Inclusion Criteria:
* Participant must be ≥ 18 years of age.
* Participant has a confirmed diagnosis of large B-cell lymphoma (including DLBCL, high-grade B-cell lymphoma \[HGBCL\], primary mediastinal B-cell lymphoma \[PMBCL\], etc), FL, MCL, marginal zone lymphoma, transformation of indolent B-cell lymphoma, or lymphoplasmacytic lymphoma, including Waldenstrom macroglobulinemia.
* Participant has relapsed or progressed on at least 2 prior lines of therapy.
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.
* All participants must have measurable disease via computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET)-CT.
* Documented CD19 or CD20 positivity of their B-cell neoplasm based on any representative pathology report from the past 3 months.
* Adequate hematologic, hepatic and renal function.
* All contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
* Capable of giving signed informed consent.
Exclusion Criteria:
* Known or suspected history of hemophagocytic lymphohistiocytosis (HLH).
* Unresolved toxicities from previous anticancer therapy.
* Primary central nervous system (CNS) lymphoma or known CNS involvement with lymphoma.
* Active medical condition requiring chronic systemic steroid use (\>10 mg/day prednisone or equivalent of \>140 mg over the last 14 days) or immunosuppressive therapy, within 6 months prior to the first dose of MDX2003.
* Known positivity with human immunodeficiency virus (HIV), known active hepatitis B or C, or uncontrolled chronic or ongoing infection requiring intravenous treatment.
* Participant has a history of allogenic tissue or solid organ transplant, with the exception of corneal transplants.
* Known hypersensitivity to allopurinol or rasburicase.
* Participant has a seizure disorder requiring therapy at the time of screening (such as steroids or anti-epileptics).
* Participant is not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions.
