The T-cell recognition of HLA-DR-peptide complexes is generally restricted by the polymorphism of the DRB molecules but pluriallelic restriction has been described. The molecular basis of restriction and promiscuity of such peptide-specific responses is poorly understood. We isolated a panel of T-cell lines specific for the tetanus toxin peptide p2 (TT830-843) exhibiting pluriallelic restriction by DR11 and DR8 alleles. Fine restriction specificity of the T-cell lines was examined in functional assays against DR oligotyped APCs expressing different variants of DR11 and DR8 alleles. Our results show that (a) polymorphisms between serologically related alleles are relevant in terms of restriction of the peptide-specific T-cell response; in some instances, a single amino acid substitution can determine the restriction of a T-cell line; (b) different patterns of restriction are not the result of specific differences in DR-p2 binding as p2 peptide binds to all DR11 and DR8 alleles tested (DRB1* 1101, -1102, -1103, -1104, 110X, -0801, -0802, -0803, and -0806); and (c) pluriallelic restriction of the peptide-specific T-cell response correlates with the presence of a DRB1 alpha-helix motif (67-71-86) shared by some DR11 and DR8 alleles. Possible implications of pluriallelic restriction of peptide-specific T-cell response in autoimmune disorders associated with DR11 and DR8 are discussed.