BCG-activation of leukocytes is sufficient for the generation of donor-independent innate anti-tumor NK and γδ T-cells that can be further expanded in vitro

Bacillus Calmette-Guerin (BCG), the nonpathogenic Mycobacterium bovis strain used as tuberculosis vaccine, has been successfully used as treatment for non-muscle invasive bladder cancer for decades, and suggested to potentiate cellular and humoral immune responses. However, the exact mechanism of action is not fully understood. We previously described that BCG mainly activated anti-tumor cytotoxic NK cells with upregulation of CD56 and a CD16(+) phenotype. Now, we show that stimulation of human peripheral blood mononuclear cells with iBCG, a preparation based on BCG-Moreau, expands oligoclonal gamma delta T-cells, with a cytotoxic phenotype, together with anti-tumor CD56(high) CD16(+) NK cells. We have used scRNA-seq, flow cytometry, and functional assays to characterize these BCG-activated gamma delta T-cells in detail. They had a high IFN gamma secretion signature with expression of CD27(+) and formed conjugates with bladder cancer cells. BCG-activated gamma delta T-cells proliferated strongly in response to minimal doses of cytokines and had anti-tumor functions, although not fully based on degranulation. BCG was sufficient to stimulate proliferation of gamma delta T-cells when cultured with other PBMC; however, BCG alone did not stimulate expansion of purified gamma delta T-cells. The characterization of these non-donor restricted lymphocyte populations, which can be expanded in vitro, could provide a new approach to prepare cell-based immunotherapy tools.

Automated summary

In this study, it was found that stimulation with iBCG can expand oligoclonal gamma delta T-cells and anti-tumor NK cells in human peripheral blood mononuclear cells. These cells exhibited cytotoxicity and can inhibit the growth of bladder cancer. The expansion of these cells in vitro may provide a new approach for cell-based immunotherapy.