Antigen presentation: structural themes and functional variations

Antigen presentation: structural themes and functional variations Thomas J. Braciale and Vivian L. Braciale T cells recognize normative processed fragments of antigens presented in association with major histocompatibility complex (MHC) class I or class H molecules. Recently, an accumulating body of evidence has provided a functional linkage between antigen presentation events and the cell biology of MHC molecule assembly and transport. In this review Thomas and Vivian Braciale synthesize these developments into a cohesive model of MHC assembly and antigen presentation pathways. The vertebrate immune system undoubtedly evolved to deal with a major problem faced by all higher eukaryotes, namely parasitism by foreign microorganisms. The general approach to this problem for vertebrates was the development of a recognition system that could both anticipate the antigenic universe and exhibit a high degree of specificity in ligand binding. To combat invasion by free-living extracellular microorganisms, vertebrates evolved a soluble receptor, the immunoglobulin molecule, which can gain access to the extracellular space, bind the antigenic moiety on the parasite and facilitate its elimination. The binding of immunoglobulin molecules to antigen is particularly sensitive to the tertiary structure of the antigens on the microorganism. Intracellular parasitism posed a more perplexing challenge for higher eukaryotes. The response to these relatively inaccessible intracellular microorganisms was the development by higher vertebrates of a highly specific cell surface receptor displayed on an effector cell, the T cell. The T-cell antigen receptor (TCR) does not recognize free antigen but rather recognizes small peptide fragments derived from the proteins of the microorganisms that are associated with products of the MHC. Fragmentation of the viral, bacterial or parasitic polypeptides is presumed to be carried out by the action of cellular proteases. The resulting fragments are sampled by the MHC molecules within the cell and the complex of peptide and MHC molecule is displayed on the cell surface. MHC-restricted recognition of foreign antigen by T cells, therefore, reflects the specificity of the TCR for the complex of MHC molecules and antigen fragments. Over the last few years, the nature of the interaction between peptides and MHC molecules has become better understood in molecular terms. The demonstration of direct binding of peptide fragments to isolated MHC molecules 1 provided the first clear-cut evidence that MHC molecules interact with processed antigen independently of the TCR engagement. Thus, the function of MHC molecules as antigen-binding moieties was directly established. With the determination of the crystal structure of the human MHC class I molecule2 and the identification of a putative peptide-binding groove or cleft in the molecule, the structural basis of this interaction was elucidated. The peptide-binding cleft can accommodate a peptide of 10-20 amino acids 2. Available evidence

suggests that a peptide of seven to ten residues, produced by processing the antigen within the cell, may normally occupy the MHC class I cleft3,4. One issue that had not been explored until recently was the relationship of antigen processing events to MHC molecule synthesis and expression. Several sets of observations have now provided a link between antigen presentation by MHC molecules and the cell biology of MHC molecule assembly, transport and the intracellular sites of antigen-MHC interaction. The aim of this review is to highlight several of these observations and to explore the potential implications of these and related findings for antigen presentation.

Defective antigen presentation and MHC class I assembly The discovery that peptide-treated target cells can be recognized by CD8 +, MHC class-I-restricted, T cells5 combined with the demonstration of electron-dense material in the binding cleft of the crystallized human MHC class I molecules 2, prompted speculation on the role of processed antigen in the assembly of MHC class I. Townsend et al.6, using the mutant cell line RMA-S, produced evidence that suggested a causal link between the peptide-MHC molecule association and the assembly and transport of nascent MHC class I molecules. The RMA-S cell line, derived from mutagenized, Rauscher-virus-transformed H-2 b lymphoma cells (RMA), was selected for low level expression of MHC class I molecules 7,8. RMA-S synthesizes both MHC class I heavy chain and [32-microglobulin ([32m)but these class I heavy chains, which primarily bear high mannose oligosaccharide, weakly associate with 132mand transit slowly to the cell surface9. Unlike its parent cell line RMA, RMA-S does not present an H-2 b haplotype minor histocompatibility antigen1° to MHC class-I-restricted cytotoxic T lymphocytes (CTL), nor can it present the influenza nucleocapsid protein (NP) product to NPspecific CTL after infection with influenza virus. Recognition of the RMA-S target by NP-specific CTL was achieved when RMA-S cells were exposed to a synthetic NP peptide corresponding to the CTL epitope on the NP. Importantly, together with target cell sensitization, exposure to specific peptide also resulted in (1) the enhanced expression of the specific MHC class I molecules

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at the cell surface, (2) enhanced association of nascent MHC class I heavy chain with endogenous f32m and (3) an increase in the more mature complex carbohydrate form of the molecule. These observations suggested that RMA-S may have a defect in its ability to process antigen or to transport antigen fragments generated endogenously within the cell. More importantly, they suggested a link between antigen fragment availability within the cell and the ability of MHC class I heavy chains and 132mto assemble and be transported to the cell surface. It thus appears that RMA-S is defective in both minor H and influenza virus presentation and is low in surface MHC class I expression because it cannot generate (or transport) the endogenous peptide fragments necessary for MHC class-I-132m association and transport. Exogenous addition of peptide presumably restored MHC class I assembly and transport to the cell surface by inducing (or stabilizing) the association of MHC class I with ]32m. Since peptide in the extracellular space can also shift nascent MHC class I molecules from a high mannose to a complex carbohydrate form, it was suggested that the synthetic peptide can enter the RMA-S cell and gain access to a pre-Golgi compartment (presumbly the endoplasmic reticulum (ER)), where it induces class I assembly and transport. These findings and speculations are consistent with previous evidence n,12 implicating the ER as the site where newly synthesized MHC class I molecules are charged by processed antigen fragments. These observations were followed by the report of peptide-induced upregulation of the mouse MHC class I molecule, L d, in normal cells 13. L d is normally expressed at low levels on H-2 d haplotype cells. This molecule is considered to exist on the cell surface in two forms which can be distinguished by antibodies 14. Exposure of an L dbinding peptide to the L
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The defect in these cells has been mapped to a gene in the human MHC region 16,18. Recently several sets of observations have provided further insight into the role of peptide in MHC class I assembly, transport and conformation. Notably, when cultured at a reduced temperature (26°C), RMA-S cells assemble, transport and express high levels of class-I[32m complexes 19. Low temperature incubation does not restore the capacity of RMA-S to present minor histocompatibility antigens or newly synthesized influenza NP to MHC class-I-restricted CTL. These class-I-J32m complexes at the cell surface are labile at 37°C but can be stabilized by exposing the cells to synthetic peptides that specifically bind the relevant MHC molecule ~9. Incubation of RMA-S or the presentation-competent RMA cell at 26°C also leads to the formation of class-I-J32m complexes that can efficiently bind specific peptides at cell surfaces or in detergent lysates in vitro 2°. These data have lead to the suggestion that peptide binding is not absolutely required for folding, assembly and transport of class-I-J32m dimers but rather that peptides may play a role in stabilizing certain MHC class I conformations. A similar conclusion was reached by an analysis of the assembly of MHC class I molecules in vitro: in cell lysates, the addition of either specific peptides or excess [32m leads to the formation of stable conformations recognized by particular antibodies 2°,21. The corollary of these data is that enhanced expression of MHC class I molecules in RMA-S cells after exposure to peptide at 3 7°C is due to peptide-mediated stabilization of unstable cell surface MHC class-I-J32m complexes by the extr> cellular peptide 19. The apparent in'duction of MHC class-I-f32m complex formation within the RMA-S cell after exposure to peptide can be attributed to residual cell-associated peptide that could induce or stabilize an M H C conformation in vitro during incubation of cell lysates 19,21. Retrograde transport of exogenous peptide to the ER need not, therefore, be invoked to account for the upregulation of MHC class I molecules on these mutant cell lines 22. However, all of these observations reinforce the view that peptide plays a critical role in maintaining MHC class I structural stability in the living cell. Whether peptide serves only to maintain certain preferred conformations or, more likely, is essential for proper class I assembly and intracellular transport during physiological conditions remains to be firmly established.

Defective antigen presentation and MHC class II structure A parallel line of investigation suggesting a link between antigen processing and MHC class II structure has evolved from the characterization of a human lymphoblastoid cell line (9.5.3) that is deficient in the presentation of protein antigens to CD4 +, class-II-restricted T cells 23. Like the RMA-S cell, the defect in 9.5.3 appears to be in processing/presentation of antigen-MHC complexes, since the cells could present preprocessed antigen, in the form of synthetic peptides, to the T cells. In contrast to the RMA-S line, this cell line expresses normal levels of MHC class II molecules. A similar defect in antigen presentation to CD4 + T cells has been described in antigen-presenting cells expressing low levels of the M H C class II invariant chain(Ii) 24. In the case of the 9.5.3

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line, a deficiency in li expression could not account for the phenotype since expression of Ii is comparable with that seen in the presentation-competent progenitor cell23. The processing-deficient 9.5.3 line differed from the progenitor line in two features. First, the HLA-DR dimers expressed by the mutant had lost the expression of two polymorphic DR determinants that are expressed by the progenitor line. Other polymorphic and monomorphic epitopes were normal. Second, the DR and DP dimers from progenitor cells retained their dimeric state after SDS polyacrylamide gel electrophoresis (PAGE) of cell extracts under nonreducing conditions, whereas dimers from mutant cells were less stable and dissociated into o~and [3 chain monomers under the same conditions. These observations were in keeping with the hypothesis that the MHC class II molecules on the surface of this processing-deficient cell line were unstable because they were empty: that is, they lacked peptide in their binding groove 23. This hypothesis is supported by the finding that culture of the progenitor line in the presence of inhibitors of endosomal antigen processing (using chloroquine) yields unstable MHC class II molecules that dissociate upon SDS-PAGE. These observations, as well as those with the RMA-S cells, suggest that processed antigen fragments may play a direct role in the induction or maintenance of a preferred MHC conformational state. Both class I and class II molecules when they are not occupied by peptide may be unstable dimers, susceptible to dissociation of [32m at 37°C, in the case of MHC class I, or as a weaker interaction between monomers in the case of MHC class II molecules. Empty MHC molecules appear to lose, to varying degrees, certain conformation-dependent serologic epitopes. The mutation leading to the endosomal processing defect in 9.5.3 has not yet been defined. It will be interesting to assess whether the defect in this, or similar, presentation/processing mutants map to the MHC locus where at least one class I presentation/processing mutation has been tentatively mapped 16. In this context it should be noted that an altered MHC class II conformation has been noted in a cell line that is deficient in Ii expression 2s. Whether defective (or the absence of) Ii interaction with MHC class II dimers results in 'empty' MHC molecules at the cell surface needs to be determined.

the physiological stabilization of the class I-[32m complex and subsequent efficient transit out of the ER through the secretory pathway. With certain notable exceptions, for example L d molecules 13 and mouse class I heavy chains expressed by the LBL721.174 mutant, it is possible that a large fraction of class I heavy-chain-[B2m complexes not occupied by peptide (and therefore not in an appropriate conformation) are retained in the ER, possibly through the action of an ER chaperone that preferentially retains such unstable complexes17.31,32. In the case of the processingdefective LBL721.174 mutant cell, complexes between mouse class I heavy chains and human [32m that lack processed antigen are expressed at high levels at the cell surface, possibly because 'empty' mouse heavy-chainhuman [32m dimers are more stable than mouse heavychain-mouse [32m dimers. However, there is evidence that in normal cells a small fraction of MHC class-I-[3zm dimers do assemble and transit from the ER to the cell surface without peptide 19,2°. Since sequence-specific targeting of proteins and peptides from the cytoplasm to specific intracellular organelles has been described 33, it is possible that 'empty' MHC class-I-f3zm dimers might also encounter, and be charged by, cytosolic peptides in post-ER compartments during transit to the cell surface. According to this emerging view of class I assembly and transport, one reason for the inefficient recognition of endosomally-processed antigen by MHC class-Irestricted T cells is that most MHC class I molecules that come into contact with this processed antigen are already charged with ER-derived peptides. In addition, recent evidence suggests that newly synthesized MHC class I molecules do not intersect with the endocytic pathway on their egress to the cell surface 34, so that even empty class-I-[~zm dimers may not have access to endocytically-derived antigen fragments. Finally, in most cell types examined to date (for example Neefjes e t al.34), MHC class I molecules at the cell surface do not internalize and recycle through an endosomal compartment. How, then, does exposure to peptide render target cells susceptible to lysis by MHC class-I-restricted CTL? One attractive hypothesis 19,2°is that the small number of empty class I molecules present at physiological temperatures on the cell surface bind to and are stabilized by exogenous peptide. Preformed dass-I-f32m-peptide complexes generated in the ER probably form highly stable complexes and the antigen fragments within such MHC assembly and presentation pathways complexes would not be expected to exchange readily Since the demonstration of differences in antigen pres- with exogenous peptide 2°,3s,36. An alternative possibility entation to MHC class-I- and class-II-restricted T cells26, is suggested by the recent reports that exogenous 132m there has been a considerable amount of speculation on may be necessary for sensitization of cells by exogenthe mechanistic basis for these differences 27-3°. Recent ous peptide 37,38. Accordingly, preformed class-I-[3zmobservations on MHC molecule assembly, transport and peptide complexes dissociate at the cell surface and reintracellular localization have begun to provide a struc- lease 132m and processed antigen into the extracellular tural framework for understanding these presentation space. Efficient reformation of complexes requires a pathways (Fig. 1). source of both specific peptide and 132m(usually provided In the case of MHC class I molecules it is likely that, by serum). It seems likely that such a mechanism would under physiological conditions (i.e. 37°C), most nascent involve only the unstable 'empty' class-I-f32m complexes MHC class I molecules arrive at the cell surface already present at the cell surface. Unstable and 'empty' MHC complexed with processed antigen fragments. As dis- class I (or class II) molecules may represent a subset of cussed above, the available evidence suggests that peptide MHC molecules that have weakly-bound peptide that charging occurs in a pre-Golgi compartment, presumably can dissociate from the MHC molecule at the cell surface. the ER H,12, and that peptide association is necessary for In this instance, a low-affinity peptide-MHC class I

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®

o ®

t

t

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Post-Golgi compartment

f

t]

IIIplll

[]

Empty

Is

Ic~-112 m

I]2m

Io~

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li

'o ti e specific transporter

.......... ~, protein

Fig. 1. A scheme for the assembly and transport of MHC class I and class II molecules is shown. MHC class I: the 45 kDa class I heavy chain (Ie0 and the 15 kDa f32-microglobulinmolecule (p2m) associate in the endoplasmic reticulum (ER). In the absence of peptide, the association of Ic~and p2m leads to an unstable complex with an altered Is conformation at physiological temperatures. These unstable complexes are inefficiently transported out of the ER and empty class Ie~heavy chains may be degraded in the ER compartment. In the presence of peptide in the binding cleft, Ie~, P2m and peptide form stable complexes (Ic~-f32m-P)which are efficiently transported to the ceil surface. At the cell surface, the small fraction of empty IR-p2m complexes may be available for charging by exogenous peptide. At the cell surface empty ic~-P2mcomplexes may dissociate and isolated heavy chains then degrade. At the cell surface a subset of'full' (Ic~-~2m-P) complexes dissociate releasing the weakly bound peptides. Empty unstable Ic~chains may be charged by exogenous peptide in the presence of exogenous p2m and may form stable complexes. Proteins present in the cytoplasm and possibly in the ER are the primary source of peptides for MHC class I charging. Peptides in the cytoplasm may gain access to the ER via a specific transporter. MHC class II: MHC class II~ (IIeOand class IIf3 (II~) normally assemble in the ER with the class II invariant chain (Ii) to form a stable trimolecular complex. Ii is thought to inhibit the binding of peptides to the IIe~-IIf3-Ii complex in the ER. The IIe~-II(3-Iicomplex is efficiently transported out of the ER and is targeted to a post-Golgi compartment. The complex then appears to traffic to an endosome where Ii is proteolytically cleaved. Loss of a portion of Ii may allow peptide to bind. Peptide binding may then signal the further cleavage of Ii resulting in the egress of the peptide charged IIc~-lIp complex to the cellsurface. Ile~-II~ complexes can assemble in the ER in the absence of li and transit to the cell surface. It is not as yet clear whether these IIe~-IIp complexes can associate with the peptide in the ER but these IIe~-I$ complexes lacking invariant chain may be in an altered conformational state. The peptides that normally charge MHC class H molecules are derived from proteins that are proteolytically processed in the endosome. interaction within the ER may allow MHC assembly and transport to the cell surface but such interactions may be too weak to maintain the stability of the complex at the cell surface. The structural constraints on MHC class II molecule assembly, transport and stability appear to be less stringent than those for MHC class I molecule assembly and transport. Cells lacking 39, or expressing low levels24 of, Ii chain assemble, transport and display normal levels of class II ~13 heterodimers. If mutant cells, deficient in endosomal processing, display predominantly 'empty' MHC class II cx[3 heterodimers at tile cell surface, then

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occupancy of the binding cleft by peptide may not be necessary for stable surface o~p association (although as noted above occupancy of the 'binding groove' by peptide appears to enhance dimer stability23). Does processed antigen play a role in the assembly and transport of MHC class II molecules in the ER ? Attempts to address this issue by using T cells as a sensitive probe for the formation of complexes between M H C class II molecules and antigen processed through a nonendosomal route have yielded conflicting results. In one report, the presentation of a cytoplasmic (but membrane° associated) protein to CD4 + T cells was sensitive to

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iiiii iiiiiii

iiiliii ii

Brefeldin A, an inhibitor of protein transport out of the ER 4°. This finding raised the possibility that newly synthesized MHC class II molecules charged with peptide in a pre-Golgi compartment contributed to the pool of MHC class II antigen complexes recognized by CD4 ÷ T cells. In another report, a newly synthesized cytoplasmic protein was processed and presented to CD4 ÷ T cells by a chloroquine-sensitive pathway consistent with an endosomal route of antigen presentation 41. In a more direct test of whether MHC class II molecules could be charged by an MHC class I presentation pathway, the product of a minigene encoding a preprocessed antigenic site (recognizable by both CD4 ÷ and CD8 ÷ T cells in the form of exogenously added peptide), when expressed in the cell cytoplasm, charged only MHC class I molecules42. Thus, MHC class I, but not class II, molecules appear to be efficiently charged in the putative class I charging compartment, the ER. Recent reports of CD4 ÷ T-cell recognition of nascent membrane glycoprotein antigens suggest that newly synthesized glycoprotein antigens must enter a post-Golgi compartment in order to be processed/presented in association with MHC class II molecules43,44. One attractive explanation for the failure of MHC class II molecules in the ER-Golgi to bind processed antigen is that the associated Ii inhibits peptide binding to the MHC class II heterodimer4s. The recent reports of the inability of class II (xf3-Ii complexes to bind specific peptide support this view44,46,47. If charging of nascent MHC molecules by antigen fragments can occur in either a pre- or post-Golgi compartment, then charging of MHC class I molecules occurs predominantly in a preGolgi compartment, whereas MHC class II charging preferentially occurs in a post-Golgi compartment. Whether Ii alone controls the availability of peptide to MHC class II cl~3heterodimers in the ER awaits further experimental verification. MHC class II o~ complexes transit through the Golgi apparatus and enter a post-Golgi compartment where the MHC class II molecules are delayed in their transit to the cell surface and where the nascent MHC molecules appear to interact with the endocytic pathway 34,48. It is at the intersection of these two pathways that nascent MHC class II molecules are presumed to contact, and be charged by, degraded exogenous antigen. This may also be the compartment where a specific set of proteolytic cleavages of the invariant chain occurs, with subsequent dissociation of the Ii fragments from the ~3 heterodimer49. One potential way to link peptide binding to Ii dissociation and MHC molecule transport is to postulate that the initial cleavage of Ii allows peptides to have access to the peptide-binding groove of the nascent o~ heterodimer. After peptide binding by the 0~f3 heterodimer, the Ii would be rendered susceptible to a second proteolytic event with subsequent further breakdown of the Ii. After the second Ii proteolytic cleavage event, the (xf3heterodimer charged with peptide is released to transit to the cell surface. Recent evidences° suggests that the Ii may itself contain a recognition sequence for targeting the (x~-Ii complex to the endosome. Thus Ii may play several roles in the class II molecule presentation pathway. Do both nascent and recycling MHC class II molecules interact with processed antigen in the endosome? The

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issues of MHC class II molecule recycling from the surface to the interior of the antigen-presenting cell and peptide exchange remain controversial 3z,34. Functional studies using various inhibitors of protein synthesis, transport or membrane recycling suggest that the contribution of recycling MHC to the pool of antigenpresenting molecules may differ for different antigenpresenting cell typessl,s2. Alternatively, differences in the techniques used to monitor recycling of MHC class II molecules may account for the discordant findings s3. Perhaps the most compelling indirect evidence for recycling of MHC class II molecules and peptide exchange within the cell comes from whole animal studies of antigenic competition s4. Conclusion These recent findings, relating MHC structure, assembly and transport to antigen presentation, have potential implications for many areas of immunology, ranging from tolerance and autoimmunity to vaccine design. For example, a requirement for peptide association with MHC class I and f32min a pre-Golgi compartment for proper MHC assembly and transport implies that only proteins that are synthesized de novo in a cell26, or that gain access to the cell cytoplasm ss, will be efficiently presented to MHC class-I-restricted T cells. Thus, as recently pointed out s6, self antigens that are not anatomically expressed in the thymus may not efficiently induce elimination of MHC class-I-reactive CD8 ÷ T-cell clones directed to these self antigens. Similarly, if access to the endosome is essential for charging nascent (or recycling) MHC class II molecules, then certain classes of self cytosolic proteins that are short lived and/or are expressed in low concentrations may not efficiently induce clonal deletion of CD4 ÷ T cells and yet could serve as targets for CD4 ÷ T-cell-mediated autoimmune responses in the periphery if expression was increased in a tissue-specific manner. Likewise, the presence of 'tight' or stable complexes between endogenous self peptide and MHC class I molecules at antigen-presenting cell surfaces would leave only a small number of unstable or 'empty' MHC class I complexes as targets for peptide vaccines. Successful priming of CD8 ÷, MHC class-I-restricted CTL by peptide-lipid complexes s7 may be due to a close or stable association of peptide-lipid complexes to the antigen-presenting cell cytoplasmic membrane which, therefore, might provide ready and/or persistent access of peptide to 'empty' MHC class I molecules at the cell surface. These findings also suggest that the use of MHCbinding peptides for therapeutic intervention, for example to displace self antigens recognized in autoimmune diseases, are more likely to be effective in disease states where CD4 ÷ T cells are implicated as the primary effector. Finally, it is likely that these emerging concepts of MHC structure and antigen presentation will not only further our understanding of the host response to infectious agents and immune surveillance of neoplastic cells but also impact on our understanding of autoimmune disease and its etiology.

Thomas J. Braciale and Vivian L. Bracialeare at the Dept of Pathology, Washington University School of Medicine, 660 South Euclid Avenue~ Box 8118, St Louis, M O 6311 O, USA.

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References

Science 250, 1421-1423

1 Babbitt, B., Allen, P.M., Matsueda, G. et al. (1985) Nature 317, 359-361 2 Bjorkman, P.J., Saper, M.A., Samraoui, B. et al. (1987) Nature 329, 506-512 3 Van Bleek, G. and Nathenson, S.G. (1990) Nature 348, 213-216 4 Ratzschke, O., Falk, K., Deres, K. et al. (1990) Nature 348, 252-254 5 Townsend, A.R.M., Rothbard, J., Gotch, F.M. et al. (1986) Cell 44, 959-968 6 Townsend, A., Ohlen, C., Bastin, J. et al. (1989) Nature 340, 443-448 7 Ljunggren, H.G. and Karre, K. (1985) J. Exp. Med. 162, 1745-1759 8 Karre, K., Ljunggren, H.G., Piontek, G. et al. (1986) Nature 319, 675-678 9 Ljunggren, H.G., Paabo, S., Cochet, M. et al. (1989) J. Immunol. 142, 2911-2917 10 (3h16n, C., Bastin, J., Ljunggren, H-G. et al. (1990) J. Immunol. 145, 52-58 11 Nuchtern, J.G., Bonifacino, J.S., Biddison, W.E. et al. (1989) Nature 339, 223-226 12 Yewdell, J.W. and Bennink, J.R. (1989) Science 244, 1072-1075 13 Lie, W.R., Myers, N.B., Gorka, J. et al. (1990) Nature 344, 439-441 14 Myers, N.B., Lie, W.R., Nett, M. et al. (1989) J. Immunol. 142, 2751-2758 15 DeMars, R., Chang, C.C., Shaw, S. et al. (1984) Hum. Immunol. 11, 77-97 16 Cerundolo, V., Alexander, J., Lamb, C. et al. (1990) Nature 345, 449-452 17 Hosken, N.A. and Bevan, M.J. (1990) Science 248, 367-370 18 Alexander, J., Payne, A.J., Shigekawa, B. et al. (1990) Immunogenetics 31, 169-178 19 Ljunggren, H-G., Stare, N.J., Ohlen, C. et al. (1990) Nature 346, 476-480 20 Schumacher, T.N.M., Heemels, M-T., Neefjes, J.J. et al. (1990) Cell 62, 563-567 21 Townsend, A., Elliott, T., Cerundolo, V. et al. (1990) Cell 62, 285-295 22 Parham, P. (1990) Nature 346, 793-794 23 Mellins, E., Smith, L., Arp, B. et al. (1990) Nature 343, 71-74 24 Stockinger, B., Pessara, U., Lin, R.H. et al. (1989) Cell 56, 683-689 25 Peterson, M. and Miller, J. (1990) Nature 345, 172-174 26 Morrison, L.A., Lukacher, A.E., Braciale, V.L. et al. (1986) J. Exp. Med. 163, 903-921 27 Germain, R.N. (1986) Nature 322, 687-689 28 Braciale, T.J., Morrison, L.A., Sweetser, M.T. et ai. (1987) Immunol. Rev. 98, 95-114 29 Bevan, M.J. (1987) Nature 325, 192-194 30 Townsend, A. and Bodmer, H. (1989) Annu. Rev. ImmunoL 7, 601-624 31 Alexander, J., Payne, A.J., Shigekawa, B. et al. (1989) Immunogenetics 29, 380-388 32 Cresswell, P. (1990) Nature 343,593-594 33 Dice, J.F. (1987) FASEB J. 1,349-357 34 Neefjes, J.J., Stollorz, V., Peters, P.J. et al. (1990) Cell 61, 171-183 35 Chen, B.P. and Parham, P. (1989) Nature 337, 743-745 36 Maryanski, J.L., Pala, P., Cerottini, J.C. et al. (1988) J. Exp. Med. 167, 1391-1405 37 Rock, K.L., Rothstein, L.E., Gamble, S.R. and Benacerraf, B. (1990) Proc. Natl Acad. Sci. USA 87, 7517-7521 38 Vitiello, A., Potter, T.A. and Sherman, L.A. (1990)

39 Miller, J. and Germain, R. (1986) J. Exp. Med. 164, 1478-1489 40 Nuchtern, J.G., Biddison, W. E. and Klausner, R.D. (1990) Nature 343, 74-76 41 Jaraquemada, D., Marti, M. and Long, E.O. (1990) J. Exp. Med. 172, 947-954 42 Sweetser, M.T., Morrison, L.A., Braciale, V.L. and Braciale, T.J. (1989) Nature 342, 180-182 43 Braciale, T.J., Sweetser, M.T., Brown, L.R. et al. (1990) Cold Spring Harbor Symp. Quant. Biol. 54, 477-485 44 Polydefkis, M., Koenig, S., Flexner, C. et al. (1990) J. Exp. Med. 171,875-888 45 Elliott, W.L., Stille, C.J., Thomas, L.J. et al. (1987) J. Immunol. 138, 2949-2952 46 Roche, P.A. and Cresswell, P. (1990) Nature 345, 615-618 47 Teyton, L., O'Sullivan, D., Dickson, P.W. et al. (1990) Nature 348, 39-44 48 Cresswell, P. (1985) Proc. Natl Acad. Sci. USA 92, 8188-8192 49 Blum, J.S. and Cresswell, P. (1988) Proc. Natl Acad. Sci. USA 85, 3975-3979 50 Bakke, O. and Dobberstein, B. (1990) Cell 63,707-716 51 Harding, C.V. and Unanue, E.R. (1989) J. Immunol. 142, 12-19 52 Adorini, L., Ullrich, S.J., Appella, E. et al. (1990) Nature 346, 63-66 53 Reid, P.A. and Watts, C. (1990) Nature 346, 655-657 54 Adorini, L., Appella, G., Doria, F. et al. (1989) Nature 342, 800-803 55 Moore, M.W., Carbone, F.R. and Bevan, M.J. (1988) Cell 54, 777-785 56 Blackman, M., Kappler, J. and Marrack, P. (1990) Science 248, 1335-1341 57 Deres, K., Schild, H., Wiesmuller, H-H. et al. (1989) Nature 342, 561-564

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