The prognosis of most cancer patients depends on the extent of primary tumor metastasis to distant organs. At the time of diagnosis a substantial percentage of patients with colorectal carcinoma have already developed liver metastases. Many patients which are free of liver metastases can be saved by surgical resection of the primary tumor, however, surgery itself can enhance metastasis development in the liver. Circulating tumor cells in the liver sinusoids interact with various resident immune cells such as Kupffer cells and iNKT cells. This interaction results in weak immunity against tumor cells due to the tumor cell tolerant environment in the liver which promotes the establishment of metastases.

Based on previous observations, the aim of the present study was to investigate the immunomodulatory role of the glycosphingolipid αgalactosylceramide (αgalcer). To assess this question we performed intravital multi-channel spinning disc confocal microscopy to visualize the interaction of iNKT cells and metastasized tumor cells in the liver in vivo as well as in vitro assays.

Repeated αgalcer treatment resulted in a major increase in the number of iNKT cells present in the liver, increased interactions with tumors, and showed significantly diminished tumor growth. Similar results were obtained when mice with established liver metastases received αgalcer. In addition, the survival rate in treated animals was longer as compared to untreated animals. We repeated experiments in NKT cell deficient mice and found that the inhibition of tumor growth is CD1d-dependent. When tumors reached a certain size the iNKT cells were no longer effective at reducing the tumor growth.

Our results suggest that iNKT cells, after being activated with αgalcer, play a pivotal role in restricting metastatic growth either via direct interaction with tumor cells (perhaps by granzymes) or indirectly by modulating the immunosuppressive tumor environment increasing Th1 immunity.