Molecular characterization of the nonspecific cytotoxic cell receptor (NCCRP-1) demonstrates gilthead seabream NCC heterogeneity Alberto Cuesta, Maria A ´ ngeles Esteban * , Jose ´ Meseguer Fish Innate Immune System Group, Department of Cell Biology, Faculty of Biology, University of Murcia, 30100 Murcia, Spain Received 3 September 2004 Available online 2 December 2004 Abstract Teleost fish NCCs (nonspecific cytotoxic cells) are thought to be the evolutionary precursors of the mammalian NK cells. A novel mechanism mediating the NCC-mediated cytotoxicity has been described in teleosts. Now, this NCC receptor protein-1 (NCCRP-1) was characterized in gilthead seabream. The NCCRP-1 full-length sequence contains 1036 bp with an open reading frame of 702 bp. A comparison of the predicted 233-amino acid protein with several fish orthologues indicates a highly conserved sequence containing the F-box associated (FBA) domain and proline-rich motifs (PRM) characteristics of this family. The phylogenetical tree shows that seabream NCCRP-1 belongs to the NCCRP subfamily within the FBA family of proteins. This is a single copy gene with a constitutive and ubiquitous expression as determined by RT-PCR and flow cytometry. The results show that lymphocytes, monocyte/macrophages and acidophilic granulocytes from lymphoid tissues express the receptor, both at gene and protein level. Immunofluorescence microscopic observations confirm the previous results. The implications of this receptor on seabream NCC activity are discussed. q 2004 Elsevier Ltd. All rights reserved. Keywords: Cytotoxic cell; Receptor; Heterogeneity 1. Introduction Mammalian natural killer (NK) and lymphokyne- activated killer (LAK) cells are responsible for spontaneous cell-mediated cytotoxicity against tumor cells, protozoa and virus-infected cells. These cell populations are well defined, and thought to have originated from ancient nonspecific cytotoxic cells (NCC) originally evolved in lower vertebrates [1]. Since then, the evolution of cell-mediated cytotoxicity has witnessed numerous rearrangements as regards cell types, specificity, mechanisms, etc. as studies on fish NCCs consistently point out. NCCs have been studied in a number of fish species, catfish and tilapia being the best-documented cases. NCCs were first described as small and non-granular lymphocytes in Developmental and Comparative Immunology 29 (2005) 637–650 www.elsevier.com/locate/devcompimm 0145-305X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dci.2004.11.003 * Corresponding author. Tel.: C34 9683 67665, fax: C34 9683 963963. E-mail address: aesteban@um.es (M.A Esteban).