Toll-like receptor signaling outcome is determined by the stoichiometry of the endogenous TRIFosome

TL;DR

This study reveals how TRIF proteins form signaling platforms in immune cells, showing both new and existing mechanisms of immune response.

Contribution

The study identifies TRIF's ability to form filamentous oligomers and reveals a previously unknown mode of TLR signaling.

Findings

TRIF forms filamentous oligomers that associate with RIP1 and RIP3 kinases.

TLR3/4 activation triggers interferon signaling before TRIFosome formation.

TRIF is predominantly monomeric in unstimulated macrophages.

Abstract

Toll-like receptors (TLRs) drive innate immunity via assembly of macromolecular signal transduction platforms [supramolecular organizing centers (SMOCs)] coordinated by adaptor proteins such as Toll/interleukin-1 receptor (IL-1R) domain–containing adaptor-inducing interferon-β (TRIF), but whether oligomeric TRIFosomes form is unknown. Here, using cryo–electron microscopy and biophysical characterization of full-length TRIF in vitro, we show that it forms filamentous oligomers, which associate with the TRIF signaling partners receptor interacting protein 1 (RIP1) and RIP3 kinases, suggesting that oligomeric TRIFosomes could form. Endogenous TRIF, however, is predominantly monomeric in the absence of ligand, only forming TRIFosome oligomers in macrophages after stimulation of TLR4 or TLR3 when large, macromolecular signaling complexes form. TRIFosomes are fully formed 45 min after TLR3 or 60 min after TLR4 stimulation, commensurate with activation of nuclear factor κB in these cells. TLR3/4 activation triggers rapid interferon signaling prior to TRIFosome formation through monomeric TRIF, unexpectedly suggesting that a macromolecular platform of TRIF is not required to drive this signaling pathway. Collectively, these data show TRIFosome macromolecular platform formation and, unexpectedly, that TLR signaling can be SMOC-independent in addition to being SMOC-dependent.

A previously entirely unknown mode of TLR signal transduction is elucidated.

Figures