The main obstacle to elucidating the role of CD4⁺ T cells in allergen-specific immunotherapy (SIT) has been the absence of an adequately sensitive approach to directly characterize rare allergen-specific T cells without introducing substantial phenotypic modifications by means of in vitro amplification.

We sought to monitor, in physiological conditions, the allergen-specific CD4⁺ T cells generated during natural pollen exposure and during allergy vaccination.

Alder pollen allergy was used as a model for studying seasonal allergies. Allergen-specific CD4⁺ T cells were tracked and characterized in 12 subjects with alder pollen allergy, 6 nonallergic subjects, and 9 allergy vaccine–treated subjects by using peptide–MHC class II tetramers.

Allergen-specific CD4⁺ T cells were detected in all of the subjects with alder pollen allergy and nonallergic subjects tested. Pathogenic responses—chemoattractant receptor homologous molecule expressed on T_H2 lymphocytes (CRTH2) expression and T_H2 cytokine production— are specifically associated with terminally differentiated (CD27⁻) allergen-specific CD4⁺ T cells, which dominate in allergic subjects but are absent in nonallergic subjects. In contrast, CD27⁺ allergen-specific CD4⁺ T cells are present at low frequencies in both allergic and nonallergic subjects and reflect classical features of the protective immune response with high expression of IL-10 and IFN-γ. Restoration of a protective response during SIT appears to be due to the preferential deletion of pathogenic (CD27⁻) allergen-specific CD4⁺ T cells accompanied by IL-10 induction in surviving CD27⁺ allergen-specific CD4⁺ T cells.

Differentiation stage divides allergen-specific CD4⁺ T cells into 2 distinct subpopulations with unique functional properties and different fates during SIT.