The goal of this clinical study is to test how well the study drug, brexucabtagene autoleucel (KTE-X19), works in participants with relapsed/refractory (r/r) mantle cell lymphoma (MCL).

Study KTE-C19-102 (NCT02601313) enrolled participants with r/r MCL who have been treated with up to 5 prior regimens including a Bruton's tyrosine kinase inhibitor (BTKi) in Cohort 1 and Cohort 2. However, to fulfill FDA Postmarketing Requirement Cohort 3 is added to the study. It will include participants with r/r MCL who have been treated with up to 5 prior regimens but have not received prior therapy with a BTKi.

The primary analysis in Cohort 1 and Cohort 2 is already completed. Data for Cohort 3 will be analyzed separately. Therefore, this separate registration is only for Cohort 3.

After the end of KTE-C19-102, subjects who received an infusion of anti-CD19 CAR T cells will complete the remainder of the 15-year follow-up assessments in a separate long-term follow-up study, KT-US-982-5968

Key Inclusion Criteria:

• Up to 5 prior regimens for MCL. Prior therapy must have included anthracycline- or bendamustine-containing chemotherapy and anti-CD20 monoclonal antibody therapy. Individuals must not have received prior therapy with a BTKi.
• At least 1 measurable lesion
• Platelet count ≥ 75,000/uL
• Creatinine clearance (as estimated by Cockcroft Gault) ≥ to 60 cc/min
• Cardiac ejection fraction ≥ 50%, no evidence of pericardial effusion as determined by an echocardiogram (ECHO) or multigated acquisition (MUGA), and no clinically significant electrocardiogram (ECG) findings
• Baseline oxygen saturation > 92% on room air

Key Exclusion Criteria:

• Known history of infection with human immunodeficiency virus (HIV) or hepatitis B (HBsAG positive) or hepatitis C virus (anti-HCV positive). Individuals with a history of hepatitis infection must have cleared their infection as determined by standard serological and genetic testing
• History of a seizure disorder, cerebrovascular ischemia/hemorrhage, dementia, cerebellar disease, cerebral edema, posterior reversible encephalopathy syndrome, or any autoimmune disease with central nervous system (CNS) involvement
• Presence of fungal, bacterial, viral, or other infection that is uncontrolled or requiring IV antimicrobials for management

Note: Other protocol defined Inclusion/Exclusion criteria may apply.