Coxiella burnetii type IVB secretion system modulates TLR3/TRIF-dependent NF-κB and IRF responses during infection.

UNLABELLED: Coxiella burnetii (Cb), the causative agent of Q fever, replicates within host macrophages by modulating innate immune responses through its type IVB secretion system (T4SS). Host cells sense pathogens via Pattern recognition receptors (PRRs), including Toll-like receptors (TLRs), to recognize pathogen-associated molecular patterns (PAMPs) and initiate signaling pathways that drive pro-inflammatory cytokine and interferon responses. Toll-like receptor 3 (TLR3) triggers the nuclear translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and Interferon regulatory factors (IRFs) resulting in pro-inflammatory cytokine production and type I interferon (IFN-I) induction. Here we demonstrate that Cb requires T4SS to suppress TLR3-induced NF-κB and IRF transcriptional responses. Furthermore, RNA purified from both virulent and avirulent Cb is sufficient to activate TLR3, identifying pathogen RNA as a relevant PAMP in this context. Using a pulmonary infection model with virulent Cb, we found that TLR3/ Toll-Interleukin-1 Receptor Domain-Containing Adapter Protein Inducing Interferon Beta (TRIF)-dependent innate immune pathway inhibits cachexia. In contrast, signaling through the interferon-α/β receptor (IFNAR) restricts bacterial dissemination, indicating that these pathways play distinct but complementary roles in host defense. Mechanistically, Cb suppresses TLR3/TRIF signaling in a T4SS-dependent manner by preventing host TLR3 recruitment to the Coxiella-containing vacuole (CCV) and disrupting TRIF-TNF Receptor-Associated Factor 6 (TRAF6) interactions, thereby selectively inhibiting impairing NF-κB activation. Consistent with this, we identify five T4SS effector proteins that attenuate TLR3-induced NF-κB signaling, including CBU1292, which suppresses both NF-κB and IRF-dependent responses downstream of TLR3.

AUTHOR SUMMARY: Pathogenic bacteria have evolved mechanisms to evade or manipulate host immune defenses. Coxiella burnetii (Cb), the causative agent of Q fever, replicates within immune cells and interferes with innate immune signaling pathways. We found that its type IVB secretion system (T4SS) express effectors that inhibit immune responses triggered by Toll-like receptor 3 (TLR3), a sensor that detects microbial RNA. Specifically, Cb suppresses signaling through the adaptor protein TRIF, thereby reducing activation of key immune regulators, NF-κB and interferon regulatory factors (IRFs), which are required for inflammatory and antiviral responses. Using a mouse model of lung infection, we found that disruption of TLR3/TRIF signaling and type I interferon receptor (IFNAR) signaling leads to different disease outcomes, indicating that these pathways play distinct roles in controlling infection and disease severity. We also identified several Cb-secreted effectors that interfere with TLR3-mediated immune activation, including one that inhibits both NF-κB and interferon responses. Together, our findings reveal how Cb evades immune detection by targeting a critical RNA-sensing pathway and highlight distinct host defense mechanisms that limit bacterial infection and disease.