CAR T cells targeting CD99 as an approach to eradicate T-cell acute lymphoblastic leukemia without normal blood cells toxicity

CAR T cell therapy has shown dramatic clinical success in relapsed or refractory B-ALL and other hematological malignancies. However, the loss of specific antigens, cell fratricide, T cell aplasia, and normal T cell separation are challenges in treating T cell leukemia/lymphoma with CAR T therapy. CD99 is a promising antigen to target T-ALL and AML as it is strongly expressed on the majority of T-ALL and AML. Here, we isolated a low-affinity CD99 (12E7) antibody, which specifically recognizes leukemia cells over normal blood cells. Moreover, T cells transduced with an anti-CD99-specific CAR that contained the 12E7 scFv expanded with minor fratricide and without normal blood cells toxicity. We observed that our anti-CD99 CAR T cells showed robust cytotoxicity specifically against CD99+ T-ALL cell lines and primary tumor cells in vitro and significantly prolonged cell line-derived xenografts (CDXs) or patient-derived xenografts (PDXs) models survival in vivo. Together, our results demonstrate that anti-CD99 CAR T cells could specifically recognize and efficiently eliminate CD99+ leukemia cells.