Use of MHC class II tetramers to investigate CD4+ T cell responses: problems and solutions

MHC-class I tetramers technology enabled the characterization of peptide-specific T cells at the single cell level in a variety of studies. Several laboratories have also developed MHC-class II multimers to characterize Ag-specific CD4+ T cells. However, the generation and use of MHC-class II multimers seems more problematic than that of MHC-I multimers. We have generated HLA-DR*1101 tetramers in a versatile empty form, which can be loaded after purification with peptides of interest. We discuss the impact of critical biological and structural parameters for the optimal staining of Ag-specific CD4+ T cells using HLA-DR*1101 tetramers, such as: (i) activation state of CD4+ T cells; (ii) membrane trafficking in the target CD4+ T cells; (iii) binding characteristics of the loaded CD4 epitope. Our data indicate that reorganization of TCR on the plasma membrane upon CD4+ T cell activation, as well as an homogenous binding frame of the CD4 epitopes to the soluble HLA-DR monomer, are critical for a stable TCR/MHC-class II tetramer interaction. These factors, together with the low frequencies and affinities of specific CD4+ T cells, explain the need for in vitro expansion or ex vivo enrichment of specific T cells for the optimal visualization with MHC-class II tetramers.