Background: Chemo-based immunotherapy improves the prognosis of TNBC. However, the dominant population and optimal chemo-backbone of neoadjuvant immunotherapy remain undefined. Aim of TREND trial was to assess the efficacy and safety of tislelizumab in combination with nab-P and EC (nab-P-EC) in neoadjuvant therapy of TNBC. Methods: In this prospective, single-arm phase 2 clinical study, untreated TNBC patients who have clinically node-positive or at least T2 disease without distant metastasis were eligible. Before surgery, enrolled patients received tislelizumab (200 mg IV Q3W) plus weekly nab-P (100 mg/m² IV Q3W) for 12 weeks, followed by tislelizumab (200 mg IV Q3W) plus epirubicin (75 mg/m² IV Q3W) and cyclophosphamide (600 mg/m² IV Q3W) for the subsequently 12 weeks. The primary endpoint was pCR (ypT0/Tis ypN0) and secondary endpoints was objective response rate (ORR, RECIST v1.1). Combined positive score (CPS) of PD-L1 expression in pre-treatment biopsy samples was tested by PD-L1 IHC 22C3 PharmDx. Single-cell RNA sequencing, single-cell TCR sequencing, mass cytometry by time of flight, whole exome sequencing, RNA-seq and olink-assay were performed on pre- and post-treatment samples from primary tumors, metastatic lymph nodes and peripheral blood. Results: Of 43 enrolled patients from Nov 2020 to Nov 2022, 21 patients completed neoadjuvant therapy and accepted surgery in per protocol set (PPS; median age 48 years; range 25-69 years), and 16 were still on treatment. In PPS, the pCR (ypT0/is ypN0) rate was 71.43% (15/21). For different target lesions, 15 patients achieved ypT0/Tis (pCR rate = 71.43%) and 20 reached ypN0 (pCR rate = 95.24%). When stratified by CPS, pCR (ypT0/is ypN0) rate were 68.4%,72.2% and 76.9% in CPS ≥ 1, CPS ≥ 5 and CPS ≥ 10 subgroups, respectively. The ORR in PPS was 90.48% (n = 19/21, 95% CI: 68.18 - 98.33) and DCR reached 95.24% (n = 20/21, 95% CI: 74.13 - 99.75), including 9 complete response and 10 partial response. Treatment emergent adverse event (TEAE) occurred in 74.4% patients (32/43), of which 69.8% were Immune-related adverse event (irAE) (30/43). Grade ≥ 3 IrAE was 7.0% (3/43) and no treatment-related deaths were found. Conclusions: Combining tislelizumab and neoadjuvant nab-P-EC achieved favorable efficacy in TNBC, supporting that platinum-free regimen was non-inferior as a chemo-backbone. For heterogeneity of efficacy to immunotherapy, metastatic tumors in lymph nodes responded better to tislelizumab than primary tumor, suggesting that node-positive patients were potentially dominant population of neoadjuvant immunotherapy. Further results from the ongoing TREND trial and subsequent analysis of mechanism are urgently awaited. Clinical trial information: ChiCTR2000035262.