Ligation of GITR (glucocorticoid-induced tumor necrosis factor (TNF) receptor related gene, or TNFRSF18) by agonist antibody has recently entered into early phase clinical trials for the treatment of advanced malignancies. Although the preclinical ability of GITR modulation to induce tumor regression is well documented, the underlying mechanisms of action, particularly its effects on CD4+ Foxp3+ regulatory T cells (Tregs), have not been fully elucidated. We have previously demonstrated that GITR ligation in vivo by agonist antibody (DTA-1) causes a >50% reduction of intra-tumor Tregs with down modulation of Foxp3 expression. Here we show that loss of Foxp3 is tumor dependent. Adoptively transferred Foxp3+Tregs from tumor bearing animals lose Foxp3 expression in the host when treated with DTA-1, whereas Tregs from naïve mice maintain Foxp3 expression. GITR ligation also alters the expression of various transcription factors and cytokines important for Treg function. Complete Foxp3 loss in intra-tumor Tregs correlates with a dramatic drop in Helios expression and is associated with upregulation of transcription factors T-Bet and Eomes. Changes in Helios corresponded with reduction in IL-10 and increase of IFNγ expression in DTA-1 treated Tregs. Together, these data show that GITR agonist antibody alters Treg lineage stability inducing an inflammatory effector T cell phenotype. The resultant loss of lineage stability causes Tregs to lose their intra-tumor immune suppressive function, making the tumor susceptible to killing by tumor specific effector CD8+ T cells.
- Received June 26, 2013.
- Revision received August 23, 2013.
- Accepted September 8, 2013.
- Copyright © 2013, American Association for Cancer Research.