Change in specificity of antibodies to a random synthetic branched polypeptide in mice tolerant to its ordered analogs

Dichotomy between the T and the B cell epitopes of the synthetic polypeptide (T,G)-A--L

Studies with the well-characterized, synthetic, random-multichain polypeptide poly(LTyr,LGlu)-poly(DLAla)-poly(LLys) (T,G)-A-L) led to the discovery of determinant-specific genetic control of the immune response, as well as to other immunological phenomena. Moreover, the tetrapeptide TyrTyrGluGlu built on the same backbone ("(T-T-G-G)-A--L") was found to represent its major B cell epitope. We have recently shown that for interaction with major histocompatibility complex class II molecules and stimulation of T cells, (T,G)-A--L requires proteolytic processing and the resulting T cell epitopes are close to the N termini of the branched polymer's side chains. Thus, we were interested to elucidate the major T cell epitope of (T,G)-A--L, by using the ordered polypeptides (T-T-G-G)-A--L and (T-G-T-G)-A--L, in which only the two internal amino acids of the tetrapeptide attached to the side chains are switched. We established T cell lines to these antigens, and found that the ordered analog (T-T-G-G-)-A--L, which was defined as the B cell epitope of (T,G)-A--L, did not represent its T cell epitope, whereas (T-G-T-G)-A--L, to which only a minor anti-(T,G)-A--L Ab response was directed, was found to be its major T cell epitope. In addition, there was no cross-reaction between (T-G-T-G)-A--L and (T-T-G-G)-A--L at the T cell level, similar to the lack of cross-reaction of their antibodies. Analysis of the repertoire of the T cell receptors used by these lines revealed that the (T,G)-A--L and the (T-T-G-G)-A--L specific T cell lines were not restricted in their V alpha and V beta TCR usage, whereas the (T-G-T-G)-A--L-specific line was restricted by both V alpha and V beta T cell receptor gene products. This difference might be due to the thymus-independent characteristics previously described for the latter antigen.

Inhibition of T-cell reactivity to myasthenogenic epitopes of the human acetylcholine receptor by synthetic analogs

The synthetic peptides p195-212 and p259-271, representing amino acids 195-212 and 259-271 of the alpha subunit of the human acetylcholine receptor, preferentially stimulate T cells of patients with myasthenia gravis and are immunodominant T-cell epitopes in SJL and BALB/c mice, respectively. We designed and synthesized analogs of these peptides that contain single amino acid substitutions. An analog of peptide p195-212, no. 455 (Met-207-->Ala), was capable of inhibiting up to 100% of the proliferative responses of a p195-212-specific T-cell line originating from the high-responder strain SJL. Similarly, an analog of p259-271, no. 306 (Glu-262-->Lys), was capable of inhibiting up to 93% of the proliferative responses of the p259-271-specific T-cell line originating from high-responder BALB/c mice. Analog 306 also inhibited up to 43% of the proliferative responses of p259-271-primed lymph node cells in an in vitro proliferation assay. To test the in vivo inhibitory activity of the analogs, mice were primed with the myasthenogenic peptides in complete Freund's adjuvant concomitant with administration of the analogs in aqueous solution. Administration of analogs 455 and 306 led to decreased proliferative responses of up to 70% by peptide p199-212-primed lymph node cells and up to 85% by peptide p259-271-primed lymph node cells. Similar results were obtained whether the analogs were administered i.v. or i.p. Thus, these analogs are good candidates for specific immunomodulatory therapy for patients with myasthenia gravis.

Cellular analysis of specificity of antibodies and of delayed type hypersensitivity responses toward some structurally related synthetic antigens: boosting is determined by specificity of T cells

The crossreactivity between the random synthetic polypeptide antigen poly(Tyr,Glu)-poly(DLAla)--poly(Lys) [(T,G)-A--L] and its ordered-sequence analogs (Tyr-Tyr-Glu-Glu)-poly(DLAla)--poly(Lys) [(T-T-G-G)-A--L] and (Tyr-Glu-Tyr-Glu)-poly(DLAla)--poly(Lys) [(T-G-T-G)-A--L] at the level of humoral and cellular responses was studied. For delayed type hypersensitivity responses, (T,G)-A--L-activated T cells could be challenged with the homologous antigen as well as with the ordered analogs. T cells activated by (T-T-G-G)-A--L could be challenged with either the homologous antigen or with (T,G)-A--L but not with (T-G-T-G)-A--L. Similarly, no cross stimulation was observed between (T-G-T-G)-A--L-activated cells and (T-T-G-G)-A--L, whereas (T,G)-A--L could challenge the latter cells to mediate significant responses. Similar but not identical cross reactions were observed when primed spleen cells or lymph nodes were transferred to irradiated recipients that were boosted for the production of antibodies. In contrast to observations at the level of cellular responses, (T-G-T-G)-A--L-primed spleen or lymph node cells could not be boosted with (T,G)-A--L for the production of detectable amounts of antibodies, although boosting with the homologous antigen resulted in significant levels of (T-G-T-G)-A--L-specific antibodies. Transfer experiments in which mixtures of T and B cells, each primed to a different ordered polypeptide antigen, were injected into irradiated recipients showed that successful cooperation occurs provided that the boost is given with the T-cell-specific antigen. The antibodies produced were specific to the antigen used for B-cell priming. The T-cell-B-cell collaboration probably occurs through specific determinants that are shared between the two antigens in which the ordered peptides are attached to the same multichain polymer and that are recognized by both the T and the B cells.