Differential effects of subcutaneous and sublingual immunotherapy on timothy grass-specific T_H2 CD4⁺ T-cell subsets

Allergen-specific CD4⁺ T cells are a highly heterogenous population. Depletion of these cells has been proposed as essential to achieve allergen desensitization in allergen immunotherapy.

The overall aim of this study was to characterize the heterogeneity of timothy grass (Phleum pratense) allergen-specific CD4⁺ T cells and determine how the frequency and phenotype of these cells change in response to sublingual (SLIT) and subcutaneous (SCIT) immunotherapy. Correlations between frequencies of these cells with Total Nasal Symptom Score and grass-specific serum immunoglobulin were also investigated.

Mass cytometry with lanthanides-tagged peptide major histocompatibility complex class II multimers and CD154 upregulation assays were used to examine changes in the frequency and phenotype of Phl p-specific CD4⁺ T cells in longitudinal peripheral blood mononuclear cell samples from a randomized, double-blind, placebo-controlled trial of SLIT and SCIT. Supervised and unsupervised clustering was used for data analysis.

Phenotypes of Phl p-specific T cells were highly heterogenous but could be categorized into two major metaclusters, CRTH2^hiCD27^lo and CRTH2^loCD27^hi, each with distinct phenotypic profiles. Weak positive correlations between Total Nasal Symptom Score and frequencies of T cells within both subsets were observed. SCIT preferentially depleted CRTH2^hiCD27^lo cells, whereas SLIT depleted CRTH2^loCD27^hi cells. CRTH2^hiCD27^lo cell frequency correlated with Phl p-specific IgE and IgG₄, but not IgA, levels.

Unsupervised clustering revealed distinct subpopulations of allergen-specific T cells that were differentially targeted and depleted by SCIT and SLIT, suggesting that SCIT and SLIT act through overlapping but distinct immunologic pathways.