MHC class II transport at a glance

MHC class II molecules comprise polymorphic α- and β-chains that are assembled in the endoplasmic reticulum (ER) together with a chaperone protein termed invariant chain (Ii) (Wolf and Ploegh, 1995). Ii serves to both stabilize the αβ heterodimer and inhibit inappropriate binding of antigen to the MHC class II peptide-binding groove. The newly formed αβ-Ii complex exits the ER, traffics through the Golgi complex and the trans-Golgi network (TGN) and is ultimately delivered to lysosome-like antigen-processing compartments. Although there is still some debate as to whether MHC class II traffics to these compartments directly from the TGN, we have proposed (and favor) a pathway that involves an intermediate step at the plasma membrane (PM) (Hiltbold and Roche, 2002): after arrival at the PM from the TGN, the αβ-Ii complex is efficiently sorted into the endocytic pathway by the recognition of two dileucine-based signals in the cytoplasmic tail of Ii by the clathrin adaptor AP-2, a scaffolding-protein complex that brings together components of the vesicle-formation machinery. The AP-2 complex sorts the αβ-Ii complex into clathrin-coated vesicles, which pinch off from the PM and fuse with endocytic compartments (Dugast et al., 2005; McCormick et al., 2005). Although Ii is not absolutely required for the localization of MHC class II molecules to endocytic structures, the efficiency with which MHC class II enters antigen-processing compartments is greatly enhanced by its association with Ii.

The endocytic pathway comprises various organelles of low pH, including early endosomes, late endosomes and lysosomes, each with varying capacities for protein degradation and protein recycling. Certain proteins, such as transferrin receptors, are sorted for retrieval from endocytic compartments and are selectively recycled back to the PM or TGN, whereas those that are targeted for degradation are retained in late endosomes and lysosomes (Piper and Katzmann, 2007). Upon reaching the late endosomes, proteins that are intended for destruction are sorted onto intraluminal vesicles that are derived from the inward budding of the limiting membrane of the endosome, thereby giving rise to multivesicular antigen-processing compartments termed multivesicular bodies (MVBs). The endosomal sorting complex required for transport (ESCRT) protein machinery, either directly through the recognition of the small molecule ubiquitin or indirectly through ubiquitin-independent targeting, sorts molecules that are intended for destruction into subdomains of the MVB limiting membrane (Piper and Katzmann, 2007). These subdomains are enriched in tetraspanins and cholesterol, molecules that form microdomains that have been implicated in intraluminal vesicle biogenesis (Piper and Katzmann, 2007). Interestingly, several other lipids, including lysobisphosphatidic acid and ceramide, directly induce the inward budding of liposomes (Matsuo et al., 2004; Trajkovic et al., 2008), suggesting that the lipid microenvironment in late endosomes also plays a role in protein sorting and intraluminal-vesicle formation.

In the case of MHC class II, once in the endocytic pathway, αβ-Ii complexes are delivered to lysosome-like multivesicular antigen-processing compartments not for degradation, but for antigenic-peptide binding. Although it is clear that both immature (Ii-associated) and mature (peptide-loaded) MHC class II is found on intraluminal vesicles of these structures (Kleijmeer et al., 2001; Kleijmeer et al., 1997; Peters et al., 1995), the mechanism that regulates the movement of MHC class II to the intraluminal vesicles of MVBs has not been fully resolved. MHC class II is ubiquitylated in immature, but not mature, dendritic cells (DCs) (Shin et al., 2006; van Niel et al., 2006), and this modification is added solely to αβ complexes that are devoid of Ii (van Niel et al., 2006). These data suggest that αβ-Ii complexes sort onto the intraluminal vesicles of MVBs through an ubiquitin-independent pathway. However, there are conflicting reports regarding the relevance of ubiquitylation to the sorting of MHC-class-II-αβ–peptide complexes onto intraluminal vesicles of MVBs (Shin et al., 2006; van Niel et al., 2006), and further study will be required to unambiguously delineate the mechanisms that regulate the movement of MHC class II in MVBs.

To direct a specific immune response, antigenic peptides that have been derived from endocytosed and degraded foreign materials must bind to MHC class II. This is accomplished with the help of the proteases and chaperone proteins that reside in the low-pH environment of the endocytic pathway. Soon after entry onto the intraluminal vesicles of multivesicular antigen-processing compartments, Ii undergoes sequential degradation by various proteases, including cathepsins S and L (Hsing and Rudensky, 2005), leaving behind a small fragment, termed CLIP (class-II-associated invariant chain peptide), that is bound to the MHC class II peptide-binding groove. Removal of CLIP is facilitated by the MHC-class-II-like chaperone protein HLA-DM via direct binding to the αβ-CLIP complex. HLA-DM then stabilizes the empty MHC class II molecule and presumably allows repeated binding and dissociation of peptides to MHC class II until primarily high-affinity antigenic peptides are bound to the peptide-binding groove (Busch et al., 2005). Thus, HLA-DM serves to edit the peptide repertoire of the APC. Interestingly, unlike MHC class II, HLA-DM is primarily retained on the limiting membrane of the MVB, and recent data suggest that an efficient interaction between MHC class II and HLA-DM requires their expression on disparate membranes (Zwart et al., 2005).

Antigen loading is modulated by endosomal protein content and the endosomal environment. In B cells, the ability of HLA-DM to interact with MHC class II is regulated by a second MHC-class-II-like protein, HLA-DO. HLA-DO binds to HLA-DM and inhibits it from catalyzing peptide exchange on the MHC class II molecule (Denzin et al., 1997). Inhibition of HLA-DM activity leads to a concomitant increase in αβ-CLIP complexes in both the intracellular antigen-processing compartments and at the PM, and results in diminished immune responses. Recently, primary DCs were shown to also express HLA-DO (Hornell et al., 2006), suggesting that this form of regulation might be more ubiquitous than previously thought. In addition to HLA-DM inhibition, the regulated expression of the endosomal proteases cathepsin S and cathepsin L leads to reduced antigen presentation by inhibiting Ii degradation and/or antigenic-peptide formation (Hsing and Rudensky, 2005). Curiously, although it might be easiest to imagine that a free peptide simply diffuses into the empty MHC class II peptide-binding site, there is increasing evidence that partially unfolded proteins actually bind to this site, with the antigen fragments that reside outside of the peptide-binding groove being `chewed-back' by lysosomal proteases (Sercarz and Maverakis, 2003), further complicating our understanding of an already complicated process.

The lysosomal microenvironment has also been implicated in the regulation of antigen loading. In immature DCs, the elevated pH in lysosome-like organelles results in the inefficient degradation of endocytosed material into antigenic peptides (Trombetta et al., 2003). Upon DC maturation, the ATP-dependent vacuolar proton pump (V-ATPase) is activated, leading to acidification of these compartments, increased antigen proteolysis and efficient peptide loading onto MHC class II molecules (Trombetta et al., 2003). The absolute amount of proteinases is lower in professional APCs than in highly destructive cells such as macrophages (Delamarre et al., 2006; Delamarre et al., 2005), suggesting that APCs possess mechanisms that `control' proteolysis to limit the destruction of antigenic peptides.

The mechanisms that regulate the movement of peptide-loaded MHC class II from antigen-processing compartments to the PM remain unresolved. Maturation of DCs induces the loss of intraluminal vesicles from MVBs (Kleijmeer et al., 2001) and, although it has been proposed that this loss is due to the back-fusion of the peptide-loaded MHC-class-II-containing intraluminal vesicles with the MVB limiting membrane, their loss owing to degradation or release as exosomes has not been ruled out. Once MHC class II is redistributed to the limiting membrane of the MVB, it is moved into tubulovesicular structures that appear to fuse directly with the PM (Chow et al., 2002). As peptide-loaded MHC class II is abundant on the surface of immature DCs as well, it is likely that this mode of transport is simply enhanced upon DC maturation, allowing for more expression at the cell surface after activation.

Interestingly, MHC class II associates with lipid rafts, membrane microdomains that are rich in cholesterol and sphingolipids, in both the multivesicular antigen-processing compartment as well as at the PM (Poloso et al., 2004). Disruption of APC lipid-raft integrity leads to antigen-specific defects in T-cell activation that can be overcome by loading the APCs with large amounts of antigen (Anderson et al., 2000). This suggests that clustering of the MHC-class-II–peptide complexes in membrane microdomains serves to potentiate the activation of T cells. In addition to lipid rafts, a pool of MHC class II is also present in a membrane microdomain termed the tetraspan web (Vogt et al., 2002). Indeed, lipid rafts, MHC-class-II–peptide complexes and tetraspan-web proteins concentrate at the site of the immunological synapse that exists at the interface between APCs and T cells (Hiltbold et al., 2003; Vogt et al., 2002). These findings indicate that the lipid and/or membrane microenvironment in which MHC class II is trafficked plays a key role in determining the antigenicity of a given MHC-class-II–peptide complex.

MHC-class-II–peptide complexes are found at the surface of both immature and mature DCs. However, their relative abundance is markedly different, with mature DCs having significantly higher surface levels of peptide-bound MHC class II. Recent evidence implicates a role for ubiquitylation in mitigating the surface expression of MHC-class-II–peptide complexes in immature DCs. Specifically, peptide-loaded MHC class II is ubiquitylated by the E3 ubiquitin ligase MARCH I (also known as MARCH1) in B cells and DCs (Matsuki et al., 2007), and it has been proposed that ubiquitylation enhances MHC class II endocytosis (Shin et al., 2006; van Niel et al., 2006). Supporting this finding, overexpression of MARCH I leads to downregulation of MHC-class-II–peptide complexes at the surface (Matsuki et al., 2007). Unlike MHC-class-II–Ii complexes, the internalization of peptide-loaded MHC class II occurs independently of clathrin, AP-2 or the GTPase dynamin. Instead, these MHC-class-II–peptide complexes internalize and recycle in tubules containing Arf6 and Rab35, which are small GTPases that are thought to be involved in clathrin-independent protein sorting (Walseng et al., 2008). Upon DC maturation, expression of MARCH I is downregulated, inhibiting the ubiquitylation of MHC-class-II–peptide complexes (De Gassart et al., 2008). Internalization of MHC-class-II–peptide complexes in mature DCs is then reduced, thereby leading to retention of peptide-bound MHC class II at the surface. Thus, there is a balance struck between the surface transport and endocytosis of MHC-class-II–peptide complexes in DCs, with the result being relatively lower surface expression of MHC class II in immature DCs than in mature DCs.

Although recent work has begun to shed light on the regulatory mechanisms that are involved in MHC class II trafficking in the endocytic pathway, much still remains to be elucidated. For example, the work that forms the basis for our model is from a diverse variety of sources including transformed DC and B-cell lines, professional primary APCs, and even MHC-class-II-transfected non-professional APC cell lines. It is therefore important to validate all models of MHC class II distribution and trafficking in primary APCs to reveal a true, if not more complicated, picture of what is going on in the cell. More specifically, what role, if any, ubiquitylation plays in regulating the movement of MHC class II onto the intraluminal vesicles of MVBs remains an open question. Additionally, the nature of the molecules that mediate the transport of peptide-loaded MHC class II from the MVB to the PM have not been identified. Recent findings that the surface expression of MHC-class-II–peptide complexes is modulated by ubiquitin ligases and clathrin-independent endocytosis and recycling pathways leave open the possibility that these two seemingly unrelated mechanisms of MHC class II expression could be interrelated. Future studies will undoubtedly advance our understanding of the mechanisms that regulate MHC-class-II–peptide expression that are essential for the initiation of a robust acquired immune response to foreign pathogens.