2533 PROGNOSTIC SIGNIFICANCE OF ARTERIOLAR C4D POSITIVITY AS A MARKER OF MICROANGIOPATHY IN ANCA-ASSOCIATED VASCULITIS

Abstract Background and Aims ANCA-associated vasculitis (AAV) represent a group of systemic autoimmune diseases characterized by necrotizing inflammation of small to medium-size blood vessels frequently associated with anti-neutrophil cytoplasmic antibody (ANCA) positivity. Another term they are commonly referred with is “paucimmune vasculitis”, reflecting the lack of immune complex deposition on histological samples, in contrast with other small vessel vasculitis, e.g. cryoglobulinemic vasculitis. The absence or the paucity of deposition of immunoglobulins and complement components was interpreted in the past as a lack of complement cascade involvement in the pathogenesis of AAV. Only in relatively recent times the possible role of complement in AAV was investigated. A few studies proved the relationship between C3 hypocomplementemia and worse renal outcome[1]. Moreover, the overactivation of the alternative complement pathway (ACP) in AAV could be responsible for thrombotic microangiopathy (TMA). A strong association between low serum C3 and histologic signs of thrombotic microangiopathy on renal biopsy was shown in patients with AAV, with a worse renal prognosis. Currently, there are no clear early histological markers of endothelial damage that allow to identify high-risk patients, other than the seldom not identified or misidentified presence of TMA. The association between renal arteriolar C4d deposition (as a vicariate of TMA) has been already described in several glomerulonephritis, including IgA nephropathy [2], but its prognostic meaning has not yet been explored [3]. Our study aims to explore the predictive role of arteriolar C4d in ANCA-associated GN. Method All patients with ANCA-associated GN who underwent a kidney biopsy between January 1990 and June 2021 at the Renal Unit of Parma University Hospital, Parma, Italy with at least 6 months follow-up data were included. All histology slides were retrieved and reviewed by experienced nephropathologists. Percent IFTA, percent global glomerulosclerosis, Brix score, the presence or absence of TMA, the positivity or negativity of C4d (immunohistochemical staining on paraffin-embedded formalin-fixed slides) were taken note of. C4d positivity was correlated with the presence of TMA and patients outcome (Fig. 1). Figure 1: C4d immunohistochemistry and microangiopathy. On the left, C4d IHC is shown. The asterisks highlight arteriolar positivity, which corresponds to a microangiopathic lesion (pin hole l