Activation of natural killer cells by Bacillus Calmette-Guérin.

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.