A Novel Bispecific T Cell Engager (UMG2-BTCE) Targeting CD1a-CD3ε Is Effective Against Cortical-Derived Acute Lymphoblastic Leukemia

Caterina Riillo*and Daniele Caracciolo*equally contributed to the work.

Background: T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive and orphan hematological disease derived from malignant transformation of thymic T cell precursors. At present, the prognosis of relapsed/refractory patients remains poor. While immunotherapy has significantly improved the outcome of B cell acute lymphoblastic leukemia (B-ALL), the lack of tumor-restricted T cell antigens hampers its efficacy in T-ALL. (Caracciolo D, Riillo C, et al. Therapeutic afucosylated monoclonal antibody and bispecific T-cell engagers for T-cell acute lymphoblastic leukemia. J Immunother Cancer. 2021) Therefore, the development of novel immune-therapeutics is eagerly awaited.

CD1a is a cell surface glycoprotein restricted to cortical T-ALL subtype expressed only by cortical thymocytes and Langerhans's cells among human healthy tissue and might represent a valuable therapeutic target for the treatment of T-ALL. On this basis, we develop an asymmetric monovalent 2 +1 bispecific T cell engager (BTCE) derived from a novel humanized UMG2 mAb directed against an original CD1a epitope selectively expressed by cortical T-ALL cells.

Methods.

To evaluate if UMG2 recognizes a specific CD1a epitope, HEK293T cell line, negative for CD1a expression, has been transfected with a plasmid encoding for CD1a (HEK293T/CD1a) and with a negative control vector and UMG2 reactivity has been evaluated by flow cytometry. A competitive binding assay between UMG2 and commercially available CD1a antibodies was performed. The UMG2 expression profile was evaluated on healthy donor peripheral blood cells and on a panel of cortical T-ALL cell lines. To develop a UMG2 targeting immunotherapeutic construct with a limited unspecific T cell activation, an asymmetric 2+1 UMG2-CD3 bispecific T cell engager (BTCE) was generated using knobs into holes technology. UMG2-CD3 BTCE in vitro T cell-mediated activity was evaluated on HEK293T CD1a antigen-negative cell line, on HEK293T/CD1a, on patient-derived and T-ALL cell lines, co-cultured with healthy donors derived peripheral blood mononuclear cells (PBMCs), CD4/CD8 depleted and CD56 enriched lymphocytes at 10:1 E:T ratio.

T cell activation, degranulation, proliferation, and pro-inflammatory cytokine secretion were assessed by flow cytometry on primary blasts and on T-ALL cells with effector lymphocytes.

To evaluate UMG2-CD3 BTCE anti-tumor activity against CD1a expressing T-ALL cell line in vivo, Hu-PBMCs NSG mouse model was generated, and tumor growth was assessed by fluorescent imaging probe.

Results

UMG2 mAb recognizes a previously uncharacterized CD1a epitope and does not compete with any of the commercially available anti-CD1a mAbs. While a strong UMG2 reactivity is observed on both patient-derived samples and T-ALL cells, no binding is found on normal blood cells, indicating the tumor-restricted pattern of reactivity of UMG2.

UMG2-CD3 BTCE specifically binds CD1a on leukemic cells and activates CD3ε downstream signaling pathway on T lymphocytes, as demonstrated by the concentration-dependent increase of T cell proliferation, cytotoxic degranulation (CD107a), expression of cell surface activation markers (CD25, CD69), and pro-inflammatory cytokine secretion (IL-2, TNF-α, IFN-γ).

UMG2-CD3 BTCE mediates strong and concentration-dependent specific T cell re-directed cytotoxicity only on CD1a expressing leukemic cells in the presence of T lymphocytes. Minimal UMG2-CD3 BTCE residual anti-tumor activity is observed in CD4/CD8 depleted and CD56 enriched lymphocytes, while CD56 depleted and Fc-blocked PBMCs are able to induce an anti-T-ALL activity comparable to total PBMCs, demonstrating that UMG2-BTCE could not recruit monocytes and NK cells through Fc-FcyR interaction by reducing the risk of immune-mediated adverse events. Most importantly, in an in vivo of immune-humanized NSG mice engrafted with human T-ALL cells, UMG2-BTCE significantly inhibits tumor growth translating into the survival advantage of treated animals.

Conclusion: Taken together, all these results provide a framework for the clinical development of UMG2-CD3 BTCE potentially offering a novel therapeutic path for cortical-derived T-ALL.