The Cell Biology of Antigen Presentation

Experimental Cell Research 272, 1–7 (2002) doi:10.1006/excr.2001.5402, available online at http://www.idealibrary.com on

MINIREVIEW The Cell Biology of Antigen Presentation Amy W. Hudson and Hidde L. Ploegh 1 Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Boston, Massachusetts 02115

Immunity to both extracellular and intracellular pathogens is achieved in part when T lymphocytes recognize foreign antigen in the context of antigenpresenting molecules. Antigen recognition by T cells occurs via binding of the T-cell receptor (TCR) to MHC class I, class II, or CD1 molecules. Extracellular bacterial or parasitic challenges to the immune system are mediated in part by antibody recognition of extracellular antigens. Antibodies are produced by B lymphocytes, which require the help of activated antigen-specific CD4 ⫹ T cells to proliferate and secrete highaffinity antibodies. Viruses, and some bacterial pathogens and parasites, replicate within host cells, where they cannot be detected by antibodies. The destruction of virus-infected cells is the task of cytotoxic T lymphocytes that possess T-cell receptors specific for viral peptides presented in the context of MHC class I molecules. This review will focus on the three modes of antigen presentation to T cells and the cell biological aspects of assembly and trafficking of the molecules involved.

achieved their proper conformation. Another ER chaperone, ERp57, assists with disulfide bond formation. Assembly of class I heavy-chain molecules requires association with its light chain, ␤ 2-m. Folded class I heavy chains assembled with ␤ 2-m are retained within the ER in a complex with calnexin and ERp57 until they are bound to antigenic peptide [1]. The order in which the different chaperones bind and the presence of additional protein products in this peptide-loading complex may vary subtly between different species (e.g., mouse, human). Further, it is possible that still more ER resident proteins play a role in this folding process (for review, see [2]). Peptide loading of class I molecules does not proceed efficiently unless the class I heavy chain/ calnexin/␤ 2 -m/ERp57 complex binds to tapasin, a class I-specific chaperone required for association of the complex with the peptide transporter TAP (transporter associated with antigen processing) [3]. Peptides generated in the cytoplasm enter the ER via the TAP complex and are then loaded into the the peptide-binding groove of heavy-chain molecules. The mechanism of peptide loading the class I heavy chain is not entirely clear. Transport of peptides into the ER is not sufficient for the class I molecules to become loaded with peptide; physical association of the class I assembly complex with the TAP transporter seems to be necessary for efficient peptide binding within the class I peptide-binding groove, since class I molecules in cells that lack tapasin are not loaded with peptide efficiently. Whether tapasin serves mainly to detain empty class I molecules at the site of loading, linking the source of peptides with empty class I molecules, or whether it takes a more active role in the loading of peptides is poorly understood. The final step in readying MHC class I molecules for transport to the plasma membrane is the departure of properly folded peptide-loaded class I molecules from the peptide-loading complex. How the chaperones sense proper conformation of a protein remains to be determined, and MHC class I molecules would appear an attractive model to pursue this question. The peptide-bound class I/␤ 2-m complexes then travel to the plasma membrane via the secretory pathway.

CLASS I MOLECULES—ASSEMBLY AND PEPTIDE LOADING

MHC class I heavy chain molecules are translated into the ER and glycosylated cotranslationally. They assemble with a light chain, ␤ 2-microglobulin (␤ 2-m), and become stable complexes only when antigenic peptide is bound within the peptide-binding groove of the heavy chain (see Fig. 1). Since all nucleated cells are prone to virus infection, MHC class I molecules are present or can be induced on almost all cells in the body. Proper folding and assembly of the peptide-bound class I complex requires the help of molecular chaperones. The single N-linked glycan on class I heavy-chain molecules serves as a point of recognition for the ER membrane chaperone calnexin and soluble chaperone calreticulin, both of which aid in the proper folding and retention of the class I molecules until they have 1

To whom correspondence and reprint requests should be addressed. E-mail: [email protected]. 1

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FIG. 1. Antigen presentation pathways. (Red) MHC class I heavy-chain molecules (red) are translated into the ER and glycosylated cotranslationally. They assemble with a light chain, ␤ 2-m (black) and become stable complexes when antigenic peptide is bound within the peptide-binding groove of the heavy chain. Folded class I heavy chains assembled with ␤ 2-m are retained within the ER in a complex with calnexin and ERp57 until they are bound to antigenic peptide (gray). (Blue) MHC class II ␣ and ␤ chains (blue) are translated into the ER and glycosylated cotranslationally, relying on chaperones for proper folding (ERp57 and calnexin shown in black). They assemble with invariant chain (blue) in a nonameric complex of (␣/␤/Ii) 3 held together by three invariant chain molecules (not illustrated). Upon arrival in the peptide-loading compartment, invariant chains are proteolyzed, dissolving nonamers to leave a small fragment of the invariant chain, called CLIP (blue fragment), within the peptide-binding groove of the ␣/␤ class II heterodimer. HLA-DM (black) catalyzes release of the CLIP peptide from the peptide-binding groove of class II molecules until stability is conferred upon the class II molecule by high-affinity binding of peptide (gray) within the peptide-binding groove. (Green) CD1 molecules (green) are translated into the ER, glycosylated, and bound by the chaperones calnexin (black) and calreticulin (not shown) to facilitate folding and assembly of CD1 molecules with ␤ 2-m (black). CD1 molecules may bind to and present endogenous or “self” glycosphingolipids (black) present in the ER. The trafficking of CD1 molecules is isoform dependent, but all CD1 molecules travel to the plasma membrane and are internalized, where they may sample different intracellular compartments for lipid antigens.

CLASS II MOLECULES—ASSEMBLY, PEPTIDE LOADING, AND TRAFFICKING

While class I antigen presentation is devoted primarily to antigens generated by proteasomal proteolysis in the cytosol, the class II pathway of antigen presentation is geared toward the presentation of extracellularly derived antigens, or antigens generated within the endolysosomal pathway, such as might be introduced by a macrophage that has engulfed bacteria. MHC class II expression is confined to a small subset of

“professional” antigen-presenting cells (APCs), which include macrophages, B cells, and especially so-called dendritic cells. All of these cell types excel at endocytosis of extracellular antigens. Like MHC class I molecules, MHC class II molecules are also translated into the ER and glycosylated cotranslationally (see Fig. 1). MHC class II molecules consist of an ␣ and a ␤ chain, each possessing immunoglobulin (Ig)-like folds. The two proteins assemble together to form a peptide-binding groove very similar in structure to that of MHC class I molecules (see Fig.

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FIG. 2. Alignment of X-ray crystal structures derived from recombinant soluble forms of MHC class I, MHC class II, and CD1 molecules. The human MHC class I HLA-A2 molecule complexed with the Tax8 peptide from human T cell lymphotrophic virus-1 (HTLV-1) was aligned with the human class II MHC HLA-DR1 molecule complexed with an influenza virus peptide and the mouse CD1d1 molecule, using the structural alignment program MOLMOL [33]. The ␤-sheets of the MHC class I and CD1 heavy chains and the ␣ chain of MHC class II are shown in turquoise. ␤ 2-Microglobulin strands are blue. The ␤ chain of the MHC class II molecule is purple. The ␣-helices of the heavy chains that comprise the peptide-binding groove are red and yellow when encoded in the heavy chain and red and white when encoded by the ␤ chain of MHC class II. The peptides are shown with their side chains in black.

2). Both class II chains are glycosylated and hence can associate with the chaperone calnexin as they assemble and achieve their proper conformation. Unlike MHC class I molecules, class II molecules do not acquire peptides in the ER. Rather, they assemble as a heterotrimer with a third transmembrane glycoprotein, invariant chain (Ii), which binds within the peptide-binding groove of the class II chains and prevents binding of ER peptides intended for class I molecules. Proper assembly of the ␣/␤/Ii heterotrimer cues release from calnexin and subsequent formation of a nonameric complex of (␣/␤/Ii) 3 held together by three invariant chain molecules. The ␣/␤/Ii nonamer then departs from the ER on its way to the peptide-loading compartment. In addition to acting as a molecular chaperone for class II heterodimers during assembly and blocking improper peptides from binding in the peptide-binding groove, invariant chain also contains targeting information in its cytoplasmic N terminus that dictates the next stop for class II molecules. Two pairs of hydrophobic amino acid sequences in the cytoplasmic tail of Ii, leu-ile and leu-met, direct targeting of the (␣/␤/Ii) 3

nonamer to its next destination, an acidic endolysosomal compartment where antigenic peptides derived from the endocytic pathway are generated by lysosomal proteases [4, 5]. Upon arrival in the peptide-loading compartment, invariant chains are proteolyzed by cathepsins S, L, and F, as well as other proteases that remain to be identified, dissolving nonamers to leave a small fragment of the invariant chain, called CLIP (class IIassociated invariant chain peptide), within the peptide-binding groove of the ␣/␤ class II heterodimer (for review, see [6]). CLIP functions to hold the ␣/␤ heterodimer together and continues to block the peptidebinding groove. Of note, the invariant chain not only serves as a chaperone for class II molecules; it also interacts with cathepsin L (CatL) in a chaperone-like manner and helps control the levels of active Cat L in APCs [7]. Just as with tapasin and class I peptide loading, loading of peptides in the class II binding groove is tightly regulated and occurs in close association with specific molecular chaperones. A heterodimeric protein with a structure similar to that of class II molecules,

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called HLA-DM (DM), catalyzes release of the CLIP peptide from the peptide-binding groove of class II molecules. Like the association of tapasin with class I molecules, DM retains class II molecules until stability is conferred upon the class II molecule by high-affinity binding of peptide within the peptide-binding groove. A further MHC class II-like accessory protein, HLA-DO, may play a role as well, although its function is less well understood. CD1 MOLECULES—ASSEMBLY, PEPTIDE LOADING, AND TRAFFICKING

The MHC class I and class II antigen-presentation pathways are part of the adaptive immune response, characterized by the ability of T cells to recognize a tremendous number of peptide–MHC combinations. To present such a diverse array of peptides, class I and II molecules must be fairly flexible in the subset of antigenic peptides that can bind within their peptide-binding grooves. Generation of diversity of the class I and class II antigen-presenting molecules is aided by the existence of multiple MHC class I and II genes, all of which are highly polymorphic. Once triggered by the appropriate peptide–MHC complex, T cells proliferate and, through clonal expansion, generate armies of effector cells specific for that particular antigen. This process can take days; defense against the initial wave of infection is handled by the innate immune system. The innate immune response is characterized by a rapid response to common constituents of microorganisms, such as the lipid components of bacterial cells walls. CD1 is a protein whose structure is remarkably similar to that of MHC class I and class II molecules (see Fig. 2). Rather than presenting peptides to T cells, CD1 has been shown to present lipid components in its antigen-binding groove. Unlike MHC class I and class II molecules, CD1 molecules are not encoded within the major histocompatibility complex and are not polymorphic. Instead, the CD1 gene family encodes antigenpresenting molecules that combine features of the innate and adapative immune systems. Considerable differences exist between species in number of CD1 loci. In humans, the CD1 gene family comprises 5 isoforms (CD1a– e), while mice and rats express only CD1d (for review, see [8]). The CD1 assembly mechanism has not yet been examined as thoroughly as it has for class I and class II molecules, but like class I molecules, CD1 is translated into the ER, glycosylated, and bound by the chaperones calnexin and calreticulin to facilitate folding and assembly of CD1 molecules with ␤ 2-m. In fact, ␤ 2-m binds more tightly to CD1b than it does to class I molecules [9]. Since CD1 possesses an Ig fold, the thiol reductase ERp57 might be predicted to be involved in CD1 ER assembly as well, but this association has yet to be demonstrated. Unlike class I molecules, CD1 assembly

complexes are not predicted to associate with tapasin or TAP, since CD1 does not present peptide antigens. Antigen loading of CD1 molecules combines principles of peptide loading from the class I and class II pathways. Like class II molecules, CD1 molecules are mainly expressed on professional antigen-presenting cells, although their patterns of expression are fairly broad. CD1 molecules can bind to and present lipid antigens from mycobacteria as well as endogenous or “self” glycosphingolipids and glycosylphosphatidylinositols. They can also associate with other phospholipids, glycosylated mycolates, free mycolic acids, and glycosphingolipids (for review see [8]). Like the class II ␣/␤/Ii heterotrimer, which is targeted to an endolysomal compartment where it binds to peptides derived from the extracellular medium and processed by proteolysis in lysosomes, CD1 molecules also acquire antigens in the endolysosomal pathway derived from endocytosed bacterial products. Moreover, like MHC class II-bound antigens, CD1d-bound lipid antigens also appear to be processed by lysosomal hydrolases before binding to CD1d molecules. An example of glycolipid antigen processing was recently demostrated to depend on lysosomal hydrolases that remove sugars linked to the primary galactose molecule on the glycosphingolipid ␣-galactosyl ceramide [10]. Not surprisingly, these lysosomal hydrolases are pH dependent; incubation of dendritic cells in the presence of the vacuolar ATPase/proton pump inhibitors blocked antigen presentation of ␣-galactosyl ceramides possessing additional galactose moieties on the primary galactose of the glycolipid [10]. The presence of multiple CD1 gene products is observed in humans and most other mammals, with the notable exception of mice and rats, apparently ensuring that a variety of intracellular compartments can be sampled for lipid antigens. The CD1b, c, and d molecules possess a tyrosine-based sorting signal in their cytoplasmic tail, responsible for targeting them to endocytic antigen-loading compartments, but all four of these isoforms apparently localize to and sample lipid products in different regions of the endolysosomal pathway [11–15]. CD1b is targeted to lysosomes by its tyrosine-based sorting signal and colocalizes with HLA-DM in a MHC class II peptide-loading compartment [13]. However, CD1b molecules arrive at the late endosomal loading compartment via a different pathway than do class II molecules. MHC class II molecules are sorted via the cytoplasmic signal in the invariant chain directly from the TGN to the acidic MHC class II-loading compartment, where they acquire peptide antigen before then proceeding to the cell surface. This process is slow; newly synthesized peptide-bound class II molecules require 2– 4 h to reach the cell surface. HLA-DM, on the other hand, contains a tyrosine-based endocytosis signal that results in the sorting of DM to lysosomes from the plasma membrane. Because CD1b

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molecules contain a similar tyrosine-based sorting signal and CD1b molecules are reported to arrive at the cell surface at the faster rates characteristic of transferrin receptors and MHC class I molecules [16], CD1b and CD1d molecules probably arrive at their lysosomal destination via internalization from the plasma membrane. Like CD1b, CD1d localizes to endosomes/lysosomes and is dependent on its tyrosine-based sorting signal for this localization, but CD1d might also acquire lipid antigen in the secretory pathway. Chiu et al. have isolated two populations of CD1d-reactive T cells— one dependent and one not dependent on the tyrosine-based internalization signal for recognition of CD1d, suggesting that CD1d molecules that are not directed to lysosomes can still acquire antigen, perhaps in the secretory pathway [15]. Of all the isoforms of CD1, CD1c has the broadest distribution within the cell, labeling plasma membrane, small vesicles, lysosomes, and the recycling pathway of the early endocytic compartment [11, 14]. CD1a, the only member of the family lacking a tyrosine-based sorting signal, localizes to recycling endosomes. Sugita et al. postulate that localization of CD1a in the recycling compartment may allow CD1a to sample lipids derived from endocytosed mycobacteria that are present in recycling endosomes and not in later parts of the endolysosomal pathway [12]. In support of this hypothesis, data from Mukherjee et al. describe endocytic sorting of three fluorescent lipid-mimetic derivatives differing in the length or degree of saturation of their acyl chains. Lipids with short unsaturated acyl chains were found to localize in recycling endosomes, while long saturated lipids were present in late endosomes [17]. How the antigen binding grooves of CD1 are kept free of endogenous lipid products is not known. Is there a molecular chaperone similar to invariant chain for class II molecules that prevents endogenous lipids from binding within the antigen-binding groove? Could this chaperone be Ii itself? Can endogenous lipids be readily displaced by microbe-derived materials, and if so, are specific catalysts and transfer proteins involved? How are the hydrophobic acyl chains maintained within the endolysomal pathway? These questions are just beginning to be addressed. Many autoreactive T cells react with CD1 molecules that present “self” lipid products, suggesting that CD1 molecules do bind to and present endogenous lipids. One report postulates that endogenous glycophosphatidylinositol (GPI) lipids in the ER may act as chaperones for CD1d, since GPI was the predominant low-molecular-weight ligand that was eluted from purified CD1d molecules [18]. This proposal would require that an HLA-DM equivalent act to remove GPI once in the proper loading compartment. Likewise, chaperones for mycobacterial lipids to enter the appropriate antigen--

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loading endocytic compartment have been proposed. The macrophage mannose receptor appears to be responsible for proper trafficking of the mycobacterial lipoglycan lipoarabinomannan (LAM) from the plasma membrane to the lipid antigen-loading compartment. Blocking mannose receptor function with either the mannose receptor ligand ␣-mannan or an antiserum directed against the mannose receptor resulted in reduced LAM internalization or reduced IFN-␥ production by CD1b-LAM-specific T cells [19].

STRUCTURALLY SIMILAR ANTIGEN-PRESENTATION MOLECULES

Considering the structural similarity between class I, class II, and CD1 molecules (see Fig. 2), it is conceivable that a protein associating with class I molecules might also be able to associate with class II or CD1 molecules. This appears to be the case for the human cytomegalovirus (HCMV) viral glycoproteins US2 and US3. US2 binds to and causes the dislocation of class I heavy-chain molecules through the ER membrane into the cytosol, where they are ubiquitinated and degraded by the proteasome [20]. US3 associates transiently with MHC class I molecules, resulting in retention of class I molecules in the ER [21, 22]. Recently, US2 was shown to bind to the ␣ chains of MHC class II and HLA-DM molecules and cause their degradation, also in a proteasome-dependent manner [23]. The structure of HCMV US2 in complex with the class I heavy chain and ␤ 2-m has been solved, and the contact sites between US2 and the class I heavy-chain HLA-A2 allele have been identified [24]. The region of the class I heavy-chain molecule that contacts US2 is poorly conserved in class II␣ and HLA-DM␣ molecules. Further, purified US2 does not associate with recombinant soluble class II or HLA-DM ␣ chains [25]. These data suggest that US2 may bind to class II and DM ␣ chains at a site distinct from the regions corresponding to the class I heavy chain, or that another factor may be required to stabilize the interaction between US2 and the ␣ chains of class II and HLA-DM. Promiscuous association of the class II invariant chain with class I molecules has been described as well. Invariant chain, primarily responsible for chaperoning class II molecules to the endocytic peptide-loading compartment, could be crosslinked to a small subset of MHC class I molecules in a human lymphocytic cell line [26], which raises the question whether Ii might act physiologically to deliver a fraction of class I molecules to the endolysosomal class II loading compartment for peptide loading. If so, this could explain how class I and CD1 molecules sample exogenously derived antigen.

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IMMUNE EVASION BY DOWNREGULATION OF MHC CLASS II OR CD1

Viruses have provided excellent tools for the study of cell biological processes involved in antigen presentation and processing. Many different viral protein products have evolved to help viruses avoid detection by the immune system. Since immunity to virus infection is generally achieved when peptides from viral proteins are presented to cytotoxic T lymphocytes on MHC class I molecules, one of the most frequently observed targets of these viral proteins is the appearance of viral peptides in the context of class I molecules on the cell surface. We have already discussed HCMV US2 and US11, which cause proteasome-dependent degradation of class I molecules. Certainly for bacterial infections, evasion of the immune response might better be accomplished by downregulating class II or CD1 antigen presentation by APCs. Immunity to bacterial agents is mediated primarily by the humoral immune response, in which bacterial products are phagocytosed or internalized by an antigen-presenting cell. These antigens enter the endolysosomal pathway, where protein and lipid products are digested and can load onto class II and CD1 molecules for presentation to T cells. One example of immune evasion carried out in this manner is the bacterium Helicobacter pylori, which encodes a 95-kDa protein toxin designated VacA (for review, see [27]). VacA is a bacterially secreted protein that causes vacuolation of acidic endocytic compartments, resulting in an elevation of endolysosomal pH sufficient to account for general decreased activity of lysosomal proteases as well as mistargeting of the lysosomal protease cathepsin D [28]. In the absence of proper lysosomal protease function, the generation of class II-specific peptides might be impaired, as may the proper proteolytic cleavage of the invariant chain molecules. Molinari and colleagues have demonstrated that VacA toxin released by H. pylori lowered the number of T-cell epitopes generated in the antigen-processing compartment, reducing T-cell proliferation induced by antigenpresenting cells [29]. Similar to VacA, several viral proteins have been shown to bind to the vacuolar H ⫹ ATPase, possibly inhibiting its function and resulting in increased endolysosomal pH. The bovine and human papillomavirus E5 proteins [30, 31] and the human T-cell leukemia/lymphotropic virus (HTLV) type I p12 I protein [32] both bind to the 16-kDa subunit of the vacuolar H ⫹ ATPase. Expression of the papillomavirus E5 protein resulted in alkalinization of the Golgi, suggesting that E5 binding to the proton pump interfered with ATPase function [33]. Additionally, the HIV Nef protein was shown to interact indirectly with the 56-kDa catalytic subunit of the vacuolar H ⫹ ATPase, via the Nef-binding protein NBP-1 [34]. Why the vacuolar proton pump

subunits are a common target of these viral proteins is not known. Although virus infection is normally handled by the class I antigen-presenting pathway, virus particles released by an infected cell might enter neighboring antigen-presenting cells via the endocytic pathway, and peptides generated in the endocytic compartment might then be presented on class II molecules. It therefore seems plausible that viral proteins, too, could affect the formation of peptide-loaded class II molecules in a manner similar to that of H. pylori. Can lipid antigen presentation on CD1 molecules be downregulated by pathogenic agents in the same way as MHC class I and class II molecules? Just as for class II antigen presentation, chemical inhibitors of the vacuolar proton pump were shown to completely block CD1b-mediated antigen presentation to T cells, demonstrating absolute requirement for endosomal acidification in some aspect of CD1 antigen presentation. [35, 36]. Low pH may also contribute to lipid antigen loading by causing a conformational change within the antigen-binding groove of CD1b that exposes a hydrophobic lipid binding site [9]. Thus, interference with establishment of proper intraorganellar pH could very well interfere with antigen presentation by CD1 molecules. Likewise, if invariant chain were to fulfill a role as chaperone to CD1 molecules, bacterial pathogens that cause elevation of lysosomal pH might have an effect on invariant chain degradation, making the lipid antigen-binding groove inaccessible. Perhaps, then, a search for surface-downregulated CD1 molecules might help to uncover new principles of bacterial immune evasion. The authors thank Dr. Kylie Walters for help with preparation of Fig. 2 and Dr. Susanne Wells for critical reading of the manuscript.

REFERENCES 1.

Lindquist, J. A., Jensen, O. N., Mann, M., and Hammerling, G. J. (1998). ER-60, a chaperone with thiol-dependent reductase activity involved in MHC class I assembly. EMBO J. 17(8), 2186 –2195.

2.

Cresswell, P. (2000). Intracellular surveillance: Controlling the assembly of MHC class I-peptide complexes. Traffic 1(4), 301– 305.

3.

Ortmann, B., Copeman, J., Lehner, P. J., Sadasivan, B., Herberg, J. A., Grandea, A. G., Riddell, S. R., Tampe, R., Spies, T., Trowsdale, J., and Cresswell, P. (1997). A critical role for tapasin in the assembly and function of multimeric MHC class I-TAP complexes. Science 277(5330), 1306 –1309.

4.

Bremnes, B., Madsen, T., Gedde-Dahl, M., and Bakke, O. (1994). An LI and ML motif in the cytoplasmic tail of the MHC-associated invariant chain mediate rapid internalization. J. Cell Sci. 107(Pt. 7), 2021–2032.

5.

Odorizzi, C. G., Trowbridge, I. S., Xue, L., Hopkins, C. R., Davis, C. D., and Collawn, J. F. (1994). Sorting signals in the MHC class II invariant chain cytoplasmic tail and transmembrane region determine trafficking to an endocytic processing compartment. J. Cell Biol. 126(2), 317–330.

CELL BIOLOGY OF ANTIGEN PRESENTATION 6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

Turk, V., Turk, B., and Turk, D. (2001). New EMBO Members’ Review: Lysosomal cysteine proteases: Facts and opportunities. EMBO J. 20(17), 4629 – 4633. Lennon-Dumenil, A. M., Roberts, R. A., Valentijn, K., Driessen, C., Overkleeft, H. S., Erickson, A., Peters, P. J., Bikoff, E., Ploegh, H. L., and Wolf Bryant, P. (2001). The p41 isoform of invariant chain is a chaperone for cathepsin L. EMBO J. 20(15), 4055– 4064. Matsuda, J. L., and Kronenberg, M. (2001). Presentation of self and microbial lipids by CD1 molecules. Curr. Opin. Immunol. 13(1), 19 –25. Ernst, W. A., Maher, J., Cho, S., Niazi, K. R., Chatterjee, D., Moody, D. B., Besra, G. S., Watanabe, Y., Jensen, P. E., Porcelli, S. A., Kronenberg, M., and Modlin, R. L. (1998). Molecular interaction of CD1b with lipoglycan antigens. Immunity 8(3), 331–340. Prigozy, T. I., Naidenko, O., Qasba, P., Elewaut, D., Brossay, L., Khurana, A., Natori, T., Koezuka, Y., Kulkarni, A., and Kronenberg, M. (2001). Glycolipid antigen processing for presentation by CD1d molecules. Science 291(5504), 664 – 667. Briken, V., Jackman, R. M., Watts, G. F., Rogers, R. A., and Porcelli, S. A. (2000). Human CD1b and CD1c isoforms survey different intracellular compartments for the presentation of microbial lipid antigens. J. Exp. Med. 192(2), 281–288. Sugita, M., Grant, E. P., Van Donselaar, E., Hsu, V. W., Rogers, R. A., Peters, P. J., and Brenner, M. B. (1999). Separate pathways for antigen presentation by CD1 molecules. Immunity 11(6), 743–752. Sugita, M., Jackman, R. M., Van Donselaar, E., Behar, S. M., Rogers, R. A., Peters, P. J., Brenner, M. B., and Porcelli, S. A. (1996). Cytoplasmic tail-dependent localization of CD1b antigen-presenting molecules to MIICs. Science 273(5273), 349 – 352. Sugita, M., Van Der Wel, N., Rogers, R. A., Peters, P. J., and Brenner, M. B. (2000). CD1c molecules broadly survey the endocytic system. Proc. Natl. Acad. Sci. USA 97(15), 8445– 8450. Chiu, Y. H., Jayawardena, J., Weiss, A., Lee, D., Park, S. H., Dautry-Varsat, A., and Bendelac, A. (1999). Distinct subsets of CD1d-restricted T cells recognize self-antigens loaded in different cellular compartments. J. Exp. Med. 189(1), 103–110. Briken, V., Moody, D. B., and Porcelli, S. A. (2000). Diversification of CD1 proteins: Sampling the lipid content of different cellular compartments. Semin. Immunol. 12(6), 517–525. Mukherjee, S., Soe, T. T., and Maxfield, F. R. (1999). Endocytic sorting of lipid analogues differing solely in the chemistry of their hydrophobic tails. J. Cell Biol. 144(6), 1271–1284. Joyce, S., Woods, A. S., Yewdell, J. W., Bennink, J. R., De Silva, A. D., Boesteanu, A., Balk, S. P., Cotter, R. J., and Brutkiewicz, R. R. (1998). Natural ligand of mouse CD1d1: Cellular glycosylphosphatidylinositol. Science 279(5356), 1541–1544. Prigozy, T. I., Sieling, P. A., Clemens, D., Stewart, P. L., Behar, S. M., Porcelli, S. A., Brenner, M. B., Modlin, R. L., and Kronenberg, M. (1997). The mannose receptor delivers lipoglycan antigens to endosomes for presentation to T cells by CD1b molecules. Immunity 6(2), 187–197. Wiertz, E. J., Tortorella, D., Bogyo, M., Yu, J., Mothes, W., Jones, T. R., Rapoport, T. A., and Ploegh, H. L. (1996). Sec61mediated transfer of a membrane protein from the endoplasmic

Received September 24, 2001

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

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reticulum to the proteasome for destruction. Nature 384(6608), 432– 438. Gruhler, A., and Fruh, K. (2000). Control of MHC class I traffic from the endoplasmic reticulum by cellular chaperones and viral anti-chaperones. Traffic 1(4), 306 –311. Jones, T. R., Wiertz, E. J., Sun, L., Fish, K. N., Nelson, J. A., and Ploegh, H. L. (1996). Human cytomegalovirus US3 impairs transport and maturation of major histocompatibility complex class I heavy chains. Proc. Natl. Acad. Sci. USA 93(21), 11327– 11333. Tomazin, R., Boname, J., Hegde, N. R., Lewinsohn, D. M., Altschuler, Y., Jones, T. R., Cresswell, P., Nelson, J. A., Riddell, S. R., and Johnson, D. C. (1999). Cytomegalovirus US2 destroys two components of the MHC class II pathway, preventing recognition by CD4⫹ T cells. Nat. Med. 5(9), 1039 –1043. Gewurz, B. E., Gaudet, R., Tortorella, D., Wang, E. W., Ploegh, H. L., and Wiley, D. C. (2001). Antigen presentation subverted: Structure of the human cytomegalovirus protein US2 bound to the class I molecule HLA-A2. Proc. Natl. Acad. Sci. USA 98(12), 6794 – 6799. Gewurz, B. E., Wang, E. W., Tortorella, D., Schust, D. J., and Ploegh, H. L. (2001). Human cytomegalovirus US2 endoplasmic reticulum–lumenal domain dictates association with major histocompatibility complex class I in a locus-specific manner. J. Virol. 75(11), 5197–5204. Sugita, M., and Brenner, M. B. (1995). Association of the invariant chain with major histocompatibility complex class I molecules directs trafficking to endocytic compartments. J. Biol. Chem. 270(3), 1443–1448. Reyrat, J. M., Pelicic, V., Papini, E., Montecucco, C., Rappuoli, R., and Telford, J. L. (1999). Towards deciphering the Helicobacter pylori cytotoxin. Mol. Microbiol. 34(2), 197–204. Satin, B., Norais, N., Telford, J., Rappuoli, R., Murgia, M., Montecucco, C., and Papini, E. (1997). Effect of Helicobacter pylori vacuolating toxin on maturation and extracellular release of procathepsin D and on epidermal growth factor degradation. J. Biol. Chem. 272(40), 25022–25028. Molinari, M., Salio, M., Galli, C., Norais, N., Rappuoli, R., Lanzavecchia, A., and Montecucco, C. (1998). Selective inhibition of Ii-dependent antigen presentation by Helicobacter pylori toxin VacA. J. Exp. Med. 187(1), 135–140. Goldstein, D. J., Finbow, M. E., Andresson, T., Mclean, P., Smith, K., Bubb, V., and Schlegel, R. (1991). Bovine papillomavirus E5 oncoprotein binds to the 16K component of vacuolar H(⫹)-ATPases. Nature 352(6333), 347–349. Straight, S. W., Herman, B., and Mccance, D. J. (1995). The E5 oncoprotein of human papillomavirus type 16 inhibits the acidification of endosomes in human keratinocytes. J. Virol. 69(5), 3185–3192. Franchini, G., Mulloy, J. C., Koralnik, I. J., Lo Monico, A., Sparkowski, J. J., Andresson, T., Goldstein, D. J., and Schlegel, R. (1993). The human T-cell leukemia/lymphotropic virus type I p12I protein cooperates with the E5 oncoprotein of bovine papillomavirus in cell transformation and binds the 16-kilodalton subunit of the vacuolar H⫹ ATPase. J. Virol. 67(12), 7701– 7704. Koradi, R., Billeter, M., and Wu¨thrich, K. (1996). MOLMOL: A program for display and analysis of macromolecular structure. J. Mol. Graphics 14, 51–55.