Cytokine – induced killer (CIK) is a new class of CD3-activated killer cells (anti-CD3 monoclonal antibody activated killer (CD3AK)) prepared by Schmidt-Wolf et al. in 1991. CIK cells are a heterogeneous group of cells obtained from human peripheral blood mononuclear cells (PBMC) in vitro after stimulation with a variety of cytokines (IFN-γ, rIL-2, CD3McAb and IL-1α, etc.), and they possess both the powerful anti-tumor activity of T lymphocytes and the non-MHC-restricted tumor-killing characteristics of NK cells. At present, CIK cell therapy has been used in clinical studies of various tumors such as renal cancer, malignant melanoma, colon cancer, lymphoma and so on, and has achieved certain therapeutic effects. Studies have shown that compared with lymphokine activated killer cells (LAK), tumor infiltration lymphocyte (TIL) and CD3AK cells, CIK cells have fast proliferation speed, high tumor-killing activity and wide tumor-killing spectrum, CIK cells have the advantages of high proliferation speed, high tumor killing activity, wide tumor killing spectrum, same sensitivity to multi-drug-resistant tumor cells, low cytotoxicity to normal bone marrow hematopoietic precursor cells, resistance to tumor cell-induced effector cell Fas-FasL apoptosis, and the severity and metastasis of the disease do not affect the anti-tumor activity of CIK cells, thus CIK cells have a promising prospect of application in the treatment of tumors with over-immunization. 1, the induction and biological properties of CIK cells The effector cells in CIK cells are extremely rare in normal peripheral blood, only 1%~5%. At present, there are various methods to induce CIK cells in vitro, the main difference lies in the different combinations of cytokines. CD3McAb and IFN-γ are the necessary components, CD3 McAb plays the role of mitogen activity, which can cross-link with the CD3 on the surface of the T cell and induce cell activation, while IFN-γ can induce the synthesis of cytokines such as IL-1. In the process of CIK cell culture, the order of factor addition also has a certain effect on its toxicity, IFN-γ can only improve CIK cytotoxic activity if it is added 24 h before IL-2 is added (if it is added at the same time with IL-2 or later than it is added then it loses the enhancement of cytotoxicity), and this effect is related to the up-regulation of IL-2 receptor on the surface of the cell. In addition to the selection of cytokines, the following cytokines are commonly used in CIK cell culture: IL-2, exogenous phytohemagglutinin (PHA), IL-7, IL-12, and so on. CIK cells are a heterogeneous population of cells, most of which bear T-cell markers (TCRα/β 86.5% ± 5.7%, TCRγ/δ 4.5% ± 2.6%, CD4 45.4% ± 3.2%, CD8 47.7% ± 11.0%), and some of which bear NK-cell markers (CD16 10.4% ± 4.9%, CD56 28.5% ± 8.6%), and 88% of which bear NK-cell markers (CD16 10.4% ± 4.9%, CD56 28.5% ± 8.6%). ), with 88% of CD56+ cells co-expressing CD3, and this fraction of CD3+CD56+ cells proved to be the major effector cells in the CIK cell population.CD3+CD56+ cells accounted for about 1%-5% of uncultured PBMC, and could gain significant growth in both absolute and relative numbers after culture (30 d of expansion, 100-fold expansion of the cell number, and 10-fold expansion in the population proportion expanded 10-fold), CD3+CD56+ cells were mainly derived from T cells (CD3+CD56-) rather than NK cells (CD3-CD56+) in PBMC, and it was further found that CD56 expression did not occur in the CD4+CD8- cell subpopulation of the T cells during culture, whereas there was 1/4 of the CD4-CD8+ subpopulation, 1/3 of the CD4+CD8+ subpopulation had 1/3, and more than half of the CD4-CD8- subpopulation showed CD56 expression. Since the ratio of the two subpopulations, CD4+CD8+ and CD4-CD8-, was small in PBMC, the CD4-CD8+ T-cell subpopulation was the main source of CIK effector cells. However, in the expanded CD3+CD56+ cells, the expression of CD8 or not had no effect on their cytotoxic effects. The mechanism of CIK cells The principle of CIK killing target cells has not been fully elucidated yet, and the possible mechanisms are as follows: ① Direct killing of tumor cells. Mehta et al. believe that there are two pathways for CIK to kill target cells: First, CIK cells are activated by lymphocyte function associated antigen-1 (lymphocyte function associated antigen-1, LFA-1) recognition structure, resulting in cytoplasmic toxicity particle-dependent cytolysis, this pathway has nothing to do with the concentration of cyclic AMP (cAMP) in the cytoplasm; this pathway has nothing to do with the concentration of cyclic AMP (cAMP) in the cytoplasm. This pathway is independent of intracytoplasmic cyclic adenosine monophosphate (cAMP) concentration; secondly, the CD3-like receptor on the surface of CIK cells binds to the CD3-like receptor and activates CIK cells to produce cytotoxic granule-mediated cytolysis, which is related to the intracytoplasmic cAMP level. ② Tumor-suppressive and tumor-killing effects of the large amount of inflammatory cytokines produced after activation. A large number of experiments have proved that cultured CIK cells can secrete a variety of cytokines, such as TNF-α, IL-2, GM-CSF, IFN-γ, etc., which not only have a direct inhibitory effect on the tumor cells, but also indirectly kill the tumor cells through the regulation of the immune system reactivity of the body. ③Induction of tumor cell apoptosis. The experiments of Verneris et al. suggest that CIK cells expressing FasL in the process of culture can resist Fas-FasL apoptosis of effector cells triggered by FasL+ tumor cells, and can induce apoptosis of Fas+ tumor cells, thus exerting a chronic killing effect on tumor cells and ensuring the long-term persistence of anti-tumor activity. Sun et al. found that CIK cells induced tumor cell death in early stage and tumor cell necrosis in late stage by down-regulating the expression of p53, C-myc and Bcl-2 and up-regulating the expression of Bax. ④Promote T cell proliferation and activation. Ren Huan et al. concluded that the anti-tumor effect of CIK cells in vivo may be related to the promotion of T cell proliferation and activation in the host. It was also hypothesized that primitive CD3+ CD56+ T cells, after being infused into the body with CIK, were transformed into cytotoxic T cells (cytotoxic T-lymphocyte (CTL)) with tumor-killing activity under the state of the host organism or the stimulation of tumor antigens, and exerted anti-tumor effects.