Intracellular Bacterial Infection-Induced IFN-γ Is Critically but Not Solely Dependent on Toll-Like Receptor 4-Myeloid Differentiation Factor 88-IFN-αβ-STAT1 Signaling

Abstract

Infection of murine bone marrow-derived macrophages (BMMφ) with Chlamydia pneumoniae induces IFN-αβ-dependent IFN-γ secretion that leads to control of the intracellular bacterial growth. Enhanced growth of C. pneumoniae in Toll-like receptor (TLR) 4−/− and myeloid differentiation factor (MyD) 88−/− (but not TLR2−/−, TLR6−/−, or TLR9−/−) BMMφ is shown in this study. Reduced accumulation of IFN-α and IFN-γ mRNA was also observed in TLR4−/−- and MyD88−/−-infected cells. IL-1R and IL-18R signaling did not account for differences between MyD88−/− and wild-type BMMφ. Surprisingly, infection-induced NF-κB activation as well as TNF-α, IL-1, or IL-6 mRNA expression were all normal in TLR4−/− and MyD88−/− cells. Phosphorylation of the transcription factor STAT1 during bacterial infection is IFN-αβ dependent, and necessary for increased IFN-γ mRNA accumulation and chlamydial growth control. Signaling through common cytokine receptor γ-chain and RNA-dependent protein kinase both mediated IFN-αβ-dependent enhancement of IFN-γ mRNA levels. Accumulation of IFN-γ mRNA and control of C. pneumoniae growth required NF-κB activation. Such NF-κB activation was independent of IFN-αβ, STAT1, and RNA-dependent protein kinase. In summary, C. pneumoniae-induced IFN-γ expression in BMMφ is controlled by a TLR4-MyD88-IFN-αβ-STAT1-dependent pathway, as well as by a TLR4-independent pathway leading to NF-κB activation.