A multi-center, open-label, phase Ib study to evaluate the safety and efficacy of the administration of tisagenlecleucel in combination with pembrolizumab in patients with r/r DLBCL who have received 2 or more lines of systemic therapy, including an anti-CD20 and anthracycline based chemotherapy and having failed to or are not candidates for ASCT. The study will consist of 2 parts: dose timing selection part and expansion part.

Inclusion Criteria:

• Confirmed DLBCL per local histopathology assessment.
• Relapsed or refractory disease after having recieved 2 or more lines of systemic therapy, including anti-CD20 and anthracycline based chemotherapy, and either having progressed after (or relapsed after) ASCT, or being not candidates for or not consenting to ASCT.
• Measurable disease at time of enrollment
• ECOG performance status that is either 0 or 1 at screening.

Exclusion Criteria:

• Patients with Richter's transformation, and Burkitt lymphoma, and primary DLBCL of CNS.
• Prior treatment with any prior anti-CD19/anti-CD3 therapy, or any other anti-CD19 therapy.
• Patients with active CNS involvement are excluded, except if the CNS involvement has been effectively treated and provided that local treatment was >4 weeks before enrollment.
• Prior allogeneic HSCT.
• Unstable angina and/or myocardial infarction and/or coronary artery bypass graft (CABG), or stroke within 6 months prior to screening, and/or impaired cardiac function or clinically significant cardiac disease
• Patients with a history of prior treatment with anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CTLA-4 antibodies, other immune checkpoint inhibitors.
• History of interstitial lung disease or (non-infectious) pneumonitis that required oral or intravenous steroids (other than COPD exacerbation) or current pneumonitis.

Other protocol-defined inclusion/exclusion criteria may apply.

None (Open label)