Dissecting the immunobiological effects of Bacillus Calmette-Guérin (BCG) in vitro: evidence of a distinct BCG-activated killer (BAK) cell phenomenon.

Several immunobiological effects of intravesical bacillus Calmette-Guérin (BCG) during immunotherapy of superficial bladder cancer have been suggested as possible mediators of the mode of action. In an attempt to elucidate which of these effects is relevant to tumoricidal activity, an in vitro cytotoxicity assay was employed in which the direct effects of BCG and of cytokines against four transitional carcinoma cell lines were studied. Furthermore, peripheral blood mononuclear cells (PBMNC) were analyzed for their cytotoxic potential against these target cells. We found that none of the cytokines interleukin-1, interleukin-2, interferon-gamma, tumor necrosis factor alpha, lymphotoxin, or BCG alone were cytotoxic against the bladder carcinoma cell lines. However, a pronounced cytotoxicity against these targets resistant to natural killer cells could be induced in PBMNC by coincubation with viable BCG. We termed this the BCG-activated killer (BAK) cell phenomenon. In contrast to lymphokine-activated (LAK) cells, these BAK cells needed prolonged activation for 7 days and did not enhance the cytotoxicity against K562 target cells sensitive to natural killer cells. Nonviable, heat-inactivated BCG was significantly less effective, and sonificated fractions of BCG were not effective in stimulating PBMNC towards BAK cell activity. In vitro dissection of effects observed during BCG intravesical therapy may give more insight into the mode of action of BCG and may help to separate primary tumoricidal effector mechanisms from secondary concomitant phenomena. Further characterization of the BAK cell may result in an improvement of intravesical BCG immunotherapy.