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 characterised by a more pronounced infiltration with alternatively activated CD163+ macrophages and ongoing osteoproliferation. This study aimed to investigate whether these differences were related to a differential expression and/or function of TNF between both diseases.
Methods Expression of transmembrane TNF (tmTNF) and soluble TNF (sTNF) was measured in IFN-γ, IL-4 or IL-10 polarised macrophages obtained from healthy donors. Expression of TNF and its receptors was measured in synovial fluid (SF) and synovial tissue biopsies (ST) of actively inflamed knee joints of SpA and RA patients. Mice transgenically overexpressing tmTNF (TgA86) were evaluated for spondylitis and arthritis.
Results In vitro polarisation with IL-10 specifically induced the expression of CD163 on macrophages, mimicking the phenotype in SpA synovitis. IL-10 and IL-4 polarised macrophages secreted less TNF than classically, IFN-γ-polarised macrophages (p < 0.05). In contrast, IL-4 polarised macrophages expressed more tmTNF compared to the IFN-γ polarised macrophages (p < 0.05), with a similar trend for IL-10 polarised macrophages. 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 higher expression of TNF receptors as both TNFR1 and TNFR2 were similarly expressed in ST, both at protein and mRNA levels. On the contrary, the SF levels of sTNFR1 and sTNFR2, which are both cleaved from the cell membrane by the same enzyme as tmTNF, were even decreased in SpA versus RA. Investigating the potential pathophysiological role of relative overexpression of tmTNF versus sTNF in vivo, clinical analysis revealed that tmTNF transgenic mice developed arthritis, resulting in deformation and loss of grip strength, and spondylitis as evidenced by crinkled tails with a 100% incidence. Axial and peripheral joint inflammation was confirmed by histology. In contrast to mouse strains overexpressing sTNF, tmTNF tg mice did not develop systemic disease and weight loss and showed clear signs of osteoproliferation on histology.
Conclusions tmTNF is relatively overexpressed by CD163+ alternatively polarised macrophages in SpA synovitis and leads to an axial and peripheral SpA phenotype in transgenic mice.