BACKGROUND: Studies show that proallergic TH 2-cells decrease after successful allergen-specific immunotherapy (AIT). It is likely that iatrogenic administration of allergens drives these cells to exhaustion due to chronic T cell receptor stimulation. This study aimed to investigate the exhaustion of T cells in connection with allergenexposure during AIT in mice and two independent patient cohorts. METHODS: OVA-sensitized C57BL/6J mice were challenged and treated with OVA, and the development of exhaustion inlocal and systemic TH 2-cells was analyzed.In patients, the expression of exhaustion-associated surface markers on TH 2-cells was evaluated using flow cytometry in a cross-sectional grass pollen allergy cohort with and without AIT.The treatment effect was further studied in PBMC collected from a prospective long-termAIT cohort. RESULTS: The exhaustion-associated surface markers CTLA-4 and PD-1 were significantly upregulated on TH 2-cells upon OVA aerosol exposure in OVA-allergic compared to non-allergic mice. CTLA-4 and PD-1decreased after AIT, in particular on the surface oflocal lung TH 2-cells. Similarly, CTLA-4 and PD-1 expression were enhanced on TH 2-cells from patients with allergic rhinitis with an even stronger effect in those with concomitant asthma. Using an unbiased Louvain clustering analysis, we discoveredalate-differentiated TH 2 population expressing both markers that decreased during up-dosing but persisted long-term during the maintenance phase. CONCLUSIONS: This study shows that allergen exposure promotes CTLA-4 and PD-1 expression onTH 2-cells, andthat the dynamic change in frequencies of exhausted TH 2-cells exhibits a differential pattern during the up-dosing versus the maintenance phases of AIT.