Figure 1. Macrophage activation phenotypes.

Figure 2. TLRs, NLRs and RLHs: cellular localization and signalling. The localization of pattern recognition receptors depends on their ligands and signalling properties. The surface expressed TLRs include TLR4, TLR2, TLR1, TLR5 and TLR6 whose ligands include bacterial surface proteins and lipids. These receptors may signal from the cell surface or from the phagosome after endocytosis. The intracellular TLRs include TLR3, TLR7 and TLR9 which recognize viral or bacterial nucleic acids. With the exception of TLR3 all TLRs share the adaptor protein MyD88 and through recruitment of a variety of adaptor proteins and signalling molecules, activate the transcription factor NF-B to initiate transcription of various pro-inflammatory genes. The TLRs which are localized to the endosome also activate the transcription factors IRF-3 and IRF-7 which are essential for the induction of the type I interferons (IFN-/), crucial components of the anti-viral response. TLR4 is unique among the surface-expressed TLRs because it is associated with additional adaptor proteins (e.g. TRAM, TRIF) and as a consequence is also able to induce IRF-3 and type I interferon production. The cytoplasmic sensors, the RLH and the NLRs recruit additional adaptor proteins via protein–protein interactions, upon binding to their ligands. The NLRs have been demonstrated to induce NF-B activation whereas the RLHs have been demonstrated to activate both NF-B and IRF3/7. As a result of the different patterns of receptor localization and adaptor protein recruitment, a tailored immune response can be initiated in response to a range of pathogens.

Figure 3. Phagocytosis of TLR ligands. (a) Microbes are coated with a variety of opsonins including complement, pentraxin 3 and antibodies. A number of receptors are involved in initial recognition of microbes and induction of pro-inflammatory signalling (e.g. the toll-like receptors (TLRs), and especially TLR2, 4 and 5) but these receptors are not phagocytic. Receptors involved in phagocytosis include the complement receptors, Fc receptors and others. (b) Phagocytosis mediated by this method occurs via the ‘zipper’ mechanism (i.e. sequential binding between the Fc receptors and their ligands along the circumference of the microbe). Phagocytic receptor ligation initiates a signalling cascade that results in activation of the Rho-GTPase family of signalling proteins, actin polymerization and extension of the plasma membrane. TLR signalling from the phagosome results in activation of p38 and this activation results in endosome maturation. (c) Fusion with the early endosome results in a slight drop in pH that results in the uncoupling of receptors with their ligands. Receptor recycling is facilitated by the Rab proteins, which also confer the ability to undergo subsequent fusion. The developing phagosome now contains MHC class II and TLRs, which signal from within the developing endosome. (d) Fusion with late endosomes results in the addition of LAMP proteins, the accumulation of acid-resistant phospholipids and a subsequent drop in pH. (e) Upon fusion with lysosomes the low pH results in the activation of a number of proteolytic enzymes. These are necessary for both direct antimicrobial activity as well as the creation of peptides for presentation via MHC class II.

Figure 4. Phagocytosis of apoptotic cells. (a) Apoptotic cells express lipids such as PS that are normally confined to the inner membrane of the plasma membrane in addition to glycoproteins that have distinctive modifications of their charges. This results in binding by a number of ‘bridging molecules’ (e.g. C1q, MFGE8 (milk fat globule-EGF factor 8)) that are normally found within the plasma. These are recognized by receptors (e.g. complement receptor 3, possibly integrins and PS receptor) present on phagocytic cells and initiate phagocytosis via a ruffling mechanism. (b) The vacuole containing the apoptotic cell undergoes acidification and (c) the cell is degraded but antigens are not loaded onto MHC class II molecules. (d) Although it is not clear by which signalling mechanism, the anti-inflammatory cytokines TGF- and IL-10 are produced. These prevent the macrophage from becoming activated and from initiating an adaptive immune response.

Figure 5. The autophagy pathway. Autophagy is a homeostatic process that can be further enhanced in macrophages in the presence of interferons or by starvation. Cytosolic proteins and organelles are found in single or double membrane vesicles that may be derived from the endoplasmic reticulum or from de novo synthesis. These vesicles fuse with lysosomes and the proteins and organelles are degraded and recycled. Conventional wisdom states that endogenous and cytoplasmic proteins are presented by MHC class I molecules whereas exogenous peptides are presented by MHC class II molecules, however, it has now become clear that peptide presentation is altered considerably upon induction of autophagy. The presentation of peptides from intracellular and lysosomal source proteins is increased on MHC-II.