A structural, genetic and clinical comparison of CAR-T cells and CAR-NK cells: companions or competitors?

Abstract

In recent years, following the groundbreaking achievements of chimeric antigen receptor (CAR) T cell therapy in hematological cancers, and advancements in cell engineering technologies, the exploration of other immune cells has garnered significant attention. CAR-Therapy extended beyond T cells to include CAR natural killer (NK) cells and CAR-macrophages, which are firmly established in the clinical trial landscape. Less conventional immune cells are also making their way into the scene, such as CAR mucosal-associated invariant T (MAIT) cells. This progress is advancing precision medicine and facilitating the development of ready-to-use biological treatments. However, in view of the unique features of natural killer cells, adoptive NK cell immunotherapy has emerged as a universal, allogenic, "off-the shelf" therapeutic strategy. CAR-NK cytotoxic cells present targeted tumor specificity but seem to be devoid of the side effects associated with CAR-T cells. CAR-NK cells appear to be potentially promising candidates for cancer immunotherapy. However, their application is hindered by significant challenges, particularly the limited persistence of CAR-NK cells in the body, which poses a hurdle to their sustained effectiveness in treating cancer. Based upon the foregoing, this review discusses the current status and applications of both CAR-T cells and CAR-NK cells in hematological cancers, and provides a comparative analysis of the structure, genetics, and clinical outcomes between these two types of genetically modified immune cells.

Keywords: CAR-NK cells; CAR-T cells; Cytokines; Hematological malignancies; Off-the-shelf Immunotherapies.

Figure 1

A schematic diagram of the structure of the successive generations of CAR-T and CAR-NK cells. First generation CAR-T/NK cells have only CD3ζ in their signaling domain and lack co-stimulatory molecules. Second generation CAR-T/NK cells include CD3ζ and one co-stimulatory molecule, enabling dual signaling pathways. Third generation CAR-T/NK cells combine CD3ζ with multiple co-stimulatory molecules for enhanced signaling. Fourth generation CAR-T cells, also known as T cells redirected for universal cytokine-mediated killing (TRUCKs), are similar to third generation, but they are a specific type of armored CAR-T cells that produce and secrete cytokines to promote tumor killing. Fifth generation CAR-T cells have an additional intracellular domain compared to earlier generations. These CARs include truncated intracellular domains from cytokine receptors (such as fragments of the IL-2R chain) that feature motifs for binding transcription factors like STAT-3/5.

Figure 2

A schematic diagram of CAR-T and CAR-NK cell armoring with IL15 [Adapted from Zhou Y et al., 2022 (86)]. (A) The signaling cascade of IL-15 and its receptor complex involve IL-15 being presented on antigen-presenting cells through IL-15Rα, which binds with the β chain (IL-2/15Rβ) and the common γ chain (γc) complex on effector cells. Activation of the β and γc receptors initiates intracellular signaling via the Janus kinase pathway, activating signal transducer and activator of transcription (STAT) proteins downstream. Phosphorylated STATs then translocate to the nucleus, altering gene expression. (B) T or NK cells can be armored with IL-15 to serve as therapeutic cells. They can be equipped, beside the CAR, with either secretory IL-15 (sIL-15), membrane-bound IL-15 (mbIL-15), or IL-15, IL-15Ra fusion protein.

Figure 3

Manufacturing processes in CAR-NK and CAR-T-cell-based therapy. NK cells can be obtained from peripheral blood (PB-NK), CD34-positive hematopoietic stem cells (HSCs), umbilical cord blood (UCB-NK) induced hematopoietic stem cells (iPSCs), or in vitro cultured cell lines (NK-92). T cells can be obtained from peripheral blood apheresis (not shown). T and NK cells can be modified to express specific CAR using viral transduction with γ-retrovirus, lentivirus or adeno-associated virus, or non-viral strategies such as transposon system, CRISPR-Cas base editing. Non-viral gene transfer/delivery techniques are based on lipofection or electroporation/nucleofection.