Background/Purpose: 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. 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 membrane-bound TNF (mTNF) 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 fluid (SF) and synovial tissue biopsies (ST) of actively inflamed knee joints of SpA and RA patients. Mice transgenically overexpressing mTNF (TgA86) were evaluated clinically and histologically for spondylitis and peripheral arthritis.

Results: The expression of mTNF was increased in IL-10 and IL-4 polarized macrophages (alternatively activated macrophages) in comparison with IFN-γ polarized cells (classical activated macrophages). Moreover, the production of sTNF was clearly impaired in the IL-10 polarized CD163+ macrophages and IL-4 polarized macrophages (p<0.01 versus IFN-γ polarized macrophages), indicating a relative shift from sTNF to mTNF 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, the enzyme 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 mTNF could be relevant in SpA pathophysiology, we characterized mTNF 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. Interestingly, also bone remodeling was observed both in axial and peripheral joints. None of the non-transgenic littermates developed signs of arthritis and/or spondylitis.

Conclusion: mTNF is relatively overexpressed by CD163+ alternatively polarized macrophages in SpA synovitis and leads to an axial and peripheral SpA phenotype in transgenic mice, including osteoproliferation.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/relative-overexpression-of-membrane-bound-versus-soluble-tnf-in-human-and-experimental-spondyloarthritis/