Signaling of apoptosis through TLRs critically involves toll/IL-1 receptor domain-containing adapter inducing IFN-beta, but not MyD88, in bacteria-infected murine macrophages

Abstract

TLRs are important sensors of the innate immune system that serve to identify conserved microbial components to mount a protective immune response. They furthermore control the survival of the challenged cell by governing the induction of pro- and antiapoptotic signaling pathways. Pathogenic Yersinia spp. uncouple the balance of life and death signals in infected macrophages, which compels the macrophage to undergo apoptosis. The initiation of apoptosis by Yersinia infection specifically involves TLR4 signaling, although Yersinia can activate TLR2 and TLR4. In this study we characterized the roles of downstream TLR adapter proteins in the induction of TLR-responsive apoptosis. Experiments using murine macrophages defective for MyD88 or Toll/IL-1R domain-containing adapter inducing IFN-beta (TRIF) revealed that deficiency of TRIF, but not of MyD88, provides protection against Yersinia-mediated cell death. Similarly, apoptosis provoked by treatment of macrophages with the TLR4 agonist LPS in the presence of a proteasome inhibitor was inhibited in TRIF-defective, but not in MyD88-negative, cells. The transfection of macrophages with TRIF furthermore potently promoted macrophage apoptosis, a process that involved activation of a Fas-associated death domain- and caspase-8-dependent apoptotic pathway. These data indicate a crucial function of TRIF as proapoptotic signal transducer in bacteria-infected murine macrophages, an activity that is not prominent for MyD88. The ability to elicit TRIF-dependent apoptosis was not restricted to TLR4 activation, but was also demonstrated for TLR3 agonists. Together, these results argue for a specific proapoptotic activity of TRIF as part of the host innate immune response to bacterial or viral infection.