THU0317 IDENTIFICATION OF ENDOTHELIAL PROTEIN C RECEPTOR AND SCAVENGER RECEPTOR CLASS B TYPE 1 AS MAJOR AUTOANTIGENS IN TAKAYASU ARTERITIS
Authors: Tsuyoshi Shirai, Tomoyuki Mutoh, Tomonori Ishii, Hiroshi Fujii, Hideo Harigae
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background:</h3> Takayasu arteritis (TAK) affects the aorta and its major branches. It has been recognized that high numbers of patients with TAK possessed antiendothelial cell antibodies (AECA), which have potential to induce vascular lesion. However, their major target antigens remain unclear. The target antigens of AECA are plasma membrane proteins, and traditional methods to identify autoantigens do not differentiate between cell-surface molecules and intracellular molecules. To overcome this problem, we have developed an expression cloning system to identify cell-surface antigens: serological identification system for autoantigens using a retroviral vector and flow cytometry (SARF)<sup>1-3</sup>. Because there exists no disease-specific testing for TAK, identification of autoantigens would be extremely important. <h3>Objectives:</h3> To identify major autoantigens in TAK using SARF and analyze clinical significance of autoantibodies <h3>Methods:</h3> Two hundred and seventy-eight patients with collagen diseases were enrolled: 80, TAK; 10, giant cell arteritis (GCA); and 188, other collagen diseases. A cDNA library of human umbilical vein endothelial cells (HUVECs) was retrovirally transfected into a rat myeloma cell line. Cells expressing the cDNA library were stained with prototype AECAs and fluorescent-conjugated secondary anti-human IgG, and cells with fluorescence were sorted with flow cytometry. Autoantigen identification was performed by analyzing the cDNA inserted into the sorted cells. Cells expressing the identified autoantigens were generated, and the presence of autoantibodies was confirmed. The autoantibodies against identified autoantigens were measured in TAK and other collagen diseases, and the prevalence and clinical characteristics of each autoantibody were evaluated. <h3>Results:</h3> AECA activity against HUVECs were measured in patients with TAK and nine AECA serum samples were selected for subsequent SARF. Four distinct AECA-positive clones were successfully isolated using serum IgG from TAK patients. Two clones were identical to the cDNA of <i>PROCR</i> encoding protein C receptor (EPCR), and others, to <i>SCARB1</i> encoding scavenger receptor class B type 1 (SR-BI). A validation experiment involving 52 patients with TAK confirmed disease specificity in TAK; autoantibodies against EPCR or SR-BI accounted for 34.6% or 36.5% cases, respectively, with minimal overlap (3.8%). Measurement of these autoantibodies in other collagen diseases was performed, and the sensitivity and specificity of these two autoantibodies were 67.3% and 96.5%, respectively. Importantly, these autoantibodies were not detected in patients with GCA who were positive for temporal artery biopsy. TAK was classified into three subtypes based on the profile of these autoantibodies. Anti-EPCR positive group showed high prevalence of stroke, ulcerative colitis, and type II artery lesion. Anti-SR-BI positive group presented higher levels of inflammatory markers, type V artery lesion, and older age at onset. Aortic regurgitation was rare in anti-SR-BI positive group. Double-negative group presented higher rates of vascular surgery. <h3>Conclusion:</h3> We identified EPCR and SR-BI as novel autoantigens in TAK. Autoantibodies against EPCR or SR-BI were observed in 66.7% of patients, and different types of autoantibodies showed distinct clinical characteristics. These autoantibodies would aid in clinical application and elucidating pathomechanisms. <h3>References:</h3> [1] Shirai T et al. Arthritis Res Ther 2012;14:R157. [2] Shirai T et al. Clin Dev Immunol 2013;2013:453058. [3] Shirai T et al. J Neuroinflammation. 2013;10:128. <h3>Disclosure of Interests:</h3> Tsuyoshi Shirai: None declared, Tomoyuki Mutoh: None declared, Tomonori Ishii Grant/research support from: GSK, Janssen, Consultant for: GSK, Janssen, Speakers bureau: Mitsubishi-Tanabe, Janssen, Chugai, Ono, Sanofi, Abbvie, Eisai, Astellas, UCB, Teijin, Daiichi-Sankyo, Pfizer, Takeda, Asahi, and Kasei Pharma, Hiroshi Fujii: None declared, Hideo Harigae: None declared