Importantly, allogeneic T cells are known to induce graft-versus-host disease (GvHD) while NK cells do not
[95][53]. Autologous T cell therapy may not be amenable for all patients since those with significant lymphopenia due to prior anti-cancer treatments or those with a rapidly progressive disease will not be eligible due to insufficient numbers of T cells to expand into therapeutic doses of CAR T cells. This is important for GBM, as both treated and treatment-naïve GBM patients experience lymphopenia
[98][56]. The protracted time required for CAR T cell production is also a bottleneck
[97][55]. The first CAR NK cell approaches focused on primary peripheral blood-derived NK cells transfected with the CAR construct
[99][57], but NK cells are more difficult to transfect than T cells
[100][58]. To circumvent difficulties in introducing gene edits to NK cells, a recent publication has outlined a method for efficient electroporation of CRISPR-associated Cas9 ribonuclear protein (RNP) complexes in primary NK cells
[101][59]. A similar protocol has been used to efficiently knock out the immune checkpoint receptor T-cell immunoglobulin mucin family member 3 (
TIM3) in primary NK cells from healthy donors
[102][60], PD-1
[103][61], and CD38
[104][62]. However, NK cells only represent approximately 5–15% of circulating lymphocytes in healthy adult humans
[105][63] and require extensive ex vivo expansion. Additionally, they have a much shorter life span than T cells
[106][64], although the persistence of CD-19-directed, IL-15 expressing CAR NK cells from cord blood were shown to survive for at least 4 weeks in mouse models
[107][65]. In humans, multiple or higher doses of engineered NK cells might be required for a sustained treatment effect. NK cells may be obtained from various tissue sources, however, such as umbilical cord blood, induced pluripotent stem cells (iPSCs) or cell lines, such as NK-92
[108][66]. This characteristic provides the opportunity for “off-the-shelf” cells that can be applied to any patient, which is less readily achieved with autologous T cell therapies
[100][58]. However, the possibility of a host-versus-graft scenario, where the implanted cells are systematically destroyed by the host immune system, must also be considered. Research into the production of reliable, “off-the-shelf” alternatives for cancer immunotherapy is currently in progress for both T and NK cells (reviewed in
[109,110][67][68]). Strategies to avoid GvHD include knockout of the TCR in engineered T cells
[111[69][70],
112], while host attacks and graft rejection may be mitigated through knockout of MHC or application of lymphodepleting chemotherapeutics prior to engineered T/NK cell administration
[113,114,115,116][71][72][73][74]. In GBM, NK cells have been shown to comprise only a small portion of immune cells in the tumour microenvironment, and to have a CD56
+ CD16
- phenotype
[33][12], indicating that these cells may not be capable of killing CAR-expressing cells. Another opportunity may lie in NK cells’ inherent expression of KIR2DL4, which can bind HLA G to inhibit host NK cells
[117,118][75][76].
The heterogeneity of GBM creates a significant challenge for the identification of a ubiquitously expressed tumour specific target for CAR constructs. In addition, GBM cells grown in stem cell media, which most closely preserve the original tumour phenotype, tend to exhibit reduced expression of both classical and non-classical MHC class I ligands for inhibitory NK receptors
[33,119][12][77]. Therefore, these conditions, if present within patient tumour microenvironments, may provide additional opportunities for receptor mediated cytotoxicity in CAR NK cells
[120,121][78][79] via both KIR and natural cytotoxicity receptors against stress-induced ligands known to be expressed by GBM tumour cells
[87,88,122][45][46][80]. Bispecific TanCAR NK cells could be used to mitigate possible tumour escape by targeting two antigens
[57][81]. As such, the choice of either CAR T or NK cell therapy depends on the strategy utilised, as the two provide different advantages and disadvantages, as summarised in
Table 1. Taken together, CAR NK cells represent potentially superior effectors for solid tumours such as GBM.
Table 1. Comparisons between CAR T and NK cell therapies.
Comparisons to Consider
|
CAR T
|
CAR NK
|
Cell source |
Autologous setting |
Allogeneic setting or cell lines [108][66] |
Off-the-shelf |
Not eligible |
Eligible [100][58] |
GvHD |
Induces GvHD |
Not known to induce GvHD |
Gene delivery |
Effective by transfection or transduction |
Effective by Cas9 RNP |
Patients as candidates |
Restricted to patient’s condition |
All patients are potential candidates |
Off-target effects |
ICANS/CRS [70,71][82][83] |
No/minimal ICANS or CRS |
Response to antigen loss |
No alternative mechanism |
Response by existing NK cell receptors |
Infusion dose |
Single/few doses |
Multiple or high |
In vivo persistency |
Longer (several months) [123][84] |
Shorter (days to weeks) [106][64] |
FDA/EMA approval |
Approved [63,64][85][86] |
Not yet approved |
Cancer indication |
Haematological malignancies [61][87] |
Alternative for solid tumours |