Fig 1. Spleen-resident natural killer (NK) cells are directly activated by Mycobacterium bovis BCG and purified protein derivative (PPD) antigen.

NK cells isolated from spleens of naïve mice were cultured (1.5 × 10⁶ /mL) in the presence or absence of BCG (multiplicity of infections (MOI) = 1) (A) or PPD (50 μg/mL) (B) at 37°C for 24 h, and then the culture supernatants were harvested (n = 5 per group). The production level of IFNγ was measured by an enzyme-linked immunosorbent assay (ELISA). The data are presented as mean ± standard deviation, and p values < 0.05 were considered statistically significant. Similar results were obtained in three independent experiments. ***p < 0.0001.