Article
Siglec-1-positive plasmacytoid dendritic cells (pDCs) in human peripheral blood: A semi-mature and myeloid-like subset imbalanced in SLE
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Published: | August 29, 2016 |
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Background: Plasmacytoid dendritic cells (pDCs) are considered a crucial element in SLE pathogenesis due to their potency to produce high levels of IFN-α. This innate immunological function of pDCs is lost by terminal differentiation into a professional antigen-presenting cell, thereby upregulating costimulatory molecules and downregulating innate characteristics, e.g. IFN-α expression. Siglec-1 (sialoadhesin, CD169) is an adhesion molecule first characterized on cells of the macrophage-monocyte lineage. Expression of Siglec-1 on monocytes was identified as an IFN-α regulated marker for active disease in SLE. Siglec-1 is suggested to play a role in the regulation of adaptive immune responses. The impact of Siglec-1 in adaptive immunity together with its nature as IFN-α regulated molecule prompted us to study Siglec-1 in pDCs with focus on SLE pathogenesis.
Methods: An 8-color-flowcytometric analysis was performed on whole blood of healthy donors and SLE patients. PDCs were identified by CD3-/CD19-/CD14-/CD123high//BDCA-2+/HLA-DR+ expression and characterized for Siglec-1, CD86 and CD83. In vitro stimulation of intracellular IFN-α expression by TLR7 ligand Imiquimod-R837 and TLR9 ligand CpG-A was studied in MACS-sorted pDCs. Surface Siglec-1 induction by recombinant IFN-α and influenza virus vaccine (Begripal 2011/2012, Novartis) was studied in pDCs, too.
Results: We found a small subpopulation of Siglec-1 expressing pDCs in human peripheral blood. Compared to Siglec-1 negative pDCs, Siglec-1 positive pDCs express significantly higher CD86 (Wilcoxon matched pairs test:p=0.0006, n=16) but not CD83. Functionally, Siglec-1 positive pDCs fail to express intracellular IFN-α via TLR-7/TLR-9. SLE patients reveal higher percentages of Siglec-1-positive pDCs (7.69 ± 0.6 vs. 5.96 ± 0.51; p=0.04), but lower Siglec-1 expression levels in Siglec-1-positive pDCs (7.83 ± 0.61 vs. 11.7 ± 0.8, p = 0.001) than healthy donors. Siglec-1 positive pDCs in SLE correlate significantly with disease activity (rS=0.55, p=0.008, n=22). Furthermore, Siglec-1 expression is not induced in vitro by IFN-α but by influenza virus in human pDCs.
Conclusion: Human blood pDCs can be subdivided into two distinct subsets according to their surface expression of Siglec-1. Siglec-1-positive cells might arise from differentiation of Siglec-1-negative pDCs. Semi-mature Siglec-1-positive pDCs might have an adaptive, potentially tolerogenic immunological function with impact on SLE pathogenesis.