Dectin-1 : receptor internalisation, trafficking and biological effects in macrophages
In host defence, pattern recognition plays an essential role by enabling the immune system to discriminate self from pathogenic non-self. Pattern recognition is mediated by leukocyte expressed pattern recognition molecules (PRMs), which recognise pathogen associated molecular patterns (PAMPs) on pathogens. Phagocytosis is a critical event for anti-microbial defence and its contribution is not limited to the clearance and killing of pathogens, but extends to the activation of adaptive immunity through production of pro-inflammatory mediators and antigen presentation. Anti- fungal immunity is extremely efficient and operates via recognition, phagocytosis and killing of fungal pathogens by leukocytes. We have examined Dectin-1, a non-opsonic pattern recognition receptor that recognises live fungi and fungal derived particles and that is highly expressed on various leukocyte populations. We wanted to establish whether Dectin-1 contributes to anti-fungal defence by analysing various aspects of the receptor biology. Using both confocal microscopy and flow-cytometry, we demonstrate that Dectin-1 is a phagocytic receptor. Furthermore, using cell lines expressing receptor mutants, we show that this capacity is mediated by the membrane proximal tyrosine residue located in the ITAM-like motif. This makes Dectin-1 the first described phagocytic leukocyte expressed receptor for unopsonised fungi and fungal derived particles, and the first pattern recognition receptor that mediates phagocytic uptake through a tyrosine based motif. We demonstrate that the mechanisms by which Dectin-1 mediates cytoskeletal activation and actin polymerisation are novel, and not shared with the canonical IT AM containing Fc(gamma)Rs. In particular the observation that Syk kinase plays not role in Dectin- 1 mediated phagocytosis in macrophages. We show that Dectin-1 mediates cellular activation in response to zymosan particles and that these (beta)-glucan dependent biological effects require collaboration with toll-like receptors (TLRs) at the cell surface. We also show that ligand size determines intracellular receptor trafficking following internalisation. Furthermore, we show that when biologically active soluble glucans are internalised by Dectin-1, the receptor is retained intracellularly yet, when biologically silent glucans are used, Dectin-1 is recycled. Dectin-1 is thus established as both an important phagocytic fungal pattern recognition receptor with pro- inflammatory abilities and an additional tool with which to study the diversity of signalling processes associated with leukocyte expressed receptors.