BCG-induced trained immunity in NK cells: Role for non-specific protection to infection

Abstract

Adaptive features of innate immunity, also termed 'trained immunity', have recently been shown to characterize monocytes of BCG vaccinated healthy volunteers. Trained immunity leads to increased cytokine production in response to non-related pathogens via epigenetic reprogramming of monocytes. Recently, memory-like properties were also observed in NK cells during viral infections, but it is unknown if memory properties of NK cells contribute to trained immunity due to BCG vaccination. BCG vaccination of healthy volunteers increased proinflammatory cytokine production following ex vivo stimulation of NK cells with mycobacteria and other unrelated pathogens up until at least three months after vaccination. In addition, in a murine model of disseminated candidiasis, BCG vaccination led to an increased survival in SCID mice, which was partially dependent on NK cells. These findings suggest that NK cells may contribute to the non-specific (heterologous) beneficial effects of BCG vaccination.

Keywords: BCG; Innate immunity; Trained immunity; Vaccination.

Figure 1. BCG vaccination does not increase interferon-gamma production by NK cells

NK cells isolated from naïve (non-exposed) volunteers, before and after (2 weeks and 3 months) vaccination with BCG were stimulated in vitro with sonicated M. tuberculosis, heat-killed S. aureus and C. albicans blastoconidia. INFγ production was assessed by ELISA in the supernatants. Data are presented as mean ± SEM (n = 9).

Figure 2. BCG vaccination increased the non-specific production of pro-inflammatory cytokines

NK cells isolated from naïve (non-exposed) volunteers before and after (2 weeks and 3 months) vaccination were stimulated in vitro with sonicated M. tuberculosis (A-C), C. albicans blastoconidia (D-F) and heat-killed S. aureus (G-I). Pro-inflammatory cytokine production (IL-1β (A,D,G), IL-6 (B,E,H) and TNFα (C,F,I) was assessed by ELISA in the supernatants. *p<0.05, **p<0.01, ***p<0.005. Data are presented as mean ± SEM (n = 9). Friedman test was used to detect significant differences.

Figure 3. BCG-induced protection against disseminated Candida albicans infection is partially NK cell dependent

(A) Survival rate of SCID and NSG mice to an infection with live C. albicans (2×10⁶ CFU/mouse) injected intravenously. Mice were either vaccinated intravenously with PBS (Control) or BCG, 14 days prior to inoculation of lethal C. albicans dose (n≥15 per group, 2 independent experiments). ***p<0.005. vs control (PBS) animals.