Immune Cell Activation by Bacterial Cpg-DNA through Myeloid Differentiation Marker 88 and Tumor Necrosis Factor Receptor–Associated Factor (Traf)6

Transition of immature antigen presenting cells (APCs) to the state of professional APCs is essential for initiation of cell-mediated immune responses to pathogens. Signal transduction via molecules of the Toll-like receptor (TLR)/interleukin 1 receptor (IL-1R) pathway is critical for activation of APCs either by pathogen-derived pattern ligands like lipopolysaccharides (LPS) or by CD40 ligation through T helper cells. The capacity of bacterial DNA (CpG-DNA) to induce APCs to differentiate into professional APCs represents an interesting discovery. However, the signaling pathways involved are poorly understood. Here we show that CpG-DNA activates the TLR/IL-1R signaling pathway via the molecules myeloid differentiation marker 88 (MyD88) and tumor necrosis factor receptor–associated factor 6 (TRAF6), leading to activation of kinases of the IκB kinase complex and the c-jun NH₂-terminal kinases. Moreover, cells of TLR2- and TLR4-deficient mice are activated by CpG-DNA, whereas cells of MyD88-deficient mice do not respond. The data suggest that CpG-DNA initiates signaling via the TLR/IL-1R pathway in APCs in a manner similar to LPS and to T helper cell–mediated CD40 ligation. Activation of the TLR/IL-1R signaling pathway by foreign bacterial DNA may be one way to initiate innate defense mechanisms against infectious pathogens in vivo.