Background Systemic sclerosis (SSc) is an inflammatory and fibrotic disease characterized by vascular dysfunction, excessive extracellular matrix deposition and immune dysregulation. Recent observations suggest that monocytes and dendritic cells (DCs) might be involved in SSc; including cell recruitment, trafficking, activation and an enhanced pro-fibrotic phenotype. Hence these cells might be important contributors to the disease pathogenesis. However, detailed analysis of circulating monocytes and DCs in SSc in relationship to disease activity has not been performed so far.
Objectives To investigate the ex vivo pro-inflammatory properties of classical and non-classical monocytes as well as myeloid dendritic cells (mDCs) in SSc patients in relationship to disease activity.
Methods This study enrolled 43 SSc patients, 30 classified as limited cutaneous SSc (lcSSc) and 13 as diffuse cutaneous (dcSSc). The healthy control group (HC) included 20 age- and gender- matched individuals. The Spontaneous production of CXCL10, CCL4, CXCL4 and IL-6 was intracellularly evaluated in classical and non-classical monocytes and Siglec-3-expressing mDCs from peripheral blood using flow cytometry. In addition, the production of these cytokines was determined upon toll like receptor 4 (TLR4) plus Interferon-γ (IFN-γ) in vitro stimulation.
Results The frequency of non-classical monocytes spontaneously producing CXCL10 was increased in both lcSSc and dcSSC subsets of SSc patients (p<0.05) and CCL4 was augmented in the dcSSc patient subset (p<0.05). The proportion of CCL4 producing- mDCs were also elevated in dcSSc patients (p<0.01) and the percentage of mDCS producing CXCL10 only in lcSSc patients (p<0.05 compared to HC, but p<0.01 comparing to dcSSc). Upon in vitro stimulation the frequency of non-classical monocytes expressing CXCL8 was increased in both patient groups (p<0.01) and mDCs expressing CXCL8 only in lcSSc (p<0.01). SSc patients characterized by the presence or history of lung fibrosis, displayed a higher frequency of non-classical monocytes expressing CCL4 and CXCL10 in dcSSc patients as compared to those without this clinical manifestation (p<0.01 and p<0.05 respectively). Strikingly, the percentage of classical monocytes producing CXCL8 was augmented upon in vitro stimulation in lcSSc patients with lung fibrosis as compared to those without (p<0.01). No differences were found in the percentage of IL-6 producing cells.
Conclusions These data point towards a role of activated non-classical monocytes and mDCs producing enhanced levels of proinflammatory cytokines in SSc, potentially contributing to lung fibrosis.
Acknowledgements TC is supported by a grant from the Portuguese national funding agency for science, research and technology: Fundação para a Ciência e a Tecnologia [SFRH/BD/93526/2013].
Disclosure of Interest None declared