TLR4, but not TLR2, signals autoregulatory apoptosis of cultured microglia: A critical role of IFN-β as a decision maker

TLRs mediate diverse signaling after recognition of evolutionary conserved pathogen-associated molecular patterns such as LPS and lipopeptides. Both TLR2 and TLR4 are known to trigger a protective immune response as well as cellular apoptosis. In this study, we present evidence that TLR4, but not TLR2, mediates an autoregulatory apoptosis of activated microglia. Brain microglia underwent apoptosis upon stimulation with TLR4 ligand (LPS), but not TLR2 ligands (Pam₃Cys-Ser-Lys₄, peptidoglycan, and lipoteichoic acid). Based on studies using TLR2-deficient or TLR4 mutant mice and TLR dominant-negative mutants, we also demonstrated that TLR4, but not TLR2, is necessary for microglial apoptosis. The critical difference between TLR2 and TLR4 signalings in microglia was IFN regulatory factor-3 (IRF-3) activation, followed by IFN-β expression: while TLR4 agonist induced the activation of IRF-3/IFN-β pathway, TLR2 did not. Nevertheless, both TLR2 and TLR4 agonists strongly induced NF-κB activation and NO production in microglia. Neutralizing Ab against IFN-β attenuated TLR4-mediated microglial apoptosis. IFN-β alone, however, did not induce a significant cell death. Meanwhile, TLR2 activation induced microglial apoptosis with help of IFN-β, indicating that IFN-β production following IRF-3 activation determines the apoptogenic action of TLR signaling. TLR4-mediated microglial apoptosis was mediated by MyD88 and Toll/1L-IR domain-containing adaptor-inducing IFN-β, and was associated with caspase-11 and -3 activation rather than Fas-associated death domain protein/caspase-8 pathway. Taken together, TLR4 appears to signal a microglial apoptosis via autocrine/paracrine IFN-β production, which may act as an apoptotic sensitizer.