Quantitation of antigen-presenting cell MHC class II/peptide complexes necessary for T-cell stimulation.
The number of specific complexes formed between peptide and the class II major histocompatibility complex (MHC) molecules expressed by an antigen-presenting cell (APC) after exposure to protein antigens is unknown, as is the number that activates T cells. Presentation of foreign peptides by APC takes place when many class II molecules may be occupied by autologous peptides. We have now estimated the number of specific peptide/class II complexes per APC by quantitative immunoprecipitation of I-Ak after pulsing the APC with stimulatory levels of a radioactive immunogenic peptide derived from hen egg-white lysozyme protein. T cells were activated by APC that expressed as few as 210-340 specific peptide/class II complexes (0.1% of the I-Ak molecules). These figures were confirmed using anti-CD3 antibody bound to latex beads as an alternative activating ligand. This low number explains the simultaneous presentation of multiple foreign antigens, even in the face of peptide competition.
Harding
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Department of Pathology, Washington University School of Medicine, St Louis, Missouri 63110.
Unanue
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D000938:Antigen-Presenting Cells / Q000276:immunology*; D002460:Cell Line; D005802:Genes, MHC Class II; D000949:Histocompatibility Antigens Class II / Q000276:immunology*; D006801:Humans; D008213:Lymphocyte Activation; D046911:Macromolecular Substances; D009113:Muramidase / Q000276:immunology*; D010446:Peptide Fragments / Q000276:immunology*; D013601:T-Lymphocytes / Q000276:immunology*
D000949:Histocompatibility Antigens Class II; D046911:Macromolecular Substances; D010446:Peptide Fragments; D009113:Muramidase