Efficient detection of human antigen-specific CD4+ T cells with peptide-exchangeable HLA-DP4 tetramers and multiplexed magnetic enrichment.

The tremendous variation in human MHCII alleles is a barrier to using fluorophore-labeled peptide:MHCII tetramers to study antigen-specific CD4+ T cells. This is because matching of the MHCII molecule in the tetramer with each human subject is required. We show that fluorophore-labeled tetramers of peptides complexed to the most common human MHCII molecule-encoded by DPA1*01:03 and DPB1*04:01-detect antigen-specific cells in most humans. New peptides can be successfully exchanged into these tetramers. Combining these novel exchangeable DP4 tetramers with a multiplexed magnetic enrichment step allows the detection of antigen-specific CD4+ T cell responses in ≥60% of humans without forehand knowledge of HLA type making pilot experiments with new tetramers more practical and efficient. This system could be adopted by the field to study CD4+ T cell responses to multiple independent antigens or multiple epitopes within an antigen (i.e., as a system for epitope discovery).