Comparative In Vitro Apoptotic Activity of Two Chimeric Anti-CD99 Antibodies Recognizing Distinct CD99 Regions in T-ALL Models

Background/Objectives: Therapeutic antibodies have become an important modality for cancer treatment; however, effective targeted antibody therapies for T-cell acute lymphoblastic leukemia (T-ALL) remain limited. CD99 is a promising therapeutic target due to its high expression in leukemic T cells and its capacity to transmit apoptotic signals. However, the efficacy of antibody-mediated apoptosis may depend on the CD99 region recognized by the antibody. This study compared the in vitro apoptotic activity of two chimeric anti-CD99 antibodies, ChAbMT99/1 and ChAbMT99/3, which recognize distinct regions of CD99 in T-ALL models. Methods: ChAbMT99/1 and ChAbMT99/3 were generated as human IgG1 antibodies. Antibody reactivity and binding site specificity were characterized using peptide-based ELISA. Binding to native CD99, apoptosis induction in two-dimensional (2D) Jurkat E6.1 and MOLT-4 suspension cultures and three-dimensional (3D) Jurkat E6.1-derived spheroid models, and cytotoxic effects on peripheral blood mononuclear cells (PBMCs) were assessed by flow cytometry. In vitro hematologic effects were assessed using hemagglutination and platelet aggregation assays. Results: ChAbMT99/1 and ChAbMT99/3 specifically recognized peptides corresponding to distinct CD99 regions, with peptide-binding EC50 values of 0.813 and 0.819 μg/mL, respectively. Both antibodies exhibited binding reactivity to native CD99 on the tested cells. Functionally, both antibodies induced Annexin V/7-AAD-defined cell death in both 2D and 3D T-ALL models compared with controls in a crosslinking-dependent manner. In contrast, both antibodies induced low levels of cell death (<10%) in bulk PBMCs, with no visible hemagglutination or platelet aggregation observed in samples from five selected donors under the stated assay conditions. Conclusions: Both chimeric anti-CD99 antibodies demonstrated in vitro apoptotic activity against T-ALL models despite recognizing distinct regions of CD99. These findings provide a rationale for further investigation of their other mechanisms of action to support the future development of CD99-targeted antibody therapy for T-ALL.