Background and objectives During chronic inflammation, immune cells, notably Th17 cells, infiltrate the inflammatory site and interact with local mesenchymal cells. Applied to rheumatoid arthritis (RA) and extended to psoriasis (Pso), the aim is to study the interactions between mesenchymal cells (MC) from different origins and peripheral blood mononuclear cells (PBMC) with a focus on the Th17 pathway and to identify a mechanism which leads to high IL-17 secretion. In this way, our interest is focussed on podoplanin, a transmembrane protein which is increased in inflammatory sites and known to modulate IL-8 secretion during synoviocyte-platelet interaction.
Materials and methods A co-culture system with MC (RA synoviocytes or skin fibroblasts) and PBMC (healthy or autologous) was developed. MC were cultured overnight and PBMC were seeded at a 5:1 ratio for 48h, in the presence or not of TCR activation with PHA. Transwell system was used to study cell-cell contact. An antibody against podoplanin was pre-incubated 4h with PBMC before co-culture. Supernatants were collected at 48h and IL-6, IL-1β and IL-17 production measured by ELISA. Extracellular (CD3, CD4) and intracellular (IL-17) staining of PBMC was analysed by flow cytometry at 48h.
Results In control conditions, IL-8, IL-6 and IL-1β production was increased in PBMC-MC co-culture compared to PBMC alone (p ≤ 0.05). No additional effect was observed with PBMC activation. Flow cytometry showed no difference in the percentage of Th17 cells in activated-PBMC alone or co-cultured with MC (p = 0.4). Conversely, IL-17 production was highly increased only in co-cultures with activated-PBMC (p ≤ 0.02). Transwell experiments confirm that cell-cell contact was critical for IL-17 secretion. Addition of an anti-podoplanin antibody decreased significantly IL-17 secretion by 60%. Furthermore, all results were confirmed in autologous system.
Conclusions The interactions between resting PBMC and MC alone is sufficient to induce pro-inflammatory cytokine production (IL-8, IL-6 and IL-1β). Both PBMC activation and cell interactions with MC are needed to induce a high IL-17 secretion. Podoplanin contributes largely to the massive IL-17 secretion and given its involvement in different diseases, it could be a potential therapeutic target to block Th17 cell activity during chronic inflammation.