Thursday, June 21, 2007

TNF-R family member 4-1BBL keeps TNF production “on”

The bacterial cell wall component LPS (lipopolysaccharide) binds to Toll-like receptor 4 (TLR4) on macrophages and stimulates them to produce the strong pro-inflammatory cytokine TNF (tumor necrosis factor). To identify potential signaling proteins, Kang and colleagues used the intracellular domain of TLR4 in a two-hybrid screen of binding proteins. They isolated 4-1BBL (CD137L), a transmembrane protein previously best known as a T cell costimulator, and the well-known TLR signaling protein MyD88. 4-1BBL binding was not abrogated by a P712H substitution in the TLR4 TIR (Toll-IL-R) motif that is required for MyD88 binding, indicating that they bind to different parts of TLR4.

LPS induced less TNF production at 24 h from 4-1BBL “knockout” (4-1BBL KO) macrophages than from wild-type (WT) macrophages, though equivalent amounts of IL-1beta were produced. Moreover, kinetic analysis demonstrated that TNF production was equivalent soon after stimulation but ceased after about 6 hours in 4-1BBL-KO macrophages while continuing in WT macrophages (Figure, panel b), accounting for the difference at 24 h. Although 4-1BBL was cloned as the ligand for 4-1BB, which is also expressed by macrophages, it does not contribute to this response (Figure, panel c).

LPS induces a gradual appearance of 4-1BBL on the cell surface, with marked accumulation after 2 hours, which accounts for the delayed influence on TNF production. Indeed, surface 4-1BBL alone at high levels of expression, or cross-linked at lower levels of expression, is sufficient induce TNF expression. Finally, in the ultimate test of relevance, they showed that 4-1BBL-KO mice survived a dose of LPS that killed all WT mice. Their discovery of 4-1BBL's role in sustained TNF production provides a new target for therapeutic intervention in the development of inflammatory diseases.

Young Jun Kang et al. Nat Immunol. 2007 Jun;8(6):601-9 "Cell surface 4-1BBL mediates sequential signaling pathways ‘downstream’ of TLR and is required for sustained TNF production in macrophages"