CAR NK Cell Therapy for the Treatment of Metastatic Melanoma: Potential & Prospects

Abstract

Melanoma is among the most lethal forms of cancer, accounting for 80% of deaths despite comprising just 5% of skin cancer cases. Treatment options remain limited due to the genetic and epigenetic mechanisms associated with melanoma heterogeneity that underlie the rapid development of secondary drug resistance. For this reason, the development of novel treatments remains paramount to the improvement of patient outcomes. Although the advent of chimeric antigen receptor-expressing T (CAR-T) cell immunotherapies has led to many clinical successes for hematological malignancies, these treatments are limited in their utility by their immune-induced side effects and a high risk of systemic toxicities. CAR natural killer (CAR-NK) cell immunotherapies are a particularly promising alternative to CAR-T cell immunotherapies, as they offer a more favorable safety profile and have the capacity for fine-tuned cytotoxic activity. In this review, the discussion of the prospects and potential of CAR-NK cell immunotherapies touches upon the clinical contexts of melanoma, the immunobiology of NK cells, the immunosuppressive barriers preventing endogenous immune cells from eliminating tumors, and the structure and design of chimeric antigen receptors, then finishes with a series of proposed design innovations that could improve the efficacy CAR-NK cell immunotherapies in future studies.

Keywords: CAR-NK cell; CAR-NK cell therapy; cancer immunotherapy; melanoma; metastasis; natural killer cell.

Figure 1

Natural killer cell signaling receptor repertoire. Natural killer cells are equipped with a diverse assortment of activating and inhibitory signaling receptors that mediate NK cell activity through combined signal propagation. Activating receptors, including NCRs, NKG2D, DNAM-1, CD16, and aKIRs transmit the signals to promote NK cell activation and cytotoxicity, while inhibitory receptors, including CD96, TIGIT, LIR1 and iKIRs, induce the inhibition signaling to block the NK cell activation. KIR2DL4 could introduce activating or inhibitory signaling into NK cells based on its content and ligands.

Figure 2

Functional shifts between anti-tumor immunity and immunosuppression within the tumor microenvironment. Several non-malignant immune and stromal cells undergo phenotypic alterations through direct cell-cell interactions with melanoma cells and through the influence of chemical conditions within the tumor microenvironment. M1: phagocytic macrophage phenotype. M2: pro-inflammatory macrophage phenotype. N1: cytotoxic neutrophil phenotype. N2: pro-inflammatory neutrophil phenotype. APC-DC: functional antigen-presenting dendritic cell. Tol-DC: dysfunctional tolerogenic dendritic cell. Teff: CD8+ cytotoxic T cell. Th: CD4+ pro-cytotoxic helper T cell. Treg: CD4+ immunosuppressive regulatory T cell. NK: natural killer cell. P-Fib: primary fibroblast. CAF: cancer-associated fibroblast.

Figure 3

The four main mechanisms of immune escape. (A) Disruption of NK cell activation through at least three mechanisms: Tumor cells can express molecules such as IDO1 and PGE2 to mediate the inhibition of NKAR expression in NK cells. Also, tumor cells often downregulate the NKAR ligand expression on tumor cell surfaces or/and shed their NKAR ligands proteolytically. (B) Promotion of NK-inhibiting receptor-ligand interactions via upregulation of inhibitory ligand expression on melanoma cells. (C) Immune escape assisted by non-malignant constituent cells within the tumor microenvironment through secreted immunosuppressive cytokines and chemokines. (D) Immunological dysfunction resulting from hypoxic and acidic chemical conditions within the tumor microenvironment.

Figure 4

A diagram of future CAR-NK cell development for treatment of melanoma. These studies will focus strengthening CAR-NK cytotoxic activation for maximized to killing the metastatic and drug resistant melanoma, including structural optimization of CAR with NKG2D or DNAM1 transmembrane domain and 2B4 or DAP10 intracellular signaling domain, integration of TCR structural elements in CAR-NK cells, targeting receptor tyrosine kinase signaling with sLRIG1, integration of CXCR3 to enhance CAR-NK cells infiltration, targeting markers of non-malignant cells in TME, corporation of blocking NK cell inhibitory receptors, as well as optimization with drug therapies.