REVIEW 53 Rivett, A.J. (1989) Arch. Biochem. Biophys. 268, 1-8 54 Orlowski, M. (1990) Biochemistry 29, 10289-10297 55 Driscoll, J. and Goldberg, A.L. (1990) J. Biol. Chem. 265, 4789--4792 56 Fujiwara, T., Tanaka, K., Orino, E. et al. (1990) .l. Biol. Chem. 265, 16604-16613 57 Ordz-Navarrete, V., Seelig, A., Frentzel, S. et al. ( 1991 Nature 353,662-664 58 Glynne, R., Powis, S.H., Beck, S. etal. (199l) Nature ~'"7 353, ,~,>~-360 59 Kelly, A., Powis, S.H., Glynne, R. etal. (1991) Nature 353,667-668 60 Monaco, J.J. Curr. Opin. lmmunol. (in press) 61 Dick, L.R., Moomaw, C.R., DeMartino, G.N. and Slaughter, C ( 199 I) Biochemistry 30, 2725-2734 62 McGuire, M.J., McCullough, M.L., Croall, D.E. and DeMartino, G.N. (1989) Biochim. Biophys. Acta 995, 181-186
63 Carbonc, F.R, Moore, M,W., Shell,J.M. and Bevan, M.J. L1988)./. Exp. Med. 167, 1767-1779 64 Schild, H., R6tzschke, O., Kalbacher, H. and Rammensee, H-G. {1990) Science 247, 1587-1589 65 -Fiv~ari, J.L. and Terasaki, P.1. (1985) HLA and Disease Associations, Sprmger-Verlag 66 Livingstone, A.M., Powis, S.J., Diamond, A.G., Butcher, G.W. and Howard, J.C. (1989).1. Exp. Med. 170~ 777-795 67 Powis, S.J., Howard, J.C. and Butcher, (;.W. ~1991) J. Exp. Med. 173, ~)13-921 68 Pazmany, L, Rowland-Jones, S., Hum, S. et al. (199")) J. Exp. Med. 175,561-569 69 Faustman, I)., li, X., Lin, H.Y. et al. ( 1991 ) Science 254, 1756-1761 Reference added in proof
70 Henderson, R.A., Michel, H., Sakaguchi, K. et al. (1992)
Science 255, 1264-1266
Intracellular transport of MHC class II molecules Jacques J. Neefjes and Hidde L. Ploegh MHC class II molecules associate, during biosynthesis, with peptides derived from endocytosed antigen. Here, Jacques Neefjes and Hidde Ploegh describe the intracellular transport of MHC class I1 molecules and its relationship to the binding of peptides in endosomal compartments. They discuss alternative routes for the delivery of antigen to sites at which peptides associate with MHC class II molecules and raise the possibility of cell type-specific differences in the handling of MHC class II molecules, and hence in antigen presentation. One of the 'atomic facts' of immunology is that the T-cell receptor (TCR) recognizes fragments of protein (peptides) in the context of major histocompatibility complex (MHC) molecules, rather than the intact antigen from which these fragments are derived I. There are two basic types of MHC molecules, class I and class II. Despite their rather similar three-dimensional structure 2, the modus operandi of these two classes of MHC molecules is different. This is reflected in the T cells that interact with MHC molecules, where a parallel bisection in the expression of cell surface accessory molecules exists: the CD4 glycoprotein, through its interaction with class 1I molecules, predetermines the use of class II molecules as a restriction element, whereas the CD8 glycoprotein associates with, and dictates the use of, class 1 molecules 1. The peptide-binding portion of MHC class II molecules has been proposed to be similar to that of MHC class I molecules2, the structure of which has been established at atomic resolution 6,7. In this model, the most polymorphic positions of the class 1I [3 chains (the chains are less polymorphic) cluster in and around the peptide-binding groove. However, there are differences. The peptide-binding groove of class 1 and class I! mol-
ecules may have a different sizeL The groove of class I molecules is closed at both ends ~, a feature that presumably dictates the binding of peptides of restricted size (810 amino acids) l°- 12. Class-II-bound peptides are slightly larger ( - 1 4 amino acids) and show heterogeneity at the carboxy terminus, suggesting that the class I1 peptidebinding groove may be open at one end ~. The functional dichotomy between class I and class !I molecules is also reflected in the intracellular route taken by MHC molecules and antigen. Roughly speaking, antigen that enters an antigen-presenting cell (APC) from the external milieu (via the 'exogenous route') is presented by class II molecules. For antigens synthesized within the cell itself and degraded in the cytosol and/or the endoplasmic reticulum (ER) (the 'endogenous route'), the resulting peptides can be presented by class I molecules-~,although this does not preclude presentation by class II molecules. Any protein that gains access to the cytosol can, in principle, be presented via the class-l-restricted pathway4. This distinction between the endogenous and exogenous route of antigen presentation does not hold true only for the cellular phenomena described by immunologists, but is also reflected in the route of intracellular transport of MHC molecules. This article summarizes the results
© 1992, ElsevierScience Publisht'r~ltd, UK.
l,,tmunology Today
179
rot
N,, S t',92
REVIEW A
t
Antigen
A
(h)
Coated pit Early endosome ~(~
(e) I j
(i)
Late endosome
Lysosome Autophagosome
Trans-Golgi MIIC
Lysosome ~ "~i~;)
reticulum
~' G°lg2 ( b ) ~
C
F-I T Endoplasmic reticulum
(a)
0~ 13
Fig. 1. Schematic representation of intracellular transport of MHC class II molecules and antigen. Class II molecules are assembled from cq f3 and ,/ chains in the ER (a). They are transported as a nine-subunit complex from the ER through the Golgi (b) to the trans-Golgi reticulum (c). Here, the class II molecules are sorted to the endocytic pathway by the y chain, which is degraded during transport. Internalized antigen arrives via coated pits (g), early (f) and late (e) endosomes, in a compartment identified in human B cells: MIIC (d) and lysosomes. Antigen is degraded in the endocytic pathway and fragments of this antigen will associate with class II molecules after release of the ,t chain. The peptide-class II a[3 complex is then transported to the cell surface via a pathway that has not been defined. Cytosolic proteins, or fragments derived from them, can enter the class II pathway either in the ER, by direct translocation over lysosomal membranes or by the formation of autophagosomes. Autophagosomes can form by vesicularization of ER membranes and consequent engulfment of cytoplasmic proteins. After fusion of autophagosomes with lysosomal structures (i), antigen is delivered to the class II pathway. pathway and for preventing the binding of peptides to the dimers in the ER. The -/chain is a type II transmembrane protein (that is, the amino terminus is intracellular) that contains an ER retention signal in the amino-terminal 15 amino acids of the cytoplasmic taiP 5J6. Some y chains leave the ER in the absence of the class II (~13dimer, but Assembly and intracellular transport of MHC class II the y chain is more efficiently transported when associmolecules MHC class II molecules are assembled in the ER -s ated with the oqB dimer 17. Similarly, association with the (Fig. 1 (a)). The functional unit, as expressed at the cell y chain is not an absolute requirement for assembly and surface, consists of an (x[3 heterodimer. Only properly surface expression of the cxl3 heterodimerlSJ9; rather, it assembled class II ~x[3heterodimers leave the ER 13. Free c~ increases the efficiency of the process 14. In the ER, the y and 13 chains are retained by the ER-resident immuno- chain forms a homotrimer, identifiable by chemical globulin heavy chain binding protein (BIP) (authors' crosslinking 2°. Class II molecules are transported from unpublished observations), and perhaps by additional the ER in a complex consisting of a scaffold of three y ER retention signals. During biosynthesis of class II chains, onto which three c43 dimers assemble 21. On arrival in the trans-Golgi reticulum (TGR) (Fig. molecules a third chain, the invariant or y chain, associates transiently with the class II cq3 dimer 14. It is respon- 1 (c)), MHC class II molecules are sorted to the endocytic sible for targeting the (*13 dimers to the endocytic route 22,23, directed by a targeting signal for delivery to from biochemical and cell biological experiments as they relate to the intracellular distribution and trafficking of MHC class II molecules, and sets out the major problems that remain to be solved.
Immunology Today
180
Vol 13 No. 5 1992
REVIEW endosomes which is located in the cytoplasmic tail of tile y chain is,16,24. While in transit from the TGR to the endocytic route, the class-II-associated y chain is degraded bv endosomal proteases 2s-2v. This process is required fo'r class I1 molecules to bind exogenously added peptide -'s ~(L The exact location in the endocytic route where class II molecules bind peptide has not been established and could differ between cell types (see below). In human B-lymphoblastoid cells, MHC class II molecules take 1-3 h to traverse the endocytic route and appear at the cell surface22,2~; exit is linked to the rate of endosomal proteolysis and is inhibited by the protease inhibitor leupeptin 2-s,~l. Since the binding of peptide by MHC class I1 molecules is probably not essential for cell surface expression (some mutant cell lines express 'empty' MHC class II molecules at the cell surface,~2), the rate of breakdown of the y chain largely determines the rate of MHC class II transport through the endocytic route to the cell surface. Whether the proteases involved in the breakdown of the y chain are also involved in the generation of presentable peptides remains to be established, but it is quite likely. The acidic pH in endosomes/ lysosomes contributes to both an increase in antigen degradation and to the efficiency of peptide binding by MHC class 1I molecules, in view of the kinetic parameters of peptide binding at different pH values ~,~4. The type of peptide that ends up complexed with MHC class I1 molecules is influenced by a large number of variables, including (1) the protein composition of the APC itself, (2) the organization of the endocytic compartments and their functional properties in different types of APC, (3) the levels of MHC class I1 molecules available for peptide binding in the different endocytic compartments, (4) the mode of delivery of antigen to the endocytic pathway, that is, by fluid-phase endocytosis, internalization via immunoglobulin or Fc receptors ~-s, phagocytosis ~
63 Carbonc, F.R, Moore, M,W., Shell,J.M. and Bevan, M.J. L1988)./. Exp. Med. 167, 1767-1779 64 Schild, H., R6tzschke, O., Kalbacher, H. and Rammensee, H-G. {1990) Science 247, 1587-1589 65 -Fiv~ari, J.L. and Terasaki, P.1. (1985) HLA and Disease Associations, Sprmger-Verlag 66 Livingstone, A.M., Powis, S.J., Diamond, A.G., Butcher, G.W. and Howard, J.C. (1989).1. Exp. Med. 170~ 777-795 67 Powis, S.J., Howard, J.C. and Butcher, (;.W. ~1991) J. Exp. Med. 173, ~)13-921 68 Pazmany, L, Rowland-Jones, S., Hum, S. et al. (199")) J. Exp. Med. 175,561-569 69 Faustman, I)., li, X., Lin, H.Y. et al. ( 1991 ) Science 254, 1756-1761 Reference added in proof
70 Henderson, R.A., Michel, H., Sakaguchi, K. et al. (1992)
Science 255, 1264-1266
Intracellular transport of MHC class II molecules Jacques J. Neefjes and Hidde L. Ploegh MHC class II molecules associate, during biosynthesis, with peptides derived from endocytosed antigen. Here, Jacques Neefjes and Hidde Ploegh describe the intracellular transport of MHC class I1 molecules and its relationship to the binding of peptides in endosomal compartments. They discuss alternative routes for the delivery of antigen to sites at which peptides associate with MHC class II molecules and raise the possibility of cell type-specific differences in the handling of MHC class II molecules, and hence in antigen presentation. One of the 'atomic facts' of immunology is that the T-cell receptor (TCR) recognizes fragments of protein (peptides) in the context of major histocompatibility complex (MHC) molecules, rather than the intact antigen from which these fragments are derived I. There are two basic types of MHC molecules, class I and class II. Despite their rather similar three-dimensional structure 2, the modus operandi of these two classes of MHC molecules is different. This is reflected in the T cells that interact with MHC molecules, where a parallel bisection in the expression of cell surface accessory molecules exists: the CD4 glycoprotein, through its interaction with class 1I molecules, predetermines the use of class II molecules as a restriction element, whereas the CD8 glycoprotein associates with, and dictates the use of, class 1 molecules 1. The peptide-binding portion of MHC class II molecules has been proposed to be similar to that of MHC class I molecules2, the structure of which has been established at atomic resolution 6,7. In this model, the most polymorphic positions of the class 1I [3 chains (the chains are less polymorphic) cluster in and around the peptide-binding groove. However, there are differences. The peptide-binding groove of class 1 and class I! mol-
ecules may have a different sizeL The groove of class I molecules is closed at both ends ~, a feature that presumably dictates the binding of peptides of restricted size (810 amino acids) l°- 12. Class-II-bound peptides are slightly larger ( - 1 4 amino acids) and show heterogeneity at the carboxy terminus, suggesting that the class I1 peptidebinding groove may be open at one end ~. The functional dichotomy between class I and class !I molecules is also reflected in the intracellular route taken by MHC molecules and antigen. Roughly speaking, antigen that enters an antigen-presenting cell (APC) from the external milieu (via the 'exogenous route') is presented by class II molecules. For antigens synthesized within the cell itself and degraded in the cytosol and/or the endoplasmic reticulum (ER) (the 'endogenous route'), the resulting peptides can be presented by class I molecules-~,although this does not preclude presentation by class II molecules. Any protein that gains access to the cytosol can, in principle, be presented via the class-l-restricted pathway4. This distinction between the endogenous and exogenous route of antigen presentation does not hold true only for the cellular phenomena described by immunologists, but is also reflected in the route of intracellular transport of MHC molecules. This article summarizes the results
© 1992, ElsevierScience Publisht'r~ltd, UK.
l,,tmunology Today
179
rot
N,, S t',92
REVIEW A
t
Antigen
A
(h)
Coated pit Early endosome ~(~
(e) I j
(i)
Late endosome
Lysosome Autophagosome
Trans-Golgi MIIC
Lysosome ~ "~i~;)
reticulum
~' G°lg2 ( b ) ~
C
F-I T Endoplasmic reticulum
(a)
0~ 13
Fig. 1. Schematic representation of intracellular transport of MHC class II molecules and antigen. Class II molecules are assembled from cq f3 and ,/ chains in the ER (a). They are transported as a nine-subunit complex from the ER through the Golgi (b) to the trans-Golgi reticulum (c). Here, the class II molecules are sorted to the endocytic pathway by the y chain, which is degraded during transport. Internalized antigen arrives via coated pits (g), early (f) and late (e) endosomes, in a compartment identified in human B cells: MIIC (d) and lysosomes. Antigen is degraded in the endocytic pathway and fragments of this antigen will associate with class II molecules after release of the ,t chain. The peptide-class II a[3 complex is then transported to the cell surface via a pathway that has not been defined. Cytosolic proteins, or fragments derived from them, can enter the class II pathway either in the ER, by direct translocation over lysosomal membranes or by the formation of autophagosomes. Autophagosomes can form by vesicularization of ER membranes and consequent engulfment of cytoplasmic proteins. After fusion of autophagosomes with lysosomal structures (i), antigen is delivered to the class II pathway. pathway and for preventing the binding of peptides to the dimers in the ER. The -/chain is a type II transmembrane protein (that is, the amino terminus is intracellular) that contains an ER retention signal in the amino-terminal 15 amino acids of the cytoplasmic taiP 5J6. Some y chains leave the ER in the absence of the class II (~13dimer, but Assembly and intracellular transport of MHC class II the y chain is more efficiently transported when associmolecules MHC class II molecules are assembled in the ER -s ated with the oqB dimer 17. Similarly, association with the (Fig. 1 (a)). The functional unit, as expressed at the cell y chain is not an absolute requirement for assembly and surface, consists of an (x[3 heterodimer. Only properly surface expression of the cxl3 heterodimerlSJ9; rather, it assembled class II ~x[3heterodimers leave the ER 13. Free c~ increases the efficiency of the process 14. In the ER, the y and 13 chains are retained by the ER-resident immuno- chain forms a homotrimer, identifiable by chemical globulin heavy chain binding protein (BIP) (authors' crosslinking 2°. Class II molecules are transported from unpublished observations), and perhaps by additional the ER in a complex consisting of a scaffold of three y ER retention signals. During biosynthesis of class II chains, onto which three c43 dimers assemble 21. On arrival in the trans-Golgi reticulum (TGR) (Fig. molecules a third chain, the invariant or y chain, associates transiently with the class II cq3 dimer 14. It is respon- 1 (c)), MHC class II molecules are sorted to the endocytic sible for targeting the (*13 dimers to the endocytic route 22,23, directed by a targeting signal for delivery to from biochemical and cell biological experiments as they relate to the intracellular distribution and trafficking of MHC class II molecules, and sets out the major problems that remain to be solved.
Immunology Today
180
Vol 13 No. 5 1992
REVIEW endosomes which is located in the cytoplasmic tail of tile y chain is,16,24. While in transit from the TGR to the endocytic route, the class-II-associated y chain is degraded bv endosomal proteases 2s-2v. This process is required fo'r class I1 molecules to bind exogenously added peptide -'s ~(L The exact location in the endocytic route where class II molecules bind peptide has not been established and could differ between cell types (see below). In human B-lymphoblastoid cells, MHC class II molecules take 1-3 h to traverse the endocytic route and appear at the cell surface22,2~; exit is linked to the rate of endosomal proteolysis and is inhibited by the protease inhibitor leupeptin 2-s,~l. Since the binding of peptide by MHC class I1 molecules is probably not essential for cell surface expression (some mutant cell lines express 'empty' MHC class II molecules at the cell surface,~2), the rate of breakdown of the y chain largely determines the rate of MHC class II transport through the endocytic route to the cell surface. Whether the proteases involved in the breakdown of the y chain are also involved in the generation of presentable peptides remains to be established, but it is quite likely. The acidic pH in endosomes/ lysosomes contributes to both an increase in antigen degradation and to the efficiency of peptide binding by MHC class 1I molecules, in view of the kinetic parameters of peptide binding at different pH values ~,~4. The type of peptide that ends up complexed with MHC class I1 molecules is influenced by a large number of variables, including (1) the protein composition of the APC itself, (2) the organization of the endocytic compartments and their functional properties in different types of APC, (3) the levels of MHC class I1 molecules available for peptide binding in the different endocytic compartments, (4) the mode of delivery of antigen to the endocytic pathway, that is, by fluid-phase endocytosis, internalization via immunoglobulin or Fc receptors ~-s, phagocytosis ~
