Multipronged effects of FOXP3 mutations

Mutations in the transcription factor forkhead box P3 (FOXP3) are responsible for a rare autoimmune disease in young boys known as IPEX (immunodysregulation, polyendocrinopathy and enteropathy, X-linked syndrome). At least in mice, FOXP3 has a crucial role in the development and function of CD4⁺CD25⁺ regulatory T (T_Reg) cells. Therefore, it has been suggested that a lack of these regulatory cells might contribute to disease in humans, as has been shown in the mouse model of the disease, Scurfy mice. However, Maria Grazia Roncarolo and colleagues now report that the human disease is not necessarily the result of a deficiency of T_Reg cells but instead because these cells are dysfunctional. Moreover, they show that effector T cells from patients with IPEX also have a functional defect.

To carry out their studies, four children with IPEX of varying disease severity and with various FOXP3 mutations were studied, and so correlations could be drawn between different FOXP3 mutations and T_Reg-cell generation and function. The authors were surprised to see that T_Reg-cell numbers were normal in most of the patients and were comparable to those of age-matched control donors. Moreover, these T_Reg cells had a normal phenotype, including expression of GITR (glucocorticoid-induced tumour-necrosis-factor-receptor-related protein) and CTLA4 (cytotoxic T-lymphocyte antigen 4), and they displayed normal T_Reg-cell characteristics, such as anergy and lack of interferon-γ (IFNγ) production after activation.

However, the ability of T_Reg cells from the IPEX patients to suppress the proliferation of CD4⁺CD25⁻ effector T cells was reduced compared with T_Reg cells from normal donors. The degree of defective suppression varied depending on the type of FOXP3 mutation, the strength of the activation stimuli and the genotype of the target effector T cells. For example, when provided with a 'weak' stimulus (CD3-specific antibody presented by allogeneic antigen-presenting cells), T_Reg cells from IPEX patients who express the mutant FOXP3 protein could suppress in vitro proliferation of effector T cells from control donors. But when provided with a 'strong' stimulus (beads coated with CD3- and CD28-specific antibody), T_Reg cells from these same patients failed to suppress effector T-cell proliferation. Notably, activated T_Reg cells from a patient with a FOXP3 mutation that completely ablated FOXP3 expression were unable to suppress effector T-cell proliferation.

During these studies the authors also noted that, when autologous effector T cells were used as targets for suppression, activated T_Reg cells from all four patients were unable to suppress in vitro proliferation, independent of the activation conditions used. This led the authors to investigate whether the effector T cells from IPEX patients were defective. Despite their disparate clinical phenotypes, effector T cells from all four patients had a markedly reduced ability to secrete interleukin-2 and IFNγ following T-cell receptor (TCR)-mediated activation compared with cells from normal donors. This defect was not apparent when the cells were activated with phorbol ester and ionomycin, which activate T cells in a TCR-independent manner. This finding indicates that FOXP3 could have a role in regulating the effector T-cell functions that depend on TCR signalling, as well as in regulating T_Reg-cell function.

So, these findings indicate that, in contrast to the disease in Scurfy mice, human IPEX is not necessarily due to the absence of T_Reg cells but rather to their impaired suppressive function, together with altered effector T-cell function.