FOXP3

FOXP3 is a master transcription factor expressed in CD4+CD25+ regulatory T cells (Tregs), which are critical for maintaining peripheral immune tolerance and preventing autoimmunity. Tregs suppress effector T cell activation and proliferation through mechanisms including IL-2 consumption, secretion of inhibitory cytokines (IL-10, TGF-beta), and direct cell-cell contact via CTLA-4. FOXP3 is located on the X chromosome, which explains the X-linked inheritance pattern of IPEX syndrome. Loss-of-function mutations in FOXP3 result in absent or dysfunctional Tregs, leading to unchecked autoimmune activation. IPEX syndrome classically presents in male infants with a triad of enteropathy (severe watery diarrhea), endocrinopathy (most commonly type 1 diabetes mellitus, also thyroiditis), and eczema/dermatitis, often accompanied by other autoimmune manifestations (hemolytic anemia, thrombocytopenia, nephropathy) and failure to thrive. Without treatment (immunosuppression or hematopoietic stem cell transplantation), IPEX is typically fatal in infancy. This contrasts with other Treg-related pathways: CTLA-4 and IL-2/IL-2 receptor signaling also support Treg function, and mutations there cause similar but distinct autoimmune syndromes. FOXP3 is also used as an immunohistochemical marker to identify Tregs in tissue samples and tumor microenvironments (where increased Treg infiltration can indicate immune evasion by tumors). Understanding FOXP3 is high-yield for USMLE Step 1 in the context of primary immunodeficiencies with autoimmune features, and it is often contrasted with other X-linked immunodeficiencies (e.g., Wiskott-Aldrich syndrome, X-linked agammaglobulinemia) that lack the autoimmune component.