e20549 Background: OH2 is an attenuated oncolytic Herpes Simplex-2 virus expressing GM-CSF. OH2 replicates only in cancer cells, causing their lysis, which reverses the immunosuppressive tumor microenvironment, leading to tumor regression. OH2 monotherapy is safe and efficacious in phase I-III trials of 300+ patients across several cancer indications. We previously showed that OH2 treated briefly with UV light (UV-OH2) is unable to form plaques yet still able to infect cancer cells and to potently activate human PBMCs, leading to increased antitumor activity in vitro and in animal models. Methods: Translating these findings in humans, 12 stage IV lung cancer patients with malignant pleural or peritoneal effusions were enrolled in an Investigator-Initiated Trial (NCT05565014). Lymphocytes from each patient’s effusions were isolated, incubated in vitro with UV-OH2 for 36-48h, and then re-infused into the patient's pleural or peritoneal cavity. Each patient received 2 cycles of 3 or up to 6 infusions of this Virus Activated Killer (VAK) Autologous Cell Therapy. Results: Effusion volume (EV) measurement and local and whole-body tumor assessments were performed at the beginning and 28 days post VAK treatment. On D28, EV remained stable in 3 patients (SD), whereas 4 of 12 patients recorded a reduction of EV (PR). Notably, subject S004’s PR was maintained for 210 days post-treatment. Evaluation of target lesions per RECIST revealed 4 subjects with PR and 7 with an SD in target lesions. There was no toxicity or treatment-related adverse events. The median patient OS was 384 days (12.8 months), which is up to 4 times the average published OS for stage IV lung cancer with malignant pleural effusions. RNA-Seq analysis indicated increased expression of many T and NK cell effectors and NK cell activating receptors. Importantly, average autologous CD8+ T lymphocytes rose from 53% to 74% in the effusions post VAK-treatment, thereby confirming activation of the patient’s immune system by UV-OH2 (preliminary clinical PoC). Conclusions: In sum, our VAK therapy offers a cost-effective, extremely safe, and patient-friendly autologous treatment that dramatically increases overall survival in patients with essentially no options. Larger scale studies are in the works. Clinical trial information: NCT05565014 .