Class I MHC recognition

It has become apparent that the CD8 molecule directly contributes to the development of cytotoxic activity by CD8+ cells. Antibodies to CD8 block the

Figure 1 Three-dimensional molecular model of the N-terminal region of a CD8 homodimer consisting of two a chains; (A) ribbon presentation, (B) stick presentation. The structural data for the N-termlnal 113 amino acids are based on crystallography (protein data base: 1CQ8). The three-dimensional model for each monomer was generated with RasMol version 2.5 (Roger Sayle, Greenford, Middlesex, UK), and the two monomers were combined as described (Leahy et al. (1992) Cell 68: 1145). The color coding represents groups; blue corresponds to the CDR1-like loop, light blue to the CDR2-like loop, and lime to the CDR3-like loop. The monomers are distinguished in brightness of color. (See also color Plate 10.)

Figure 1 Three-dimensional molecular model of the N-terminal region of a CD8 homodimer consisting of two a chains; (A) ribbon presentation, (B) stick presentation. The structural data for the N-termlnal 113 amino acids are based on crystallography (protein data base: 1CQ8). The three-dimensional model for each monomer was generated with RasMol version 2.5 (Roger Sayle, Greenford, Middlesex, UK), and the two monomers were combined as described (Leahy et al. (1992) Cell 68: 1145). The color coding represents groups; blue corresponds to the CDR1-like loop, light blue to the CDR2-like loop, and lime to the CDR3-like loop. The monomers are distinguished in brightness of color. (See also color Plate 10.)

generation and effector phases of the CTL response specific for class I MHC antigens without regard to function. Functional and cell-binding studies have shown that the ligands for the CD 8 receptor are encoded by MHC class I genes. A role for CD8 in the interaction with class I molecules was demonstrated from the ability of CHO cells transfected with the

CD8 gene to form conjugates with class I-positive but not class I-negative cells. Similar to the interaction between CD4 and MHC class II, CD8 binds with weak affinity to MHC class I and contributes to cell adhesion. CDR-like loops (homologous to the complementarity-determining region 2 loop of antibodies) in the N-terminal end of CD8a are the most likely binding site for MHC class I as judged by mutational analyses, crystal structure and orientation in the cell membrane. In addition, an A/B strand surface may interact with a region on the a2 domain of MHC class I. The primary binding site on MHC class I is the a3 loop, a nonpolymorphic region in the membrane proximal domain and distant to the antigen-binding cleft. Furthermore, the structures of CD 8 and MHC class I make it possible that one CD 8 dimer could simultaneously bind two MHC class I molecules.

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