Response to 'Plasma proteins present in osteoarthritic synovial fluid can stimulate cytokine production via Toll-like receptor 4'

issue of Arthritis Research & Th erapy [1]. Using proteomic analysis, the authors were able to show that plasma proteins present in osteoarthritic synovial fl uid are able to induce infl am-matory cytokine production via inter action with Toll-like receptor 4, thus contributing to the low-grade infl am ma-tion associated with osteoarthritis. We would like to add some observations concerning our data on the immunomodulatory eff ect of plasma on infl ammation induced by calcium pyrophosphate crystals, which are considered danger signals to the innate immune system and, in osteoarthritis, provoke infl ammation inducing the production and the release of proinfl ammatory mediators [2]. When calcium crystals are used in vitro, cells need to be preactivated with phorbol myristate acetate or, less frequently, with lipopolysaccharide because pure crystals alone are unable to induce IL-1β. In vivo, various plasma proteins coating onto the crystal surface have been shown to enhance phagocytic recognition of crystals by cells [3], leading to activation of some infl ammatory pathways such as NALP-3 infl ammasomes [2]. In our experimental settings, using low doses of phorbol myristate acetate to prime a human monocytic cell line (THP-1) to crystals, we observed that healthy human plasma is able to enhance the infl ammatory response to crystals in a dose-dependent manner. Interestingly, the eff ect on the release of IL-1β and IL-8 was stronger when plasma was replaced by serum collected from the same blood sample (Figure 1). Hypothesizing the presence of some inhibitory components in plasma, we decided to stimulate cells by adding to the medium physiological plasmatic concentrations of fi brinogen, the clotting factor not contained in serum. While fi brinogen alone had no evident eff ect on IL-1β production, it was able to enhance the infl am-matory action of calcium pyrophosphate crystals in a dose-dependent manner. Th e eff ect was quite diff erent with regard to the release of IL-8, however, which was enhanced even in the absence of calcium pyrophosphate crystals (Figure 2). Fibrinogen has been shown to possess both infl am-matory and anti-infl ammatory properties depend ing on its concentration, and the eff ect is mediated by Toll-like receptor 4 [4]. In our case, fi brinogen showed a direct eff ect both on cells and on crystals, with an overall eff ect on the modulation of infl ammation. Our results prompt us to speculate that fi brinogen is able to stimulate/modulate the infl ammatory …