Cbl-b Deficiency Mediates Resistance to Programmed Death-Ligand 1/Programmed Death-1 Regulation.
Casitas B-lineage lymphoma-b (Cbl-b) is an E3 ubiquitin ligase that negatively regulates T cell activation. Cbl-b-/-T cells are hyper-reactive and co-stimulation independent, and Cbl-b-/-mice demonstrate robust T cell and NK cell-mediated antitumor immunity. As a result of these murine studies, Cbl-b is considered a potential target for therapeutic manipulation in human cancer immunotherapy. The PD-L1/PD-1 pathway of immune regulation is presently an important therapeutic focus in tumor immunotherapy, and although Cbl-b-/-mice have been shown to be resistant to several immuno-regulatory mechanisms, the sensitivity of Cbl-b-/-mice to PD-L1-mediated suppression has not been reported. We now document that Cbl-b-/-T cells and NK cells are resistant to PD-L1/PD-1-mediated suppression. Using a PD-L1 fusion protein (PD-L1 Ig), this resistance is shown for bothin vitroproliferative responses and IFN-γ production and is not associated with decreased PD-1 expression on Cbl-b-/-cells. In coculture studies, Cbl-b-/-CD8+, but not CD4+T cells, diminish the PD-L1 Ig-mediated suppression of bystander naïve WT CD8+T cells. Using anin vivomodel of B16 melanoma in which numerous liver metastases develop in WT mice in a PD-1 dependent manner, Cbl-b-/-mice develop significantly fewer liver metastases without the administration of anti-PD-1 antibody. Overall, our findings identify a new mode of immuno-regulatory resistance associated with Cbl-b deficiency and suggest that resistance to PD-L1/PD-1-mediated suppression is a novel mechanism by which Cbl-b deficiency leads to enhanced antitumor immunity. Our results suggest that targeting Cbl-b in cancer immunotherapy offers the opportunity to simultaneously override numerous relevant "checkpoints," including sensitivity to regulatory T cells, suppression by TGF-β, and immune regulation by both CTLA-4 and, as we now report, by the PD-L1/PD-1 pathway.
Department of Immunology, University of Connecticut Health Center , Farmington, CT , USA.
Mai
M
Fujiwara
Department of Immunology, University of Connecticut Health Center , Farmington, CT , USA.
Emily J
EJ
Anstadt
Department of Immunology, University of Connecticut Health Center, Farmington, CT, USA; Department of Medicine, University of Connecticut Health Center, Farmington, CT, USA.
Robert B
RB
Clark
Cbl-b; NK cells; PD-L1/PD-1; T cell regulation; immune checkpoint; tumor immunity