REQUIREMENT FOR PEPTIDE IN ALLOREACTIVE CD4(-CELL RECOGNITION OF CLASS-II MHC MOLECULES() T)

To examine the role oi peptide in alloreactive class II MHC-restricted responses, we transfected I-A molecules into the Ag-processing defect ive mutant cell line, T2. Consistent with their defective Ag-processin g phenotype, the T2 transfectants predominantly express SDS-unstable I -A molecules on their surface. These cells and phenotypically normal A PCs were used to study primary and secondary alloreactive T cell respo nses in limiting dilution assays. The results demonstrate that the maj ority of CD4 T cells that participate in primary alloresponses and ess entially all the CD4 T cells that participate in secondary allorespons es recognize I-A conformers that depend on the presence of peptide and do not recognize the SDS-unstable I-A expressed by T2 transfectants. To further investigate the requirement for peptide in these responses, we incubated the T2 transfectants with E alpha(52-68) peptide and gen erated SDS-stable I-A-peptide complexes on the cell surface. The I-A(b )-E alpha peptide complexes expressed on T2 cells are stimulatory in s econdary alloresponses ii the T cells were exposed to the same I-A pep tide complex during the priming step. These studies demonstrate that p eptide-containing class II MHC is the relevant ligand for alloreactive T cells, and identify an alloreactive response to a peptide (E alpha( 52-68)) derived from a highly expressed cell surface ''self'' Ag, the I-E molecule.