Background: PD-1/PD-L1 inhibitors are efficient in head and neck squamous cell cancer (HNSCC). Combination with anti-CTLA4 agents may enhance anti-tumor activity compared to anti-PD-1/PD-L1 monotherapy in different tumor types. In the CheckRad-CD8 trial the typical induction treatment consisting of Cisplatin/Docetaxel was combined with Durvalumab/Tremelimumab. Patients with pathological complete response (pCR) in the re-biopsy after induction treatment or at least 20% increase of intratumoral CD8 density in the re-biopsy compared to baseline entered radioimmunotherapy with concomitant Durvalumab/Tremelimumab. Methods: In this prospective multicenter phase II trial, patients with HNSCC stage III-IVB received a single cycle of Cisplatin 30mg/m² d1-3, Docetaxel 75mg/m² d1, Durvalumab 1500mg fix dose d5 and Tremelimumab 75mg fix dose d5. Objectives of this interim analysis were to quantify the effect of the induction treatment on intratumoral CD8 density and the pCR rate and to generate safety data. Results: Between Sep 2018 and Dec 2019, 57 patients were enrolled. Median age was 59 years, 22 patients (37%) were current smokers, 27 patients (47%) had oropharyngeal tumors (52% p16 positive). The median pre-treatment intratumoral CD8 density was 335 CD8+ cells/mm². After induction treatment 27 patients (47%) had a pCR in the re-biopsy and further 25 patients (44%) had a relevant increase of intratumoral CD8+ cells (median increase by factor 3.0). Response according to RECIST criteria was CR in 1 (2%), PR in 19 (33%) and SD in 20 patients (35%) (17 patients not evaluable). Adverse events (AE) grade 3-4 appeared in 39 patients (68%) and mainly consisted of leucopenia (43%) and infections (28%). 6 patients (11%) developed grade 3-4 immune-related AEs. In multivariable analysis the intratumoral CD8 density was the only independently significant predictor of pCR (odds ratio 1.0013 per cell/mm², 95%-CI 1.00023-1.0023, p=0.017). 42 patients (74%) continued with Durvalumab/ Tremelimumab concomitant to radiotherapy. Conclusions: Single cycle induction treatment with Cisplatin/Docetaxel/Durvalumab/Tremelimumab is feasible and achieves a high pCR rate. CD8 density may have a predictive role for further treatment planning in locally advanced HNSCC. Clinical trial information: NCT03426657.