Background Tumor necrosis factor (TNF)-a, a potent pro-inflammatory cytokine, is generated in a precursor form called transmembrane (m)TNF-a that is expressed on the surface TNF-a producing cells. TNF-a was shown to act both as a ligand by binding to TNF-a receptors, as well as a receptor that transmits outside-to-inside (reverse) signals back into the mTNF-a bearing cells. Anti-TNF-a targeting molecules which neutralize TNF-a with the same efficiency show disease specific effectiveness.

Objectives The aim of the present study was to characterize mTNF-a signaling and to test whether known anti-TNF-a targeting molecules trigger mTNF-a signaling in human macrophages.

Methods TNF-a expression and localization was studied in mouse bone marrow derived macrophages by RT-QPCR or FACS analysis and confocal microscopy. mTNF-a signaling was triggered by coated anti-TNF-a antibodies and tested by a Phospho-MAPK array. The cytokine induced was determined by RT-QPCR analysis and ELISA. The effect of mTNF-a signaling on lipopolysaccharide (LPS)-induced pro-inflammatory cytokine formation was tested with a cytokine array.

Results Non-activated macrophages express basal levels of mTNF-a and respond to anti-TNF-a antibodies by triggering the mitogen-activated protein kinase kinase 4 signaling pathway leading transforming growth factor (TGF)-b production. Exposure to LPS further induced the expression of mTNF-a, and mTNF-a signaling suppressed the LPS-induced pro-inflammatory response via TGF-b. Since apoptotic cells downregulate their TNF receptors, they do not trigger mTNF-a for TGF-b production. Etanercept, a soluble TNF-a receptor, does not, while golimumab and infliximab, which are anti-TNF-a antibodies, do trigger TGF-b production in human macrophages.

Conclusions Induction of TGF-b production by mTNF-a or by receptors triggered by apoptotic cells could be considered in the therapy of inflammatory diseases and may, in part, also account for differences in the actions of various TNF-a inhibitors.

Disclosure of Interest None declared