Role of NK cells in adoptive immunotherapy of metastatic colorectal cancer in a syngeneic rat model.

This article reviews our immunotherapy research with natural killer (NK) cells in a syngeneic rat colorectal cancer liver and lung metastasis model. Using adoptive transfer of interleukin (IL)-2-activated NK cells, NK cells were shown to selectively infiltrate the tumors. More NK cells were found in tumors when the NK cells were directly injected into tumor-draining blood vessels than when the cells were injected in systemic blood vessels. Under optimal conditions, a limited, though significant, effect of adoptively transferred NK cells on tumor growth was shown. We observed that both endogenous and adoptively transferred NK cells were predominantly present in the stroma surrounding the tumor cell nodules. It is possible that they did not penetrate the nodules containing the tumor cells because of the presence of a basal membrane-like structure around these nodules. Adoptively transferred NK cells may initiate elimination of tumor cells by activating other effector cells, whereas some may eliminate tumor cells by direct cell-cell contact. A diverse array of molecules was shown to be involved in this process. CD45 on NK cells was found to be important in initiating the lysis-inhibitory signal upon binding of 'self' major histocompatibility complex (MHC) class I on potential target cells. Our results indicate that NK-cell cancer therapy is still promising and needs improvement.