How macrophages ring the inflammation alarm.

Inflammation is the immediate response of our tissues to clear and present danger: microbial invasion, injury, or a serious malfunction of our body’s internal workings. Whereas all cells can respond to danger, innate immunity cells have that as their mission: They deploy a vast array of receptors to detect telltale molecules from pathogens or ailing cells and secrete an equally vast number of soluble proteins to communicate among themselves and with the resident cells of the tissue. In PNAS, Lu et al. report on howmyeloid cells use a key word in this cellular conversation: high-mobility group box 1 (HMGB1) (1). HMGB1 is a component of eukaryotic chromatin, and within the nucleus, it bends DNA and facilitates its interaction with transcription factors and othermolecules, including histones, to form nucleosomes (2). However, HMGB1 is also the archetypal damage-associated molecular pattern (DAMP): molecules that, when released from a cell’s interior following its untimely death, alert immune cells to danger (3). Notably, cells that die by apoptosis (and therefore kill themselves presumably after careful consideration) do not release HMGB1 to activate the immune system (4), but rather expose “eat-me” signals to request swift disposal. Interestingly, the meaning of extracellular HMGB1 goes beyond a simple “I’m dead”: Most cells can secrete HMGB1 before they die and therefore can cry out for help before it is too late for them. In fact, cells can secrete most of the HMGB1 they contain without necessarily being committed to die and can resume their normal occupations (and replenish HMGB1 by resynthesis) once the danger is over. Examples are HMGB1 secretion by cardiomyocytes in hypoxic conditions following ischemia (5) or by neural cells following the depolarization of their plasma membrane (6). Myeloid cells secrete HMGB1 once they are activated by the presence of pathogens, by the detection of injured cells, or by cytokines, in order to rebroadcast the danger message. Extracellular HMGB1 plays a central role in inflammation, both acute and chronic, and indeed in sepsis when innate immunity goes awry (7). HMGB1 both recruits and activates myeloid cells; however, these are separable processes, which depend on different redox states of HMGB1 and on different receptors. HMGB1 contains three cysteines (at positions 23, 45, and 106): When all are in the reduced (thiol) state, HMGB1 forms a heterocomplex with the chemokine stromal cell-derived factor (SDF-1/CXCL12) and binds to the CXCR4 receptor, acting as a potent chemoattractant for all motile cells, including innate immunity cells (8, 9). When cysteines 23 and 45 form a disulfide bond and the cysteine 106 is in the thiol state, HMGB1 binds to the TLR4 receptor, activating NF-κB to drive the transcription of genes involved in inflammation, including chemokine and cytokine genes; when one of the cysteines is terminally oxidized to a sulfonate, HMGB1 loses activity both as a chemoattractant and as an inflammatory ligand (9, 10). HMGB1 also binds to other surface molecules, including receptor for advanced glycation end products (RAGE), thrombomodulin, CD24, and TIM-3; RAGE can cooperate both with HMGB1’s chemoattractant and inflammatory activities, whereas the other molecules limit HMGB’s effects (7). Given HMGB1’s key role in inflammation, understanding how it secreted is of central importance. HMGB1 secretion is unconventional, as may be expected for a nuclear protein that has no leader peptide to direct it to the endoplasmic reticulum and the exocytosis route. Rather, HMGB1 is endowed with nuclear localization and nuclear export signals IFN