Abstract
Rheumatoid arthritis (RA) is a multifactorial autoimmune disease whose main hallmark is inflammation and destruction of the joints. Two cell types within the synovium that play an important role in RA are fibroblast-like synoviocytes (FLS) and macrophages. The latter innate immune cells show a high plasticity in their phenotype and are central in inflammatory processes. Low-dose radiotherapy (LD-RT) with particularly a single dose of 0.5 Gy has been demonstrated to have a positive impact on pain, inflammation, and bone in inflamed joints. We now examined for the first time how LD-RT influences FLS and bone marrow-derived macrophages in co-culture systems of an experimental model of RA to reveal further mechanisms of immune modulatory effects of low and intermediate dose of ionizing radiation. For this, the bone marrow of hTNF-α tg mice was differentiated either with cytokines to obtain key macrophage phenotypes (M0, M1, and M2) or with supernatants (SN) of untreated or irradiated FLS. Flow cytometry analyses were used to analyse the impact of radiation (0.1, 0.5, 1.0, and 2.0 Gy) on the phenotype of macrophages in the presence or absence of SN of FLS. LD-RT had no impact on cytokine-mediated macrophage polarization in M0, M1, or M2 macrophages. However, SN of irradiated FLS particularly reduced CD206 expression on macrophages. Macrophage phenotype was stable when being in contact with SN of nonirradiated FLS, but significantly increased surface expression of CD206 and slightly decreased CD80 and CD86 expression were observed when macrophage themselves were irradiated with 0.5 Gy under these microenvironmental conditions, again highlighting discontinuous dose dependencies in the low and intermediate dose range. One can conclude that FLS-dependent microenvironmental conditions have a slight influence on the modulation of macrophage phenotype under radiation exposure conditions. Future studies are needed to reveal the impact of radiation exposure on the functions of treated macrophages under such microenvironmental conditions.
Highlights
Rheumatoid arthritis (RA) is a multifactorial autoimmune disease that is associated with inflammatory infiltration of the joints leading to an advancing destruction of the bone and cartilage accompanied by chronic inflammation [1, 2]
As we have already observed that macrophages are very radioresistant [31, 39] and Low-dose radiotherapy (LD-RT) reduced the inflammatory phenotype of fibroblast-like synoviocytes (FLS) in an experimental mouse model of RA [28], we aimed to investigate for the first time the effects of LD-RT in a conditioned medium system of FLS and bone marrow-derived macrophages in an experimental model of RA on macrophage phenotype
HTNF-α tg bone marrow cells were stimulated with macrophage colony-stimulating factor (M-CSF) for 7 days and 24 h before analysing the macrophage phenotype; the cells were treated with three different cytokine cocktails: M-CSF only was continuatively added to the culture (M0 macrophages); a cocktail that consisted of IFN-γ, granulocyte macrophage colony-stimulating factor (GM-CSF), and LPS was used for generation of M1 macrophages; M-CSF and IL-4 were used for generation of M2 macrophages
Summary
Rheumatoid arthritis (RA) is a multifactorial autoimmune disease that is associated with inflammatory infiltration of the joints leading to an advancing destruction of the bone and cartilage accompanied by chronic inflammation [1, 2]. The exact cause of RA is still unknown, but the progression of the disease is in part mediated by infiltrating immune cells, fibroblast-like synoviocytes (FLS), and osteoclasts (OCs) While the latter are responsible for final bone destruction, FLS are considered to be the primary cell type involved in joint destruction as these cells contribute to disease initiation, progression, and maintenance of inflammation of the joints and secrete enzymes such as matrix metalloproteinases (MMPs) that digest the cartilage [4,5,6,7,8]. To FLS, various effector cells such as macrophages are found in the synovial membrane, and cytokines, such as macrophage colony-stimulating factor (M-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF), are present These factors further mediate the infiltration of the synovium and the maturation of immune cells, e.g., monocytes to macrophages
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