Background Juvenile-onset Systemic Lupus Erythematosus (JSLE) is a chronic, severe, multi-systemic autoimmune disease. Impaired apoptotic cell clearance is implicated as the initial step in the pathogenesis of JSLE, leading to exposure of autoantigens (1). We have investigated the phenotype of the macrophages and their precursors monocytes, important phagocytes whose function is impaired in adult-onset SLE (2), to determine the extent of their involvement in JSLE. This has been assessed by measurement of the gene expression of monocyte and macrophage receptors important in apoptotic cell phagocytosis. CD36 is a member of the Scavenger Receptor Class B receptor group, considered especially important in the recognition of apoptosing neutrophils (3). MER is a tyrosine kinase receptor important in the internalisation of apoptotic cells (4). CR3 is a complement receptor whose principle action is through recognition of C3bi, a complement component that attaches to apoptotic cells (5).
Objectives To investigate for evidence of phagocytosis dysregulation in JSLE by determining if ineffective macrophage and monocyte function is associated with abnormal expression of phagocytic receptors.
Methods Monocytes were separated using CD14 MicroBeads (MACS Miltenyi Biotec) from processed whole blood, cultured in medium (RPMI + 10% FCS + 1% PenStrep + M-CSF) over 6 days into macrophages, and RNA extracted. The RNA was assessed for quantity and quality using NanoDrop, converted to cDNA and RT-qPCR performed using SYBR Green with primers for CD36, MERKT, and CR3; with expression compared against β2M as reference gene. Statistical analyses were performed using IBM SPSS v19 (Mann-Whitney U). We compared the relative expression ratios between JSLE patients and healthy controls (HCs; median [IQR]).
Results In monocytes, CD36 expression was significantly increased in JSLE (n=6): (0.96 [0.68-1.34]) vs HCs (n=6): (0.53 [0.39-0.70]), (p=0.03), with a trend towards greater expression of MERKT: (0.47 [0.30-0.74]) vs (0.30 [0.19-0.44]), (p=0.20). Conversely, CR3 was more highly expressed in HC monocytes: (0.65 [0.49-0.96]) vs (0.77 [0.71-1.32]), although not significantly (p=0.15).
Relative expression ratios for macrophages showed similar trends, with increased expression of CD36 and MERKT in JSLE (n=2 to date) approaching statistical significance compared with HCs (n=9): CD36 (1.86 [N/A]) vs (1.14 [0.35-2.23]), (p=0.06); MERKT: (2.14 [N/A]) vs (1.14 [0.78-1.73]), (p=0.10), but no difference in CR3 expression: (0.98 [N/A]) vs (1.40 [0.35-2.23]), (p=0.64).
Conclusions Our preliminary findings indicate that there are features that distinguish JSLE from HC monocytes and macrophages, reflected in the observed increased expression of receptors associated with apoptotic cell clearance. Work is on-going to consolidate these data with increased sample size and investigations of other markers of phagocytosis, correlated with macrophage phagocytic function.
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Disclosure of Interest None Declared