Cbl-b deficiency renders T cells resistant to PD-L1/PD-1 mediated suppression

The PD-L1/PD-1 pathway is a critical regulator of T cell responses that has become a significant focus in cancer immunotherapy. Cbl-b is an E3 ubiquitin ligase that regulates many aspects of T cell activation. In humans, polymorphisms in the CBLB gene are associated with autoimmunity. Cbl-b deficient (Cbl-b −/− ) mice demonstrate spontaneous autoimmunity and enhanced anti-tumor T cell responses. Thus, there is now great interest in manipulating Cbl-b to enhance anti-tumor T cell responses. We now report that, in contrast to WT T cells, in vitro TCR-stimulated proliferative responses of Cbl-b −/− CD4 + and CD8 + T cells are not suppressed by a recombinant PD-L1 fusion protein (PD-L1 Ig) (% suppression in CD4 + : WT 40%, Cbl-b 0%, p + : WT 32%, Cbl-b 5%, p −/− CD4 + T cells is less suppressed by PD-L1 Ig than that of WT cells (% Suppression: WT 68%, Cbl-b 20%, p −/− and WT T cells. To confirm this PD-L1 resistance in vivo, we used a model of B16 melanoma in which liver metastases develop only when PD-L1/PD-1 immune regulation is functionally intact. We confirmed that WT mice develop numerous liver metastases which are significantly reduced by anti-PD-1 antibody treatment. Strikingly, Cbl-b −/− mice develop only rare liver metastases even in the absence of anti-PD-1. In sum, we report for the first time that Cbl-b −/− T cells are resistant in vitro and in vivo to suppression by PD-L1/PD-1. Our finding of Cbl-b −/− T cell resistance to PD-L1/PD-1-mediated inhibition broadens our understanding of Cbl-b’s role in autoimmunity and suggests a new mechanism by which manipulation of Cbl-b function may lead to enhanced anti-tumor T cell responses.