Background Macrophages and their pro-inflammatory cytokines, including TNF, are pivotal mediators of chronic synovitis in rheumatoid arthritis (RA) as well as spondyloarthritis (SpA). Despite similar levels of synovial macrophage infiltration and similar clinical responses to TNF blockade in both diseases, SpA is characterized by a more pronounced infiltration with alternatively activated CD163+ macrophages and ongoing osteoproliferation.
Objectives This study aimed to investigate whether these differences were related to a differential expression and/or function of TNF between both diseases.
Methods Healthy donor peripheral blood-derived monocytes were polarized in vitro by IFN-γ, IL-4 or IL-10 for 4 days. The expression of transmembrane TNF (tmTNF) and soluble TNF (sTNF) was measured by FACS and ELISA, respectively. Expression of TNF and its receptors was also measured by qPCR and ELISA in synovial tissue biopsies (ST) and synovial fluid (SF) of actively inflamed knee joints of SpA and RA patients. Mice transgenically overexpressing tmTNF (TgA86) were evaluated clinically and histologically for spondylitis and peripheral arthritis.
Results In vitro polarization with IL-10 specifically induced the expression of CD163 on macrophages, mimicking the phenotype in SpA synovitis. The expression of tmTNF was not increased in IL-10 polarized macrophages in comparison with IFN-γ and IL-4 polarized cells. However, the production of sTNF was clearly impaired in the IL-10 polarized CD163+ macrophages (p<0.01 versus IFN-γ and IL-4 polarized macrophages), indicating a relative shift from sTNF to tmTNF by alternative macrophage activation. In line with these in vitro data, the sTNF SF levels were significantly lower in SpA compared to RA (p=0.01) despite similar TNF mRNA levels in ST. This was not related to altered expression of TNF receptors as both TNF-R1 and TNF-R2 were similarly expressed in ST, both at protein and mRNA levels. The mRNA levels of TACE, which is responsible for the cleavage of TNF, TNF receptors and other molecules from the cell membrane were also similar between SpA and RA ST. To investigate whether relative overexpression of tmTNF could be relevant in SpA pathophysiology, we characterized tmTNF transgenic mice. As previously described, these mice developed a moderate peripheral arthritis with 100% incidence, resulting in deformation of the paws and loss of grip strength. Histologically, the joints were characterized by moderate synovitis and appearance of lymphoid aggregates in the bone marrow, mostly in the absence of osteoclastic infiltration or extensive destruction. Besides arthritis, all transgenic animals spontaneously developed spondylitis as evidenced by a crinkled tail, a hunchback and histological inflammation in the connective tissue next to the intervertebral disc. Occasionally, this led to disappearance of the intervertebral disc and fusion of the vertebrae. None of the non-transgenic littermates developed signs of arthritis and/or spondylitis.
Conclusions tmTNF is relatively overexpressed by CD163+ alternatively polarized macrophages in SpA synovitis and leads to an axial and peripheral SpA phenotype in transgenic mice.
Disclosure of Interest None Declared