DESCRIPTION: (Adapted from applicant's abstract): Knowledge of how cells acquire and display MHC/peptide complexes is basic to understanding of how normal and abnormal (eg autoimmune) responses work. One broad goal of the work described in this application is to identify MHC-encoded proteins that are needed for production and presentation of the complexes and to learn about the function of each protein. Mutant human B cell lines in which the formation of MHC/peptide complexes is defective will be isolated and then cloned genes will be transferred into the mutants in order to identify genes that repair the defects. Genes that encode components of a (peptide?) transporter protein and of proteosomes, as well as a gene that is needed for acquisition of class II/peptide complexes, will be emphasized. Various assays will be performed in order to identify the step in antigen processing/presentation that is defective in each kind of mutant. Mutants in which peptide complex formation is impaired produce unstable "empty" class I or class II molecules that lack bound self-peptides. Stabilization of empty MHC molecules by binding of peptide will be exploited as an assay to systematically screen for peptides that can bind to specific MHC molecules. Mutant cells that bind exogenous peptides acquire high densities of specific MHC/peptide complexes. Such cells will be used as antigen-presenting cells in attempts to stimulate the in vitro outgrowth of complex-specific T cells from non-immune individuals. The goal of the peptide-binding and T cell outgrowth experiments is to expedite ways of using peptides to elicit or suppress specific immune responses, as desired.