Challenges and strategies associated with CAR-T cell therapy in blood malignancies

Abstract

Cellular immunotherapy, particularly CAR-T cells, has shown potential in the improvement of outcomes in patients with refractory and recurrent malignancies of the blood. However, achieving sustainable long-term complete remission for blood cancer remains a challenge, with resistance and relapse being expected outcomes for many patients. Although many studies have attempted to clarify the mechanisms of CAR-T cell therapy failure, the mechanism remains unclear. In this article, we discuss and describe the current state of knowledge regarding these factors, which include elements that influence the CAR-T cell, cancer cells as a whole, and the microenvironment surrounding the tumor. In addition, we propose prospective approaches to overcome these obstacles in an effort to decrease recurrence rates and extend patient survival subsequent to CAR-T cell therapy.

Keywords: CAR-T cell therapy; Hematologic malignancies; Resistance and relapse mechanisms.

Fig. 1

CAR structure and generations. CAR T cells are composed of three functional regions from the inside out: (1) the signaling domain of the TCR, (2) the transmembrane domain, and (3) the scFv. 1st generation: CD3ζ as signal transduction domain;2nd generation: additional costimulatory signaling domains (CD28 or 4-1BB);3rd generation: both CD28 and 4-1BB as co-stimulatory domains;4th generation: genes added for cytokine expression; Next generation: IL-2R β fragment added to 2nd generation CARs, inducing JAKs and STAT-3/5 production by mRNA transcription

Fig. 2

Antigen-positive relapse mechanism. A: T Cell Quality: Patients receiving chemotherapy drugs prior to T cell collection lead to low-quality T cells, resulting in suboptimal CAR-T cell products. B: Co-Stimulatory Domain: CART cells with 4-1BB co-stimulatory molecules persist in the bloodstream for a prolonged period than CART cells that structurally contain a CD28 co-stimulatory molecule. C: Different T Cell Subsets: CAR-T cells constructed by CD8 + T cells have a stronger tumor-killing ability than CD4 + CAR-T cells

Fig. 3

Antigen-negative relapse mechanism. A. Tumor cells downregulate surface target antigen to levels below the activation threshold of CAR T cells. B. Lentiviral modification of single leukemic cell for antigen expression, masking antigen epitope from CAR T cells. C. Tumor heterogeneity leads to antigen mutation. D. Tumor cells lineage switch or receptor genetic mutations leading to lack of extracellular epitopes recognized by CAR T cells

Fig. 4

Tumor microenvironment. Immunosuppressive cells, including Treg, MDSCs, extracellular vesicles and TAMs, are present in the TME. They inhibit effector T cell function, enhance recruitment of Treg cells, and induce an immunosuppressive M2 phenotype by secreting cytokines such as IL-8, IL-6, IL-10, IL-35, and TGF-β.

Fig. 5

Metabolic fuel deprivation. The rapid growth of tumour cells consumes a large amount of glucose and oxygen, resulting in CAR-T cells lacking oxygen and glucose, which promotes cell apoptosis

Fig. 6

Optimization strategies for CAR-T cells. A: Multi-targeted CAR-T cells. B: Armored CAR-T cells. C: Universal CAR-T-Cells. D: Promoting the differentiation of CAR-T cells into central memory T cells (TCM) and stem cell memory T cells (TSCM). E: An appropriate co-stimulatory molecule. Three signaling domains of CD28, 4-1BB, and CD3zeta construct P28BBz receptor outperforms receptors containing only one or two of these domains. F: Humanized CAR-T cells

Fig. 7

Strategies for modulating the Tumor Microenvironments. A: Targeting suppressive cell types. Using monoclonal antibodies and CAR-T cells to eliminate immunosuppressive cells such as T regulatory cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs). B: Targeting suppressive cytokines. CAR-T cells targeting TGF-βinhibit the proliferative and anti-tumor abilities of cytotoxic cells. C: Targeting inhibitory signals. Genetic engineering technology that disrupts FAS signaling on T cells

Fig. 8

Combination therapy. Combination therapy using CART cells with chemotherapy drugs, radiotherapy, Bruton’s tyrosine kinase (BTK) inhibitors, PI3K inhibitors, and other agents such as Bcl-2 inhibitors enhance anti-tumor efficacy