Purpose <p>The HN1901 phase II window-of-opportunity study investigated an IDO1 (IO102) and PD-L1 (IO103) immune-modulatory therapeutic peptide cancer vaccines in monotherapy versus no preoperative treatment in patients with primary squamous cell carcinoma of the head and neck. The primary endpoint was the T-cell peptide specific response measured by INFγ-ELISpot in peripheral blood mononuclear cells. Secondary endpoints included safety, tumor modification assessment and evaluation of T-cell tumoral infiltration.</p> Patients and methods <p>Ten patients were randomized between arm A - IO102 and arm B - Control. Five unrandomized additional patients were included in arm C - IO103. Patients received vaccines for three weeks prior to curative treatment. Tumor biopsies, blood samples and radiological imaging were carried out at baseline and before curative treatment.</p> Results <p>The INFγ-ELISpot was positive in one IO103-vaccinated patient. Two IO103-vaccinated patients showed a tumor shrinkage with less cancer-related pain. Vaccine injections were safe and no grade III-IV side effects were attributed to the investigational treatment. For the IO103-vaccinated patient with a positive INFγ-ELISpot, a high baseline expression of PD-L1 and <i>CD274</i> was demonstrated with high CD8 + T-cell density in his operated tumor. Using spatial transcriptomics in surgical specimens, IO103-vaccinated patients showed high expression of gene lists involved in inflammation compared to controls, particularly in shrunken tumor.</p> Conclusion <p>Short-term treatment with the PD-L1 peptide vaccine IO103 showed tumor size reduction in two patients out of five associated with a positive INFγ-ELISpot and high CD8 + T-cells tumor density in one patient. Spatial transcriptomics analyses showed high expression of inflammation-related gene lists in IO103-vaccinated patients compared to controls. IO102 and IO103 injections were safe during the pre-operative period.</p>