SYK inhibition in experimental autoimmune vasculitis and its glomerular expression in ANCA-associated vasculitis

Abstract Background The anti-neutrophil cytoplasm antibody (ANCA)-associated vasculitides (AAV) are a group of life-threatening multisystemic diseases characterised by necrotising inflammation of small blood vessels and crescentic glomerulonephritis. Existing treatments are not always effective and are often complicated by substantial toxicity. Previous studies have shown that spleen tyrosine kinase (SYK) is phosphorylated during ANCA-induced neutrophil activation, and thus it represents a potential therapeutic target. We aimed to investigate the role of SYK in the pathogenesis of an in-vivo model of AAV, and to establish whether SYK is expressed in human disease. Methods We studied the effect of fostamatinib, a small molecule inhibitor selective for SYK, in experimental autoimmune vasculitis (EAV), which is a rodent model of AAV that is induced by immunisation of Wistar Kyoto rats with the ANCA target antigen, myeloperoxidase. Animals (eight per group) with established disease, as confirmed by onset of haematuria and proteinuria 4 weeks after immunisation, were treated with vehicle, fostamatinib 20 mg/kg, or fostamatinib 30 mg/kg by twice daily oral gavage from week 4 to 6 and then assessed for disease severity at the end of week 6. We also used immunohistochemical techniques to analyse SYK expression and activation in human renal biopsy specimens from patients with ANCA-associated glomerulonephritis. Findings In EAV, at the end of week 6, there was a dose-dependent reduction in proteinuria (median 2·88 mg/day [IQR 1·80–6·37] vs 0·13 [0·00–0·78] vs 0·23 [0·00–0·59] in vehicle, fostamatinib 20 mg/kg, and fostamatinib 30 mg/kg groups, respectively; p=0·0004), haematuria (3 arbitrary units [AU] [2–3] vs 0 [0–0·38] vs 0 [0–0·038], p=0·0018), glomerular histological abnormalities (12% [3–29] vs 5 [2–9] vs 0 [0–6], p=0·0262), glomerular macrophage infiltration (1·77 [1·28–2·58] AU vs 0·25 [0·20–0·41] vs 0·20 [0·06-0·41], p=0·0004), lung haemorrhage severity (3 AU [2·0–3·8] vs 1 [0·3–1·8] vs 0 [0·1–0·4], p vs 0·48 [0·17–0·70] vs 0·19 [0·06–0·42], p=0·0087). Immunohistochemical analysis confirmed SYK expression in human ANCA-associated glomerulonephritis. Staining for phospho-SYK demonstrated SYK activation that localised to diseased lesions (segmental necrosis, areas of extracapillary proliferation). Glomerular SYK expression correlated with histological class of disease (crescentic 3·0% [2·1–3·8], mixed 1·4% [0·92–2·4], focal 1·1% [0·7–1·8], sclerotic 0·3% [0·1–0·5]; p=0·01). Interpretation Our results suggest that, in a preclinical model, SYK inhibition with fostamatinib is an effective treatment for crescentic glomerulonephritis and lung haemorrhage, the life-threatening manifestations of AAV, even after onset of disease. SYK is activated in human ANCA-associated glomerulonephritis, and expression correlates with disease severity, suggesting that SYK contributes to the pathogenesis of human disease. These observations support the potential investigation of targeting SYK in clinical studies of AAV. Funding UK Medical Research Council.