The role of SIGNR1 and the beta-glucan receptor (dectin-1) in the nonopsonic recognition of yeast by specific macrophages

We recently demonstrated that the beta-glucan receptor Dectin-1 (betaGR) was the major nonopsonic beta-glucan receptor on macrophages (Mphi) for the yeast-derived particle zymosan. However, on resident peritoneal Mphi, we identified an additional mannan-inhibitable receptor for zymosan that was distinct from the Mphi mannose receptor (MR). In this study, we have studied the mannose-binding potential of murine Mphi and identified the dendritic cell-specific ICAM-3-grabbing nonintegrin homolog, SIGN-related 1 (SIGNR1), as a major MR on murine resident peritoneal Mphi. Both SIGNR1 and betaGR cooperated in the nonopsonic recognition of zymosan by these Mphi. When SIGNR1 was introduced into NIH3T3 fibroblasts or RAW 264.7 Mphi, it conferred marked zymosan-binding potential on these cells. However, in the nonprofessional phagocytes (NIH3T3), SIGNR1 was found to be poorly phagocytic, suggesting that other receptors such as betaGR may play a more dominant role in particle internalization on professional phagocytes. Binding of zymosan to RAW 264.7 Mphi expressing SIGNR1 resulted in TNF-alpha production. Treatment of RAW 264.7 Mphi expressing SIGNR1, which express low levels of betaGR, with beta-glucans had little effect on binding or TNF-alpha production, indicating that there was no absolute requirement for betaGR in this process. These studies have identified SIGNR1 as a major MR for fungal and other pathogens present on specific subsets of Mphi.