AUTHOR=Stocco Elena , Barbon Silvia , Piccione Monica , Belluzzi Elisa , Petrelli Lucia , Pozzuoli Assunta , Ramonda Roberta , Rossato Marco , Favero Marta , Ruggieri Pietro , Porzionato Andrea , Di Liddo Rosa , De Caro Raffaele , Macchi Veronica 

TITLE=Infrapatellar Fat Pad Stem Cells Responsiveness to Microenvironment in Osteoarthritis: From Morphology to Function

JOURNAL=Frontiers in Cell and Developmental Biology

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2019.00323

DOI=10.3389/fcell.2019.00323

ISSN=2296-634X

ABSTRACT=<p>Recently, infrapatellar fat pad (IFP) has been considered as a source of stem cells for cartilage regeneration in osteoarthritis (OA) due to their ability for differentiation into chondrocytes. However, stressful conditions, like that related to OA, may induce a pathogenic reprograming. The aim of this study was to characterize the structural and functional properties of a new population of stem cells isolated from osteoarthritic infrapatellar fat pad (OA-IFP). Nine OA patients undergoing total knee arthroplasty (TKA) were enrolled in this study [median age = 74 years, interquartile range (IQR) = 78.25-67.7; median body mass index = 29.4 Kg/m<sup>2</sup>, IQR = 31.7-27.4]. OA-IFP stem cells were isolated and characterized for morphology, stemness, metabolic profile and multi-differentiative potential by transmission electron microscopy, flow cytometric analysis, gene expression study and cytochemistry. OA-IFP stem cells displayed a spindle-like morphology, self-renewal potential and responsiveness (CD44, CD105, VEGFR2, FGFR2, IL1R, and IL6R) to microenvironmental stimuli. Characterized by high grade of stemness (<italic>STAT3</italic>, <italic>NOTCH1, c-Myc</italic>, <italic>OCT-4, KLF4</italic>, and <italic>NANOG</italic>), the cells showed peculiar immunophenotypic properties (CD73<sup>+</sup>/CD39<sup>+</sup>/CD90<sup>+</sup>/CD105<sup>+</sup>/CD44<sup>–/+</sup>/CD45<sup>–</sup>). The expression of HLA-DR, CD34, Fas and FasL was indicative of a possible phenotypic reprograming induced by inflammation. Moreover, the response to mechanical stimuli together with high expression level of <italic>COL1A1</italic> gene, suggested their possible protective response against <italic>in vivo</italic> mechanical overloading. Conversely, the low expression of <italic>CD38/NADase</italic> was indicative of their inability to counteract NAD<sup>+</sup>-mediated OA inflammation. Based on the ultrastructural, immunophenotypic and functional characterization, OA-IFP stem cells were hypothesized to be primed by the pathological environment and to exert incomplete protective activity from OA inflammation.</p>