A new therapeutic potential for cancers: One CAR with 2 different engines!

ABSTRACT

Tumor cells escape from immune recognition by several mechanisms such as down-regulating of MHC class I molecules, losing of tumor antigens, etc. The purpose of cancer immunotherapy is to robust or reconstruct the capacity of the immune system to recognize and kill tumor cells by overwhelming the mechanisms by which tumors escape the immune response. One of the novel immunotherapeutic strategies were used to potentiate NK- and T cell functions is chimeric antigen receptor (CAR). CARs are composed of an antigen-binding domain of a molecule such as an antibody (that binds to a tumor associated antigens expressed on the surface of tumor cells) and an intracellular T cell activation domain. The CARs provide the recognition of target antigen in a MHC-independent manner. CAR-armed T cells may be unable to kill their targets in the absence of co-stimulators like NK cells. On the other hand, CAR-armed NK cells may also be unable to destroy their targets without receiving help signals from Th cells. Thus, if CAR-armed NK cells use together with CAR-armed T cells, NK cells will be aggregated to the tumor site. Thus, not only CAR T cells will obtain the necessary cytokines/costimulators from NK cells, but also other tumor specific T cells will be primed by recognition of tumor specific antigen (TSA) associated with MHC class I. These new specific primed T cells probably combat against tumor cells which have lost their TAAs that CAR-T cells are redirected to them.

Dear editor,

The immune system plays an essential role in defense against tumor cells so that the patients with compromised or suppressed immune function have an extremely increased incidence of cancer.¹ According to the immune surveillance hypothesis, tumor associated antigens (TAA) are considered as “non-self” by the immune system, and an important function of the immune system is to examine carefully the body for the development of malignancy and to eliminate tumor cells as they arise.² The elements of the both innate and adaptive immunity, such as natural killer (NK) cells, NKT cells, macrophages, neutrophils, eosinophils, specific cytotoxic T lymphocytes (CTLs), antibodies and some cytokines exhibit antitumor activity.^1-4 However, the tumor cells escape from immune recognition/killing by several mechanisms such as down-regulating of MHC class I molecules, losing of tumor antigens, defective death receptor signaling, lacking of costimulation, producing of immunosuppressive cytokines and inducing of suppressive cells.^5-6

Surgery, chemotherapy and radiotherapy have been used for decades as primary strategies for treatment of cancer patients, however, the development of resistance to medication or radiation led to a subsequent incidence of tumor relapse.⁷ After many years of disappointing efforts, the application of cancer immunotherapy has recently received a significant consideration.⁸ The purpose of cancer immunotherapy is to robust or reconstruct the capacity of the immune system to recognize and kill tumor cells by overwhelming the mechanisms by which tumors avoid and inhibit the immune response.⁹ Some immunotherapeutic strategies have received more attention for treatment of cancers such as cancer vaccines, adoptive transfer of ex vivo activated T and natural killer (NK) cells, genetic engineered NK- and T cells, oncolytic viruses and administration of antibodies or recombinant proteins.⁸

The natural killer (NK) cells are characterized by the lack of TCR molecules and by the expression of CD16 and CD56 markers. Approximately 90% of circulating NK cells are CD56^dim, described by their strong capacity to mediate cytotoxicity against target cells.¹⁰ T cells express a T cell receptor (TCR) which binds a peptide originated from pathogens or malignant in association with the major histocompatibility complex (MHC). In this way, T cells are activated, leading to killing of target cells or the releasing of pro-inflammatory cytokines that recruit other immune effector cells.¹¹

The novel immunotherapeutic strategies were used to potentiate NK- and T cell functions and enhance their antitumor activity, including chimeric antigen receptor (CAR)-genetic engineered NK- and T cells.^10-11 CARs are composed of an antigen-binding domain of a molecule such as an antibody (that binds to a TAA expressed on the surface of tumor cells) and an intracellular T cell activation domain. The CARs provide the recognition of target antigen in a MHC-independent manner and can be used to improve the outcome of immunotherapy.¹¹

Both T and NK cells expressing CARs have been separately used for cancer immunotherapy. Due to the immune evasion pathways of tumor cells using of the each T cell or NK cell expressing CARs alone, may don't lead to the complete tumor regression. Therefore, CAR-armed T cells may be unable to kill and eliminate their targets in the absence of co-stimulators like NK cells. On the other hand, CAR-armed NK cells may also be unable to destroy their targets without receiving help signals from Th cells. The utilization of the CAR-T or CAR-NK cells in some clinical trials resulted in the partial and transient tumor regression, probably due to the heterogeneity of tumors at the immunological level.¹² The massive and repetitive stimulation of T cells may also render them into apoptosis (through Fas-FasL interaction) or make them exhausted or senescent. Moreover, the stimulation of T lymphocytes by tumor specific antigens, without appropriate co-stimulation signals may render them into an anergic situation.¹³

Therefore the synergistic combination strategies by using CAR-armed immune cells should consider in cancer immunotherapy. In addition to CAR-T cells, another important strategy for treatment of cancer can be the concomitant using of the genetic modification of NK cells with CARs that directly recognize tumor associated antigens. The human NK cells not only have direct cytotoxic activity against tumor cells, but also act as antigen presenting cells (APCs) and can present tumor antigen to T cells and activate specific T cells.¹⁴ Several different cytokines/co-stimulatory molecules also are expressed by NK cells, which are necessary for long-term T cell activation and effector functions. Thus, if CAR-armed NK cells use together with CAR-armed T cells, NK cells will be aggregated to the tumor site. Thus, not only CAR T cells will obtain the necessary cytokines/costimulators from NK cells, but also other tumor specific T cells will be primed by recognition of tumor specific antigen (TSA) associated with MHC class I (Fig. 1). These new specific primed T cells probably combat against tumor cells which have lost their TAAs that CAR-T cells are redirected to them. In contrast to CAR T cells, CAR-NK cells permit us to use allogeneic NK cell sources, which are less likely result in or even may inhibit graft-versus-host disease, while also potentiating an increase in cytotoxicity due to mismatched killer immunoglobulin-like receptors.^15-16

Figure 1. Combinational CAR-T plus CAR-NK cell therapy hypothesis: The CAR-T, CAR-NK and CTL are simultaneously induced to exert synergistic anti-tumor activity.

The Food and Drug Administration of US and Institutional Review Board have confirmed the use of NK-92 cells, which act as potential anti-tumor agents and ideal therapeutic research models. NK-92 cells do not express the main inhibitory receptors. These cells are highly cytotoxic against various types of tumor cells without assaulting non-malignant cells.^17-18 Thus, NK92 could be an excellent candidate to be equipped by CAR for therapeutic purposes and it can play as an APC as well.

In conclusion, although combined immunotherapy by using both CAR-NK plus CAR T-cell may be a powerful treatment strategy for malignant diseases, however, some important additional considerations such as defining the target antigen, improvement of CAR signaling and enhancing of the safety the CAR expressing NK-T (the 2 engines!) cell therapy should be consider in future investigations.

Disclosure of potential conflicts of interest

Authors declare no conflict of interest.