Harnessing natural killer T cells to stop cancer

Cancer accounted for nearly 10 million deaths worldwide in 2020 and is the leading cause of death in Japan¹. Immunotherapy primes the patient's immune system to fight cancer cells and offers a promising alternative to the usual treatments of chemotherapy, radiation therapy and surgery. Researchers at the RIKEN Center for Integrative Medical Sciences (IMS) are investigating a number of immunology and cancer therapies, and our team at the RIKEN IMS Laboratory for Developmental Genetics is spearheading efforts to develop new cancer immunotherapy strategies by harnessing the power of natural killer T (NKT) cells.

In September 2020, we started recruiting for the world's first clinical trial using NKT cells derived from induced pluripotent stem (iPS) cells to treat patients with head and neck cancer. This followed more than a decade of intensive basic research and validation studies with colleagues at Chiba University. Four to 18 patients will be enrolled in the trial and may undergo the novel treatment over the next two years. While it is too soon to evaluate efficacy, confirming the safety of iPS-derived NKT cell therapy will be a key step toward the development of this potentially groundbreaking treatment for many different types of cancer.

The unique invariant antigen receptor (Vα14Jα18 in mice) of NKT cells was discovered in 1986² by immunologist Masaru Taniguchi, then at Chiba University and now senior visiting scientist at RIKEN IMS. His work spurred research into the role of NKT cells in the regulation of immune responses, opening up the development of anticancer therapeutics. The discovery of NKT cells has been designated as a 'Pillar of Immunology' by the American Association of Immunologists³.

Because of their unique receptors, NKT cells recognize lipid antigens presented by the cell surface molecule monomorphic CD1d. They rapidly produce large amounts of cytokines (such as interferon-gamma, IFN-γ) which are critical to activating antitumor functions^4,5. Studies of NKT cell-deficient mice have demonstrated⁶ that NKT cells play an essential role in triggering antitumor immune responses and establishing immunological memory against tumor cells.

With a view to unlocking the potential of these rare cells as a cancer therapy, the search was on for a suitable molecule to trigger NKT cells⁷. Taniguchi and his team succeeded in singling out alpha-galactosylceramide (α-GalCer), a glycolipid derived from the sea sponge Agelas mauritianus, in 1997. They showed that α-GalCer binds to CD1d. The resulting complex leads to enhanced NKT cell activity and high levels of cytokine production, which mediates adjuvant activity and activates various other antitumor immune cells.

A preclinical breakthrough came in 1999 with intravenous administration of α-GalCer to mice with liver cancer⁸. The treatment, involving the use of dendritic cells pulsed with α-GalCer, resulted in complete inhibition of melanoma metastasis within just one week.

Subsequent studies have shown that after stimulation with α-GalCer, NKT cells appear to activate both CD8+ cytotoxic T cells (to destroy major histocompatibility complex (MHC)-positive tumor cells) and natural killer cells (to eliminate MHC-negative tumor cells)^9,10. As tumors frequently contain both MHC-positive and -negative cell types, this remarkable ability to attack cancer cells on two fronts has led researchers to hail NKT cells as "an ideal anti-tumor immunotherapeutic"¹¹ that could potentially lead to "complete eradication of tumors without relapse"¹².

In 2001, a series of studies to assess the effects and safety of α-GalCer-pulsed dendritic cell therapy began at Chiba University Hospital, led by Shinichiro Motohashi¹³. A clinical trial of NKT cell-targeted immunotherapy using α-GalCer for patients with advanced non-small cell lung cancer began in March 2004. A cohort of 17 patients showed increased IFN-γ production and had a median survival time of 18.6 months,¹⁴ which is considerably longer than the typical survival time of less than 12 months. The results also suggested a mechanism by which NKT cells are able to induce long-term immune memory.

While α-GalCer treatment continues to be developed and explored, both in Japan and overseas, the stark fact remains that many cancer patients are ineligible for this treatment because their NKT cell count is too low. Even in the blood of healthy individuals, NKT cells comprise only 0.1% of total lymphocytes. This makes it extremely challenging to harvest NKT cells from the body and culture them to numbers sufficient for clinical use.

To overcome this problem, a multidisciplinary team at RIKEN developed a technique to generate large quantities of NKT cells using induced pluripotent stem (iPS) cells in mice in 2010¹⁵. They succeeded in generating millions of functionally competent iPS-derived NKT cells, which secreted high levels of IFN-γ and suppressed tumor growth in live mice.

Following proof of concept¹⁶ for clinical application of human iPS-derived NKT cells in 2016, a clinical trial began at Chiba University Hospital in late 2020. iPS cells were made from NKT cells collected from the blood of healthy donors and cultured into large numbers. These iPS cells were then redifferentiated to NKT cells and injected into patients. Each patient received around 50 million iPS-derived NKT cells in one injection, followed by two further injections spaced two weeks apart.

The focus of the study is on patients with squamous cell carcinoma (a condition accounting for 80-90% of all head and neck tumors) who have previously not responded to surgery, radiation therapy, chemotherapy or a combination of these therapies. Those with cachexia (a wasting disease seen in many end-stage cancer patients) were excluded from the study, which limits participant recruitment.

The most important investigation is safety. The two biggest risk factors are iPS cells becoming cancerous themselves and patients exhibiting allogeneic responses (rejecting immunologically incompatible cells created from another person).

Provided that the preliminary results are satisfactory, a combination therapy of α-GalCer-pulsed dendritic cell treatment and iPS-derived NKT cells may be explored for tumor suppression. We will see results in 2022 at the earliest.

Devising new ways to make iPS-derived NKT cells more active presents an ongoing challenge.

Human leukocyte antigen (HLA) molecules mediate allogeneic rejection, but HLA can be knocked out at the iPS phase by using technologies such as CRISPR-mediated editing.

This process could also extend the retention period of iPS-derived NKT cells in the body. Efforts are also underway to make iPS-derived NKT cells more immunogenic through the application of chimera antigen receptor (CAR) T-cell therapy. This involves genetically engineering iPS-derived NKT cells to express a synthetic receptor that can bind tumor-specific antigens. As CD19-specific CARs are already successfully used in the treatment of patients with B cell-derived leukemia¹⁷, this is another promising avenue of research.

In order to scale up, there is a need for a simpler, more optimized protocol. It currently takes 40-50 days to generate sufficient numbers of NKT cells for treatment. However, based on evidence that NKT cells appear in mice on day 11 after fertilization, this could be shortened to as little as 2-3 weeks. A long-term goal will be to develop a cocktail of differentiation factors to facilitate the industrial generation of iPS-derived NKT cells, which would make the cells more cost effective.

Before such dreams can be realized, there is still a great deal of basic research to be done. There is much scope for informatics and data scientists to help us trace the developmental trajectory of iPS cells to NKT cells. Shin-ichiro Fujii, team leader at the RIKEN IMS Laboratory for Immunotherapy, is working on artificial adjuvant vector cell therapy, another variation of NKT cell-targeted therapy, for the treatment of relapse and refractory acute myeloid leukemia. In addition to the fruitful collaboration with Motohashi's group at Chiba University, there may be more collaborations with other university hospitals nationwide and around the globe.