MyD88 Contributes to TLR3-Mediated NF-κB Activation and Cytokine Production in Macrophages

Toll-like receptor 3 (TLR3) initiates antiviral and inflammatory responses exclusively through the adaptor protein TRIF (TIR-domain-containing adapter-inducing interferon-β). In contrast, MyD88 (myeloid differentiation primary response 88), a central adaptor for most other TLRs, is traditionally considered dispensable for TLR3 signaling. Here, we demonstrate that MyD88 directly contributes to TLR3-mediated NF-κB activation and cytokine production in macrophages. Bone marrow-derived macrophages (BMDMs) from MyD88 deficient mice exhibited significantly attenuated NF-κB activation in response to the TLR3 agonist polyinosinic-polycytidylic acid (poly(I:C)) compared to wild-type cells, as evidenced by the reduced phosphorylation of NF-κB p65 and IκBα, as well as IκBα degradation. Consistently, pro-inflammatory cytokine production, including IL-6, TNF-α, and IFN-β, was attenuated in MyD88-deficient BMDMs in vitro following stimulation by poly(I:C) or poly(A:U), another TLR3 agonist. Blood concentrations of IL-6, TNF-α, and IFN-β were significantly reduced in both TRIF-deficient mice and MyD88-deficient mice challenged by the i.p. injection of poly(I:C). Mechanistic analyses revealed that MyD88 physically associates with activated TLR3 upon poly(I:C) stimulation, and that TLR3 engagement triggered MyD88 oligomerization, which was absent in TLR3 or TRIF deficient macrophages. Our findings highlight a previously unrecognized dual-adaptor mechanism for TLR3, wherein MyD88 recruitment amplifies NF-κB signaling dynamics by bridging TLR3 to the canonical NF-κB activation cascade and robust cytokine induction. This study expands the paradigm of TLR3 signaling by establishing MyD88 as a direct contributor to TLR3-driven innate immune responses, offering new insight into cross-talk between MyD88-dependent and -independent pathways.