Dectin-1 ligands produce distinct training phenotypes in human monocytes through differential activation of signaling networks
Cells of the innate immune system retain memory of prior exposures through a process known as innate immune training. b-glucan, a Dectin-1 ligand purified from the Candida albicans cell wall, has been one of the most widely utilized and well-characterized ligands for inducing innate immune memory. However, many Dectin-1 agonists exist, and it is not known whether all Dectin-1 ligands produce the same phenotype. Using a well-established in vitro model of trained immunity, we compared two commercially available Dectin-1 agonists with the gold standard b-glucan represented in the literature. We found that depleted zymosan, a b-glucan purified from the Saccharomyces cerevisiae cell wall through alkali treatment, produced near identical training effects as C. albicans b-glucan. However, untreated zymosan produced a distinct training effect from b-glucans at both the transcript and cytokine level. Training with zymosan diminished, rather than potentiated, induction of key cytokines such as TNF, IL-12, and IL-6. Zymosan activated NFkB and AP-1 transcription factors more strongly than b-glucans. The addition of the toll-like receptor (TLR) ligand Pam3CSK4 was sufficient to convert the training effect of b-glucans to a phenotype resembling training with zymosan. We conclude that differential activation of TLR signaling pathways determines the phenotype of innate immune training induced by Dectin-1. These findings bring clarity to the specific question of which Dectin-1 agonists produce prototypical training effects and provides broader insight into how signaling networks regulate innate immune training at large. Overall design: Bulk RNA sequencing data from human monocyte-derived macrophages trained with zymosan, depleted zymosan, or beta-glucan and stimulated with LPS