Background: Although PD-1 blockade has improved survival in patients with recurrent and/or metastatic HNSCC, safety and efficacy of neoadjuvant immunotherapy with PD-L1 inhibitor with or without CTLA-4 inhibitor has not been investigated. Here, we report the updated results of the safety and efficacy of a preoperative D with or without T (D+/-T) in patients with resectable HNSCC, accompanied with high dimensional profiling of circulating immune cells. Methods: Patients with locally advanced but resectable HNSCC were eligible. Enrolled patients were randomized into D or D+T, stratified by primary site and human papilloma virus (HPV) infection status. A single dose of preoperative D (1500mg) or D+T (1500mg+75mg) was administered, with surgery planned 2 to 8 weeks later for curative resection. Postoperative (chemo) radiation was prescribed based on standard guidelines, followed by maintenance with D every 4 weeks for 1 year. Dynamic changes in circulating immune cells were tracked with mass cytometry. The primary objective was to determine the local recurrence rate. Secondary endpoints included pathologic response, safety, survival outcome, and exploration of immune dynamics. Results: As of January 25th 2022 for the interim analysis, a total of 45 patients were completely enrolled and received surgical resection (D: 21 patients, D+T: 24 patients). Oropharyngeal cancer was most common (n = 23; 51.1%) and HPV-mediated cancer was observed in 20 patients (44.4%). Neoadjuvant D+/-T had acceptable safety profiles and was not associated with delays in surgery or unexpected adverse events. Tumor shrinkage was observed in 31 patients (68.9%), with 15.6% of average tumor shrinkage (range; 100.0% to -80.0%). Major pathologic response (no more than 10% of viable tumor cells) was achieved in 3 patients (6.7%), including 2 cases with pathologic complete response (4.4%). During median follow-up duration of 407 days after surgery, local recurrence and systemic recurrence were documented in 9 patients (20.0%) and 7 patients (15.6%), respectively. Median disease-free survival and overall survival was 910 days and not reached, respectively. High dimensional immune profiling with mass cytometry revealed that D+T disproportionally increased the frequency of regulatory T cells accompanied with the upregulation of their functional markers, which was absent in patients treated with D monotherapy. Conclusions: These updated data suggested that preoperative D+/-T was safe and feasible and had the potential to provide clinical benefits for patients with resectable HNSCC. Distinct immunologic changes in circulating immune cells were induced by each treatment regimen, warranting further investigation. The trial is ongoing and the updated outcomes with immune correlates will be presented in this ASCO. Clinical trial information: NCT03737968.