Loss of FOXP3 function causes expansion of two pools of autoreactive T cells in patients with IPEX syndrome.
The monogenic autoimmune disease Immunedysregulation polyendocrynopathy entheropathy X-linked syndrome (IPEX) has elucidated the essential function of the transcription factor FOXP3 and of thymic-derived regulatory T (Treg) cells in controlling autoimmunity. However, the presence of autoreactive T cells in IPEX remains undetermined, thus representing a crucial gap in understanding the origin of autoimmunity in a FOXP3 deficient immune system. Combining epigenetic analysis as a lineage marker of Treg identity and TCR sequencing to assess the self-reactive clones, we showed that IPEX patients have two pools of expanded autoreactive T cells. The first originates from the expansion of autoreactive effector T cells (Teff), likely due to loss of Treg suppressive function since it is absent in carrier mothers, in whom Treg cells are functional. The second pool originates, unexpectedly, from Treg cells which lose their phenotypic markers, including CD25 and FOXP3. We call these loss of identity Treg cells and show that they are i) suppressed by healthy donor Treg in a patient post hematopoietic transplantation despite low donor chimerism, and ii) not detectable in patients with Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy syndrome (APECED), a monogenic autoimmune disease of thymic origin. Moreover, we demonstrate that FOXP3 knock-out in Treg cells leads to increased Treg expansion and production of Th17 and Th2 cytokines, known to be increased in IPEX patients. These results suggest that the loss of identity Treg cells could directly contribute to immune dysregulation in IPEX. Collectively, we provide a better understanding of autoimmunity and novel ways to monitor the effects of Treg cell therapies in IPEX disease or other autoimmune diseases. One Sentence SummaryMutations of FOXP3 gene in humans cause expansion of autoreactive T cells originating from both effector T cells and regulatory T cells which gain effector function.