| CD4+ helper T-cell responses tend to be directed against certain sequences in a given antigen, the phenomenon called epitope immunodominance. Unlike CD8+ T-cell epitope immunodominance, CD4+ T-cell epitope immunodominance cannot be explained by peptide affinities for the major histocompatibility antigen presenting protein. The potential influence of antigen three-dimensional structure in shaping the epitope specificity of the CD4+ T-cell response has been demonstrated in numerous studies showing that the circumstances of antigen processing alter the pattern of epitope specificities. However, there has been no general explanation for the patterns of epitope immunodominance. We have tested the hypothesis that structurally unstable segments promote the presentation and therefore the immunodominance of flanking sequences because unstable segments are preferred sites of endoproteolytic cleavage during antigen processing.; Bacteriophage T4 (T4Hsp10) was selected as a model antigen because its structure is well characterized and it is homologous to an important antigen in tuberculosis and leprosy. Limited proteolysis demonstrated that the T4Hsp10 mobile loop is the preferred site of endoproteolytic cleavage by proteases, and T-cell epitope mapping by splenocyte proliferation demonstrated that an immunodominant epitope lies on the mobile loop flank. Large deletions in the mobile loop reduced proteolytic sensitivity in the mobile loop and diversified the helper T-cell and antibody responses. Analysis of antigen presentation using two CD4+ T-cell hybridomas reveals that the efficiency of presentation of a given epitope depends on its structural context. While presentation of one epitope correlated with its accessibility within the native structure, presentation of the other epitope correlated with initial cleavage in the mobile loop by the antigen-processing enzyme, cathepsin S. Taken together these results indicate that antigen three-dimensional structure is a major determinant of helper T-cell epitope immunodominance. Thus, these findings will contribute to the pool of basic knowledge that leads to rational vaccine design. |
