Signalling and phagocytosis in the orchestration of host defence
Dendritic cells (DCs) orchestrate either tolerance or immunity. At the heart of this function lies phagocytosis, which allows DCs to sample the tissue microenvironment and deliver both its self and non-self constituents into endocytic compartments for clearance, degradation and presentation by major histocompatibility complex (MHC) molecules. Depending on the type of signalling pathways triggered during phagocytosis, DCs deliver appropriate signals to T cells that determine either their tolerance or activation and differentiation. Here I draw attention to the ability of DCs to read the contents of their phagosomes depending on the type of compartmentalized signalling pathways engaged during internalization. Toll-like receptors (TLRs) engaged during phagocytosis of microbial pathogens, but not syngeneic apoptotic cells exert phagosome autonomous control on both the kinetics and outcome of phagosome maturation. By bearing the assembly of signalling complexes on their membranes, individual phagosomes undergo separate programmes of maturation irrespective of the activation status of the DC carrying them. Phagosomes carrying microbial cargo are favoured for MHC class II presentation precluding phagosomes carrying self from contributing to the first signal delivered to T cells - the peptide-MHC complex. This mechanism prevents the potential presentation of peptides derived from self within the context of TLR-induced co-stimulatory signals.