The Important Role Played By Tumor Cells Expressed Fractalkine/CX3CR1 In Tumor Immunosurveillance Based On NK Cells

Many of the chemokine receptors are typically found on natural killer cells, including CX3CR1, the receptor for the chemokine fractalkine (FKN). These Chemokine receptors typically expressed on natural killer cells could be activated by ligands including the membrane chemokine fractalkine (mFKN).Fractalkine is a newly identified chemokine, the only member of cxxxc family, and also called as CX3C. we didn't know if fractalkine play a key role in cancer immunosurveillance.Firstly, we checked if tumor cell lines express fractalkine and CX3CR1. We found most tumor cells express fractalkine. And some of them express CX3CR1 at the same time. These results showed the importance of fractalkine in the cancer formation.Then we explored whether interactions between CX3CR1 and FKN are relevant for NK cell functions in cytotoxicity against tumor. FKN expression was examined by polymerase chain reaction and CX3CR1 expression on NK cells was analyzed by flow cytometry. NK cell cytotoxicity was examined by 4-hour ⁵¹Cr release assay. FKN was expressed on a variety of tumor cell lines such as K562 cells, a NK sensitive cell lines. Around 90% of peripheral blood NK cells and almost all NK cell line, NK-92 cells, expressed CX3CR1. Anti-CX3CR1 antibody strongly neutralized the cytotoxicity of NK cells against K562 cells, and pretreatment of NK cells with recombinant soluble FKN improved the cytolytic function on tumor cells. This study demonstrates that interaction between CX3CR1 on NK cells and FKN on tumor cells is involved in natural cytotoxicity of NK cells against tumor.Thirdly, we investigated the function of mFKN on NK cell activation for interferon-γproduction and cytotoxicity against tumors. HeLa cells were transfected with membrane human fractalkine (mhFKN)-expressing vector, and the transcription and surface expression of mhFKN in transfected HeLa cells were confirmed by an RT-PCR analysis and immunofluorescence assay, respectively. After co-culture of NK-92 cells with FKN-HeLa cells, the intracellular IFN-γin NK-92 cells significantly increased if compared to mock-HeLa cells. The concentration of IFN-γalso increased in the supernatant of NK-92 cells simulated with mhFKN-HeLa cells. More over, cytolysis activity of NK-92 cells against K562 target tumor cells was significantly enhanced at each effectontarget ratio in 4-hour ⁵¹Cr-release assays if the NK-92 cells were pretreated with mhFKN-HeLa, indicating that membrane fractalkine may activate the NK cells in killing process. This study further confirms that membrane-expressed fractalkine plays a critical role in NK cell activation.These experiments gave evidence of the participation of fractalkine in cancer formation. And these evidence told us the interaction of fractalkine/CX3CR1 could activate NK cells directly, which play an important role in innate immunity and important in regulation of adoptive immunity. These results showed the importance of fractalkine for regulation of immune system by tumor cells.