CD8(+) Regulatory T Cells, and Not CD4(+) T Cells, Dominate Suppressive Phenotype and Function after In Vitro Live Mycobacterium bovis-BCG Activation of Human Cells

Mycobacterium bovis bacillus Calmette-Guérin (M. bovis BCG), the only currently available vaccine against tuberculosis, has been reported to induce regulatory T cells in humans. The activity of regulatory T cells may not only dampen immunogenicity and protective efficacy of tuberculosis-vaccines, but also hamper diagnosis of infection of tuberculosis, when using immune (e.g. IFNγ-release) assays. Still, in settings of infectious diseases and vaccination, most studies have focused on CD4(+) regulatory T cells, and not CD8(+) regulatory T-cells. Here, we present a comparative analysis of the suppressive phenotype and function of CD4(+) versus CD8(+) T cells after in vitro live BCG activation of human cells. Moreover, as BCG is administered as a (partly) live vaccine, we also compared the ability of live versus heatkilled BCG in activating CD4(+) and CD8(+) regulatory T cell responses. BCG-activated CD8(+) T cells consistently expressed higher levels of regulatory T cell markers, and after live BCG activation, density and (co-)expression of markers were significantly higher, compared to CD4(+) T cells. Furthermore, selection on CD25-expression after live BCG activation enriched for CD8(+) T cells, and selection on co-expression of markers further increased CD8(+) enrichment. Ultimately, only T cells activated by live BCG were functionally suppressive and this suppressive activity resided predominantly in the CD8(+) T cell compartment. These data highlight the important contribution of live BCG-activated CD8(+) Treg cells to immune regulation and emphasize their possible negative impact on immunity and protection against tuberculosis, following BCG vaccination.