Immunophenotyping of immune cells
Allergic sensitization is associated with multiple changes in blood immune cells,175 and high dimensional immunophenotyping using flow- and mass spectrometry (i.e. Cytometry Time of Flight; CyTOF) has contributed greatly to this identification.237 A study on bee venom AIT provided detailed characterization of allergen-specific B cells before and after bee venom tolerance in allergic patients and bee-keepers with Api m 1 -specific B cells showing increased CCR5 expression after high-dose allergen exposure.238 In a study of OIT for milk allergy, researchers found, using flow cytometry, a significant increase in blood invariant natural killer cells and a shift from a type 2 T-helper (Th-2; i.e. IL-4, IL-13) to a Th-1 (ie IFN-γ) cytokine profile.239 Similar studies in other immunotherapy models can assist with determining potential biomarkers for monitoring AIT.
Recent evidence shows that AIT modulates the balance between circulating T follicular helper (Tfh) and regulatory cells (Tfr), with Tfr as a potential biomarker for AIT efficacy.240,241Upregulation of the activated allergen‐specific Tregs and downregulation of dysfunctional allergen‐specific Treg cell subset, associated with improved clinical response, were recently described.242 Responder status was shown to be associated with increased frequency of IgA- and IgG4-expressing allergen specific B cells, plasmablasts, and IL-10+ and/or IL-1RA+ Breg cells.243
AIT-induced T regulatory cells secreting IL-35 (iTR35) cells promote production of IL-10 from CD19+ B cells, Breg subsets and Tfr cells.244 Circulating Tfr cells share properties of memory cells and are distinct from their lymph nodes (LN) counterpart as they suppress B and Tfh cells with a much lower capacity, while circulating memory-like Tfh cells are more potent than LN effector Tfh cells.245 In addition, circulating memory-like Tfr cells persist for long periods, thus they could support the long-term immunomodulatory effect of peptide AIT. Functional evaluation of T regulatory cells via expression of Glycoprotein A repetitions predominant (GARP) and Special AT-rich sequence binding protein 1 (SATB1) is also interesting as a future potential biomarker. Tregs uniquely express GARP on their cell surface and GARP functions as a delivery system for latent TGF-β, which might augment the immunosuppressive role of Tregs on effector cells.246GARP expression was described as an activation marker of parasitic infection induced Tregs that strongly suppress allergic inflammation, thus is a novel potential mechanistic pathway for AIT. SATB1 is a genome organizer protein expressed in a lineage specific manner in CD4+ T-cells. During the early Th2 cell differentiation, IL-5 expression is repressed through direct binding of SATB1 to the IL-5 promoter. Thus, SATB1 modulation might expand the impact of AIT on eosinophilic inflammation. In addition, SATB1-dependent Treg-cell specific super-enhancers activation is crucial for Treg cell lineage specification in the thymus