Co-ordination of incoming and outgoing traffic in antigen-presenting cells by pattern recognition receptors and T cells

Abstract

Dendritic cells are innate sentinels of the immune system and potent activators of naÏve T cells. Mechanisms must exist to enable these cells to achieve maximal activation of T cells specific for microbial antigens, while avoiding activation of T cells specific for self-antigens. Here we discuss how a combination of signals from pattern recognition receptors and T cells co-ordinates subcellular trafficking of antigen with both major histocompatibility complex class I and class II molecules and T-cell costimulatory molecules, resulting in the preferential presentation of microbial peptides within a stimulatory context.

Figure 1. Three proposed pathways for cross-presentation and their regulation by PRRs

A) The phagosome-to-cytosol pathway. After phagocytosis, antigen is transported out of the phagosome, possibly through the Sec61 translocon. Once in the cytosol, antigen follows the classical MHCI route, consisting of degradation by the proteasome, TAP-mediated entry into the ER and routing to the plasma membrane via the Golgi complex. B) The ERgosome pathway. Phagosomes fuse with (components of) the ER (forming an ERgosome), which brings in TAP and pMHCI displaying self-antigen (shown in blue). Antigens are translocated to the cytosol for processing by cytosolic proteasomes, followed by re-entry into the ERgosome, preferential loading of microbial antigen (shown in green) on MHCI and migration of pMHCI complexes to the plasma membrane. C) The vacuolar pathway. Antigens can be directly processed in the early phagosome by resident cathepsins. Resultant microbial peptides (green) are loaded onto MHCI derived from the plasma membrane (presumably after exchange with self-peptides), which arrive in phagosomes via early recycling endosomes (EREs). After loading of microbial peptides, migration to the plasma membrane takes place by unknown mechanisms. PRRs (TLR and CLR) continue to be engaged (indicated by red lightning bolts) by PAMPs in phagosomes, resulting in signals that may promote these pathways via inducing Rab27a-mediated recruitment of NADPH oxidase from lysosomal structures (A) (which may help preservation of antigenic epitopes by limiting acidification), recruitment of Sec61 to facilitate translocation of antigens into the cytosol (B), enhancement of proteasomal (C) and TAP activity (D) as well as ER recruitment of TAP and MHCI to phagosomes (E).

Figure 2. Antigen processing and presentation on MHCII following phagocytosis of microbial antigen

Phagosomes containing microorganisms (in green) engage TLR and CLR signals (indicated by red lightning bolts) and undergo accelerated maturation fusing with lysosomes to become phagolysosomes. PRRs may regulate assembly and function of V-ATPases to promote phagolysosomal acidification (A). This acidic compartment favors the degradation of antigens by promoting activity of resident cathepsins (B) within individual phagosomes containing PRR ligands. Perhaps as a consequence, processing of the invariant chain is favored (C), thereby allowing MHCII to acquire peptides generated from microbial antigens (D). The assembled pMHCII complexes are then routed to the plasma membrane via endolysosomal tubules (E). Tetraspanins are also selectively recruited to phagosomes containing microbial signatures and are believed to help create microdomains with superior antigen presentation capacity (F). MHCII loaded with self-peptides (in blue) in immature DCs are ubiquitinated (indicated by Ub) on their cytoplasmic tail. Although these complexes can go to the plasma membrane (G), they are rapidly internalized and routed to MVBs and end up in lysosomes, where they are degraded. As a consequence, levels of MHCII-carrying self-peptides on the cell surface are low. Upon TLR stimulation, newly synthesized MHCII loaded with microbial peptides are not ubiquitinated, allowing their stable expression on the plasma membrane.

Figure 3. Formation of a stimulatory immunological synapse by targeted recruitment

T cells actively influence the transport of pMHCII from endocytic compartments to the plasma membrane. Interaction of antigen-specific CD4⁺ T cells with APCs stimulates reorientation of the MTOC in the APC toward the T cells, and leads to the formation of tubules containing pMHCII complexes, tetraspanins and costimulatory molecules such as CD70 and CD86. CD40L–CD40 and LFA-1–ICAM-3 interactions promote tubule formation. Molecules instructing T-cell differentiation, including IL-12 and the Notch ligands, Delta-like ligands (DLLs) and Jagged, are also projected toward the interaction site.