Mechanisms underlying CD19-positive ALL relapse after anti-CD19 CAR T cell therapy and associated strategies

Abstract

Chimeric antigen receptor (CAR) T cell therapy, especially anti-CD19 CAR T cell therapy, has shown remarkable anticancer activity in patients with relapsed/refractory acute lymphoblastic leukemia, demonstrating an inspiring complete remission rate. However, with extension of the follow-up period, the limitations of this therapy have gradually emerged. Patients are at a high risk of early relapse after achieving complete remission. Although there are many studies with a primary focus on the mechanisms underlying CD19^- relapse related to immune escape, early CD19⁺ relapse owing to poor in vivo persistence and impaired efficacy accounts for a larger proportion of the high relapse rate. However, the mechanisms underlying CD19⁺ relapse are still poorly understood. Herein, we discuss factors that could become obstacles to improved persistence and efficacy of CAR T cells during production, preinfusion processing, and in vivo interactions in detail. Furthermore, we propose potential strategies to overcome these barriers to achieve a reduced CD19⁺ relapse rate and produce prolonged survival in patients after CAR T cell therapy.

Keywords: Acute lymphocytic leukemia (ALL); CAR T cell therapy; Chimeric antigen receptor; Mechanism; Positive relapse; Strategy.

Fig. 1

Factors influencing CD19 CAR T cell therapy. The limited persistence and impaired efficacy of CAR T cells could be possible mechanisms underlying CD19⁺ relapse. This figure summarizes potential obstacles to durable remission and better CAR T cell efficacy. First, T cell collection: T cells selected for manufacturing should be of sufficient quantity and good quality and have a phenotype with memory characteristics. Second, CAR T cell manufacture: transgene rejection induced by a murine scFv results in transient in vivo persistence. Selection of the costimulatory domain, transduction technique, especially vector selection, and proliferation method also plays roles in persistence and efficacy. Third, preinfusion: the tumor burden before infusion is associated with patient long-term survival. In addition to lymphodepleting therapy, a conditioning regimen with fludarabine ameliorates T cell persistence. Finally, postinfusion: normal B cells are supposed to recover, but transient B cell aplasia may result in CD19⁺ relapse. Aberrant signaling pathways and the BM microenvironment will impair a T cell’s potential along with its in vivo persistence

Fig. 2

Main signaling pathways involved in CD19-BBζ T cells and CD19-28ζ T cells. a A high mitochondrial respiratory capacity promotes metabolism and differentiation. 4-1BB domain signaling activates the PI3K pathway and upregulates Bcl-xL and BFL-1 expression. Tonic CAR-derived 4-1BB signaling activates the NF-B pathway and enhances FAS-dependent apoptosis. CD19-BBζ T cells diminish the expression of exhaustion-associated molecules more than CD19-28ζ T cells. b The main signaling pathways involved in CD19-28ζ T cells. CAR T cell inhibition induced by regulatory T cells, IL-10 and TGF-β can be reduced by the incorporation of the CD28 domain. CD19-28ζ T cells exhibit enhanced activation of the transcription factor NF-B and promote cytokine secretion. CD19-28ζ T cells are more likely to result in the development of severe CRS than CD19-BBζ T cells. However, tonic CAR CD3ζ phosphorylation triggered by clustering of the CAR single-chain variable fragment (scFv) leads to more rapid exhaustion

Fig. 3

The mechanism of CD19+ relapse in BM microenvironment. a Main interaction between negative regulatory cells, tumor cells and immune effector cells in BM microenvironment. Tregs, MDSCs and TAMs suppress CTLs, DCs, NK cells and T cells by cytokines, enzymes and cell-cell interactions. Negative regulatory cells and tumor cells attract and improve each other’s recruitment, differentiation and expansion. Tregs: regulatory T cells; MDSCs: myeloid-derived suppressor cells; TAMs: tumor associated macrophages; IDO: indoleamine-2, 3-dioxygenase; CTLs: cytotoxic T cells; DCs: dendritic cells; NK cells: natural kill cells; TGF-β: transforming growth factor β. b The negative regulation checkpoint in BM microenvironment. The PD-1/PD-L1 pathway between tumor cells and MDSCs, T cells, TAMs inhibits the proliferating of T cells and transforms T cells into induces Tregs or induces apoptosis. The CTLA-4/B7 pathway suppresses APCs while activates Tregs. (Tregs: regulatory T cells; iTregs: induced Tregs; TAMs: tumor associated macrophages; APCs: antigen-presenting cells; MDSCs: myeloid-derived suppressor cells; PD-1: programmed death-1; PD-L: programmed cell death 1 ligand)

Fig. 4.

Some key points during the process of manufacturing CAR T cells. a The progress of T cells with the associated characteristics. As T_N cells differentiate, the proliferative capacity of T cells is gradually reduced. Except for T_E cells, the remaining subsets have a self-renewal ability, which declines from T_N to T_EM cells. Therefore, it is best to choose T_SCM cells for CAR T cell generation. b Adding IL-7/IL-15 during in vitro expansion has a positive impact. IL-7/IL-15 can increase the proportion of T_SCM cells and contribute to maintaining the ratio of CD4⁺:CD8⁺ cells.