The role of monocytes in chronic inflammatory diseases

Abstract

Monocytes and monocyte-derived cells are important players in the orchestration of inflammatory reactions in blood and peripheral tissues. However, little is known about the monocyte fate upon entry into human tissues, and current concepts are mainly based on animal models, with the addition of observational studies in humans that often do not allow determining causality. To provide additional understanding of the monocytes and the monocyte-derived cells in tissues, we developed three-dimensional (3D) co-culture models of epithelial tissues with monocytic cells implanted. These 3D tissue models in combination with clinical samples, including blood, saliva and tissue have been the platform for my thesis work on monocytes and monocytes-derived cells in chronic inflammatory diseases.

In paper I, we identified increased mRNA expression of MMP12, COX2, TNF and DCSIGN, genes associated with inflammation and tissue remodeling, in gingival tissue from individuals with Periodontitis (PD). The increased production of MMP12 was confirmed at a protein level, and flow cytometry analysis identified CD68+CD64+CD14+ monocyte-derived cells as responsible for increased MMP12 production in tissue. In addition, monocyte-derived cells from PD gingival tissue had a relatively low surface expression of the co-inhibitory molecule CD200R. Similarly, using a multicellular 3D model of oral mucosa with induced inflammation showed increased MMP12 production and reduced CD200R surface expression by monocyte-derived cells. We identified CSF2 as a potent inducer of MMP12, and that treatment of CSF2-stimulated monocyte-derived cells with a CD200 ligand reduced MMP12 production. Thus, this study identified CD200/CD200R as a potential pathway to modulate aberrant inflammatory reactions in order to reduce the subsequent immunopathology and induce resolution of chronic inflammation.

In a follow up study (paper II), a larger patient cohort (n=436) was investigated to assess the potential of MMP12, as well as the S100 proteins S100A8/A9 (calprotectin) and S100A12 as salivary biomarkers of PD. We found that MMP12 levels reflect destruction of periodontal structures, while the levels of the S100s reflect periodontal inflammation, and that smoking and age are important to take into consideration in future studies. The presence of other chronic inflammatory diseases did not influence MMP12 and S100 protein levels, however the presence of tumor was associated with an increase in the levels of MMP12 and S100A12.

Paper III was a methodological study, where we further developed the 3D lung tissue model to establish protocols for live imaging analysis of monocytes-derived cell migratory behavior in inflamed tissue. Inflammation was induced by TLR ligand stimulation at the apical side of the lung tissue models, and the level of inflammation was evaluated by flow cytometry, gene expression analysis as well as cytokine secretion. An immunofluorescence live-imagine technique was established to study the migration of the monocyte-derived dendritic cells (DC) in 4D (time, x, y, z) in inflamed lung tissue models.

In paper IV, we focused on IL-17A which is a cytokine associated with human chronic inflammatory diseases, and that has been linked to both PD and Langerhans cell histiocytosis (LCH). In LCH, DC-like cells have been described to produce IL-17A, and therefore, we investigated whether blood monocytes had IL-17A-producing capacity. These analyses led to the identification of IL-17A-producing monocytes in patients with LCH, particularly evident in patients with the highest disease activity. In contrast, IL-17A-producing monocytes could not be identified in patients with PD or healthy individuals.

In summary, these studies have contributed to the establishment of new tools to study human monocytes in a tissue milieu, and identified new disease-associated mechanisms and pathways that can be further explored to develop new immunomodulatory treatments for chronic inflammatory diseases.