Background In patients with melanoma, ipilimumab (anti-CTLA-4) prolongs overall survival and nivolumab (anti-PD-1) produced durable tumor regressions in a phase 1 trial. Based on their distinct immunologic mechanisms of action and supportive preclinical data, we conducted a phase 1 trial of nivolumab combined with ipilimumab in advanced melanoma patients. Methods Patients received nivolumab and ipilimumab every 3 weeks for 4 doses, followed by nivolumab alone every 3 weeks for 4 doses (concurrent regimen). Combined treatment was subsequently continued every 12 weeks for up to 8 doses. In a sequenced regimen, patients previously treated with ipilimumab received nivolumab every 2 weeks. Results Fifty-three patients received concurrent nivolumab/ipilimumab and 33 received sequenced treatment. The objective response rate, for all concurrent-regimen patients was 40% (modified WHO criteria). Evidence of clinical activity (conventional, unconfirmed, or immune-related response or stable disease ≥24 weeks) was observed in 65% of patients. At the maximum tolerated dose (1 mg/kg nivolumab + 3 mg/kg ipilimumab), 53% of patients achieved an objective response, all with ≥80% tumor reduction. Grade 3–4 related adverse events occurred in 53% of concurrent-regimen patients, but were qualitatively similar to historical monotherapy experience and were generally reversible. Among sequenced-regimen patients, 18% had grade 3–4 related adverse events and the objective response rate was 20%. Conclusions Concurrent nivolumab/ipilimumab had a manageable safety profile and achieved clinical activity that is distinct from published monotherapy data, with rapid and deep tumor regressions in a substantial number of patients.

Expression of programmed cell death receptor ligand 1 (PD-L1) on tumor cells has been associated with immune escape in human and murine cancers, but little is known regarding the immune regulation of PD-L1 expression by tumor cells and tumor-infiltrating macrophages in dogs. Therefore, 14 canine tumor cell lines, as well as primary cultures of canine monocytes and macrophages, were evaluated for constitutive PD-L1 expression and for responsiveness to immune stimuli. We found that PD-L1 was expressed constitutively on all canine tumor cell lines evaluated, although the levels of basal expression were very variable. Significant upregulation of PD-L1 expression by all tumor cell lines was observed following IFN-γ exposure and by exposure to a TLR3 ligand. Canine monocytes and monocyte-derived macrophages did not express PD-L1 constitutively, but did significantly upregulate expression following treatment with IFN-γ. These findings suggest that most canine tumors express PD-L1 constitutively and that both innate and adaptive immune stimuli can further upregulate PD-L1 expression. Therefore the upregulation of PD-L1 expression by tumor cells and by tumor-infiltrating macrophages in response to cytokines such as IFN-γ may represent an important mechanism of tumor-mediated T-cell suppression in dogs as well as in humans.

The co-inhibitory checkpoint molecule programmed death receptor 1 (PD-1) can trigger T cell functional exhaustion upon binding to its ligand PD-L1 expressed on tumour cells or macrophages. PD-1 blocking antibodies have generated remarkable results in human cancer patients, including inducing durable responses in a number of advanced cancers. Therefore, monoclonal antibodies specific for canine PD-1 were assessed for T cell binding and induction of functional activation. A total of 5-10% of CD4 T cells and 20-25% of CD8 T cells from healthy dogs expressed PD-1, and PD-1 expression was upregulated on T cells from dogs with cancer. Functionally, PD-1 antibodies significantly enhanced T-cell activation, as assessed by proliferation and interferon-gamma (IFN-) production. PD-1 antibodies also reversed T-cell suppression induced by canine soluble PD-L1 and by tumour cells and tumour explant fragments. These findings indicate that PD-1 antibodies have potential for use in cancer immunotherapy in dogs.