OP0078 LINKING LIPID MARKERS TO SYNOVIAL HYPERPLASIA AND VASCULARIZATION IN OSTEOARTHRITIS BY MALDI-MSI
Authors: Beatriz Rocha Loureda, Berta Cillero-Pastor, Cristina Ruiz-Romero, Ron M A Heeren, Francisco J. Blanco
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background</h3> Synovial inflammation or synovitis causes severe histological changes in the tissue affecting its normal functioning. These morphological changes are one of the most characteristic events in the OA pathology. Additionally, lipid species are recognized as important key factors in the development and regulation of inflammatory processes. Therefore, the lipid signature of the synovial membrane can provide new markers involved in the synovitis process undergoing the OA disease <h3>Objectives</h3> To define the specific lipid profile and distribution in human control and OA synovial membranes. <h3>Methods</h3> Human synovial membrane samples of OA patients (n=13) and controls (n=10) were compared by matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI). Tissue sections at 10 µm thickness were mounted on conductive slides and coated with norharmane matrix (7mg/ml, 2:1 chlorophorm/methanol) for lipid extraction. MALDI images were acquired using a MALDI TOF-TOF instrument (Rapiflex MALDI Tissuetyper, Bruker, Germany) in reflectron positive and negative ionization mode at a pixel size of 50 µm. Multivariate statistical analysis was used to search for the lipids with the highest differences between OA and control synovial membranes. Then, lipid spatial distribution was investigated with FlexImaging 4.1 software. Identifications were based on tandem mass spectrometry analyses in comparison to LIPID MAPS Structure Database. <h3>Results</h3> MALDI-MSI in combination with principal component analysis (PCA) and discriminant analysis (DA) revealed differential lipid profile between OA and control samples. Data acquired in positive ion mode showed a good separation of OA patients and controls (Figure 1A). OA tissues showed higher lipid content in the mass/charge (<i>m/z</i>) range 600-800, compared to controls (Figure 1B). Among identified lipid species, we found 35 phospholipids significant differentially expressed between OA and controls, mainly classified in phosphatidylcholines (30%), phosphatidylethanolamines (26%), phosphatidylinositols (26%), phosphatidylserines (14%) and lysophosphatylcholines (6%). Some of them showed a specific localization within tissue. For instance, the phosphatidylcholine (PC) <i>m/z</i> 734.55 showed a specific localization in the lining layer of hyperplastic OA synovium, whereas PC <i>m/z</i> 768.56 were located in the sublining layer characterized by high vascularity and inflammatory cell infiltrations (Figure 1C). The PCA-DA analysis also showed a higher presence of phosphatidylinositols (PIs) and phosphatidylethanolamines specifically in OA samples, whereas lysophosphatidylcholines were more abundant in control samples compared to OA. Most of these molecules, mainly PIs, were also spatially distributed in the lining layer of the OA synovium. <h3>Conclusion</h3> OA synovial tissues were characterized by a higher content of phospholipids, mainly PCs and PIs, compared to control synovial biopsies. Specific phospholipids related to morphological features of the synovial membrane have been described for the first time by MALDI-MSI. These molecules could have an important role in the synovitis associated with the pathogenesis of OA and constitute relevant molecular disease classifiers for the OA diagnosis. <h3>Disclosure of Interests</h3> Beatriz Rocha Loureda: None declared, Berta Cillero-Pastor: None declared, Cristina Ruiz-Romero: None declared, Ron M A Heeren: None declared, Francisco J. Blanco Consultant for: AbbVie, Bioiberica, BMS, GSK, Grünenthal, Janssen, Lilly, Pfizer, Regeneron, Roche, Sanofi, TRB Chemedica, and UCB