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Dendritic cell immunotherapy has proven to be safe and induces an immune response in humans. We aimed to establish the efficacy of dendritic cells loaded with allogeneic tumour cell lysate (MesoPher, Amphera BV, 's-Hertogenbosch, Netherlands) as maintenance therapy in patients with pleural mesothelioma. In this open-label, randomised, phase 2/3 study, patients with histologically confirmed unresectable pleural mesothelioma, aged 18 years or older, with an Eastern Cooperative Oncology Group performance status score of 0–1, and non-progressing disease after four to six cycles of standard chemotherapy (with pemetrexed 500 mg/m2 plus platinum [cisplatin 75 mg/m2 or carboplatin area under the curve of 5]) were recruited from four centres in Belgium, France, and The Netherlands. Participants were randomly assigned (1:1), using block randomisation (block size of 4), stratified by centre and histology (epithelioid vs other), to MesoPher treatment plus best supportive care or best supportive care alone. Patients received up to a maximum of five MesoPher infusions, with treatment administered on days 1, 15, and 29, and weeks 18 and 30. At each timepoint, participants received an injection of 25 × 106 dendritic cells (two-thirds of the dendritic cells were administered intravenously and a third were injected intradermally). Best supportive care was per local institutional standards. The primary endpoint was overall survival, assessed in all participants randomly assigned to treatment (full analysis set) and safety assessed in all randomly assigned participants, and who underwent leukapheresis if they were in the MesoPher group. This study is registered with ClinicalTrials.gov , NCT03610360 , and is closed for accrual. Between June 21, 2018, and June 10, 2021, 176 patients were screened and randomly assigned to the MesoPher group (n=88) or best supportive care alone group (n=88). One participant in the MesoPher group did not undergo leukapheresis. Mean age was 68 years (SD 8), 149 (85%) of 176 were male, 27 (15%) were female, 173 (98%) were White, two were Asian (1%), and one (1%) was other race. As of data cutoff (June 24, 2023), after a median follow up of 15·1 months (IQR 9·5–22·4), median overall survival was 16·8 months (95% CI 12·4–20·3; 61 [69%] of 88 died) in the MesoPher group and 18·3 months (14·3–21·9; 59 [67%] of 88 died) in the best supportive care group (hazard ratio 1·10 [95% CI 0·77–1·57]; log-rank p=0·62). The most common grade 3–4 treatment-emergent adverse events were chest pain (three [3%] of 87 in the MesoPher group vs two [2%] of 88 in the best supportive care group), dyspnoea (none vs two [2%]), anaemia (two [2%] vs none), nausea (none vs two [2%]), and pneumonia (none vs two [2%]). No deaths due to treatment-emergent adverse events were recorded. Treatment-related adverse events consisted of infusion-related reactions (fever, chills, and fatigue), which occurred in 64 (74%) of 87 patients in the MesoPher group, and injection-site reactions (itch, erythema, and induration), which occurred in 73 (84%) patients, and all were grade 1–2 in severity. No deaths were determined to be treatment related. MesoPher did not show improvement in overall survival in patients with pleural mesothelioma. Immune checkpoint therapy is now standard of care in pleural mesothelioma. Further randomised studies are needed of combinations of MesoPher and immune checkpoint therapy, which might increase efficacy without adding major toxicities. Amphera BV and EU HORIZON. [ABSTRACT FROM AUTHOR]