Investigating the lysis of small-cell lung cancer cell lines by activated natural killer (NK) cells with a fluorometric assay for NK-cell-mediated cytotoxicity

B-lymphoma cells escape rituximab-triggered elimination by NK cells through increased HLA class I expression

OBJECTIVE

Antibody-dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells is a major effector mechanism of the monoclonal anti-CD20 antibody rituximab in eliminating B-cell lymphomas. Resistance to this treatment occurs, although CD20 antigen is expressed on the tumor cells.

MATERIALS AND METHODS

A model of ADCC was established by stimulating human bulk NK cells and inhibitory killer immunoglobulin receptor (KIR)-defined NK cells from human leukocyte antigen (HLA)-typed donors. NK-cell activation was triggered via stimulation of the Fc receptor with immunoglobulin G aggregates, rituximab-labeled HLA-defined CD20-positive B-lymphoblast cell lines or CD20-positive B-lymphoma cell lines. The effect of KIR ligation by anti-KIR antibodies and HLA, the HLA expression density and rituximab concentrations on the efficacy of ADCC were analyzed in granzyme B ELISPOT measuring NK-cell activation and fluorescein-activated cell sorting cytotoxicity assay.

RESULTS

HLA, but not CD20 expression density correlated with NK-cell activity against rituximab-labeled targets. ADCC was increased or decreased following HLA shielding or KIR activation by anti-KIR antibodies, respectively. Herein we show that rituximab-induced ADCC is attenuated upon ligation of KIR by HLA molecules expressed on human B-lymphoma target cells. Moreover, anti-KIR antibodies do not only block KIR/HLA interactions, but display agonistic effects at the KIR, which has to be considered for therapeutical applications.

CONCLUSION

KIR activation and HLA expression density are critical determinants for the efficacy of rituximab treatment. An explanation for the failure of rituximab treatment may be the protection of the tumor cells from ADCC by inhibiting NK-cell function with their surface HLA.

Relevance of target cell-induced apoptosis as mechanism of resistance against natural killer cells

Natural killer (NK) cells contribute to the graft-versus-leukemia effect after allogeneic stem cell transplantation. However, the efficacy of NK cell-mediated tumor cell lysis is limited due to target cell resistance, and target cell-induced apoptosis (TiA) was proposed to contribute to differences in susceptibility to NK cells. Here we analyzed the effects of target cells on the apoptosis of cytokine-activated NK cells in vitro. We found no association of target cell susceptibility and TiA of NK cells in an array of human and murine target-effector cell combinations. Incubation of NK cells with caspase inhibitors blocked TiA incompletely, indicating that TiA is partly based on caspase-independent mechanisms. Modulating NK cell susceptibility against TiA by caspase inhibition did not influence cytotoxic efficacy. Furthermore, we found cytotoxic potential of NK cells to be markedly decreased following first target cell contact. Exhaustion of NK cell activity by first target cell contact was, however, not mediated by TiA. In addition, we found no relevant TiA by lymphoma cell lines against activated murine NK cells. We conclude that TiA represents only a minor factor of target cell resistance against NK cell-mediated cytolysis.

Resistance Against Natural Killer Cell Cytotoxicity: Analysis of Mechanisms

Target cell resistance against natural killer (NK) cell-mediated cytotoxicity obstructs NK cell-based immunotherapy of leukaemia. Several mechanisms of resistance have been described. Because of lack of simple assays for analysing these mechanisms, their relative impact on a given effector-target pair is mostly unknown. We here analysed the combination of the Granzyme B (GrB) enzyme-linked immunospot assay (ELISPOT) for the assessment of NK cell reactivity and cytotoxicity assays to estimate target cell escape mechanisms. Target cell recognition failure leads to negative GrB ELISPOT results, whereas target cell resistance shows positive GrB ELISPOT results in the absence of cytotoxicity. We confronted NK cells with the sensitive target cell line K562, and with the resistant cell lines ML2, SupB15 and Raji. ML2 cells sufficiently activated GrB-release whilst being resistant against cytotoxic granules of NK cells. Partial resistance of Raji results from the interaction of HLA class I with inhibitory killer immunglobulin-like receptors (KIR) on the NK cells. Failure of target recognition by HLA class I-KIR interaction, lacking ligands to stimulatory NK cell receptors and partial resistance to cytotoxic granules all contributed to resistance of SupB15. In conclusion, revealing the mechanisms of resistance against NK cell-mediated cytotoxicity may allow improving the results of NK-based immunotherapy.

A novel four-colour flow cytometric assay to determine natural killer cell or T-cell-mediated cellular cytotoxicity against leukaemic cells in peripheral or bone marrow specimens containing greater than 20% of normal cells

To be able to determine the cytotoxic activity of NK cells or T cells against leukaemic cells in patient samples containing >20% of normal peripheral blood cells, we have developed a four-colour flow cytometric cytotoxicity assay. The assay is based on differential immunostaining of both leukaemic cells and effector cells in combination with propidium iodide (PI). The cytometer is set for measuring the fluorescence of the monoclonal antibody (mAb) bound fluorochromes, with moderate overcompensation of the third and fourth fluorescence signals. PI-positive events were excluded from analysis by their characteristic uncompensated signal on these two detectors. Thus, all four fluorescence ranges can be used for detection of mAb-derived signals and this allows discrimination between various populations contained in effector and target cell samples. The cytotoxic activity in our assay is calculated by the absolute loss of vital leukaemic cells. For this purpose, fluorescent beads are included as an internal standard. When calculating the effector concentrations after coculture, characteristic changes can be seen which yield additional information about the presence of cytotoxic activity and the active effector cell subset. With this assay, we present a versatile tool that combines minimum cell manipulation before coculture with maximum information from a sample. The assay is suitable for the analysis of complex samples with regard to different cell subsets, their decrease or increase, and conjugate formation.

Activation of natural killer cells with interleukin 2 (IL-2) and IL-12 increases perforin binding and subsequent lysis of tumour cells

Natural killer (NK) cells can lyse a variety of different tumour cells by exocytosis of perforin, subsequent binding of perforin to the target cell membrane and formation of lytic pores. Some tumour cells, however, are resistant to cellular cytotoxicity. Using the NK-resistant tumour cell lines ML-2, MONOMAC-1, RPMI and L540Cy, we demonstrated that activation of NK cells with interleukin 2 (IL-2) and IL-12 resulted in significant lysis of these tumour targets. To investigate the underlying mechanisms, we isolated the cytotoxic granules from non-activated and IL-2-/IL-12-activated NK cells and compared the killing of K562 leukaemia cells (sensitive to NK cell-mediated lysis) and ML-2 leukaemia cells (resistant to NK cell-mediated lysis). In contrast to K562 cells, which were easily killed by NK-cell granules, ML-2 cells were resistant to granules from non-activated NK cells. However, granules from NK cells activated with IL-2 and IL-12 were able to induce significant tumour cell lysis. Cell death of both K562 and ML-2 cells by granules from activated NK cells was completely blocked by anti-perforin antibodies, indicating that perforin mainly accounts for the lysis induced by NK granules. Comparing granules from non-activated and IL-2-/IL-12-activated NK cells, the increased cell death of ML-2 cells was caused by an improved binding of perforin to the target cell membrane. Functional assays, however, indicated that the differences in perforin binding were not as a result of an augmented production of perforin by activated NK cells. We conclude that activation of NK cells results in an increased binding of perforin and subsequent lysis of tumour cells.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.