Background: Combining checkpoint blockade with a cancer vaccine may induce broader immune responses, leading to better clinical outcomes. UV1 targets the enzyme telomerase (hTERT) which is expressed in almost all cancer types and is essential for the immortality of cancer cells and a hallmark of cancer. UV1 consists of three synthetic long peptides and vaccination induces Th1 responses in most patients irrespective of HLA type. This trial explores the synergistic effect of CTLA-4 blockade and hTERT vaccination, allowing unchecked expansion of hTERT-specific T cell clones. Increased number of tumor-specific T cells is associated with a favorable clinical outcome in patients with metastatic melanoma. We investigated the safety, immunological and clinical responses of UV1 vaccine and ipilimumab in this group of patients. Methods: In a phase I/IIa, single-center trial (NCT02275416) patients with metastatic melanoma received treatment with UV1 (300 µg) + GM-CSF (75 µg) as an adjuvant, combined with ipilimumab (3 mg/kg). Safety was assessed according to CTCAE v. 4.0, and tumor responses according to RECIST v.1.1. Immune responses against UV1 peptides were monitored in peripheral mononuclear blood cells by using ³H-thymidine proliferation and IFN-γ ELISPOT assays. Tumor mutational burden (TMB) estimations were based on whole-exome sequencing. Results: 12 patients were treated from Feb to Nov 2015. Treatment was generally well tolerated. Adverse events mainly included injection site reactions and diarrhea. Immune responses occurred very early and 10/11 evaluable patients showed an immune response. Three patients obtained a partial response, and one patient a complete response. 3-year overall survival (OS) was 67%. 4-year survival outcome will be presented along with baseline characteristics and TMB estimations. Conclusions: Combining UV1 and ipilimumab is safe and induces clinical responses in melanoma. The high proportion of immunological responders and early induction of detectable immune responses suggest synergism. OS compares favorably to historical controls. Clinical trial information: NCT02275416