The present invention provides of using multimeric MHC class II/peptide complexes. In one aspect, methods provided for identifying MHC class II-restricted immune epitopes of a predetermined polypeptide antigen. Methods for identifying an immunostimulatory epitope for a predetermined polypeptide antigen are provided. In a related aspect, methods for screening a therapeutic polypeptide agent for an MHC class II epitope are provided. In other aspects, methods for modulating T cells and for determining or monitoring an MHC class II-restricted immune status of a patient are also provided.