Glu227-->Lys substitution in the acidic loop of major histocompatibility complex class I alpha 3 domain distinguishes low avidity CD8 coreceptor and avidity-enhanced CD8 accessory functions.
Cytotoxic T lymphocyte (CTL) activation requires specific T cell receptor (TCR)-class I major histocompatibility complex (MHC) antigen complex interactions as well as the participation of coreceptor or accessory molecules on the surface of CTL. CD8 can serve as a coreceptor in that it binds to the same MHC class I molecules as the TCR to facilitate efficient TCR signaling. In addition, CD8 can be "activated" by TCR stimulation to bind to class I molecules with high avidity, including class I not recognized by the TCR as antigenic complexes (non-antigen [Ag] class I), to augment CTL responses and thus serve an accessory molecule function. A Glu/Asp227-->Lys substitution in the class I alpha 3 domain acidic loop abrogates lysis of target cells expressing these mutant molecules by alloreactive CD8-dependent CTL. Lack of response is attributed to the destruction of the CD8 binding site in the alpha 3 domain which is likely to disrupt CD8 coreceptor function. The relative importance of the class I alpha 3 domain acidic loop Glu227 in coreceptor as opposed to accessory functions of CD8 is unclear. To address this issue, we examined CTL adhesion and degranulation in response to immobilized class I-peptide complexes formed in vitro from antigenic peptides and purified class I molecules containing wild-type or Glu227-->Lys substituted alpha 3 domains. The alpha 3 domain mutant class I-peptide complexes were bound by CTL and triggered degranulation, however to much lower levels than wild-type class I-peptide complexes. In further experiments, it is directly demonstrated that the alpha 3 domain mutant class I molecules, which lack the Glu227 CD8 binding site, still serve as TCR-activated, avidity-enhanced CD8 accessory ligands. However, mutant class I peptide Ag complexes failed to effectively serve as CD8 coreceptor ligands to initiate TCR-dependent signals required to induce avidity-enhanced CD8 binding to coimmobilized non-Ag class I molecules. Thus the Glu227-->Lys mutation effectively distinguishes CD8 coreceptor and avidity-enhanced CD8 accessory functions.
Shen
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L
Department of Medical Microbiology and Immunology, Faculty of Medicine, University of Alberta, Edmonton, Canada.
Potter
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TA
Kane
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KP
D001665:Binding Sites / Q000235:genetics; D016827:CD8 Antigens / Q000276:immunology*; D002448:Cell Adhesion; D015550:Cell Degranulation; D002999:Clone Cells; D003602:Cytotoxicity, Immunologic; D018698:Glutamic Acid / Q000235:genetics / Q000276:immunology*; D006183:H-2 Antigens / Q000276:immunology; D015395:Histocompatibility Antigens Class I / Q000235:genetics / Q000276:immunology*; D008024:Ligands; D008213:Lymphocyte Activation; D008239:Lysine / Q000235:genetics; D009154:Mutation; D010446:Peptide Fragments / Q000276:immunology; D011487:Protein Conformation; D011948:Receptors, Antigen, T-Cell; D011993:Recombinant Fusion Proteins / Q000276:immunology; D015398:Signal Transduction; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology*; D014758:Viral Core Proteins / Q000276:immunology
D016827:CD8 Antigens; D006183:H-2 Antigens; D015395:Histocompatibility Antigens Class I; D008024:Ligands; D010446:Peptide Fragments; D011948:Receptors, Antigen, T-Cell; D011993:Recombinant Fusion Proteins; D014758:Viral Core Proteins; C102366:nucleoprotein (366-374), influenza virus; D018698:Glutamic Acid; D008239:Lysine