Plasmacytoid Dendritic Cells and CXCL4: Key Drivers of Systemic Sclerosis

Systemic sclerosis (SSc), or scleroderma, is a rare disease where connective tissue in the skin and internal organs become thickened or hardened. When internal organs such as the lungs and heart are affected this may lead to fatal consequences. It is a complex autoimmune disease characterized by a persistent inflammation, vascular abnormalities, and fibrosis due to the accumulation of extracellular matrix (ECM) such as collagen. Owing to its complex nature and heterogeneity, SSc remains one of the greatest challenges to both investigators and physicians. There is no cure available and current treatment remains for many complications ineffective. Fibrosis is mainly attributed to the formation of myofibroblast and their production of ECM. Preceding fibrosis, immune cells infiltrate tissue and release pro-inflammatory and pro-fibrotic stimuli such as TGFβ, IL-6, and type 2 cytokines. Furthermore, in multiple autoimmune diseases, the increase of type I interferon cytokines (IFN-I), mainly produced by plasmacytoid dendritic cells (pDCs), have been implicated. In this thesis, my research aim was to investigate the dysregulation of pDCs in SSc patients and to evaluate their involvement through their production of CXCL4. The first part of the thesis focuses on the plasmacytoid dendritic cells and their underlying cause of dysfunction in SSc. In Chapter 2, we have identified CXCL4 to be secreted in a large amount by pDC from SSc patients. CXCL4 level predicts disease progression in the skin and lung of SSc patients. CXCL4 also enhances IFN-I production by pDCs and induced skin inflammation in mouse. In Chapter 3, we have found downregulation of transcription factor RUNX3 in pDCs of patients with SSc. Low Runx3 level causes pDCs to overexpress co-stimulatory molecules, and mice with DC-specific deficiency of Runx3 are more susceptible to skin fibrosis. In Chapter 4, we have identified miR-618 to be upregulated in SSc pDCs, that caused downregulation of its target gene IRF8. miR-618 upregulation potentiates pDC to produce a higher IFN-I amount upon TLR9 stimulation. In the second part, this thesis reveals that the pDC-derived CXCL4 functions as a booster for both innate and adaptive immune responses, and is a crucial mediator of fibrosis in vivo. In Chapter 5, we show that CXCL4 is able to prime human monocytes-derived dendritic cells leading to their amplified response to TLR activation, and enhanced their activation of T cells. In Chapter 6, we have discovered CXCL4 ability to directly skew CD4 T cells activation, specifically towards Th17 phenotype in healthy individuals, and patients with psoriatic arthritis, and SSc (Chapter 8). In Chapter 7, we demonstrate CXCL4 to be a key molecule driving fibrosis, by promoting myofibroblast formation leading to ECM accumulation. In experimental fibrosis models, mice lacking CXCL4 are protected from skin, lung, and heart fibrosis. This thesis reveals key roles of plasmacytoid dendritic cells and CXCL4 in the pathogenesis of systemic sclerosis that can potentially be extrapolated to other autoimmune-rheumatic diseases. This thesis highlights the necessity of combining basic and translational immunological research to uncover new avenues of therapeutic intervention for patients suffering from these disorders.