Inclusion Criteria:
* Confirmed diagnosis of any of the below according to World Health Organization (WHO) classification
1. Diffuse large B-cell lymphoma, not otherwise specified (DLBCL-NOS)
2. Diffuse large B-cell lymphoma / high-grade B-cell lymphoma (DLBCL/HG-BCL) with MYC and BCL2 rearrangements
3. High-grade B-cell lymphoma, not otherwise specified (HG-BCL, NOS)
4. Large B-cell lymphoma (LBCL) transformed from an antecedent indolent B-cell lymphoma (BCL), including Follicular lymphoma (FL) and marginal zone lymphoma (MZL)
* Must have relapsed or refractory disease with no other available therapies with established clinical benefit including anti-cluster of differentiation 20 (CD20)-based immunochemotherapy, autologous stem cell transplant \[if eligible\], chimeric antigen receptor T-cell therapy (CAR-T) \[if eligible\], or T-cell engager (TCE) therapy) that would be more appropriate in the investigator's judgment.
* Must have measurable disease by computed tomography (CT)/ magnetic resonance imaging (MRI), defined as at least 1 lymph node \> 1.5 cm in the longest diameter or 1 extranodal lesion \> 1.0 cm in the longest diameter, measurable in 2 perpendicular dimensions.
* Must have an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 2.
Exclusion Criteria:
* Prior autologous stem cell transplant \< 100 days before first dose of the study drug.
* Prior allogenic transplant or solid organ transplant at any time in the past.
* Known or suspected progressive multifocal leukoencephalopathy.
* Known or suspected history of hemophagocytic lymphohistiocytosis (HLH).
* Recent prior therapies, prior to the first dose of study drug:
1. Biologic-based anti-lymphoma therapy (eg, monoclonal antibodies, antibody-drug conjugates, radioimmunoconjugate, or TCE therapy) within 28 days
2. CAR-T therapy within 90 days
3. Systemic chemotherapy or small-molecule targeted therapy (investigational or otherwise) within 28 days or 5 half-lives (whichever is shorter)
4. Systemic corticosteroids in excess of 25 mg/day prednisone (or equivalent) within 14 days. Inhaled, topical, or systemic corticosteroids at doses of ≤ 25 mg/day prednisone (or equivalent) are permitted.
5. Radiation therapy within 14 days. If a patient has undergone radiation therapy since last progression, at least one measurable lesion must be outside the radiation field
6. T-cell suppressive therapy (eg, alemtuzumab, fludarabine) within 6 months, unless discussed with the medical monitor.
* History of severe allergic reactions or hypersensitivity to the active ingredient or excipients of BG-T0050 or other monoclonal antibodies.
* Active significant infection (bacterial, viral, fungal, parasitic) at study enrollment, or any Grade 3 or 4 infection requiring treatment with Intravenous (IV) systemic therapy or hospitalization within 4 weeks prior to the first dose of the study drug. Note: Oral antimicrobial therapy used for minor or controlled infections or for prophylactic purposes is allowed.
Note: Other protocol defined Inclusion/Exclusion criteria may apply.
