CD33-Specific Chimeric Antigen Receptor T Cells with Different Co-Stimulators Showed Potent Anti-Leukemia Efficacy and Different Phenotype

Acute myeloid leukemia (AML) is a kind of a malignant hematologic tumor caused by uncontrolled repopulation of myeloid hematopoietic stem cells (HSCs). Current therapeutic effects for AML patients are unsatisfactory. In particular, relapsed and refractory AML still have a poor prognosis. T cells modified by chimeric antigen receptor (CAR) was an immunotherapeutic strategy for malignancies, which has a broad developing prospect. Most AML cells overexpress the myeloid antigen CD33. Therefore, CD33-specific CAR-T cells with different co-stimulators (CD28, 4-1BB, or both, referred to as CD33 28z.CAR-T cells, CD33 BBz.CAR-T cells, or CD33 28BBz.CAR-T cells, respectively) were developed to evaluate their efficacy against AML. The effectiveness of three types of CD33 CAR-T cells against AML was verified by specific killing effect to AML cells and prolonged survival of a xenograft mouse model. In terms of CAR-T cell efficacy, especially when transfused into human bodies, the persistence of T cells is also an important index, as it is closely associated with the long-term effect of CAR-T cells. Therefore, the characteristics of three types of CD33 CAR-T cells related to the persistence of T cells were examined. It was found that during expansion, CD33 BBz.CAR-T cells had an increased central memory compartment, while CD33 28z.CAR-T cells were predominantly effector memory T cells. In addition, CD33 28z.CAR-T cells were more inclined to become exhausted. The study suggests that incorporation of 4-1BB in CARs may endow T cells with long-lasting survival ability, thus improving the long-term anti-leukemia effect of CAR-T cells, especially when transfused to the human body.