Recognition of aberrant cell death is a crucial function of the immune system. It seems that one way in which immune cells identify damage is by sensing actin, an abundant intracellular protein.
The immune system evolved to detect not only invading microorganisms but also non-infectious damage or alterations in the body's cells, and to mobilize reparative as well as defensive responses1. Aberrant-cell death is one form of injury that is recognized as 'dangerous' by the immune system2, but how this death is sensed, and how it activates subsequent immune responses, is incompletely understood. Writing in Immunity, Ahrens et al.3 and Zhang et al.4 provide a key piece of this puzzle by demonstrating that the immune system can detect actin, a major protein component of the cytoskeleton, the cellular scaffold.
Immune cells detect both infection and damage by using pattern-recognition receptors (PRRs), which sense molecular signatures termed pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), respectively1,2. Such motifs are found, for example, in the cell walls of bacteria or in viral nucleic acids. Many PRRs, including members of the Toll-like receptor family, can recognize both DAMPs and PAMPs, and can trigger intracellular signalling pathways that culminate in inflammatory responses. More recently, another family of PRRs, the C-type lectin receptors (CLRs), has also been implicated in sensing DAMPs5. This family includes a receptor called DNGR-1(otherwise known as CLEC9A). However, unlike other PRRs, which are expressed on many immune cells, DNGR-1 expression is limited to a small subset of a type of immune cell called dendritic cells.
Dendritic cells have pivotal roles in initiating the development of the adaptive arm of the immune response, in which T cells mount responses against specific antigens that are recognized as being foreign or dangerous. There are many subsets of dendritic cells and those that express DNGR-1, which include CD8α+ dendritic cells in mice and BDCA3+ dendritic cells in humans6,7,8, are of particular interest as they can drive T-cell responses to foreign antigens through a process called cross-presentation. Cross-presentation activates CD8+ T cells, also called cytotoxic T cells, which are crucial for protection against tumours and viral infections.
Previous research revealed6,8 that DNGR-1 can directly facilitate antigen cross-presentation and can mediate protective antitumour responses in mouse models of cancer. Shortly thereafter, it was discovered9 that DNGR-1 recognizes damaged and dead (necrotic) cells and can cross-present their associated antigens. In common with other CLRs, DNGR-1 contains an intracellular domain that triggers a signalling pathway mediated by a kinase protein called Syk7,9, and both this domain and Syk are essential for DNGR-1-mediated cross-presentation9. Surprisingly, however, DNGR-1 and Syk were found9 not to be involved in the capture and engulfment of necrotic cells by the dendritic cell, implying that other cellular receptors are involved in this process.
Although it had been shown9 that DNGR-1 recognizes an intracellular DAMP that is exposed only after cell damage, it was unclear exactly which structure, or ligand, DNGR-1 binds to. Ahrens et al.3 and Zhang et al.4 managed to identify this ligand by a nifty piece of detective work. Both groups demonstrated that the ligand can be found in cells from many species, including those of insects, suggesting that it has been conserved over a long evolutionary period. However, attempts to detect the ligand using conventional biochemical approaches proved difficult and routinely yielded cytoskeletal components, which are normally considered to be contaminants in such procedures. It was only when the researchers used microscopy to examine cells that had been stained with a soluble fluorescent probe of DNGR-1, and saw fluorescence in the same position as the actin cytoskeleton, that they realized that the ligand was likely to be a component of this structure. Indeed, both groups subsequently demonstrated that DNGR-1 recognizes filamentous actin, or F-actin — the structural form of actin found in cells — but not its monomeric form, G-actin (Fig. 1).
Actin is a highly conserved globular protein found in all cells of eukaryotes — organisms that include animals, plants and fungi. It associates with numerous actin-binding proteins and serves multiple functions ranging from maintaining cell shape to facilitating cell division. In fact, both groups3,4 found that some of these actin-binding proteins, including α-actinin and spectrin, enhance DNGR-1 recognition of F-actin, presumably owing to stabilization and/or formation of the actin-filament complexes. How DNGR-1 recognizes F-actin is unclear, although Zhang et al.4 did identify a ligand-binding site on the receptor.
These discoveries have profound implications for our understanding of the mechanisms of dead-cell recognition. Arguably the most exciting insight is that the recognition of exposed F-actin in necrotic cells by DNGR-1 can directly induce antigen cross-presentation. As an abundant protein that is exposed only upon cell damage, F-actin makes an ideal DAMP. Furthermore, because actin release is known to correlate with tissue damage, it is likely that such a ubiquitous protein is also recognized by other PRRs. The ability of F-actin to drive cross-presentation of dead-cell antigens suggests that DNGR-1 could be involved in the development of certain autoimmune diseases and in immune responses to intracellular infections that cause cell damage. Indeed, DNGR-1 was recently shown10,11 to be essential for mounting protective CD8+ T-cell responses during cell-damaging viral infections.
These studies will also enhance our understanding of the functions of the Syk signalling pathway used by CLRs. This pathway normally activates dendritic cells, triggering them to mature and to produce inflammatory mediators, as well as inducing their ability to activate the adaptive immune system. Unexpectedly, DNGR-1 does not use the signalling pathway in this way, and is not involved in driving inflammatory responses to necrotic cells3,10. Rather, the DNGR-1 receptor seems to be responsible for diverting the necrotic-cell cargo ingested by dendritic cells to an intracellular pathway that favours cross-presentation10. Curiously, Syk-mediated signalling initiated by DNGR-1 is not required for this diversion, and yet it is essential for inducing cross-presentation10 (Fig. 1). How these cellular processes fit together remains a mystery, but the identification of F-actin as the DNGR-1 ligand should help us to decipher these essential mechanisms.
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Immunological Reviews (2012)