Single-cell transcriptomic profiles reveal changes associated with BCG-induced trained immunity and protective effects in circulating monocytes

Bacillus Calmette-Guérin (BCG) vaccine is one of the most widely-used vaccines worldwide. In addition to protection against tuberculosis, BCG confers a degree of non-specific protection against other infections by enhancing secondary immune responses to heterologous pathogens, an effect termed trained immunity. To better understand BCG-induced immune reprogramming, we performed single-cell transcriptomic measurements before and after BCG vaccination using secondary immune stimulation with bacterial lipopolysaccharide (LPS). We find that BCG vaccination reduces systemic inflammation, and we identify 75 genes with an altered response to LPS, including several inflammatory mediators such as CCL3 and CCL4 which have a heightened response. Co-expression analysis reveals gene modules containing these cytokines lose coordination after BCG vaccination. Others have increased coordination, including several humanin nuclear isoforms which we confirmed induce trained immunity in vitro. Our results link in vivo BCG administration to single cell transcriptomic changes, validated in human genetics experiments, and highlight new genes which may be responsible for the non-specific protective effects of BCG. Overall design: Single cell RNA-seq of monocytes from 3 healthy donors during 3 visits. On the first visit, blood collection was performed before BCG vaccination. Follow-up samples were collected 2 weeks and 3 months after the initial visit