Two Sources of Autoreactive T Cells in Patients with FOXP3 Mutations

T-cell ontogeny in thymus gives rise to immense amount of T cells with different TCR specificities, ensuring protection against wide variety of pathogens. Although the self-reactive T cell clones are largely deleted during the development in a process of negative selection, autoreactive T cells could be readily detected in healthy individuals.  The autoreactive clones are being suppressed by various mechanisms among which CD4+FOXP3+ regulatory T cells (Treg) are the best recognized. However, to which extend Treg suppress autoreactive T cells expansion in human in vivo is unclear. Here, we analyzed TCR repertoire of patients with FOXP3 mutations, a key transcription factor of Treg, and found increased TCR autoreactivity in bona fide T effector cells. Interestingly, the increased TCR autoreactivity positively correlates with increased number of cells with Treg epigenetic signature, which loss Treg phenotypic markers including FOXP3 expression. Since Treg cells are physiologically self-reactive, we speculate that the loss of identity Treg might represent a new pool of autoreactive T cells exacerbating the autoimmunity and thus promoting expansion of autoreactive T cells.  In line with the hypothesis of pathological potential of loss of identity Treg cells, we observed increased production of proinflammatory cytokines by CRISPR/Cas9 mediated FOXP3 knock out Treg and increased proliferation in response to TCR-mediated stimulation. Collectively, these data suggest that mutation in FOXP3 causes loss of Treg suppressive function leading to of autoreactive T cells in periphery and simultaneously increased number of Treg which lose their identity and are capable of exacerbating the autoimmune reaction.