Evaluate the Safety and Efficacy of Tafasitamab and Lenalidomide in Combination With Gemcitabine and Oxaliplatin Versus Rituximab in Combination With Gemcitabine and Oxaliplatin in Patients With Relapsed/Refractory Diffuse Large B-Cell Lymphoma

Inclusion Criteria: 1. 18 Years and older. 2. One of the histologies of DLBCL confirmed by participated sites below with，not otherwise specified;T-cell/histiocyte-rich large B-cell lymphoma;Epstein-Barr virus (EBV) positive DLBCL (EBV-positive DLBCL); Composite lymphoma with a DLBCL component with a subsequent DLBCL relapse; Disease transformed from an earlier diagnosis of low-grade lymphoma into DLBCL with DLBCL treatment failure. 3. Availability of tumor tissue biopsied post last line of therapy and prior to current study treatment for the patients enrolled in safety and tolerability stage. 4. Relapsed/refractory (R/R) DLBCL, at least one (≥1) but no more than three (≤3) line of prior systemic therapies. 5. Patients who have not received high dose therapy/stem cell transplantation (HDT/SCT) must be ineligible for HDT/SCT. 6. At least one measurable site of disease per CT or magnetic resonance imaging (the longest axis of the lymph node lesion is \> 1.5 cm, and the longest diameter of the extra-nodal lesion is \> 1.0 cm). 7. ECOG PS score of 0 to 2. 8. Subject must have adequate organ functions, and the laboratory values comply with the protocol requirements. 9. Life expectancy of ≥ 3 months. 10. Informed consent before screening and can understand and comply with the requirements of the study. Exclusion Criteria: 1. Existing or prior history of other malignant tumor within 3 years, except for those who have received curative treatment. 2. Current or history of central nervous system (CNS) lymphoma. 3. Known high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements. 4. Primary mediastinal B-cell lymphoma. 5. History of allogeneic stem-cell transplantation. 6. Prior exposure to anti-CD19 treatment, and (or) failed with gemcitabine plus platinum-based agent combination therapy. 7. Current toxicity of ≥ Grade 2 from prior anti-cancer therapy (except for alopecia, neutrophil, hemoglobin and platelets). 8. Clinically significant cardiovascular disease or nervous system disease. 9. History of deep venous thrombosis/embolism, threatening thromboembolism or known thrombophilia or are at a high risk for a thromboembolic event in the opinion of the investigator and who are not willing/able to take venous thromboembolic event prophylaxis during the entire treatment period. 10. Uncontrolled systemic infection requiring parenteral intravenous anti-infective therapy. 11. Known human immunodeficiency virus (HIV), or serologic status reflecting active hepatitis B or C infection.