title: The molecular basis of the requirement for antigen processing of pigeon cytochrome c prior to T cell activation.
abstract: Antigen-induced activation of T lymphocytes that co-recognize Ia molecules has been shown to require an antigen-processing step by the presenting cell before T cell stimulation can occur. In this report, we demonstrate that antigen presentation of pigeon cytochrome c to an E kappa beta:E kappa alpha-restricted T cell hybridoma, 2C2, is inhibited by pretreatment of the antigen-presenting cells (APC) either with chloroquine or with fixation by paraformaldehyde. The chloroquine effect was partially reversible after 22 hr; the paraformaldehyde effect was not. In contrast, these treatments had little or no effect on the presentation of the carboxy-terminal cyanogen bromide cleavage fragment of pigeon cytochrome c, residues 81 to 104. There was at least a 50-fold greater potency of the fragment, as compared to that of the intact molecule, when paraformaldehyde-fixed APC were used. In addition, the fixed cells did not present synthetic fragments of the cytochrome c that were nonstimulatory when presented by unfixed cells. This observation showed that the loss of potency, demonstrated previously for analogs of pigeon cytochrome c with single amino acid substitutions at positions such as 99, was not a consequence of an alteration in the rate of antigen-processing. This result is consistent with our earlier hypothesis that these residues are contact amino acids with the antigen-specific T cell receptor or the Ia molecule. The major goal of these experiments was to define the molecular transition that occurred as a result of antigen processing. To achieve this end, we tested a variety of pigeon cytochrome c molecules and fragments for their ability to be presented by paraformaldehyde-fixed APC. Apocytochrome c, the denatured form of the molecule with the heme removed, could not be presented by the fixed cells, nor could the fragment 60-104, derived by acid cleavage of the tryptophan at position 59. Both molecules stimulated an IL 2 response from the T cell hybridoma when unfixed APC were utilized, demonstrating that the conditions used to prepare these two molecules did not destroy their antigenic determinant. In contrast, carboxy-terminal fragments, both native and synthetic, ranging in size from 16 to 39 amino acids, were capable of stimulating in the presence of paraformaldehyde-fixed APC. In particular, the partial-digest cyanogen bromide fragment, residues 66 to 104, was only twofold less potent than the pigeon fragment 81-104.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.forename: Z
authors.initials: Z
authors.lastname: Kovac
authors.forename: R H
authors.initials: RH
authors.lastname: Schwartz
mesh_terms: D000818:Animals; D000938:Antigen-Presenting Cells / Q000276:immunology*; D002738:Chloroquine / Q000494:pharmacology; D010856:Columbidae / Q000276:immunology; D003574:Cytochrome c Group / Q000032:analysis / Q000276:immunology*; D000939:Epitopes / Q000032:analysis; D005557:Formaldehyde / Q000494:pharmacology; D006825:Hybridomas / Q000145:classification / Q000276:immunology; D008213:Lymphocyte Activation / Q000187:drug effects; D008954:Models, Biological; D010446:Peptide Fragments / Q000032:analysis / Q000276:immunology; D011108:Polymers / Q000494:pharmacology; D013601:T-Lymphocytes / Q000145:classification / Q000276:immunology*
chemical_list: D003574:Cytochrome c Group; D000939:Epitopes; D010446:Peptide Fragments; D011108:Polymers; D005557:Formaldehyde; D002738:Chloroquine; C003043:paraform