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ObjectiveThis study investigated the impact of innate lymphoid cell type 2 (ILC2s) on the function of regulatory T cells (Treg) and CD8+ T cells in chronic lymphocytic leukemia (CLL) through IL-9.MethodsPeripheral blood samples were collected from CLL patients (n = 52) and healthy controls (n = 30). ILC2 proportions and IL-9 levels were assessed using flow cytometry and ELISA. Immunofluorescence staining was performed to stain GATA3, CRTH2, and IL-9 in cervical lymph nodes from CLL patients (n = 10) and control subjects with reactive lymphadenitis (n = 10). Correlation analysis between ILC2s and IL-9 was conducted using the Spearman test. ILC2s were sorted and cultured from CLL patients, followed by co-culture experiments with PBMCs of healthy controls and MEC-1 cells, with or without anti-IL-9 antibody intervention. Flow cytometry was used to measure the proportions of ILC2s, Treg cells, PD-1+/TIGIT+/CTLA-4+ Treg subsets, and granzyme B+/perforin+ CD8+ T cells, along with MEC-1 cell apoptosis.ResultsThe proportions of ILC2s and Treg, along with serum IL-9 levels, were significantly elevated in CLL patients (P < 0.05). Peripheral blood ILC2s were positively correlated with IL-9 (r = 0.609, P < 0.001). The average fluorescence intensity of GATA3, CRTH2, and IL-9 in the cervical lymph nodes of CLL patients increased significantly (P < 0.001), and IL-9 showed colocalization with GATA3 and CRTH2. In vitro, IL-9 levels in the supernatant of sorted ILC2s from CLL patients increased. Treatment with anti-IL-9 antibody significantly reduced the PD-1+ Treg and TIGIT+ Treg cells while increasing granzyme B+ CD8+ T cells (P < 0.05). However, there was no significant effect on Treg, CTLA-4+ Treg, and perforin+ CD8+ T cells (P > 0.05). Additionally, anti-IL-9 antibody significantly increased early apoptosis (P < 0.05).ConclusionILC2s affect CD8+ T cells and Treg cells through IL-9, weakening the anti-tumor effects of CD8+ T cells and enhancing the immunosuppressive effects of Treg cells, thereby contributing to CLL pathogenesis.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00262-025-04082-4.