MHC restriction of L33-specific T-cell clones. MHC restriction was determined by ELISPOT: L33-specific T-cell clones were assayed for IFNγ secretion in response to stimulation with a panel of matched, MHC class II homozygous APCs. Only STEINLIN cells pulsed with peptide L33, but not Lol P1 or P33 peptides, induced IFNγ secretion. On the basis of the combined MHC class II expression pattern of the 3 APCs, it could be deduced that L33 peptide recognition by the T-cell clones is restricted by HLA-DRB3*0101. Similar results were obtained with both D7T1 and D7T4 T-cell clones. Duplicate assays with D7T4 cells are shown. The MHC alleles expressed by the 3 APCs are shown in Table 3.