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Updated: Mar 2, 2026
Assessment of Human Natural Killer Cell Events Driven by FcγRIIIa Engagement in the Presence of Therapeutic Antibodies
Published on: May 22, 2020
Activation of natural killer cells by Bacillus Calmette-Guérin
1Division of Immunotherapy, Research Center Borstel, Medical University of Lübeck, Germany. sbrandau@fz.borstel.de
Insights
Bacillus Calmette-Guérin (BCG)-activated killer (BAK) cells are a subpopulation of natural killer (NK) cells. NK cells play a crucial role in the effectiveness of BCG immunotherapy for bladder cancer in vivo.
Area of Science:
- Immunology
- Cancer Research
- Cell Biology
Background:
- Bacillus Calmette-Guérin (BCG) immunotherapy is a standard treatment for superficial bladder cancer.
- Previously identified BCG-activated killer (BAK) cells showed potential as effector cells in BCG immunotherapy.
- The precise nature and role of BAK cells in BCG immunotherapy remained to be elucidated.
Purpose of the Study:
- To confirm that BAK cells are a subpopulation of natural killer (NK) cells.
- To investigate the role of NK cells in the efficacy of BCG immunotherapy in vivo.
Main Methods:
- Human mononuclear cells (MNCs) were stimulated with BCG.
- Lymphocyte subpopulations were depleted using magnetic cell separation.
- Cytotoxicity assays, flow cytometry, and in vivo studies comparing wild-type and NK-deficient mice were performed.
Main Results:
- BAK cells exhibited a CD3-/CD56+ NK cell phenotype with high perforin and low CD16 expression.
- BCG immunotherapy significantly improved survival in wild-type mice but not in NK-deficient mice.
- BCG treatment did not affect pulmonary metastasis frequency in either mouse group.
Conclusions:
- BCG stimulation activates cytotoxic lymphocytes with an NK cell phenotype (BAK cells).
- These NK cells express perforin and CD16, molecules critical for cytotoxicity.
- The ineffectiveness of BCG in NK-deficient mice highlights the essential role of NK cells in BCG immunotherapy.
Objectives:
Using a human in vitro model we have previously identified so-called bacillus Calmette-Guérin (BCG)-activated killer (BAK) cells as potential effector cells in BCG immunotherapy. This study was designed to prove the hypothesis that BAK cells are a subpopulation of natural killer (NK) cells and to analyze the role of NK cells during BCG immunotherapy in vivo.
Methods:
After stimulation of mononuclear cells (MNCs) with BCG for 7 days CD3+ and CD56+ lymphocytes were depleted by magnetic cell separation. Subsequently, the cytotoxicity of the marker-negative cell population was tested in a radioactive release assay. Coexpression of CD56/CD16 and CD56/perforin was assessed by flow cytometry. The importance of NK cells for effective BCG immunotherapy in vivo was analyzed by comparing BCG treatment of bladder tumors bearing 'wild-type' C57BL/6 and NK-deficient beige mice.
Results:
BAK cells were shown to have a CD3-/CD56+ NK cell phenotype. They expressed high amounts of perforin and low amounts of CD16, both of which are characteristic features of (activated) NK cells. BCG immunotherapy significantly prolonged survival in tumor-bearing C57BL/6 mice but was ineffective in NK-deficient beige mice. However, BCG treatment did not influence the frequency of pulmonary metastases in both mouse strains.
Conclusions:
Our data clearly indicate that stimulation of human MNCs with BCG leads to the activation of cytotoxic lymphocytes with NK cell phenotype. These killer cells express perforin and CD16, two molecules involved in NK cell cytotoxicity. Finally, ineffective BCG treatment of beige mice suggests a key role for NK cells during BCG immunotherapy in vivo.
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