Alleviation of monocyte exhaustion by BCG derivative mycolic acid

Monocyte exhaustion characterized by sustained pathogenic inflammatory and immune-suppressive features underlies the pathogenesis of sepsis induced by systemic polymicrobial infections. However, effective strategies in blocking monocyte exhaustion and restoring innate homeostasis are currently not available. In this study, we found that Methoxy-Mycolic Acid (M-MA), a branched mycolic acid derived from Mycobacterium Bovis Bacillus Calmette–Guérin (BCG), to be a potent agent in alleviating monocyte exhaustion and restoring immune homeostasis. Co-treatment of monocytes with M-MA can effectively block the expansion of exhausted Ly6Chi /CD38hi/PD-L1hi monocytes induced by repetitive LPS challenges, and restore the expression of immune-enhancing CD86 on co-treated monocytes. Functionally, M-MA treatment restored mitochondrial functions of exhausted monocytes and alleviated their suppressive activities on co-cultured T cells. Mechanistically, M-MA exerts its protective effects independent of cellular receptor TREM2, and relieves cellular stress signaling through blocking Src-STAT1 mediated pathogenic inflammatory polarization as well as reducing the production of compensatory immune suppressors TAX1BP1 and PLAC8. Our whole genome methylation analyses further revealed that M-MA can effectively erase methylation memory of exhausted monocytes, with validated restoration of plac8 methylation by M-MA. Together, our data reveal M-MA as a potent agent in restoring monocyte homeostasis with future therapeutic potential for the treatment of sepsis. Genome-wide DNA methylation profiling was performed using Infinium Mouse Methylation BeadChip arrays. Based on variability observed in previous studies, a minimum of 4 biological replicates was determined to be sufficient for most BMMC culture experiments, which is sufficiently powered to detect differences in 5mC as low as 5%. All experiments were performed on at least two separate days and with mice from at least 3 separate litters.