Inappropriate exposure to nucleic acids, which occurs during infection or tissue damage, stimulates the innate immune response both through transmembrane Toll-like receptors (TLRs) and through various cytosolic receptors. These different receptors sense different classes of nucleic acid. For instance, TLR3, TLR7, and TLR9 sense double-stranded RNA, single-stranded and short double-stranded RNA, and hypomethylated DNA, respectively. The cytosolic RIG-I–like receptors (RLRs) respond to both RNA and DNA, whereas cytosolic DNA-dependent activator of IRFs (DAI) and absent in melanoma 2 (AIM2) specifically respond to DNA. Yanai et al. identified high-mobility group box proteins (HMGBs, a group of proteins that regulate chromatin structure and transcription) 1, 2, and 3 as the proteins most prominently recovered in a biochemical screen for DNA recognition. In vitro assays revealed that the affinity of different types of nucleic acid for HMGBs correlated with their immunogenicity. Induction of the mRNA encoding type-I interferons and the proinflammatory cytokines interleukin-6 (IL-6) and RANTES in response to cytosolic nucleic acids was impaired in mouse embryo fibroblasts (MEFs) lacking HMGBs, although the responses to lipopolysaccharide (LPS) and cytokine stimulation were intact. Comparable defects in the innate immune response, including reduced secretion of IL-1β by macrophages (which depends on AIM2-dependent formation of the inflammasome), were found in various other cells lacking HMGBs. Furthermore, the ability of nucleic acids to elicit TLR-mediated responses in conventional and plasmacytoid dendritic cells and in a macrophage line depended on HMGBs, whereas that to LPS did not. The authors thus propose that HMGBs serve as “universal sentinels” necessary for the innate immune responses mediated by the more discriminative receptors that are activated by specific classes of nucleic acid.
H. Yanai, T. Ban, Z. Wang, M. K. Choi, T. Kawamura, H. Negishi, M. Nakasato, Y. Lu, S. Hangai, R. Koshiba, D. Savitsky, L. Ronfani, S. Akira, M. E. Bianchi, K. Honda, T. Tamura, T. Kodama, T. Taniguchi, HMGB proteins function as universal sentinels for nucleic-acid-mediated innate immune responses. Nature 462, 99–103 (2009). [PubMed]