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21 Tomonari, K., Rosenwasser, O.A. and Fairchild, S.P. (1997) in Viral

Wiley, D.C. (1996) Nature 384, 134–141

Superantigens (Tomonari, K., ed.), pp. 127–135, CRC Press

32 Garcia, K.C., Degano, M., Pease, L.R. et al. (1998) Science 279, 1166–1172

22 Scherer, M.T., Ignatowicz, L., Pullen, A., Kappler, J. and Marrack, P. (1995)

33 Luescher, I.F., Vivier, E., Layer, A. et al. (1995) Nature 373, 353–356

J. Exp. Med. 182, 1493–1504

34 Garcia, K.C., Scott, C.A., Brunmark, A. et al. (1996) Nature 384, 577–581

23 Liao, N-S., Maltzman, J. and Raulet, D.H. (1989) J. Exp. Med. 170, 135–143

35 Gao, G.F., Tormo, J., Gerth, U.C. et al. (1997) Nature 387, 630–634

24 Janeway, C.A., Jr and Bottomly, K. (1996) Semin. Immunol. 8, 109–115

36 Wu, H., Kwong, P.D. and Hendrickson, W.A. (1997) Nature 387, 527–530

25 Rojo, J.M., Saizawa, K. and Janeway, C.A., Jr (1989) Proc. Natl. Acad. Sci.

37 Sakihama, T., Smolyar, A. and Reinherz, E.L. (1995) Immunol. Today 16,

U. S. A. 86, 3311–3315

581–587

26 Mittler, R.S., Goldman, S.J., Spitalny, G.L. and Burakoff, S.J. (1989) Proc.

38 Vignali, D.A.A., Carson, R.T., Chang, B., Mittler, R.S. and Strominger, J.L.

Natl. Acad. Sci. U. S. A. 86, 8531–8535

(1996) J. Exp. Med. 183, 2097–2107

27 Beyers, A.D., Spruyt, L.L. and Williams, A.F. (1992) Proc. Natl. Acad. Sci.

39 Seibel, J.L., Wilson, N., Kozono, H., Marrack, P. and Kappler, J.W. (1997)

U. S. A. 89, 2945–2949

J. Exp. Med. 185, 1919–1927

28 Kupfer, A. and Singer, S.J. (1989) Annu. Rev. Immunol. 7, 309–337

40 Sprent, J. and Schaefer, M. (1989) Immunol. Today 10, 17–23

29 Collins, T.L., Uniyal, S., Shin, J., Mittler, R.S. and Burakoff, S.J. (1992)

41 Amadori, A., Zamarchi, R., De Silvestro, G. et al. (1995) Nat. Med. 1,

J. Immunol. 148, 2159–2162

1279–1283

30 Garcia, K.C., Degano, M., Stanfield, R.L. et al. (1996) Science 274, 209–219

42 Bendelac, A., Rivera, M.N., Park, S.H. and Roark, J.H. (1997) Annu. Rev.

31 Garboczi, D.N., Ghosh, P., Utz, U., Fan, Q.R., Biddison, W.E. and

Immunol. 15, 535–562

The role of endosomes and lysosomes in MHC class II functioning Hans J. Geuze B cells, dendritic cells and ntigen-presenting cells (APCs) that display major histocompatibility complex (MHC) class II molecules at their cell surface, present antigenic peptides to CD4⫹ T helper (Th) cells. Most of these peptides are derived from internalized antigens by proteolytic processing in the endocytic pathway. Convincing evidence has shown that the majority of peptides are loaded onto newly synthesized MHC class II molecules1 rather than onto recycling surface-derived class II molecules2. Thus, the biosynthetic pathway of MHC class II intersects the endocytic pathway3,4.

macrophages present peptides

MA lumenal domain of Ii, known as class IIassociated Ii peptide (CLIP), interacts with ⫹ the putative peptide-binding groove of MHC to CD4 T cells. After uptake, class II, rendering the groove inaccessible to antigens are processed into peptides present in the ER (Ref. 6). Once the peptides, bound to major nonameric class II–Ii complexes are formed, the calnexin/calreticulum-based quality conhistocompatibility complex (MHC) trol system at the ER–cis-Golgi interface class II molecules and transported allows egress from the ER and transport to to the plasma membrane. Here, the Golgi complex. Incorrectly folded or oligomerized ␣-, ␤- and Ii-chains are retained Hans Geuze reviews the current in the ER and degraded. In the Golgi, the ␣-, controversy surrounding the ␤- and Ii-subunits are terminally glycosylidentity of the compartments that ated and transported to the trans-Golgi network (TGN), a tubulo-vesicular organelle loplay a role in antigen processing cated at the trans-face of the Golgi stacks. and peptide loading. From here, the complexes are targeted to the The biosynthetic pathway of MHC endocytic pathway7. class II MVarious types of vesicles including constiMHC class II molecules are synthesized in the endoplasmic reticulum tutive secretory vesicles, secretory storage granules, clathrin-coated (ER) as type 1 transmembrane glycoproteins, comprising homodimer- vesicles, and other vesicles with cytoplasmic coats of unknown ized ␣- and ␤-chains of which the C-terminal lumenal domains com- composition, originate from the TGN. Clearly, the TGN is an imprise the peptide-binding groove. In the ER, the ␣ and ␤ homodimers portant sorting station in the biosynthetic pathway of proteins and associate with a non-MHC-encoded type II integral membrane glyco- lipids for transport to a variety of destinations in the cell. One of protein, the invariant chain (Ii), to assemble into nonameric complexes the best studied examples of protein sorting in the TGN is that comprising three ␣-, three ␤- and three Ii-chain subunits5. of the mannose 6-phosphate receptor (MPR). MPRs bind newly

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1 AV 2 Macroautophagy

Fig. 1. Schematic representation of the conversion of the autophagic and endocytic pathways. During macroautophagy, a cisterna, presumably of ER origin, randomly engulfs a portion of cytoplasm that may contain organelles (step 1). Subsequently, the enveloping membrane closes and the structure matures into an AV, with proteolytic capacity (step 2). AVs can fuse with endocytic structures including LE/MIICs (step 3). MIICs occur downstream of EEs in the endocytic route. The merging of the endocytic and autophagic routes at step 3 provides one way by which class II molecules could be confronted with endogenous peptides. Finally, class II–peptide complexes can be transported to the cell surface (step 4). Abbreviations: AV, autophagic vacuole; EE, early endosome; ER, endoplasmic reticulum; LE, late endosome; MIIC, major histocompatibility complex class II compartments. synthesized soluble lysosomal enzymes in the TGN, where subsequently, the receptor–ligand complexes are sorted from secretory and plasma membrane proteins for transport and enzyme delivery to endosomes. This transport is mediated by small TGN vesicles coated with adaptor protein 1 (AP-1) and clathrin. Two di-leucinebased targeting signals in the cytoplasmic domain of MPR mediate the sorting of the MPR–enzyme complexes into the AP-1 and clathrin-coated TGN-derived vesicles. Similar di-leucine motifs have been identified in the cytoplasmic domain of Ii. Therefore, AP-1 and clathrin-coated vesicles are obvious candidates to mediate MHC class II–Ii transport to the endocytic pathway. However, biochemical evidence and immunoelectron microscopy (IEM) have shown that an important fraction of newly synthesized MHC class II is incorporated in TGN-derived vesicles without detectable clathrin and AP-1 (Ref. 8). The reason for two distinct pathways to the endocytic system is unknown but indicates that class II–Ii and MPR–enzyme complexes enter the endocytic pathway at different points. Thus, the lumenal domain of Ii confers the specificity for exogenous antigens to MHC class II molecules9, whereas the cytoplasmic domain of Ii regulates specific transport of the ␣-, ␤- and Ii-complexes to the endocytic pathway10. Here, class II molecules encounter peptides that are proteolytically derived from internalized antigens.

The endocytic pathway of antigens Endocytosis of antigen varies for different types of APCs (Refs 4, 11). Macrophages and immature dendritic cells (DCs) take up antigen via

phagocytosis, (macro)pinocytosis or receptor-mediated endocytosis12. Phagocytosis (cell eating) by macrophages is initiated by binding of IgG-derived particles to surface Fc receptors, which results in polymerization of actin. This, in turn initiates the formation of pseudopods that engulf the particle, resulting in the formation of phagosomes. Pinocytosis (cell drinking) refers to the formation of small pits, often clathrin-coated, at the plasma membrane, which carry fluid and proteins specifically or non-specifically from the extracellular medium into the cell. B cells internalize specific antigens via membranebound immunoglobulin (Ig) at their surface or by nonspecific pinocytosis, the latter being relatively inefficient. Receptor-mediated antigen internalization occurs by clathrin-coated pits and requires specific sequence information in the cytoplasmic tail of the receptors11. Important endocytosis signals are tyrosine- and dileucine-based sequences, which mediate the recruitment of the plasma membrane adaptor protein 2 (AP-2). This triggers clustering of the receptors into clathrin-coated pits at the plasma membrane. After ‘pinching-off’ from the plasma membrane, clathrincoated vesicles rapidly lose their coats and fuse with endocytic compartments.

Endocytic compartments All entries into the cell converge at endosomes. The endocytic system comprises an extremely pleiomorphic interconnected array of vesicles, larger vacuoles and tubules that communicate with each other, the plasma membrane, and the TGN. The complex morphology and plasticity of the endocytic system, its dispersed intracellular distribution and the lack of reliable discriminative molecular markers have hampered defining its subcompartments. However, a number of operational terms have been introduced and have proved to be useful.

Early endosomes Early endosomes (EEs) are the first structures receiving endocytosed material. The residence time in EEs is in the order of minutes. EEs have a less acidic pH than later endocytic compartments and exhibit relatively little proteolytic activity. EEs comprise an elaborate system of 60 nm wide tubules that in many cell types extend centrally into the trans-Golgi region. In the cell periphery, the EE tubular network is connected to small vacuolar EEs, often referred to as ‘sorting endosomes’, because they are engaged in the sorting of soluble and membrane proteins for transport further down the endocytic pathway, as well as to the plasma membrane and TGN. Recycling plasma membrane proteins such as the transferrin receptor (TfR), which is commonly used as a marker for EEs, exit the EEs via small vesicles emanating from the EE tubules13. The mechanism underlying the segregation of membrane proteins from the vacuolar EEs into the EE tubules is unknown. EE tubules in the Golgi area are also referred to as ‘recycling endosomes’ because they are enriched in recycling plasma membrane proteins. Small GTPases of the Rab family are associated with specific membrane compartments and confer specificity to vesicle transport by playing

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Plasma membrane undergo a transition into lysosomes, no sharp distinction between the two compartments can be made. Undegradable material accumulates in ‘resting’ lysosomes or ‘residual bodies’, being the end points in the endocytic route (reviewed in Ref. 11).

Protein translocation Macroautophagy

Delivery of biosynthetic MHC class II to the endocytic pathway

LE/MIIC

Fig. 2. Microautophagy encompasses the internalization of small portions of cytoplasm and limiting membrane by LE/MIICs. The integrity of the internal vesicles can be destroyed in the proteolytic content of the LE/MIICs so that peptides derived from the cytosolic and membrane proteins of the vesicles (in green) can be bound to class II molecules which project with their peptide-binding grooves into the lumen of the MIIC. Alternatively, cytosolic proteins (red) can be translocated across the MIIC membrane. In B cells, exocytotic fusion of the MIIC with the plasma membrane (blue arrow) is one way by which peptide-loaded class II molecules are inserted in the plasma membrane. During exocytosis, MIIC vesicles are secreted as exosomes41. For abbreviations, see Fig. 1 legend.

a role in vesicle-target recognition. Rab4 and Rab5 interact mainly with the peripheral EEs, whereas Rab11 is enriched at the recycling EEs in the trans-Golgi region, suggesting different functions of these EE subdomains.

Late endosomes and lysosomes Internalized proteins that do not recycle from EEs are subsequently transported to late endosomes (LEs) and lysosomes. LEs develop from the vacuolar parts of the EE network and while incoming and outgoing vesicles remodulate their membrane composition, LEs mature gradually into lysosomes14. LEs can fuse with pre-existing lysosomes and autophagic vacuoles (Fig. 1). Autophagic vacuoles contain discrete portions of cytoplasm destined for degradation in lysosomes. Characteristically, LEs display numerous small membrane vesicles in their lumens that are formed by microautophagy (see Fig. 2). This process probably directs certain membrane and cytosolic constituents to the LE lumen for degradation. LE formation is initiated in the cell periphery, and continues while migrating along microtubules to the perinuclear region. Owing to their morphology, LEs are often referred to as ‘multivesicular bodies’ or ‘ multivesicular endosomes’. In some cell types LEs are enriched in MPRs, a feature that distinguishes them from EEs and lysosomes, which lack MPRs. Soon after their formation, LEs acquire lysosomal membrane proteins such as lysosome-associated membrane proteins (LAMPs) which, in contrast to MPRs, are not retrieved from LEs but accumulate in lysosomes. LEs contain a full set of acid hydrolases, have a relatively low pH, and like lysosomes, are degradative compartments. LEs progressively accumulate internal membranes and dense waste material, eventually resulting in the characteristic high densities of lysosomes. Because LEs gradually

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Considerable evidence indicates that LEs/lysosomes play a crucial role in MHC class II-mediated antigen presentation15–21. Notably, IEM has revealed that in a variety of APCs the majority of intracellular MHC class II is found in late endocytic structures with numerous internal membrane vesicles and sheets, typical for LE/lysosomes22. These structures are collectively designated MIICs (MHC class II compartments)7,22,23. In addition to high levels of MHC class II, MIICs contain lysosomal membrane proteins and enzymes, little or no TfR, and are reached by endocytic tracers ~10 min after uptake. Endocytic tracer first reaches the multivesicular MIICs and then progresses to intermediate MIICs containing vesicles and membrane sheets, and to multilaminar MIICs with membrane sheets only (Fig. 3). Until recently, it was unknown whether MIICs comprise a unique endocytic structure designed for antigen presentation, or belong to the conventional endocytic pathway. A detailed IEM analysis of the endocytic system in murine and human B cells now indicates that MIICs belong to the regular array of LEs and lysosomes comparable to those in non-APCs (Ref. 23). MIICs have been identified in B cells (Figs 3, 4), B-cell lines, DCs, Langerhans cells, monocytes and macrophages22. Newly synthesized class II molecules experience a delay of 1–3 h between their exit from the TGN and arrival at the cell surface, reflecting the time required for transport to the endocytic pathway and peptide loading1. The majority of MHC class II–Ii complexes that have departed from the TGN, are thought to arrive at the endocytic system via a direct route, by passing the plasma membrane (see Fig. 5). It is not known to which endocytic subcompartments the complexes are first delivered4,5. Following biosynthetic transport using cell fractionation techniques, both EEs (Refs 24–26) and MIICs (Refs 1, 8, 15, 27–29) have been implicated as entry sites for MHC class II–Ii complexes. Recent IEM studies identified of an early MIIC type, which is situated just downstream of EEs and contains abundant Ii (see Figs 4, 5). Upon its arrival in the endocytic system Ii is rapidly degraded, suggesting that early MIICs represent the site where the majority of TGN-derived class II–Ii complexes enter the endocytic tract8,23. Ii degradation starts at the lumenal C-terminus leaving CLIP fragments transiently associated with the MHC class II peptide-binding groove until they are exchanged for antigenic peptide. This process is facilitated by the MHC-like molecule HLA-DM (H-2M in mice)30, which can be detected in early MIICs but is most abundant in later MIICs (Fig. 3)31,32. It cannot be excluded that a fraction of the class II–Ii complexes present in early MIICs enters the endocytic system at the level of EEs. If class II–Ii complexes appear at the cell surface, the di-leucine motifs in the cytoplasmic tail of Ii promote rapid internalization via clathrincoated vesicles and delivery of the complex to EEs (Ref. 33).

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Fig. 3. Immunoelectron micrograph of an ultrathin cryosection from a human B cell. The section was double-immunogold labelled for HLA-DM and MHC class II with 15 nm and 10 nm gold particles, respectively. Several MIICs including four multivesicular MIICs and a multilaminar MIIC can be seen close to a Golgi complex. Magnification ⫽ 60 000⫻. For abbreviations, see Fig. 1 legend.

In studies using free-flow electrophoresis of cell fractions from murine B cells and DCs, Mellman and co-workers proposed a model in which class II–Ii complexes are transported to EEs and then included in so-called class II vesicles (CIIVs)26,31. CIIVs are physically and biochemically distinct from other endocytic compartments, but many of their characteristics match those of early MIICs (Ref. 23): both have internal vesicles, represent a post-EE stage, contain low levels of TfR and HLA-DM, and markedly differ from LEs and lysosomes. However, the main difference between CIIVs and early MIICs, is that only the latter are enriched in Ii. The reason for this discrepancy is unknown. Taken together, recent biochemical24 and morphological23 evidence suggests that a major biosynthetic route of class II–Ii complexes merges with the endocytic pathway at an early stage, presumably the early MIIC, just downstream of EEs (the main TfR recycling compartment) (see Fig. 5).

Peptide loading of MHC class II in the endocytic pathway Following endocytosis and transport through EEs, antigens are proteolytically processed into peptides that can be loaded onto MHC class II molecules. The class II molecules that bind antigen are primarily derived from the biosynthetic route8,15–17,34, whereas only a minor fraction comprises recycling surface class II molecules6,12. To date, it has not been possible to pin-point the precise sites at which class II molecules are loaded with peptide. It seems likely that more than one compartment is involved, depending on the source of the peptide, the class II haplotype and the individual APC type. Oligomeric class II–Ii complexes dissociate in the presence of sodium dodecyl sulphate (SDS) at elevated temperatures, whereas most high affinity class II–peptide complexes are stable at these conditions35. These characteristics are often used to determine the peptide-

Fig. 4. Immunoelectron micrograph from a B cell that had internalized bovine serum albumin (BSA)-coated 5 nm gold particles as an endocytic tracer for 10 min. The section was double-immunogold labelled for C-terminal invariant chain (Ii) and major histocompatibility complex (MHC) class II with 15 nm and 10 nm gold particles, respectively. C-terminal Ii, class II and BSA-gold particles all co-localize to early MIICs, while only class II labelling is present in a multivesicular MIIC that had not yet been reached by the internalized BSA–gold (seen to the left) and at the plasma membrane. Magnification ⫽ 60 000⫻. For abbreviations, see Fig. 1 legend.

loaded state of class II in subcellular fractions16,17,26,29. In addition, peptide-loaded class II can be located by IEM using the antibody YAe, which selectively recognizes an abundant class II–peptide complex23. Both approaches indicate that MIICs in B cells and macrophages represent the primary sites for peptide loading15–17,24,29,36. Other observations that are in line with this view are: (1) the late appearance of antigen-presenting capacity after antigen uptake18,22; (2) the acquisition of SDS stability of class II in MIIC-enriched fractions15–17; (3) the loss of presentation capacity by blocking transfer of antigen to late endocytic compartments at 18°C; (4) the efficient generation of antigen-presenting capacity by administration of antigen to the cells in liposomes that lose their integrity at only low pH (Ref. 21); and (5) the fact that reduction of disulfide bonds as in lysosomes promotes presentation of certain peptides20. Although MIICs are clearly involved in peptide loading, this does not exclude the possibility that some peptides, perhaps those that are formed under mild proteolytic conditions, are already bound to class II in EEs (Refs 27, 37). This may involve recycling surface-derived mature class II molecules that have lost their peptide or that exchange lowaffinity peptides for higher affinity ones24. In what types of MIICs does peptide loading take place? Early MIICs exhibit low levels of cathepsin D and HLA-DM, and quantitative IEM reveals that of the intracellular class II–peptide complexes recognized by the YAe antibody, 10% are seen in the early MIICs (Ref. 23). Moreover, the YAe epitope is not found in EEs, suggesting that for the formation of this class II–peptide complex early MIICs may represent the first site in the endocytic route23. Mellman and co-workers found that CIIVs represent the major site of peptide-loaded class II molecules in murine B cells26, which again suggests that early MIICs and CIIVs are related.

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?

?

LE MVE, MVB Multivesicular MIIC

TfR ± Class II + HLA-DM ± Ii-COOH +++ Ii-NH2 +++ Cathepsin D ± YAe +

Class II +++ MPR ± HLA-DM + Ii-COOH + Ii-NH2 ++ LAMP-1 + CD63 + Cathepsin D + YAe ++

CIass II / Ii

Intermediate MIIC

Class II +++ MPR − HLA-DM +++ Ii-NH2 + LAMP-1 ++ CD63 ++ Cathepsin D ++ YAe +++

TGN

Lysosome Multilaminar MIIC Residual body Class II ++ to − MPR − HLA-DM +++ to − LAMP-1 +++ CD63 +++ Cathepsin D ++ to − YAe ++ to −

selective portions of cytoplasm and can fuse with LEs (Ref. 39, Fig. 1); (2) microautophagy (as opposed to the above macroautophagy) occurring during the formation of internal vesicles of MIICs (Fig. 2); (3) cytosolic antigens present in the internal vesicles may be processed in the MIIC lumen and become available for binding to class II, and (4) cytosolic proteins can be translocated across the MIIC membrane by a receptor-mediated process requiring ATP and heat shock proteins40 (Fig. 2).

How do class II–peptide complexes get to the cell surface?

As yet, this question cannot be answered fully. One pathway for MHC class II delivery to the plasma membrane found in B Fig. 5. Comprehensive scheme depicting the post-Golgi and endocytic transport routes of MHC cells involves the fusion of multivesicular class II molecules, the abundance (from ⫺ to ⫹⫹⫹) of molecular markers as found by immunoelecMIICs with the plasma membrane (exocytron micrograph (IEM), and the conventional and class II-specific terminology of endocytic comtosis), resulting in the incorporation of class partments. The model proposes that a major fraction (bold arrow) of newly synthesized class II, after II present in the limiting membrane of the having egressed from the TGN, travels to early MIICs and is then transported further down the MIIC into the plasma membrane41,42. Interendocytic pathway. A small amount of the class II–Ii may be transported by default to the plasma estingly, MIIC exocytosis is accompanied membrane from where it is endocytosed and delivered to EEs. The dashed arrow indicates that some by the release of the internal MIIC vesicles biosynthetic class II molecules are transported directly from the TGN to EEs. The requirements for as so-called exosomes (Fig. 2), which bear antigen processing, Ii degradation and peptide loading probably start to be fulfilled in early MIICs SDS stable class II–peptide complexes at and become more pronounced in later MIICs (see lists of markers). CIIVs as defined in the murine their surface. Exosomes of human and B-cell line A20 by free-flow electrophoresis26, may correspond to early MIICs or represent a separate mouse B cells can stimulate specific post-EE compartment. Class II–peptide complexes probably exit the endocytic pathway at multiple T cells41. The relative contribution of MIIC sites (red arrows), one of which is by direct fusion of multivesicular MIICs with the plasma memexocytosis to class II transport to the cell brane. In addition to delivering class II to the cell surface, fusion of MIICs results in the exocytosis surface is unclear. Kinetic data suggest the of the internal vesicles of MIICs as exosomes. See text for further details. Abbreviations: CIIV, class existence of other pathways41 most likely II vesicle; EE, early endosome; Ii, invariant chain; LAMP-1, lysosome-associated membrane protein; originating at a post-EE site43. In this reLE, late endosome; MIIC, major histocompatibility complex (MHC) class II compartment; MPR, spect, it is of interest that early MIICs show mannose 6-phosphate receptor; MVB, multivesicular body; MVE, multivesicular endosome; TfR, an irregular morphology of vesicles and transferrin receptor; TGN, trans-Golgi network. attached tubules reminiscent of the ‘recycling’ tubules of EEs, suggesting that pepThe organization of the endocytic system and the relative distri- tide-loaded class II molecules, perhaps those formed quickly after bution of MHC class II within it are not necessarily constant during antigen uptake, exit early MIICs by means of vesicles detaching the cell’s lifetime. Interestingly, new data indicate that when DCs from the tubular extensions of early MIICs. Finally, in murine B are stimulated by environmental factors to mature from ‘detector’ cells, CIIVs have been proposed to function as transport vehicles for cells – devoted to endocytosis and processing of antigens – towards peptide-loaded class II to the plasma membrane. very potent APCs, they reorganize their endosomal/lysosomal system to accommodate for this functional shift31,38. Golgi

Concluding remarks Possible pathways for endogenous antigens to be loaded on MHC class II In addition to exogenous peptides, MHC class II molecules can also bind endogenous, cytosolically derived peptides. Possible mechanisms that render such peptides available for binding to class II molecules include: (1) autophagy – autophagic vacuoles contain non-

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Ignoring the many differences in terminology, definitions of endocytic compartments, methodologies and cell systems used, and in an attempt to reconcile an impressive body of information, a model arises in which APCs exploit their endosomal/lysosomal system for antigen processing and peptide binding to MHC class II (Fig. 5). There is no evidence for the existence of a specialized compartment fulfilling this function and that uniquely developed in APCs (Refs 23, 24). Instead,

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APCs display normal endocytic compartments including EEs, LEs and lysosomes. The latter two harbour by far the most of the intracellular MHC class II, and thus give reason to call them MIICs. MIICs are characterized by the presence of internal membranes, contain a low pH, lysosomal hydrolases and lysosomal membrane proteins, and are enriched in HLA-DM. Newly synthesized class II molecules probably enter the endocytic pathway in early MIICs situated just downstream of EEs. Peptide loading of class II probably starts in these early MIICs and proceeds in MIICs further down the endocytic route. A challenge for future research will be to discover the mechanisms by which the loaded class II molecules escape the endocytic pathway and are directed to the cell surface for presentation to T cells.

16 West, M.A., Lucocq, J.M. and Watts, C. (1994) Nature 369, 147–151 17 Qiu, Y., Wandinger-Ness, A., Dalke, D.P. and Pierce, S.K. (1994) J. Cell Biol. 125, 595–605 18 Harding, C.V. and Geuze, H.J. (1992) J. Cell Biol. 119, 531–542 19 Peters, P.J., Raposo, G., Neefjes, J.J. et al. (1995) J. Exp. Med. 182, 325–334 20 Collins, D.S., Unanue, E.R. and Harding, C.V. (1991) J. Immunol. 147, 4054–4059 21 Harding, C.V., Collins, D.S., Slot, J.W., Geuze, H.J. and Unanue, E.R. (1991) Cell 64, 393–401 22 Kleijmeer, M.J., Raposo, G. and Geuze, H.J. (1996) Methods 10, 191–207 23 Kleijmeer, M.J., Morkowski, S., Griffith, J.M., Rudensky, A.Y. and Geuze, H.J. (1997) J. Cell Biol. 139, 639–649 24 Castellino, F. and Germain, R.N. (1995) Immunity 2, 73–88 25 Romagnoli, P., Layet, C., Yewdell, J. et al. (1993) J. Exp. Med. 177, 583–596 26 Amigorena, S., Drake, J.R., Webster, P. and Mellman, I. (1994) Nature

Hans Geuze ([email protected]) is at Utrecht University, Dept of Cell Biology, AZU H02.314, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.

369, 113–119 27 Harding, C.V. and Geuze, H.J. (1993) Curr. Opin. Cell Biol. 5, 596–605 28 Pieters, J., Horstmann, H., Bakke, O., Griffiths, G. and Lipp, J. (1991) J. Cell Biol. 115, 1213–1223

References

29 Bénaroch, P., Yilla, M., Raposo, G. et al. (1995) EMBO J. 14, 37–49

1 Neefjes, J.J., Stollorz, V., Peters, P.J., Geuze, H.J. and Ploegh, H.L. (1990)

30 Denzin, L.K. and Cresswell, P. (1995) Cell 82, 155–165

Cell 61, 171–183

31 Pierre, P., Turley, S.J., Gatti, E. et al. (1997) Nature 388, 787–792

2 Pinet, P., Vergelli, M., Martin, R., Bakke, O. and Long, E.O. (1995) Nature

32 Sanderson, F., Kleijmeer, M.J., Kelly, A. et al. (1994) Science 266,

375, 603–606

1566–1569

3 Wolf, P.R. and Ploegh, H.L. (1995) Annu. Rev. Cell Dev. Biol. 11, 267–306

33 Wang, K., Peterson, P.A. and Karlsson, L. (1997) J. Biol. Chem. 272,

4 Watts, C. (1997) Annu. Rev. Immunol. 15, 821–850

17055–17060

5 Cresswell, P. (1994) Annu. Rev. Immunol. 12, 259–293

34 Davidson, H.W., Reid, P.A., Lanzavecchia, A. and Watts, C. (1991)

6 Roche, P.A., Teletski, C.L., Stang, E., Bakke, O. and Long, E.O. (1993)

Cell 67, 105–116

Proc. Natl. Acad. Sci. U. S. A. 90, 8581–8585

35 Sadegh-Nasseri, S. and Germain, R.N. (1991) Nature 353, 167–170

7 Peters, P.J., Neefjes, J.J., Oorschot, V., Ploegh, H.L. and Geuze, H.J. (1991)

36 Niebling, W.L. and Pierce, S.K. (1993) J. Immunol. 150, 2687–2697

Nature 349, 669–676

37 Escola, J-M., Grivel, J-C., Chavrier, P. and Gorvel, J-P. (1995) J. Cell Sci.

8 Glickman, J., Morton, P.A., Slot, J.W., Kornfeld, S. and Geuze, H.J. (1996)

108, 2337–2345

J. Cell Biol. 132, 769–785

38 Cella, M., Engering, A., Pinet, V., Pieters, J. and Lanzavecchia, A. (1997)

9 Teyton, L., O’Sullivan, D., Dickson, P.W. et al. (1990) Nature 348, 39–44

Nature 388, 782–787

10 Lotteau, V., Teyton, L., Peleraux, A. et al. (1990) Nature 348, 600–605

39 Liou, W., Geuze, H.J., Geelen, M.J.H. and Slot, J.W. (1997) J. Cell Biol. 136,

11 Mellman, I. (1996) Annu. Rev. Cell Dev. Biol. 12, 575–625

61–70

12 Sallusto, F., Cella, M., Danieli, C. and Lanzavecchia, A. (1995) J. Exp.

40 Cuervo, A.M. and Dice, J.F. (1996) Science 273, 501–503

Med. 182, 389–400

41 Raposo, G., Nijman, H.W., Stoorvogel, W. et al. (1996) J. Exp. Med. 183,

13 Geuze, H.J., Slot, J.W., Strous, G.J. et al. (1984) Cell 37, 195–204

1161–1172

14 Stoorvogel, W., Strous, G.J., Geuze, H.J., Oorschot, V. and Schwartz,

42 Wubbolts, R., Fernandez-Borja, M., Oomen, L. et al. (1996) J. Cell Biol.

A.L. (1991) Cell 65, 417–427

135, 611–622

15 Tulp, A., Verwoerd, D., Dobberstein, B., Ploegh, H.L. and Pieters, J.

43 Pond, L. and Watts, C. (1997) J. Immunol. 159, 543–553

(1994) Nature 369, 120–126

44 Reid, P.A. and Watts, C. (1992) Immunology 77, 539–542

Coming soon in IT • Helicobacter pylori: a possible role initiating autoimmune gastritis and pernicious anaemia • CD1 peptides – lipid antigen presentation to T cells • Allergy and asthma: the genetics of complex disease • Heat shock proteins – immunoregulation of the T-cell response in inflammatory disease • The effects of virion-bound host-derived proteins on HIV-1 infection • Regulation of transcription of MHC class I and class II genes – a role for CIITA and RFX • Potent allergenicity of Der p 1 due to cleavage of CD23 and CD25

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