Oral administration of yeast, Saccharomyces cerevisiae, enhances the cellular innate immune response of gilthead seabream Sparus aurata L.) Jesu Âs Ortun Äo, Alberto Cuesta, Alejandro Rodrõ Âguez, M. Angeles Esteban, Jose  Meseguer * Faculty of Biology, Department of Cell Biology, University of Murcia, 30071 Murcia, Spain Received 29 June 2001; received in revised form 16 October 2001; accepted 16 October 2001 Abstract The effects of including lyophilised whole yeast, Saccharomyces cerevisiae, in the diet on the seabream innate immune response were investigated. Gilthead seabream Sparus aurata L.) specimens were fed four different diets for 4 weeks: a commercial diet as control and the same diet supplemented with 1, 5 or 10 g/kg yeast. After 1, 2 and 4 weeks, serum complement titres, as a humoral parameter, and phagocytic, respiratory burst, myeloperoxidase and natural cytotoxic activities of head-kidney leucocytes, as cellular parameters, were evaluated. The results showed that yeast supplements enhanced all the latter responses, but not the humoral response. This enhancement was dose-dependent except for the cytotoxic activity that was only stimulated by the lower dose of yeast assayed. As yeast cell walls are able to enhance the seabream cellular innate immune response, these results support the possible use of whole yeast as natural inmunostimulants in common ®sh diets. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Yeast; Saccharomyces cerevisiae; Diet; Immunomodulation; Innate immune system; Leucocytes; Gilthead seabream Sparus aurata L.) 1. Introduction Natural immunostimulants are valuable for activat- ingthe®shimmunesystemandprotecting®shagainst adverse conditions Anderson, 1992; Sakai, 1999). Among them, yeast Saccharomyces cerevisiae has beenfoundtobeagoodenhancerofthetroutimmune system Siwicki et al., 1994). Yeast cell walls are constructed almost entirely of two classes of polysaccharides,mannosepolymerscovalentlylinked to peptides or mannoprotein) and glucose polymers orglucans).Theglucansandmannoproteinsoccurin roughly equal amounts in the wall. A third sugar polymer of N-acetylglucosamine, chitin, is also an important component of fungal walls, but is present only in minor amounts about 1%) in yeasts Cabib and Roberts, 1982). b-Glucans and chitin have been described as powerful immunostimulants in ®sh and mammals, while the use of yeast mannoproteins as immunostimulants have only been studied in mam- mals Vecchiarelli, 2000). b-Glucans have generally been demonstrated to activate ®sh macrophages in vitroandtoenhancethenon-speci®cdefenceof®shto Veterinary Immunology and Immunopathology 85 2002) 41±50 * Corresponding author. Tel.: 34-68-364-965; fax: 34-968-363-963. E-mail address: meseguer@um.es J. Meseguer). 0165-2427/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0165-242701)00406-8