Association of CD169 Positive Monocytes with Disease Activity in Rheumatoid Arthritis
Eakin AJ 1, McGeough CM 1, Ahmed T 1, Alexander HD 1, Wright GD 2, Small D 3, Gardiner PV 3, Bjourson AJ 1, Gibson DS 1
1. Ulster University, Northern Ireland Centre for Stratified Medicine, Altnagelvin Hospital, Londonderry, BT47 6SB, United Kingdom. 2. Department of Rheumatology, Musgrave Park Hospital, Belfast, United Kingdom. 3. Department of Rheumatology, Altnagelvin Hospital, Londonderry, United Kingdom.
CD169 (Siglec-1) is expressed by monocytes and has been shown to correlate with disease activity in RA patients . This cell-surface protein drives pro-inflammatory processes including the suppression of Tregs through its cognate ligand (CD169-L). The suppression of Tregs leads to loss of control in the immune response as it prevents damage-causing effector T cells from being reduced.
Treatment of RA includes small molecule DMARDs, which have immunosuppressive and anti-inflammatory mechanisms. 30% of patients have no response to traditional DMARDs for unknown reasons . Additionally, patients starting on DMARD therapy undergo a three month treatment regimen before response is determined. This pilot study investigates the relationship between CD169 and CD169-L density on peripheral cells and disease activity.
Peripheral blood mononuclear cells (PBMCs) were isolated from RA patients (ACR diagnosis criteria fulfilled) who have failed DMARD treatment (n=9) and healthy controls (n=9) using Ficoll density gradient separation. FACS was used to both immunophenotype and isolate CD169 positive (+ve) monocytes and CD169-L +ve Tregs using five and eight colour antibody panels, respectively. Isolated CD169 and CD169-L +ve cells were stored for downstream qPCR and analysis of proteins including FoxP3 and TNF-α.
RA patients have monocytes with significantly higher median fluorescence intensity (MFI) of CD169 compared to healthy controls (1441.6 ± 1201.1 vs 198.0 ± 87.3, (mean ±SD), p=<0.01). Also, the MFI of CD169-L +ve Tregs is significantly decreased in RA patients relative to healthy controls (1395.3 ± 345.6 vs 3797.6 ± 863.6, p=<0.01). FACS data indicates FoxP3, an intracellular transcription factor of Tregs, is expressed at lower levels in RA patients compared with healthy controls. A positive association was observed between increased CD169 MFI and DAS28-ESR, whereas the opposite was true for CD169-L MFI and DAS28-ESR.
CD169 plays a significant role in autoimmune disease  and here we show its elevation in patients with high disease activity. The CD169 counter receptor, CD169-L is significantly reduced in RA compared to health. Low levels of FoxP3 in RA patients indicates reduced Treg activation, which may subsequently lead to increased disease activity. We postulate that this balance of cells is key in the immune response and could be a surrogate measure of disease activity.