A beta-glucan inhibitable zymosan receptor on channel catfish neutrophils

In mice and humans zymosan binds to the complement receptor three/Mac-1 receptor; however, identification of this receptor in channel catfish (Ictalurus punctatus) has not been accomplished. Soluble fluorescein isothiocyanate (FITC) conjugated beta-glucan, a component of zymosan, was found to bind to channel catfish anterior kidney (AK) neutrophils but not to B-lymphocytes. Serum activated zymosan (SAZ)-mediated chemiluminescence responses of channel catfish AK neutrophils could be inhibited by beta-glucan but not by mannan, and inhibition of chemiluminescence responses by beta-glucan was dose dependent. Similarly, phagocytosis of FITC-SAZ could be inhibited by beta-glucan in a dose-dependent manner. Treatment of channel catfish AK neutrophils with various concentrations of trypsin resulted in inhibition of phagocytosis of FITC-SAZ but not of Aeromonas hydrophila indicating that A. hydrophila phagocytosis was mediated by a trypsin-resistant receptor. Deleting serum or using heat-inactivated serum in the mixtures for the chemiluminescence and FITC-SAZ phagocytosis assays resulted in baseline readings. These data indicate that the beta-glucan component of zymosan is responsible for zymosan phagocytosis. Furthermore, the recognition of zymosan by a specific receptor is evident based on trypsin sensitivity assays. Based on these results it is proposed that a complement receptor 3, Mac-1-like receptor, is present on channel catfish AK neutrophils.