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.