Hapten-specific T cell response to azobenzenearsonate-N-acetyl-L-tyrosine in the Lewis rat. III. Effects of peptide-spacer structure on eliciting ABA-specific helper activity with TNP-haptened ABA-peptide-Ficoll.

A series of antigens was synthesized in which peptide spacers were inserted between the ABA group and TNP-Ficoll. When these antigens were used to assess helper activity in Lewis rats immunized with ABA-tyr, it was found that an increase in the ABA-epitope density and an increase in the peptide spacer size both increased the efficacy of the antigen in eliciting ABA-specific help, manifest by an enhancement of anti-TNP PFC. Substitution of D- for L-amino acids progressively decreased the ability of these conjugates to elicit help until D-tyr-D-ala-D-ala, which when used for coupling ABA to the TNP-Ficoll resulted in a nonimmunogenic molecule. When these same Ficoll conjugates were used to study ABA-specific in vitro proliferative responses, it was found that introduction of even a single D-amino acid into the spacer greatly reduced reactivity. By contrast the ABA-peptides, free of the Ficoll backbone, were all equivalent on a molar basis in their ability to elicit ABA-specific in vitro proliferation, regardless of their content of D-amino acids. These results suggest some form of digestion is required in the processing of these antigens before they can elicit helper activity. This processing can occur only if one or more L-amino acid residues are present. If the ABA-peptides are free of the Ficoll backbone, they are all capable of stimulating T cell proliferation without apparent further processing.