Protein antigenic structures recognized by T cells: potential applications to vaccine design.

In summary, our results using the model protein antigen myoglobin indicated, in concordance with others, that helper T lymphocytes recognize a limited number of immunodominant antigenic sites of any given protein. Such immunodominant sites are the focus of a polyclonal response of a number of different T cells specific for distinct but overlapping epitopes. Therefore, the immunodominance does not depend on the fine specificity of any given clone of T cells, but rather on other factors, either intrinsic or extrinsic to the structure of the antigen. A major extrinsic factor is the MHC of the responding individual, probably due to a requirement for the immunodominant peptides to bind to the MHC of presenting cells in that individual. In looking for intrinsic factors, we noted that both immunodominant sites of myoglobin were amphipathic helices, i.e., helices having hydrophilic and hydrophobic residues on opposite sides. Studies with synthetic peptides indicated that residues on the hydrophilic side were necessary for T-cell recognition. However, unfolding of the native protein was shown to be the apparent goal of processing of antigen, presumably to expose something not already exposed on the native molecule, such as the hydrophobic sides of these helices. We propose that such exposure is necessary to interact with something on the presenting cell, such as MHC or membrane, where we have demonstrated the presence of antigenic peptides by blocking of presentation of biotinylated peptide with avidin. The membrane may serve as a short-term memory of peptides from antigens encountered by the presenting cell, for dynamic sampling by MHC molecules to be available for presentation to T cells. These ideas, together with the knowledge that T-cell recognition required only short peptides and therefore had to be based only on primary or secondary structure, not tertiary folding of the native protein, led us to propose that T-cell immunodominant epitopes may tend to be amphipathic structures. An algorithm to search for potential amphipathic helices from sequence information identified 18 of 23 known immunodominant T-cell epitopes from 12 proteins (p less than 0.001). Another statistical approach confirmed the importance of amphipathicity and also supported the importance of helical structure that had been proposed by others. It suggested that peptides able to form a stable secondary structure, especially a helix, more commonly formed immunodominant epitopes. We used this approach to predict potential immunodominant epitopes for induction of T-cell immunity in proteins of clinical relevance, such as the malarial circumsporozoite protein and the AIDS viral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen conformation determines processing requirements for T-cell activation.

We studied the difference in requirements for processing and presentation to a single T-cell clone of four different forms of the same epitope of sperm whale myoglobin--namely, on the native protein, on two conformationally altered forms of the protein, or as a 22-residue antigenic peptide fragment. The T-cell clone was I-Ed-restricted and specific for an epitope on the CNBr fragment 132-153 involving Lys-140. As inhibitors of macrophage processing of antigen, we used several agents that inhibit lysosomal function: the weak bases chloroquine and NH4Cl, the cationic ionophore monensin, and the competitive protease inhibitor leupeptin. When these agents were used to inhibit processing of antigen by presenting cells and then washed out before T cells were added to culture, they inhibited the presentation of native antigen but not of fragment 132-153. To our surprise, the intact but denatured form, S-methylmyoglobin, behaved like the fragment not like the native protein. Apomyoglobin was intermediate in susceptibility to inhibition. Thus, native myoglobin requires a processing step that appears to involve lysosomal proteolysis, which is not required by fragment 132-153 or the denatured unfolded forms. For an antigen the size of myoglobin (Mr 17,800), it appears that unfolding of the native conformation, rather than further reduction in size, is the critical parameter determining the need for processing. Since a major difference between native myoglobin and the other forms is the greater accessibility in the latter of sites, such as hydrophobic residues, buried in the native protein, we propose that processing may be necessary to expose these sites, perhaps for interaction with the cell membrane or the Ia of the antigen-presenting cell.