A bispecific single-domain antibody boosts autologous Vg9Vd2-T cell responses toward CD1d in chronic lymphocytic leukemia

Purpose: Although considerable progress has been made with autologous T cell-based therapy in B-cell malignancies, application in chronic lymphocytic leukemia (CLL) lags behind due to disappointing response rates as well as substantial toxicity that is of particular concern in the elderly CLL population. Vg9Vd2-T cells form a conserved T-cell subset with strong intrinsic immunotherapeutic potential, largely because of their capacity to be triggered by phosphoantigens that can be overproduced by CLL and other malignant cells. Specific activation of Vg9Vd2-T cells by a bispecific antibody may improve the efficacy and toxicity of autologous T-cell-based therapy in CLL. Experimental Design: We evaluated CD1d expression in a cohort of 78 untreated patients with CLL and generated and functionally characterized a CD1d-specific Vg9Vd2-T cell engager based on single-domain antibodies (VHH). Results: CD1d was expressed by CLL in the majority of patients, particularly in patients with advanced disease. The CD1d-specific Vg9Vd2-T cell engager induced robust activation and degranulation of Vg9Vd2-T cells, enabling Vg9Vd2-T cells from patients with CLL to lyse autologous leukemic cells at low effector-to-target ratios. Expression of CD1d on CLL cells is upregulated by all-trans retinoic acid, and sensitizes the malignant cells to bispecific VHH-induced lysis. Furthermore, we provide evidence that the Vg9Vd2-T cell receptor retains responsiveness to phosphoantigens when the bispecific VHH is bound, and aminobisphosphonates can therefore enhance bispecific Vg9Vd2-T cell engager-mediated tumor-specific killing. Conclusions: Collectively, our data demonstrate the immunotherapeutic potential of this novel CD1d-specific Vg9Vd2-T cell engager in CLL.