Transforming Growth Factor‐β (TGF‐β)–Dependent Iron Homeostasis and Altered TGF‐β Responsiveness in FLVCR1‐Deficient CD4 T Cells

ABSTRACT Transforming growth factor‐β (TGF‐β) regulates CD4 T‐cell quiescence, activation, and regulatory T‐cell differentiation, but its relationship with T‐cell iron homeostasis remains incompletely understood. We investigated how TGF‐β affects iron‐associated parameters during CD4 T‐cell activation and how FLVCR1 deficiency, a chronic perturbation of heme export associated with altered cellular iron distribution, affects functional responsiveness to TGF‐β. During activation, TGF‐β enhanced cell survival but markedly reduced proliferation, CD71 expression, and cytosolic labile iron while increasing mitochondrial labile iron. FLVCR1‐deficient CD4 T cells exhibited greater functional sensitivity to exogenous TGF‐β, elevated total TGF‐β in culture supernatants, and failure to downregulate TGFβRI after activation. Pharmacological inhibition of TGFβRI partially restored proliferation, CD71 expression, and cytosolic labile iron in TGF‐β‐treated FLVCR1‐deficient cells. Although TGF‐β increased total and mitochondrial reactive oxygen species (ROS)‐associated signal in FLVCR1‐deficient cells, antioxidant and Nox2‐inhibitor experiments did not establish ROS as a causal mediator. Acute FeSO 4 exposure reproduced selected functional effects of FLVCR1 deficiency but decreased TGFβRI expression and total TGF‐β accumulation, demonstrating that acute extracellular iron exposure and chronic FLVCR1 deficiency are biologically distinct perturbations. Finally, in vitro‐generated regulatory T cells displayed a distinct iron‐associated profile, and FeSO 4 exposure reduced Foxp3 induction. Together, these findings identify context‐dependent relationships among TGF‐β responsiveness, labile iron homeostasis, and FLVCR1‐dependent heme regulation in activated CD4 T cells.