Page 124 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

122 CHAPTER 5 Cell therapy

suppress tumors through various effective mechanisms such as the IFNγ-mediated
pathway. Therefore, NK cells are able to distinguish normal cells from virus-infected
cells and malignant cells and are able to kill them after attaching to these abnormal
cells [25].
The function of these cells is due to the very complex balance between the inhibitory
and activating signals that are transmitted into the cell by the inhibitory and activating
receptors. When NK cells are exposed to normal cells, class I major histocompatibility
complex (MHC) molecules present at the surface of normal cells bind to the inhibitory
receptor of NK cells and NK cells are not activated and do not lyse normal cells. In
contrast, when NK cells encounter virus-infected cells and malignant cells, the activat-
ing ligands present on the surface of these abnormal cells bind to the activating recep-
tors of the NK cells and activate the NK cells and thereby lyse the virus-infected cells
and malignant cells [26]. The function of these activating and inhibitory receptors in
exposure to normal host cells and malignant cells is simply illustrated in Fig. 5.2. But
in fact, the function of these receptors is much complex, because in addition to having

FIGURE 5.2 The function of inhibitory and activating receptors in NK cells in exposure to
normal and tumoral cells.