T-cell recognition of human haemoglobin. Localization of the full T-cell recognition profile of the beta-chain by a comprehensive synthetic strategy

Characteristics of peptides which compete for presented antigen-binding sites on antigen-presenting cells

The T cell recognition of globular protein antigens requires the cell surface presentation of the protein by Ia-expressing antigen-presenting cells (APC). The mechanisms by which APC function remain to be elucidated. To gain a better understanding of association of antigen with APC surfaces, a large panel of peptides of diverse physicochemical properties was assayed for the ability to compete with presented antigen for binding sites on the APC surface. Competition was measured by the ability of a peptide to block the I-Ek-restricted T cell response to pigeon cytochrome c (Pc) as presented by APC. The panel assayed included overlapping peptides representing the entire length of sperm whale myoglobin and the alpha and beta chains of human adult hemoglobin as well as synthetic conformational peptides of lactate dehydrogenase C4 exhibiting stable secondary, alpha-helical structures. The results presented here show that several peptides of this group compete with the presented form of Pc for binding sites on the APC. However, there is no single biochemical property or amino acid sequence algorithm which predicts the blocking ability. The peptides which compete with presented Pc are not predicted to assume the amphipathic alpha-helical conformation hypothesized by De Lisi and Berzofsky (Proc. Natl. Acad. Sci. USA 1986. 82: 7048) for T cell antigenic peptides. However, peptides designed and synthesized to adopt a stable alpha-helical secondary structure show more potent blocking activity than the corresponding linear peptides, suggesting that the secondary structure may indeed be a contributing factor in the ability of presented antigenic peptides to be bound by the APC. The results with the myoglobin and hemoglobin peptides show no connection between any particular secondary structure of the peptide in the native proteins and the ability of the peptides to block presentation. Further, there is no correlation between the major histocompatibility complex restriction of the competing peptides and their ability to block the I-Ek-restricted Pc-specific T cell response. This suggests that antigen presented by the APC may be bound to APC structures other than Ia prior to association with Ia. Such additional binding sites for presented antigen may be necessary to facilitate association with Ia.

Recognition of inter-transmembrane regions of acetylcholine receptor alpha subunit by antibodies, T cells and neurotoxins. Implications for membrane-subunit organization

Three regions of the alpha chain of Torpedo californica acetylcholine receptor (AChR), corresponding to residues alpha 262-276, alpha 388, 408 and alpha 427-437 were synthesized, purified and characterized. The first two peptides have been proposed to occupy inter-transmembrane regions while the third represented the C-terminal segment, proposed by various models to be either extracellular or intracellular. Peptide alpha 388-408 stimulated a good response in the AChR-primed T cells of H-2s haplotype mice, a low response in the H-2q haplotype and no response in the H-2b haplotype. Peptide alpha 427-437 stimulated AChR-primed T cells of the H-2s haplotype, but caused no response in the q and b haplotypes. Peptide alpha 262-276 evoked no in vitro stimulation in any of the s, q or b haplotypes. In antibody binding studies, peptide alpha 388-408 bound antibodies raised against free AChR or against membrane-bound AChR. The other two peptides showed little or no binding activity. Further, peptide alpha 388-408 bound specifically both 125I-labelled bungarotoxin and cobratoxin, while the other two peptides had no binding activity. These results were consistent with only one of the models for subunit organization within the membrane.

T lymphocyte recognition of acetylcholine receptor: localization of the full T cell recognition profile on the extracellular part of the alpha chain of Torpedo californica acetylcholine receptor

A series of eighteen consecutive overlapping synthetic peptides, of uniform size and overlaps, which encompass the entire extracellular part (residues 1-210) of the Torpedo californica acetylcholine receptor alpha chain were examined in vitro for their ability to stimulate lymph node cells from acetylcholine receptor-primed C57BL/6 (H-2b), C3H/He (H-2k), SWR (H-2q) and SJL (H-2s) mice. The recognition sites (T sites) by acetylcholine receptor-primed lymph node cells from these mouse strains resided within six regions on the extracellular part of the alpha chain. Three of the regions recognized by T cells coincided with regions recognized by antibodies (i.e. B cells) and one of these three regions also coincided with an alpha-neurotoxin-binding region. It is noteworthy that, in addition to sites recognized by both T and B cells, the protein has at least two sites which are recognized exclusively by T cells and to which no detectable antibody responses are directed.

Non-specific peptide size effects in the recognition by site-specific T-cell clones. Demonstration with a T site of myoglobin

Six regions (T sites) of myoglobin (Mb) were found by a comprehensive synthetic strategy to stimulate Mb-primed lymph-node cells. To define precisely the N-terminal boundary of the immunodominant T site (residues 107-120) with site-specific T-cell clones and to determine the effects of peptide size on their stimulation, two sets of peptides were employed. In one set, the peptides were elongated to the left from His-113 by one-residue increments of the Mb sequence. The other set represented an identical stepwise elongation by one-residue increments of the Mb sequence, but which were extended by additional unrelated ('nonsense') residues to a uniform size of 14 residues. Examination of the proliferative responses of eight T-cell clones, derived from Mb-primed DBA/2 (H-2d) or SJL (H-2s) mice, revealed a dramatic non-specific size requirement. In every clone, the longer nonsense-extended peptides achieved maximum stimulating activity at a lower optimum peptide dose than its natural-sequence, but shorter, analogue. In addition, slight (one-residue) differences in the N-terminal boundaries among the clones was observed. Thus, the fine specificity of each clone was mapped to the region from residue 111 or 112 to about residue 120 of Mb, which coincides with the site of B-cell recognition and resides in a small discrete surface region of the protein chain.

The continuous antigenic regions in the second domain of the beta chain of human MHC DR2 antigen: antigenic profile of the entire extracellular part of the chain

Ten consecutive 15-residue peptides (except for peptide 181-198) that spanned the entire second domain of the HLA DR2 beta chain and overlapped by 5 residues were synthesized, purified and characterized. The antibody-binding activities of the peptides with human alloantisera and with antisera raised in other species were determined by radioimmunosorbent titrations. This established the full profile of peptides having specific antibody-binding activity with these various antisera. The continuous antigenic sites in this domain were localized within four regions. The localization of the antigenic regions on the second domain completes the description of the antigenic profile of the entire extracellular part of the beta chain of HLA DR2. Thus, the continuous antigenic sites on the extracellular part of the DR2 beta subunit reside within seven regions with an eighth site being recognized somewhat weakly with some antisera. As was found for the first domain, the immunodominance of a given site varied with the antisera and boundary shifts were present. The submolecular regions recognized on the second domain of the DR2 beta were independent of the host species from which the antisera were obtained.

The regions of T-cell recognition on the extracellular part of the alpha chain of Torpedo californica acetylcholine receptor

A comprehensive synthetic approach was employed to identify the continuous regions of T-cell recognition on the alpha-chain of Torpedo californica acetylcholine receptor (AChR). Eighteen synthetic consecutive overlapping peptides, of uniform size and overlaps, that spanned the entire extracellular part (residues 1-210) of the alpha chain were examined for their in vitro stimulation of lymph node cells from AChR-primed C57BL/6 (H-2b), C3H/He (H-2k), SWR(H-2q) and SJL (H-2s) mice. The T-cell recognition sites (T sites) in the AChR-primed mouse strains resided within six regions on the extracellular part of the alpha-chain. Three of the regions recognized by T cells coincided with regions recognized by antibodies (i.e. B cells) and one of these three regions also coincided with an alpha-neurotoxin binding region. It is noteworthy that, in addition to sites recognized by both T and B cells, the protein has at least two sites which are recognized exclusively by T cells and to which no detectable antibody responses are directed.

T-cell recognition and antigen presentation of myoglobin

Determination of the T-cell recognition profile of Mb by the overlapping peptide strategy revealed that the protein has six T sites. Five of these coincide with the antigenic sites while one site was recognized exclusively by T cells and not by any detectable levels of antibody. Recognition of the synthetic T sites by protein-primed T cell lines or clones indicated that T cells display an unusual peptide size requirement beyond the essential contact residues of the T site. The antigen presentation of Mb has been examined with the significant advantage of knowing the full profiles of T- and B-cell recognition of this multi-determinant antigen. Significant differences in the patterns of T-cell recognition were observed following protein-priming as compared to peptide-priming. The absence of a clear relationship between these patterns of recognition presents a strong evidence against a mechanism of antigen presentation which is dependent on the generation of peptide fragments with the latter being the 'presented' species. From this new perspective, the protein molecule must be predominantly presented in its intact form.

Site recognition by protein-primed T cells shows a non-specific peptide size requirement beyond the essential residues of the site. Demonstration by defining an immunodominant T site in myoglobin

In previous studies, six T sites within myoglobin (Mb) were localized. To define precisely the boundaries of the T sites, a new approach is introduced and applied here to the T site residing within residues 107-120 of Mb. Two sets of peptides were synthesized. One set represents a stepwise elongation by one-residue increments of the Mb sequence. The other set represents an identical stepwise addition of one-residue increments of the Mb sequence, but which were extended by additional unrelated (nonsense) residues to a uniform size of 14 residues. The longer peptides (nonsense-extended) usually gave higher proliferative responses than did their shorter counterparts having the same Mb region. Thus a minimum peptide size is required for optimal T-cell stimulation. The T site subtends, in three high-responder mouse strains, residues 109-119 or 110-120, depending on strain, and, in three low-responder strains, maps to residues 108-120. Thus, in this case, the T site coincides with the site of B-cell recognition and resides in a small discrete surface region of the protein chain.