Immune checkpoint blockade impairs immunosuppressive mechanisms of regulatory T cells in B-cell lymphoma

In malignant disease, CD4(+)Foxp3(+) regulatory T cells (Tregs) hamper antitumor immune responses and may provide a target for immunotherapy. Although immune checkpoint blockade (ICB) has become an established therapy for several cancer entities including lymphoma, its mechanisms have not been entirely uncovered. Using endogenously arising λ-MYC-transgenic mouse B-cell lymphomas, which can effectively be suppressed by either Treg ablation or ICB, we investigated which mechanisms are used by Tregs to suppress antitumor responses and how ICB affects these pathways. During tumor development, Tregs up-regulated Foxp3, CD25, CTLA-4 and IL-10, which correlated with enhanced immunosuppressive functions. Thus, in contrast to other tumors, Tregs did not become dysfunctional despite chronic stimulation in the tumor microenvironment and progressive up-regulation of PD-1. Immunosuppression was mediated by direct contacts between Tregs and effector T cells and by IL-10. When λ-MYC mice were treated with ICB antibodies, Tregs revealed a less profound up-regulation of Foxp3, CD25 and IL-10 and a decreased suppressive capacity. This may be due to the shift towards a pro-inflammatory milieu fostered by ICB. In summary, an ICB-induced interference with Treg-dependent immunosuppression may contribute to the success of ICB.