T cell subsets in allergy and tolerance induction.

Antigen-specific T lymphocytes are the central regulators of tolerance versus immune pathology against otherwise innocuous antigens and key targets of antigen-specific immune therapy. Recent advances in the understanding of T cells in tolerance and allergy resulted from improved technologies to directly characterize allergen-specific T cells by multi-parameter flow cytometry or single cell sequencing. This unravelled phenotypically and functionally distinct populations such as Th2a, follicular T helper cells (Tfh), regulatory T cells (Treg), Tr1 cells and follicular T regulatory cells (Tfr). Here we will discuss the role of the different T helper cell subsets in the healthy state, during sensitisation and development of allergy, and in tolerance induction by allergen immunotherapy. To date, the mechanisms of allergen immunotherapy as only causal treatment of allergy are not completely understood. The analyses of allergen-specific T cells directly ex vivo during allergen immunotherapy support the concept of specific-Th2(a) cell deletion rather than an expansion of allergen-specific Tr1 or Treg cells as underlying mechanism. This article is protected by copyright. All rights reserved.