Abstract 1546: ADCC enhanced anti-CD25 mAb (TST010) demonstrated antitumor activity via depleting Treg cells and increasing CD8+T/Treg ratio in preclinical tumor models

Regulatory T (Treg) cells are a distinctive lineage of CD4+ T cells as characterized by expression of FoxP3 and CD25 markers. Treg cells are involved in tumor development and progression by inhibiting antitumor immunity. Accumulation of Treg cells in the tumor microenvironment (TME) is often associated with an adverse prognosis in multiple cancers and also has implication for predicting response to immunotherapy. Treg cell depletion strategy has been explored in preclinical and clinical settings to induce effective antitumor immune responses in TME. CD25, a high-affinity binding subunit alpha of the IL-2 receptor, is constitutively and highly expressed on Treg cells but also transiently upregulated on effector T cells. The high level of CD25 expression on Treg cells could deprive of IL-2 for effector T cells and inhibit their proliferation. Thus, an effective approach to selectively suppress Treg cell function is to deplete Treg cells while avoid blocking IL-2 binding to CD25 for allowing T effect cell proliferation and activation. Using our immune tolerance breaking antibody platform, we generated a humanized IgG1 subtype anti-CD25 monoclonal antibody (TST010). TST010 bound to human CD25 with high affinity and selectivity but did not block IL-2 binding to CD25. No blocking activity to IL-2 signaling on human PBMC was further confirmed by the phosphorylation of Stat5 using FACS analysis. By reducing fucosylation during cell culture process, TST010 mAb gained the enhanced ADCC activity. In an ADCC reporter assay, TST010 showed its sub-nanomolar ADCC activity against CD25-overexpression SU-DHL-1 cell line and human induced Treg cells, but much less to human activated CD8+ T effect cells due to lower CD25 expression. Similarly, in a NK cytotoxicity assay, TST010 selectively killed human induced Treg cells in contrast to activated CD8+ T cells. In vivo antitumor activity of TST010 was evaluated in the syngeneic mouse tumor models. The mouse colon cancer MC38 model was developed on the human CD25 knock-in mice. In the MC38 tumor model, TST010 markedly reduced the Treg (FoxP3+CD4+) population in both peripheral blood and TME. Importantly, the CD8+T/Treg ratio in tumor was increased starting at 2 weeks post TST010 treatment, and it resulted in a significant single agent tumor growth inhibition at a 10 mg/kg dose. Furthermore, in the LLC tumor model which does not respond well to checkpoint inhibitors, the anti-CD25 mAb showed a good combination effect with a PD-L