Renaissance of armored immune effector cells, CAR-NK cells, brings the higher hope for successful cancer therapy

Abstract

In recent decades, a new method of cellular immunotherapy was introduced based on engineering and empowering the immune effector cells. In this type of immunotherapy, the immune effector cells are equipped with chimeric antigen receptor (CAR) to specifically target cancer cells. In much of the trials and experiments, CAR-modified T cell immunotherapy has achieved very promising therapeutic results in the treatment of some types of cancers and infectious diseases. However, there are also some considerable drawbacks in the clinical application of CAR-T cells although much effort is in progress to rectify the issues. In some conditions, CAR-T cells initiate over-activated and strong immune responses, therefore, causing unexpected side-effects such as systemic cytokine toxicity (i.e., cytokine release syndrome), neurotoxicity, on-target, off-tumor toxicity, and graft-versus-host disease (GvHD). To overcome these limitations in CAR-T cell immunotherapy, NK cells as an alternative source of immune effector cells have been utilized for CAR-engineering. Natural killer cells are key players of the innate immune system that can destroy virus-infected cells, tumor cells, or other aberrant cells with their efficient recognizing capability. Compared to T cells, CAR-transduced NK cells (CAR-NK) have several advantages, such as safety in clinical use, non-MHC-restricted recognition of tumor cells, and renewable and easy cell sources for their preparation. In this review, we will discuss the recent preclinical and clinical studies, different sources of NK cells, transduction methods, possible limitations and challenges, and clinical considerations.

Keywords: CAR; Immunotherapy; NK cells; T cells.

Fig. 1

The fundamentals of CAR engineering of three generations of CAR-NK cells. CAR molecules on NK cells consist of three main parts: an antigen detection domain (e.g., ScFv), a transmembrane domain, and the signaling domain. First-generation CARs consist of CD3ζ as the signaling domain. Second-generation CARs have the CD-28 or a combination of CD-28 and a second additional signaling molecule: 4-1BB. As an example of 3rd generation, a CAR comprising an activating receptor; NKG2D can be constructed containing the signaling molecules: CD3ζ and DAP10/12. It was found that CD3ζ has better signaling properties than DAP10 and also it seems that DAP12 may activate NK cells better than both CD3ζ and DAP10. NK cells antigen binding or receiving activation signal by a bunch of activator receptors such as NKG2D leads to signal transduction, NK cell activation, the release of cytolytic enzymes, cytokine production, and also NK cells expansion and maintenance

Fig. 2

Different approaches for delivery of CAR-containing genes/entire CARs into NK cells. There are two main categories of methods, viral and non-viral. In the viral method, the lentiviral or retroviral vectors carrying CAR genes are delivered into the NK cells. Non-viral methods include various approaches including trogocytosis, the piggyBac transposon, mRNA-electroporation, mRNA transfection, and nanoparticles. The goal of all methods is to equip primary NK cells with CARs

Fig. 3

Engineered activatory receptors of NK cells strengthen NK cells in tumor killing. CAR redirect tumor cell-specific NK cells to bind to their specific target on the surface of the cancer cell, resulting in NK cell activation, cytolytic factors secretion, and tumor lysis