124. Investigating the contribution of platelets to inflammation in ANCA Associated Vasculitis.
Creators: name: Mathew1, David
name: Antonelou1, Marilina
name: Salama1, Alan
Publisher: name: Zenodo
Year: 2022
Subjects: Vasculitis
ANCA
Abstracts
Giant Cell Arthritis
MPA
GPA
EGPA
Takayasu
IgA vasculitis

description: <strong>Background:</strong> The release of Neutrophil Extracellular Traps (NETs) causes endothelial damage in numerous inflammatory pathologies including ANCA associated vasculitis (AAV). The mechanisdriving these cellular processes are yet to be fully characterised. Autoantibodies are likely to make a significant contribution, however, other factors must be considered given that ANCA titre does not correlate well with NET production<sup>1</sup> and an ANCA negative subpopulation comprise up to 10% of cases.<sup>2</sup> Thrombocytosis is well described in acute AAV, and a raised platelet-to-lymphocyte ratio is significantly associated with disease severity.<sup>3</sup> Platelets are implicated neutrophil activation and NETosis in both infective and sterile inflammation including AAV, with TLR 9 and monomeric CRP being pathways of interest.<sup>4,5 </sup>Platelets themselves become activated by NETs in the circulation creating a positive feedback loop, propagating the inflammatory state. We have used descriptive methods to examine the inflammatory phenotype of acute AAV and have established an <em>in vitro</em> Platelet-Neutrophil co-culture model to characterise platelet-induced NETosis. <strong>Methods:</strong> GraphPad PRISM was used to analyse biochemical data related to 103 acute AAV presentations. Tests used included multilinear regression, T-Test and Mann Whitney-U test. Neutrophils and platelets were isolated from whole blood using density gradient and centrifugation techniques. Results were analysed using immunofluorescence and ELISA. <strong>Results:</strong> Multilinear regression analysis of biochemical parameters of 103 initial presentations of AAV to our centre identified a significant positive association between platelet count and ESR (p=0.0008). A subgroup of thrombocythaemic patients (platelet count &gt;400 x10<sup>9</sup>/L) (n=26) had significantly raised ANCA titre (p=0.009), ESR (p=0.0000006), CRP (p=0.000005) and neutrophil count (p=0.000002) when compared to those with normal platelet counts. <em>In vitro</em> work using platelet-neutrophil co-culture has demonstrated a reduction in supernatant myeloperoxidase (MPO) concentration when inhibitors of platelet activation (Prostaglandin E1 or Apyrase) are added to culture. Additionally, we found reduced supernatant High Mobility Group Box 1(HMGB1) concentration when aspirin or heparin are added to the culture (Figure 1). HMGB1 serum levels have been implicated as biomarkers of AAV disease activity. These readily available therapeutic agents provide an interesting avenue for future investigation in AAV patients, with potential to interrupt the positive inflammatory feedback loop between platelets and neutrophils. <strong>Conclusions:</strong> Elevated platelet counts are associated with elevated markers of inflammation and autoimmunity in AAV patients. This positive feedback loop between platelet activation and NETosis may have therapeutic implications for AAV patient management. <strong>Disclosures:</strong> None Figure 1. Supernatant HMGB1 Concentration Abbreviations (PMN: Polymorphonuclear leucocyte, PRP, platelet rich plasma)