HLA class II peptide tetramers vs allergen‐induced proliferation for identification of allergen‐specific CD4 T cells

Background Fluorescence-labeled MHC class II/peptide tetramer complexes are considered as optimal tools to characterize allergen-specific CD4+ T cells, but this technique is restricted to frequently expressed HLA class II molecules and knowledge of immunodominant epitopes. In contrast, allergen-stimulated proliferation assessed by CFSE dilution is less sophisticated and widely applicable. The major mugwort allergen, Art v 1, contains only one single, immunodominant, HLA-DR1-restricted epitope (Art v 125-36). Thus, essentially all Art v 1-reactive cells should be identified by a HLA-DRB1*01:01/Art v 119-36 tetramer. Methods We compared specificity and sensitivity of tetramer+ and allergen-induced proliferating (CFSElo) CD4+ T cells by flow cytometry. Results The frequency of tetramer+ CD4+ T cells determined ex vivo in PBMC of mugwort-allergic individuals ranged from 0 to 0.029%. After 2–3 weeks of in vitro expansion, sufficient tetramer+ T cells for phenotyping were detected in 83% of Art v 125-36-reactive T-cell lines (TCL) from mugwort-allergic individuals, but not in TCL from healthy individuals. The tetramers defined bona fide Th2 cells. Notably, Art v 125-36-reactive TCL depleted of tetramer+ T cells still reacted to the peptide, and only 44% of Art v 125-36-specific T-cell clones were detected by the tetramer. CFSElo CD4+ T cells contained only 0.3–10.7% of tetramer+ T cells and very low proportions of Th2 cells. Conclusion Allergen-specific T cells can be identified by HLA class II tetramers with high specificity, but unexpected low sensitivity. In contrast, allergen-stimulated CFSElo CD4+ T cells contain extremely high fractions of bystander cells. Therefore, for T-cell monitoring, either method should be interpreted with caution.