Downstaging Protocol Containing Immunotherapy for HCC Beyond the Milan Criteria Before Liver Transplantation

This will be a single-arm, open-label, non-randomized phase II study aiming to access the efficacy and safety of the downstage protocol containing immunotherapy for HCC beyond Milan Criteria. This study contains 4 phases: 1. Screening and Downstaging phase Eligible patients (See inclusion and exclusion criteria) will undergo downstaging treatment containing Anti-PD-1 inhibitor (tislelizumab, pembrolizumab, nivolumab et al). Anti-PD-1 inhibitor will be given every 3-4 weeks intravenously. Other combination regimens, such as locoregional therapies and targeted therapies will be given to patients according to the protocol decided by the multidisciplinary team of Centers. Patients that have reached the Criteria for Successful Downstage within 12 cycles of downstaging treatment will proceed to the observation phase, while those who fail to meet the Criteria for Successful Downstage after the maximum length of downstaging procedures will drop out from the study. Criteria for Successful Downstage Patients that meet 1-4 criteria are considered to have achieved the downstaging goal, otherwise, the downstaging treatment is considered to have failed within 12 cycles of downstaging treatment: * CR or PR evaluated by iRECIST/mRECIST after at least 2 cycles of downstaging treatment. * Patients with onset AFP\<400ng/ml: AFP \<400ng/ml at the end of downstaging therapy; Patients with onset AFP\>400ng/ml: AFP levels decrease more than 30%, and must be less than 1000ng/ml at the end of downstage therapy. * Estimated 5-year overall survival ≥60% at the end of downstaging therapy according to the Metroticket model (http://www.hcc-olt-metroticket.org/calculator). * For patients with portal vein tumor thrombus (PVTT1-3), tumor thrombus needs to retract above the bifurcation of the main portal vein, and portal vein anastomosis is feasible during liver transplantation according to the surgeons' judgment. 2. Observation phase Patients that have reached the Criteria for Successful Downstage within 12 cycles of downstaging treatment will proceed to the observation phase ≥ 3 months. Patients at this phase can optionally receive TKIs (sorafenib, lenvatinib et al), and other anti-tumor treatments (such as locoregional therapies, surgery, and immunotherapies et al) are not allowed at this phase. Patients with a sustained response and whose tumors still meet the Criteria for Successful Downstage at the end of the observation phase will be enrolled in the waiting list for liver transplantation, while patients with tumor progression (PD) and their tumors fail to meet the Criteria for Successful Downstage at the end of observation phase will drop out from the study. 3. Waiting and Bridging phase During the waiting period before liver transplantation, patients can optionally receive a non-immunotherapy protocol as bridging treatment determined by the multidisciplinary team (MDT). Surgery is not allowed at this phase. 4. Liver transplantation and post-transplantation phase Patients will undergo liver transplantation unless major medical or oncological contraindications should occur during the waiting phase.