Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) is a primary immunodeficiency with autoimmunity caused by mutations in forkhead box P3 (FOXP3), which encodes a transcription factor involved in regulatory T (Treg) cell function. The mechanistic basis for how different mutations in FOXP3 cause distinct manifestations of IPEX remains unclear.

To determine whether 3 different point mutants of FOXP3 that cause severe or mild IPEX differ in their ability to reprogram conventional T cells into Treg cells.

Human CD4⁺ T cells were transduced with wild-type or point mutant forms of FOXP3, and changes in cell surface marker expression, cytokine production, proliferation and suppressive capacity were assessed. Ex vivo T_H17 cells were also transduced with different forms of FOXP3 to monitor changes in IL-17 production.

The forkhead mutant F373A failed to upregulate CD25 and CCR4, did not suppress cytokine production, and induced suppressive activity less effectively than wild-type FOXP3. In contrast, although the forkhead mutant R347H was also defective in upregulation of CD25, it suppressed the production of cytokines, conferred suppressive capacity on CD4⁺ T cells, and suppressed IL-17 production. F324L, a mutant outside the forkhead domain associated with mild IPEX, was equivalent to wild-type FOXP3 in all aspects tested.

Mutations in FOXP3 that cause IPEX do not uniformly abrogate the ability of FOXP3 to regulate transcription and drive the development of Treg cells. These data support the notion that factors in addition to functional changes in Treg cells, such as alterations in conventional T cells, are involved in the pathogenesis of IPEX.