RecruitingDrug trialStudy of Pomalidomide Combination with Rituximab and Anti-PD-1 Antibody (PPR) in Third or Later Line Therapy of DLBCLThis study aims to observe and explore the efficacy and safety of pomalidomide combination with rituximab and Anti-PD-1 Antibody in Third or Later Line Therapy of diffuse large B-cell lymphoma (DLBCL)