IMMUNOHISTOCHEMICAL DEMONSTRATION OF CD35 AND CD16 RECEPTORS IN DEVELOPMENTAL STAGES OF SEA BASS (D. LABRAX)

The study of teleosts immune system is useful to better understand its involvement inthe prevention of infectious diseases. Additionally, such studies are important from aphylogenetic point of view, since fish are the first animals that, in the evolutionaryprocess, show aspects of innate and adaptive immune response comparable to thosepresent in higher vertebrates. In mammals, phagocytosis is a prerogative ofmacrophages and neutrophil granulocytes. At the moment, some fish cell populationsinvolved in this activity are still under investigation, due to the evident morphologicalheterogeneity of leucocytes among different species and to the lack of specific surfacemarkers for fish phagocytes. A crucial role in the phagocytosis activation can beattributed to the membrane receptors for complement and immunoglobulin Fc. Inmammals four types of receptors for C3 have been identified, namely CR1, CR2, CR3and CR4. CR1 (CD35) is expressed by neutrophils, eosinophils and some B and Tsub-populations. It promotes the link to and phagocytosis of particles opsonised byC3b and C4b fractions. FcR" (CD16) is expressed by several hemopoietic cells(macrophages and lymphocytes) and its role is fundamental for the cellular immuneresponse mediated by immunoglobulins (removing of immune-complexes oropsonised particles by phagocytosis). Investigations dealing with the identification ofthese receptors in teleosts are recent and concern a limited number of species:rainbow trout, catfish, carp, zebrafish. These studies have mainly focused on aspectsof CR1 and FcR phylogenesis, as well as structural/functional homologies withmammalians. From the literature there is no evidence of investigations dedicated tothe identification of CR1 and FcR in marine teleosts, and also there are no dataregarding the immunohistochemical localization of cells presenting these CDs. Thepresent paper is aimed to the demonstrations of CD16 and CD35 immune-receptors intissues of sea bass (Dicentrarchus labrax) at different developmental stages.Histological sections obtained from tissue specimens collected from sea bassfingerlings were submitted to immunohistochemical analysis by use of the followingantibodies: goat polyclonal to CD35 (CR1); rabbit polyclonal to CD16 (FcR). Thereaction was developed by ABComplex peroxidase and DAB. As a result of theantibodies employed, even if specific for mammalian receptors, allowed the detectionof cell populations in different tissues, and also the chronology of appearance wasdescribed. In synthesis, CD16 was detectable in thymus, gills and skin starting from50dph, and in the digestive system starting from 90 dph. CD35 was detectable inseveral organs (thymus, gills, pancreas, skin, stomach, gut) starting from 47 dph, andlater in spleen and pyloric caeca. Moreover, the morphology of CD16+ and CD35+cells colonizing various tissues was described, allowing the discrimination of at leastthree different cell populations: CD16+ cells, morphologically similar to macrophagesor dendritic cells (APCs), with non granular cytoplasm; CD35+ cells,morphologically similar to macrophages or dendritic cells (APCs), with non granularcytoplasm; CD35+ cells, morphologically similar to eosinophilic granulocytes orEGCs, with granular cytoplasm. These populations were particularly consistent inthymus (among the primary lymphoid organs), skin and digestive tract (mucosaeassociate lymphoid tissue). Based on their morphology and specific localization, ahypothesis concerning their role as phagocytes and antigen presenting cells will bediscussed.[...]