The Role of the Selenoprotein Glutathione Peroxidase-1 in T Cell Activation and Differentiation

Glutathione peroxidase-1 (GPx-1) is an antioxidant enzyme that plays an important role in reducing cellular oxidative stress. The expression of GPX1 has previously been shown to be upregulated upon T cell activation in vitro and CD4+ T cells that lack GPx-1 have also been shown to preferentially differentiate into the TH1 effector subtype. These observations suggest that the activity of GPx-1 may play a role in T cell activation and differentiation. In order to determine the relationship between GPx-1 expression and T cell activation, we used a combination of in vitro and in vivo experiments to correlate GPx-1 expression with the activation marker CD44 and to address whether inhibition of GPx-1 impacts the differentiation of induced regulatory T cells.^ Gene expression analysis done on naïve CD4+ T cells stimulated in vitro reveals that the number of GPX1 transcripts is reduced in cells cultured in the presence of recombinant PD-L1 when compared to cells cultured in the presence of control human Ig-Fc. In contrast to these data, flow cytometry data does not reveal any significant differences between the overall abundance of intracellular GPx-1 over the course of 72 hours when comparing CD4+ T cells cultured in the presence and absence of recombinant PD-L1. Using in vivo immunization and tumor models, we demonstrate that the overall expression of GPx-1 is higher both in CD44+ T cells derived from the draining lymph node and tumor infiltrating lymphocytes when compared to CD44- T cells from each microenvironment. To assess the role of GPx-1 during activation and differentiation of naïve CD4+ T cells in vitro, we used a small molecule inhibitor of GPx-1, mercaptosuccinate (MS).^ Our data reveal that the addition of MS reduces the overall number of CD44+ T cells and that the presence of MS increases the oxidative state of CD4+ T cells in a dose dependent manner. These data support the role of GPx-1 as an important cytosolic and mitochondrial antioxidant enzyme. Our data also reveal that the presence of MS in cell culture media induces the expression of the transcription factor Forkhead box P3 (FoxP3) in a dose-dependent manner that is independent of the presence of TGF-β. Overall, these data suggest that GPx-1 is an important antioxidant enzyme that may play a role in regulating T cell activation and differentiation. Additionally, we provide preliminary evidence to support the idea that the redox tone of the cell may be important for fate determination. Further work is needed to assess the functionality and durability of these FoxP3 expressing T cells, and to address whether or not these cells exhibit the typical characteristics of induced regulatory T cells.