Exploring CD26 -/lo subpopulations of lymphocytes in asthma phenotype and severity: A novel CD4 + T cell subset expressing archetypical granulocyte proteins.

Background: Asthma pathology may induce changes in naïve/memory lymphocyte proportions assessable through the evaluation of surface CD26 (dipeptidyl peptidase 4/DPP4) levels. Our aim was to investigate the association of asthma phenotype/severity with the relative frequency of CD26 ^-/lo , CD26 ^int and CD26 ^hi subsets within different lymphocyte populations.
Methods: The proportion of CD26 ^-/lo , CD26 ^int and CD26 ^hi subsets within CD4 ⁺ effector T cells (T _eff ), total CD4 ^- lymphocytes, γδ-T cells, NK cells and NKT cells was measured in peripheral blood samples from healthy (N = 30) and asthma (N = 119) donors with different phenotypes/severities by flow cytometry. We performed K-means clustering analysis and further characterised the CD4 ⁺ CD26 ^-/lo T _eff cell subset by LC-MS/MS and immunofluorescence.
Results: Cluster analysis including clinical and flow cytometry data resulted in four groups, two of them with opposite inflammatory profiles (neutrophilic vs. eosinophilic). Neutrophilic asthma presented reduced CD4 ^- CD26 ^hi cells, which negatively correlated with systemic inflammation. Eosinophilic asthma displayed a general expansion of CD26 ^-/lo subsets. Specifically, CD4 ⁺ CD26 ^-/lo T _eff expansion was confirmed in asthma, especially in atopic patients. Proteomic characterisation of this subset with a T _EM /T _EMRA phenotype revealed upregulated levels of innate (e.g. MPO and RNASE2) and cytoskeleton/extracellular matrix (e.g. MMP9 and ACTN1) proteins. Immunofluorescence assays confirmed the presence of atypical proteins for CD4 ⁺ T cells, and an enrichment in 'flower-like' nuclei and MMP9/RNASE2 levels in CD4 ⁺ CD26 ^-/lo T _eff compared to CD4 ⁺ T lymphocytes.
Conclusion: There is an association between CD26 levels in different lymphocyte subsets and asthma phenotype/severity. CD4 ⁺ CD26 ^-/lo T _EMRA cells expressing innate proteins specific to eosinophils/neutrophils could be determinant in sustaining long-term inflammation in adult allergic asthma.
(© 2024 The Author(s). Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)