Immunobiology

Dendritic cells (DC) are leucocytes that play a key role in the initiation and regulation of acquired immune responses (Steinman and Banchereau Nature 2007; 449:419-426). Therefore, DC manipulation offers great potential for cancer immunotherapy and for vaccination against cancer-promoting infectious agents (Tacken et al. Nat Rev Immunol 2007; 7:790- 802). In the resting state, DC have only a limited ability to prime naïve T cells. However, in response to specific disturbances, such as recognition of microbes or dead cells, DC can undergo a number of phenotypic and functional changes, collectively termed DC activation, that render them competent to promote adaptive immunity (Reis e Sousa, C. Nat Rev Immunol 2006; 6:476-483).

Dectins

Over the last year we have made significant progress in understanding the molecular pathways involved in DC activation by fungi. Innate recognition of fungal particles by myeloid cells, including DC, can lead to two distinct events, namely phagocytosis and production of pro-inflammatory mediators. Multiple receptors participate in particle phagocytosis, including Dectin-1, a C-type lectin receptor (CLR) that binds β-glucans present on fungal cell walls. In contrast, the induction of pro-inflammatory genes requires signalling to the nucleus and was first demonstrated for members of the toll-like receptor (TLR) family of pattern-recognition receptors (PRRs). A few years ago we reported that yeast-induced production of IL-2 and IL-10 by DC can occur independently of TLR signalling but requires the kinase Syk (Rogers et al. Immunity 2005; 22:507-517). We went on to show that Dectin-1, in addition to mediating yeast phagocytosis, can promote Syk-dependent DC activation (Rogers et al. Immunity 2005; 22:507- 517). Dectin-1 couples to Syk directly via a phospho-tyrosine within a 'hemITAM' motif in its tail (Rogers et al. Immunity 2005; 22:507-517). Activated Syk then promotes the activation of downstream pathways such as NF-κB (via the adaptor CARD9), MAPKs and NFAT, all of which coordinately induce pro-inflammatory gene expression (Reid et al. Curr Opin Immunol 2009; 21(1): 30-7). However, we also found that DC must express an additional Syk-coupled yeast receptor besides Dectin-1 because TNF, IL-10 and IL-2 responses to fungal particles are abrogated in Syk-deficient but only partially reduced in Dectin-1-deficient cells.

This year we reported that the second receptor is Dectin-2, a myeloid CLR that recognises high mannose structures present on fungal cell walls (Robinson et al. J Exp Med 2009; 206:2037-2051). Despite its name, Dectin-2 differs from Dectin-1 in that it does not possess a hemITAM or intracellular tyrosine residue. It does, however, associate with the Fc receptor gamma (FcRγ) chain, which contains a bona fide ITAM (Sato et al. J Biol Chem 2006; 281:38854-38866). In collaboration with Phil Taylor at Cardiff University, we showed that triggering of Dectin-2 by crosslinking with agonistic antibodies induces DC activation dependent on Syk, CARD9 and FcRγ chain (Robinson et al J Exp Med 2009; 206:2037-2051). Blocking antibodies against Dectin-2 can prevent Candida albicans-induced Syk-dependent responses in Dectin-1-deficient DC and we further found that Dectin-2, together with Dectin-1, account for the Th17 response to Candida albicans in infected mice (Robinson et al J Exp Med 2009; 206:2037-2051). Thus, Dectin-2 is another fungal PRR operating via Syk, coupling indirectly to the kinase by association with an ITAM-bearing signalling chain. The finding that Dectins selectively regulate Th17 responses to fungal infection indicates that CLR/Syk pathways can be a key point of control in the translation of innate into adaptive immunity. Given the recentlyestablished usefulness of Th17 cells for anti-tumour immunity (Martin-Orozco et al. Immunity 2009; 31: 787-798 ), agonists of Dectin-1 that act as selective adjuvants for Th17 induction may constitute useful tools for immunotherapeutic approaches against cancer.

DNGR-1

DNGR-1 is another CLR that is the subject of intense study by our laboratory. DNGR-1 resembles Dectin-1 and possesses a hemITAM motif that permits coupling to Syk kinase. However, DNGR-1 does not recognise fungal particles. Rather, it binds to a ligand(s) that is ubiquitously expressed inside all cells but hidden from DNGR-1 recognition (Sancho et al. Nature 2009; 458:899-903). Indeed, DNGR-1 binding to cells is only detectable when the integrity of the plasma membrane is experimentally compromised, for example, by chemical permeabilisation or by induction of cell death (Sancho et al. Nature 2009; 458:899-903). The role of DNGR-1 in dead cell recognition remains somewhat mysterious. Unlike Dectin-1 and -2, DNGR-1 is not expressed widely among myeloid cells but is restricted to DC, in particular a subset known as CD8α⁺ DC in the mouse and its putative equivalent in human. CD8α⁺ DC are particularly effective at 'crosspresenting' antigens from dead cells to CD8⁺T cells and DNGR-1- deficiency significantly impairs this process (Sancho et al. Nature 2009; 458:899-903). The ability of DNGR-1 to regulate crosspresentation to dead cell-associated antigens is dependent on the key tyrosine residue within the hemITAM motif that mediates association with Syk kinase (Sancho et al. Nature 2009; 458:899-903). Antigens associated with dying cells can induce immunological tolerance or immunity depending on experimental protocol. So far, we have tested cell death induction protocols that induce immunity when those cells are subsequently injected into mice (Figure 1). Notably, we found that CD8⁺T cell responses to antigens within those cells is partly dependent on DNGR-1 (Sancho et al. Nature 2009; 458:899-903). Thus, DNGR-1 signalling via Syk kinase regulates the immune response to dead cell associated antigens. Whether this reflects an activity of DNGR-1 in retrieving antigenic information from cell corpses or whether DNGR-1 acts by decoding the intrinsic adjuvanticity of dead cells has not yet been determined. The latter possibility would place DNGR-1 in the category of a 'Danger' receptor, defined as a DC activatory receptor that senses preformed cellular components that are sequestered in healthy cells but released upon cell damage and/or necrosis (Matzinger Annu Rev Immunol 1994; 12:991-1045). Such 'Danger' receptors may underlie the occurrence of spontaneous or chemotherapyinduced immunity to cancer and efforts are ongoing to establish the contribution of DNGR-1 to resistance from malignancy.

For a list of refereed research papers, see Publications (in navigation on left).