PURPOSE: CD4 effector T cells (Teffs) play a key role in immune responses by infiltrating the sites of inflammation and modulating local leukocyte activity. In turn resident immune cells shape their response. This study aimed to investigate the influence of mast cells (MCs) on Teff biological responses. METHODS: This study examined human MC-Teff interactions, focusing on how MCs shape Teff responses. Flow cytometry, qRT-PCR, and cytokine assays were used to analyze the impact of primary human MCs on the Teff phenotype and function. MC-Teff crosstalk within Crohn's disease patient tissues was assessed using confocal microscopy and advanced image analysis. RESULTS: MCs promoted the differentiation of Th17 cells, particularly the inflammatory Th17.1 subset, that secretes IFN-γ and GM-CSF. This differentiation was driven by the PGE and IL-1β axis. Additionally, MCs disrupted the phenotype and impaired the suppressive function of regulatory T cells (Tregs) through PGE, skewing the Th17/Treg balance. The analysis of biopsies from patients with Crohn's disease indicated that this MC/Teff crosstalk may play a role in the pathogenesis of auto-inflammatory processes. CONCLUSION: MCs influence CD4 T cell responses by fostering pro-inflammatory Th17 differentiation while impairing Treg function. This interaction underpins a Th17/Treg imbalance, which is significant in auto-inflammatory diseases such as Crohn's disease, positioning MCs as critical drivers of disease pathogenesis.