Metabolic-epigenetic rewiring of CCR5hi monocytes sustains long-term trained immunity against lethal sepsis.

Trained immunity enhances innate host defense by endowing monocytes with memory-like properties, yet the underlying integrated metabolic and epigenetic mechanisms remain elusive. Here, we demonstrate that coimmunization with Bacille Calmette-Guérin (BCG) and bacterial lipoprotein (BLP) induces a durable form of trained immunity that provides robust, long-term protection against polymicrobial sepsis from early life into adulthood. Single-cell RNA sequencing revealed that this effect is mediated by an expansion of CCR5hi memory-like monocytes with enhanced antimicrobial capacity. Mechanistically, BCG + BLP vaccination activated the AKT-mTOR-HIF-1α axis, driving glycolytic reprogramming and lactate accumulation. Elevated lactate enhanced KAT2B-dependent histone H3K18 lactylation, an epigenetic mark directly facilitating the transcription of phagocytic and inflammatory genes. In addition, BCG + BLP stimulation of human cord blood mononuclear cells induced CCR5hi monocytes that recapitulated trained immunity features. These findings identify a lactate-KAT2B-H3K18la epigenetic axis that orchestrates the long-term reprogramming of CCR5hi monocytes, highlighting CCR5hi monocytes as a promising therapeutic target for modulating innate immunity against lethal sepsis.