Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Diffuse Large B-cell Lymphoma Histologically proven DLBCL and associated subtypes, according to the WHO 2016 classification including DLBCL not otherwise specified (NOS) High-grade B-cell lymphoma (HGBL) with MYC and BCL2 and/or BCL6 rearrangements with DLBCL histology (double hit lymphoma/triple hit lymphoma) High-grade BCL, NOS Primary (thymic) large mediastinal BCL Disease transformed from an earlier diagnosis of low-grade lymphoma (e.g. an indolent pathology such as follicular lymphoma, marginal zone lymphoma) into DLBCL with DLBCL disease progression subsequent to DLBCL directed systemic treatment. 2. Relapsed or refractory disease after first-line chemoimmunotherapy Refractory disease is defined as no CR to first-line therapy PD as best response after at least 4 full cycles of R-CHOP (rituximab, cyclophosphamide, hydroxydaunorubicin, oncovin and prednisone) cycles as first-line therapy Stable disease (SD) after 6 R-CHOP cycles as first-line therapy PR as best response after at least 6 R-CHOP cycles and biopsy-proven disease progression (except where prohibited due to comorbidities) within ≤ 12 months from the completion of the first-line therapy  Contraindications for R-GemOx and BR plus polatuzumab vedotin as judged by the treating physician. 2. Prior chimeric antigen receptor therapy or other genetically modified T-cell therapy. 3. ECOG performance status > 2. 4. Absolute neutrophil count < 1,000/μL (unless secondary to bone marrow involvement by DLBCL as demonstrated by bone marrow biopsy required for screening). 5. Platelet count < 50,000/μL (unless secondary to bone marrow involvement by DLBCL as demonstrated by bone marrow biopsy required for screening). 6. Absolute lymphocyte count < 100/μL. 7. Participants who have CNS lymphoma involvement in present or past medical history. 8. Known history of infection with human immunodeficiency virus or active infection with hepatitis B (hepatitis B surface antigen positive). 9. Known history of infection with hepatitis C virus unless treated and confirmed to be polymerase chain reaction negative. 10. Active infection with SARS-CoV-2. 11. Known history or evidence of severely immunocompromised state; i.e. corticosteroid treatment > 10 mg/day for more than 6 months. 12. Has received vaccination with live virus vaccines within 6 weeks prior to randomisation. 13. Prior CD19 targeted therapy 14. Known history or presence of seizure activities or on active antiseizure medications within the previous 12 months. 15. Presence of CNS disease that, in the judgement of the investigator, may impair the ability to evaluate neurotoxicity. 16. Known history or presence of autoimmune CNS disease, such as multiple sclerosis, optic neuritis or other immunologic or inflammatory disease. 17. Known history or presence of cerebral vascular accident (CVA) within 12 months prior to randomisation. Note: In case of history of CVA > 12 months prior to leukapheresis, then the participant must not have any unstable or life-threatening neurological deficits. 18. Participants with Richter's transformation or Richter's syndrome. 19. Participants who are concurrently on any other experimental treatments or during the previous 4 weeks or 5-half-lives. 20. Clinical heart failure with New York Heart Association class ≥ 2 or LVEF < 30%. 21. Resting peripheral oxygen saturation < 90% on room air. 22. Liver dysfunction as indicated by total bilirubin, aspartate aminotransferase and/or alanine aminotransferase > 5 × institutional upper limit of normal (ULN). 23. Serum creatinine ≥ 2.0 × ULN or CrCl < 30 mL/min calculated according to the modified formula of MDRD. 24. Pregnant or breast-feeding woman. 25. Prior history of malignancies other than DLBCL, unless the participant has been free of the disease for ≥ 3 years prior to screening. Exceptions to the ≥ 3-year time limit history of the following Basal cell carcinoma of the skin Squamous cell carcinoma of the skin Carcinoma in situ of the cervix Carcinoma in situ of the breast Carcinoma in situ of the bladder Incidental histological finding of untreated localized (T1a or T1b) prostate cancer under surveillance. 26. History of severe immediate hypersensitivity reaction against any drug or its ingredients/impurities that is scheduled to be given during study participation e.g. as part of the mandatory lymphodepletion protocol, premedication for infusion, or rescue medication/salvage therapies for treatment-related toxicities. 27. Any medical condition likely to interfere with assessment of safety or efficacy of study treatment. 28. Refusal to participate in CAR T long-term follow-up (LTFU)