEGF-A Antibody, 2G11-2A05, Biolegend) on day 15 and 18 post-immunisation. (b) VEGF levels (pg/mL) were detected in vitreoretinal space of corresponding EAU mice in (a) and compared to the healthy control mice. Each bar was drawn based on the mean value ± SD of each score (n = 8–17 mice). Unpaired t-test P values compared to vehicle. * p < 0.05, ** p < 0.01, *** p < 0.001.