HMGB proteins bind to all immunogenic nucleic acids and have recently been identified to serve as universal sentinels for nucleic-acid-mediated innate immune responses. 24 Activation of macrophages or DCs with microbial cytosinephosphate-guanosine (CpG)-DNA, resulting in secretion of proinflammatory cytokines, involves TLR9 initially localized in the endoplasmic reticulum (ER). The mechanism of TLR activation by CpG-DNA was recently discovered and HMGB1 was found to have a crucial role. HMGB1 binds to CpG-DNA and receptor for advanced glycated endproducts (RAGE), the first identified receptor for HMGB1, and this DNA-protein complex preassociates with TLR9 in the ER-Golgi intermediate compartment, thus accelerating the delivery of microbial DNA to TLR9 and leading to the recruitment of the TLR adaptor molecule MyD88. 25 Thus, HMGB1 and RAGE are pivotal for TLR9-dependent induction of genes encoding type I interferon after stimulation of DCs with DNA-containing complexes. Ablation or depletion of HMGB1 impairs redistribution of TLR9 to early endosomes in response to CpG-oligodeoxynucleotides (ODN), leading to a decreased response to CpG-ODN, but which could be complemented by extracellular HMGB1. 25 In addition, HMGB1 was found to be a key factor in immune complex-triggered activation of autoreactive B cells and the induction of type I interferon by plasmacytoid DCs, thus possibly contributing to SLE. 25 HMGB1, a cytokine that initiates host defense. HMGB1 is a nuclear protein and, to function as a cytokine, it must be released in the extracellular milieu. This occurs either via passive release from necrotic cells, 26, 27 or by active secretion by cells of the innate immune system. Wang et al. 9 first reported that HMGB1 was liberated from macrophages stimulated with LPS, and that HMGB1 had an important role in experimental sepsis. HMGB1 secretion from LPS-primed macrophages was shown to require the inflammasome components apoptotic speck protein containing a caspase recruitment domain (ASC), caspase 1 and NALP3. 28 Monocytes, macrophages and immature DCs (mDCs) secrete HMGB1 in response to LPS, TNF-a, or IL-1b stimulation. 29 IFN-g can induce HMGB1 release from macrophages that, at least in part, requires induction and signaling through TNF-a. 30 Secretion of a nuclear protein requires a tightly controlled relocation program. Several forms of post-translational modifications, such as acetylation, phosphorylation and oxidation result in the accumulation of HMGB1 in the cytosol. [31] [32] [33] [34] Upon activation with LPS, monocytes and macrophages acetylate HMGB1 extensively, allowing its relocalization from the nucleus to the cytosol, and further concentration into secretory lysosomes. 35 Thus, because it does not contain a leader sequence, HMGB1 is secreted via a non-classical vesiclemediated secretory pathway.