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UBC Theses and Dissertations

Mechanism of leukemic cell killing by IL-2 activated natural killer cells : role of cell adhesion molecules Maki, Guitta


Natural killer (NK) cells and lymphokine activated NK (LAK) cells, contribute to the elimination and growth control of malignant and virally infected cells. The binding of killer cells to their targets is a prerequisite for the lysis of malignant cells by NK cells, which utilize cell adhesion molecules (CAMs) to establish initial attachment to target cells. This thesis examined the possibility that defective expression of CAMs on some leukemic cells may be the primary cause of resistance to NK cell-mediated killing. To elucidate the mechanisms by which some leukemic cells are resistant to NK cytotoxicity, a model system was established with the human NK cell line NK-92, and the NK resistant leukemic cell line SR-91 which were established and characterized. SR-91 cells express very low levels of ICAM-1 and they failed to bind to NK-92 cells. NK-92 is highly cytotoxic and kills virtually all leukemic cell lines with the only exception being SR-91. Pre-treatment of SR-91 cells with TNF-α or IFN-γ, two cytokines known to upregulate ICAM-1 expression, increased both ICAM-1 expression on SR-91 cells and binding to NK-92 cells. However, only TNF-α treated SR-91 cells became sensitive to killing by NK-92 cells. The increased binding to NK-92 cells and sensitivity to their killing were abrogated by anti-LFA-1 antibody or a combination of antibodies against ICAM-1, ICAM-2 and ICAM-3, indicating that LFA-1 interaction with the three ICAMs is essential for effector-target cell binding, which is a prerequisite for subsequent target cell lysis. These results underline the importance of ICAM-1 expression on the target cell SR-91 to allow adequate conjugate formation. However, this is, on its own, insufficient to allow target cell lysis by NK-92 cells. TNF-α, but not IFN-γ, also induced the activation of LFA-1, CD44 and β1 integrins on SR-91 cells. Based on these observations, it was hypothesized that the differential effect of TNF-α and IFN-γ could be due to the TNF-α activation of LFA-1 and CD44 on the surface of SR-91 cells that bind to their counter-receptors and activate NK-92 cells. Preliminary experiments showed that engagement of ICAM-3 and CD44 on NK-92 cells induced tyrosine phosphorylation of several proteins including the tyrosine kinase p56*. Further confirmation of these results would not only suggest a role for these adhesion molecules in signal transduction events in NK-92 cells, but perhaps implicates the protein tyrosine kinase p56fc* as an early intermediate in the subsequent lysis of SR-91 cells. These data suggest that NK resistance of leukemic cells can be overcome by some cytokines. Although increased conjugate formation is induced by both TNF-α and IFN-γ, only TNF-α functionally activates LFA-1 and CD44 on target cells that may, upon interaction with counter-receptors on NK-92 cells induce signal transduction events in the latter that lead to target cell lysis. Therefore, treatment of patients with cytokines to overcome NK cell resistance and to eradicate tumor cells may not only activate and stimulate immune effector cells function but may also have direct effects on leukemic cells to make them more susceptible to the lytic effects of NK cells.

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