Loss of the actin remodeling protein Flightless-1 impairs CD8 and regulatory T cell function.

T cell immunity depends on the precise coordination of signaling networks with actin cytoskeleton remodeling, yet the molecular regulators of these processes remain incompletely defined. Flightless-1 (FLII) is a gelsolin-family actin regulator with unique leucine-rich repeats that can couple cytoskeletal dynamics to diverse signaling pathways. Here, using conditional knockout mice, we identify essential roles for FLII in both CD8⁺ and regulatory T cells. Loss of FLII in CD8⁺ T cells caused a profound loss of naive cells from the spleen, impaired CCR7-dependent migration, and defective accumulation in the lung parenchyma during antigen-specific responses to respiratory vesicular stomatitis virus infection, despite largely preserved activation, effector differentiation, and cytotoxic function. FLII-deficient Foxp3⁺ regulatory T cells maintained normal numbers but exhibited diminished CD25 expression, defective IL-2 signaling, and failed to restrain spontaneous, tissue-specific autoimmunity. These findings identify FLII as a critical and previously unrecognized orchestrator of T cell trafficking and immune regulation, which may link chemokine receptor signaling to actin remodeling and is essential for proper T cell migration and function.