CD4⁺CD25⁺Foxp3⁺ Regulatory T cells (Tregs) play a critical role in immune tolerance. The plasticity and functional adaptability of Tregs in an inflammatory microenvironment has been demonstrated in autoimmunity. Here, using a double transgenic mouse model that permits Foxp3 lineage tracing, we investigated the phenotypic plasticity of Foxp3⁺ Tregs in a well-characterized murine model of corneal transplantation. In order to subvert the normal immune privilege of the cornea and foster an inflammatory milieu, host mice were exposed to desiccating stress prior to transplantation. Treg frequencies and function were decreased following desiccating stress, and this corresponded to decreased graft survival. A fraction of Tregs converted to IL-17⁺ or IFNγ⁺ ‘exFoxp3’ T cells that were phenotypically indistinguishable from effector Th17 or Th1 cells, respectively. We investigated how Foxp3 expression is modulated in different Treg subsets, demonstrating that neuropilin-1⁻ peripherally-derived Tregs are particularly susceptible to conversion to IL-17⁺/IFNγ⁺ exFoxp3 cells in response to cues from their microenvironment. Finally, we show that IL-6 and IL-23 are implicated in the conversion of Tregs to exFoxp3 cells. This report demonstrates that the pathological conversion of Tregs contributes to the loss of corneal immune privilege.

Th17 cells are critical effectors mediating the ocular surface autoimmunity in dry eye disease (DED). Increased IFN-γ has also been implicated in DED; however, it remains unclear to what extent Th1 cells contribute to DED pathogenesis. In this study, we investigated the cellular source of IFN-γ and assessed its contribution to corneal epitheliopathy in DED mice. We discovered a significant IL-17A⁺IFN-γ⁺ (Th17/1) population and determined that these cells are derived from Th17 precursors. Adoptive transfer of Th17/1, but not Th1, cells confers the disease to naive recipients as effectively as do Th17 cells alone. DED-induced IL-12 and IL-23 are required for in vivo transition of pathogenic Th17 cells to IFN-γ producers. Furthermore, using IFN-γ–deficient Th17 cells, we demonstrate the disease-amplifying role of Th17-derived IFN-γ in DED pathogenesis. These results clearly demonstrate that Th17 cells mediate ocular surface autoimmunity through both IL-17A and IFN-γ.

NK cells have been increasingly reported to be an important effector in autoimmune diseases. However, nothing is known in this regard in DED, the most common eye pathology, which is characterized by sustained inflammation on the ocular surface. In the present study, we have examined the profile of NK cells on the ocular surface as well as in the draining lymphoid tissues during the development of this disease. Our data demonstrate activated NK cells during the disease-induction phase. Moreover, in vivo depletion of NK cells in mice results in reduced disease severity and diminished proinflammatory cytokines. Furthermore, we show that NK cells are also able to modulate the maturation of APCs, which is correlated with IFN-γ from NK cells. Together, our findings provide new in vivo evidence that IFN-γ-secreting NK cells can promote induction of DED via direct target tissue damage and indirect influence on the priming phase of an adaptive immune response in secondary lymphoid tissue.