8536d_ch08_185-199 8/2/02 10:08 AM Page 195 mac79 Mac 79: 45_Bwppldsby et al./ Immunology Se Antigen Processing and Presentation CHAPTER8 195 Antigen A nonclassical class II MHC molecule called HLA-DM is aili s yy pepide (figure hh ae l ens modin HLA have been identified in the mouse and rabbit, indicating 获感的 Digested invariant chain pH60-6.5 ∠ Released clip H45-5.0 pH5.0-6.0 8·—8-8288 HLA-DM Class Invariant l MHc chain FIGURE8-9Generation of antigenic peptides in the endocytic HLA-DO processing pathway. Internalized exogenous antigen moves through several acidic compartments, in which it is degraded into peptides that ultimately associate with class ll MHC molecules transported in (b) vesicles from the Golgi complex. The cell shown here is a B cell. which internalizes antigen by receptor-mediated endocytosis, with the membrane- bound antibody functioning as an antigen-specific B chains, their exit from the RER, and the subsequent routing of class II molecules to the endocytic processing pathway from the trans-Golgi network. The role of the invariant chain in the routing of class II mol- cules has been demonstrated in transfection experiments with cells that lack the genes encoding class II MHC molecules and the invariant chain Immunofluorescent labeling of such cells transfected only with class II MHC genes revealed class Il mol ecules localized within the Golgi complex. However, in cell transfected with both the class II mHc genes and invariant- FIGURE 8-10 (a) Assembly of class Il MHC molecules.Within the chain gene, the class II molecules were localized in the cytoplas- rough endoplasmic reticulum, a newly synthesized class ll MHC nic vesicular structures of the endocytic pathway. The molecule binds an invariant chain. The bound invariant chain pre- invariant chain contains sorting signals in its cytoplasmic tail vents premature binding of peptides to the class ll molecule and that directs the transport of the class II MHC complex from the helps to direct the complex to endocytic compartments containing trans-Golgi network to the endocytic compartments peptides derived from exogenous antigens Digestion of the invariant chain leaves CLIP, a small fragment remaining in the binding groove Peptides assemble with Class II MHC of the class ll MHC molecule HLA-DM. a nondassical mhc class ll Molecules by Displacing CLIP molecule expressed within endosomal compartments, mediates ex- change of antigenic peptides for CLIP. The nonclassical class ll mol Recent experiments indicate that most class II MHC-invari- ecule HLA-DO may act as a negative regulator of class ll antigen ant chain complexes are transported from the RER, where processing by binding to HLA-DM and inhibiting its role in the dis- they are formed, through the golgi complex and trans-Golgi sociation of CLiP from class ll molecules. ( b)Comparison of three- network, and then through the endocytic pathway, moving dimensional structures showing the binding groove of HLa class ll from early endosomes to late endosomes, and finally to lyso- molecules(al and B1)containing different antigenic peptides or somes. As the proteolytic activity increases in each successive CLIP. The red lines show DR4 complexed with collagen Il peptide compartment, the invariant chain is gradually degraded. yellow lines are DRI with influenza hemagglutinin peptide, and blue However, a short fragment of the invariant chain termed lines are DR3 associated with CLIP (N indicates the amino terminus CLIP(for class II-associated invariant chain peptide)remains and C the carboxyl terminus of the peptides. No major differences in bound to the class ii molecule after the invariant chain has the structures of the class ll molecules or in the conformation of the been cleaved within the endosomal compartment. CLIP bound peptides are seen. This comparison shows that CLIP binds physically occupies the peptide-binding groove of the class ll the class ll molecule in a manner identical to that of antigenic pep- MHC molecule, presumably preventing any premature bind- tides. (Part(b) from Dessen et al., 1997, Immunity 7: 473-481; cour. ng of antigenic peptide(see Figure 8-8) tesy of Don Wiley, Harvard University. 1 chains, their exit from the RER, and the subsequent routing of class II molecules to the endocytic processing pathway from the trans-Golgi network. The role of the invariant chain in the routing of class II mol￾ecules has been demonstrated in transfection experiments with cells that lack the genes encoding class II MHC molecules and the invariant chain. Immunofluorescent labeling of such cells transfected only with class II MHC genes revealed class II mol￾ecules localized within the Golgi complex. However, in cells transfected with both the class II MHC genes and invariant￾chain gene, the class II molecules were localized in the cytoplas￾mic vesicular structures of the endocytic pathway. The invariant chain contains sorting signals in its cytoplasmic tail that directs the transport of the class II MHC complex from the trans-Golgi network to the endocytic compartments. Peptides Assemble with Class II MHC Molecules by Displacing CLIP Recent experiments indicate that most class II MHC–invari￾ant chain complexes are transported from the RER, where they are formed, through the Golgi complex and trans-Golgi network, and then through the endocytic pathway, moving from early endosomes to late endosomes, and finally to lyso￾somes. As the proteolytic activity increases in each successive compartment, the invariant chain is gradually degraded. However, a short fragment of the invariant chain termed CLIP (for class II–associated invariant chain peptide) remains bound to the class II molecule after the invariant chain has been cleaved within the endosomal compartment. CLIP physically occupies the peptide-binding groove of the class II MHC molecule, presumably preventing any premature bind￾ing of antigenic peptide (see Figure 8-8). A nonclassical class II MHC molecule called HLA-DM is required to catalyze the exchange of CLIP with antigenic peptides (Figure 8-10a). MHC class II genes encoding HLA￾DM have been identified in the mouse and rabbit, indicating Antigen Processing and Presentation CHAPTER 8 195 Recycling of receptors Early endosome pH 6.0–6.5 Clathrin￾coated vesicle Antigen Late endosome pH 5.0–6.0 Lysosome pH 4.5–5.0 Golgi complex FIGURE 8-9 Generation of antigenic peptides in the endocytic processing pathway. Internalized exogenous antigen moves through several acidic compartments, in which it is degraded into peptides that ultimately associate with class II MHC molecules transported in vesicles from the Golgi complex. The cell shown here is a B cell, which internalizes antigen by receptor-mediated endocytosis, with the membrane-bound antibody functioning as an antigen-specific receptor. (a) + Released CLIP CLIP Digested invariant chain Peptides HLA-DM HLA-DO Class II MHC Invariant chain α β N C β1 α1 (b) FIGURE 8-10 (a) Assembly of class II MHC molecules. Within the rough endoplasmic reticulum, a newly synthesized class II MHC molecule binds an invariant chain. The bound invariant chain pre￾vents premature binding of peptides to the class II molecule and helps to direct the complex to endocytic compartments containing peptides derived from exogenous antigens. Digestion of the invariant chain leaves CLIP, a small fragment remaining in the binding groove of the class II MHC molecule. HLA-DM, a nonclassical MHC class II molecule expressed within endosomal compartments, mediates ex￾change of antigenic peptides for CLIP. The nonclassical class II mol￾ecule HLA-DO may act as a negative regulator of class II antigen processing by binding to HLA-DM and inhibiting its role in the dis￾sociation of CLIP from class II molecules. (b) Comparison of three￾dimensional structures showing the binding groove of HLA class II molecules (1 and 1) containing different antigenic peptides or CLIP. The red lines show DR4 complexed with collagen II peptide, yellow lines are DR1 with influenza hemagglutinin peptide, and blue lines are DR3 associated with CLIP. (N indicates the amino terminus and C the carboxyl terminus of the peptides.) No major differences in the structures of the class II molecules or in the conformation of the bound peptides are seen. This comparison shows that CLIP binds the class II molecule in a manner identical to that of antigenic pep￾tides. [Part (b) from Dessen et al., 1997, Immunity 7:473–481; cour￾tesy of Don Wiley, Harvard University.] 8536d_ch08_185-199 8/2/02 10:08 AM Page 195 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: