The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.Background T cells that recognize tumor antigens are the foun - dation of current immunotherapy, on which the effi - cacy of checkpoint inhibitors (CPIs) relies. CPIs have Open AccessJournal of Translational Medicin e *Correspondence: espen.ellingsen@ultimovacs.com 1 Department of Tumor Biology, Institute for Cancer Research, The Norwegian Radium Hospital, Oslo, Norway Full list of author information is available at the end of the article Page 2 of 13 Ellingsen et al. Journal of Translational Medicine (2022) 20:419 revolutionized the treatment of many cancers, most notably malignant melanomas [1, 2]. Although many patients experience deep and durable clinical responses to CPIs, unfortunately, many patients progress and require additional treatment. CPIs work by inhibit - ing axes that restrict T cell activation and proliferation, releasing spontaneously primed antitumor immune responses [3]. Therefore, predictive biomarkers for CPI efficacy are largely related to tumor immunogenicity, such as tumor mutational burden [4], neoantigen bur - den [5], PD-L1 positivity [6], tumor-infiltrating lympho - cytes [ 7], and IFN-γ gene signature [8]. Conversely, a lack of CPI efficacy is associated with inadequate antitumor T cell responses and immunosuppressive factors in the tumor microenvironment (TME) [9]. Although incre - mental benefits are achieved by simultaneously inhibiting multiple immune checkpoints [10], the quality and quan - tity of T cells specific for tumor antigens remains a limit - ing factor for further advancements in immunotherapy. Therapeutic cancer vaccines (TCVs) aim to direct the expansion of T cells targeting relevant tumor antigens, providing a new wave of tumor-specific T cells to the TME [11]. This approach thereby serves to supplement and reinforce the anti-tumor immune response, synergiz - ing with immunotherapies that depend on the presence of tumor-specific T cell responses (e.g. CPIs). Strate - gies applied to TCVs include targeting neoantigens and shared tumor-associated antigens (TAAs). Tumor-spe - cific somatic mutations may give rise to aberrant peptides that are sufficiently different from their normal counter - parts, allowing recognition of these neoantigens by the patient’s T cells. In contrast, TAAs are non-mutated anti - gens with a selective expression pattern that is preferen - tially limited to tumors. Personalized neoantigen cancer vaccines require tumor tissue harvesting and next-gener - ation sequencing for in silico neoantigen prediction, and subsequent personalized vaccine production. TAA-based vaccines, however, can forego personalized production and can be delivered directly off-the-shelf to the patient, assuming that the tumor expresses the target antigen. TAA-based vaccines may also be more relevant in can - cers with low tumor mutational burden, where fewer neoantigens are presented. Telomerase reverse transcriptase (hTERT) is a TAA activated in 85–90% of all tumor types [12, 13]. Telom - erase is expressed by cancer cells to maintain telomere replication supporting unconstrained cancer cell pro - liferation and metastasis [14, 15]. Therefore, telomerase activation is considered a hallmark of cancer [16]. Mela - nomas frequently harbor hTERT promoter mutations and gene copy number amplification which are associ - ated with increased hTERT expression [17–19]. These genomic aberrations collectively lead to an increased antigen presence providing a scientific rationale for tar - geting hTERT in melanomas. High tumor telomerase activity is a well-established negative prognostic factor across multiple cancer indications [20–24], whereas anti- telomerase CD4 T cell immune responses are emerging as independent positive prognostic factors validated in several malignancies [25–27]. Based on these character - istics, hTERT is considered a promising TAA for thera - peutic vaccination [28]. UV1 is a TCV composed of three synthetic long pep - tides derived from the active site of hTERT and has been proven to establish robust, long-lasting T cell responses across an HLA-unselected population in three completed phase I clinical trials [29]. Immune responses induced by UV1 have been identified as CD4 Th1-polarized effector memory cells with inflammatory cytokine profiles (tumor necrosis factor-α and IFN-γ). Expanding a population of CD4 Th1 cells targeting a shared tumor antigen could lead to intratumoral re-activation of these cells, inducing an inflammatory TME and immune-mediated cancer cell death [30–32]. CD4 T cells enhance antitumor immunity by licensing dendritic cells for effective antigen presen - tation and by secretion of inflammatory cytokines, pro - moting immune cell infiltration, and effector functions. Immune checkpoints maintain immune responses within a desired physiological spectrum, and their block - ing is expected not only to disinhibit spontaneously primed anti-tumor T cell responses, but also de novo T cell responses induced by vaccination. This provides a therapeutic rationale for combining checkpoint inhibi - tion with TCVs. The cytotoxic T lymphocyte-associated protein 4 (CTLA-4) immune checkpoint competitively inhibits the binding of CD28 on T cells with CD80/CD86 on antigen-presenting cells, thereby reducing T cell acti - vation by preventing the co-stimulation of primed T cells. Ipilimumab is a monoclonal antibody that blocks this immune checkpoint and disrupts negative regula - tion imposed by CTLA-4. We wanted to explore whether combining ipilimumab with the UV1 vaccine would lead to synergy in terms of expanding vaccine-specific T cells, improving anti-tumor immune responses, and clinical outcomes. We have previously reported safety and feasibility data of this phase I/IIa clinical trial evaluating combined UV1 vaccination and ipilimumab in patients with meta - static melanoma [33]. A parallel phase 4 trial evaluating ipilimumab monotherapy at Norwegian hospitals during the same period has since been published, demonstrat - ing clinical outcomes aligned with previously reported data on ipilimumab monotherapy [34, 35]. Considering the combination study yielded comparatively superior progression-free survival, overall survival, and objective responses rate, we sought to further investigate whether Page 3 of 13 Ellingsen et al. Journal of Translational Medicine (2022) 20:419 this cohort comprised patients with favorable baseline characteristics and explore the dynamics of the vaccine- induced immune response. Herein, we report an updated survival analysis and extensive translational research from this clinical trial. Methods Study design, patients, and treatments The study design, eligibility criteria, and treatments have been previously described [33]. The UV1/hTERT-MM study was an open-label, single-arm, single-center, phase I/IIa clinical trial (NCT02275416). The primary objective of this study was to assess the safety of ipilimumab com - bined with UV1 vaccination in patients with malignant melanoma. The secondary objectives included immune response assessment, objective response rate (ORR) per RECIST 1.1, overall survival (OS), and progression-free survival (PFS). Key eligibility criteria were age ≥ 18 years and a histologically confirmed diagnosis of unresect - able stage III/IV cutaneous malignant melanoma. An Eastern Cooperative Oncology Group (ECOG) perfor - mance status ≤ 1 and any previous therapies for mela - noma were permitted. The study participants provided written informed consent prior to enrolment.