Efficient capture of Candida albicans and zymosan by SIGNR1 augments TLR2-dependent TNF-α production

SIGNR1, a mouse C-type lectin, binds various pathogens, including Candida albicans. In this study, we explore the impact of SIGNR1 in the recognition of C. albicans/zymosan and the subsequent tumor necrosis factor (TNF)-α production using SIGNR1-transfected RAW264.7 (RAW-SIGNR1) cells and resident peritoneal macrophages. Compared with RAW-control cells, RAW-SIGNR1 cells dramatically enhanced TNF-α production upon the stimulation with heat-killed C. albicans and zymosan. Recognition of microbes via carbohydrate recognition domain (CRD) of SIGNR1 was crucial for the enhanced TNF-α production. Consistently, such an enhancement was significantly decreased by anti-SIGNR1 mAb. Laminarin, antagonistic Dectin-1 ligand, cooperated to further diminish the response, although no effect was observed by itself in RAW-SIGNR1 cells. However, it moderately reduced the response of RAW-control cells. Zymosan depleted of toll-like receptor (TLR) ligands decreased the response, even though it was recognized by SIGNR1 and Dectin-1. Moreover, antagonistic anti-TLR2 abolished the response, suggesting that TNF-α production largely relies on TLR2-mediated signaling. Resident peritoneal macrophages expressing SIGNR1 predominantly captured zymosan injected intra-peritoneally and produced TNF-α, which was dependent on TLR2 and partly inhibited by anti-SIGNR1 mAb. Finally, physical association of SIGNR1 with the extracellular portion of TLR2 through CRD was confirmed by immunoprecipitation using various deletion mutants. These results suggest that SIGNR1 recognizing microbes participates in the enhanced TNF-α production by M in cooperation with TLR2.