Outside-to-Inside Signaling Through Transmembrane Tumor Necrosis Factor Reverses Pathologic Interleukin-1β Production and Deficient Apoptosis of Rheumatoid Arthritis Monocytes

Objective. Monocytes are a major source of proinflammatory cytokines in rheumatoid arthritis (RA), and inhibitors of monocytic cytokines are highly efficient agents for treatment of the disease. The aim of this study was to analyze the effects of a therapeutic antitumor necrosis factor a (anti-TNF alpha) antibody on monocytes from patients with RA and healthy control subjects. Methods. Peripheral blood monocytes from patients with RA and healthy control subjects were incubated in the presence of anti-TNF alpha antibody or IgG. Annexin V staining, caspase activation, poly(ADP-ribose) polymerase cleavage, and DNA staining with propidium iodide were used to analyze apoptosis. The signaling events elicited in monocytes by infliximab were analyzed by Western blotting and electromobility shift assay. Results. Peripheral blood monocytes from patients with RA were characterized by increased expression of transmembrane TNF alpha, spontaneous in vitro production of interleukin-1 beta (IL-1 beta), and a decreased rate of spontaneous ex vivo apoptosis. Incubation with infliximab induced significantly increased apoptosis in monocytes from patients with RA but not in monocytes from healthy control subjects. This apoptosis was triggered by reverse signaling of transmembrane TNF after ligation by infliximab and was independent of caspase activation. Instead, transmembrane TNF reverse signaling inhibited the constitutive NF-kappa Beta activation in RA monocytes, suppressed IL-1 beta secretion, and normalized spontaneous in vitro apoptosis. This normalization was reversible by the addition of exogenous IL-1 beta. Conclusion. This study demonstrates that outside-to-inside signaling through transmembrane TNF after ligation by infliximab inhibits constitutive NF-kappa B activation and suppresses spontaneous IL-1 beta production by monocytes from patients with RA. Besides the induction of monocyte apoptosis, this inhibition could also contribute to the therapeutic effects observed during treatment with TNF alpha inhibitors.