Study of the hooking behaviour of Lepeophtheirus salmonis (Krøyer, 1837) copepodids on Atlantic salmon, Salmo salar L., using a novel in vivo test system

The screening of putative semiochemicals involved in the interaction between Lepeophtheirus salmonis copepodid and the Atlantic salmon could be used to develop methods to prevent infestation. A bioassay was designed to evaluate the attractiveness of Atlantic salmon for L. salmonis copepodids by counting the number of hooked parasites. A salmon was bathed in a tank with 60 copepodids during 45 min. The salmon was humanely killed and its body was scrubbed above a plastic bag. The content of the bag was filtered to count the number of hooked parasites. This procedure was tested at different water temperatures and for different body mass. The temperature significantly influenced the number of hooked copepodids (p < .05). Smolts presented significantly more hooked copepodids at a higher water temperature (24.6 at 13.8°C) compared to smolts at a lower temperature (18.6 at 6.1°C; p = .011). No correlation was found between the body mass and the number of hooked parasites (p = .65). This bioassay was used to successfully measure the attractiveness of Atlantic salmon for L. salmonis copepodids, making it an interesting tool to screen putative semiochemicals designed to prevent the infestation.

Human thymic dendritic cell-thymocyte association: ultrastructural cell phenotype analysis

In rodent thymus, associations between dendritic cells (DC) and thymocytes have been suggested to be implicated in differentiation and/or maturation processes. In this study, we report intimate associations formed between human thymic DC and thymocytes in culture and we analyze their ultrastructural cell phenotype. Observations by phase contrast microscopy showed that DC present long and thin dendrites and bind many thymocytes. Transmission (TEM) and scanning electron microscopy (SEM) revealed that both cellular populations were in close connection and tight membrane contact could be observed. The phenotype of DC and attached thymocytes was characterized with a series of monoclonal antibodies by protein A-gold TEM and SEM immunolabelings. Quantitative evaluation of immunolabeling (number of gold granules/microns of cellular membrane) suggests the presence of two subpopulations of CD1+ thymic DC (strong and weak), whereas this discrepancy is not observed in DR+ and CD4+ DC populations. On the other hand, the majority of thymocytes bound to DC strongly express the CD1, CD4, CD8 and CD2 antigens and weakly the CD3 antigen, indicating that they represent double-positive immature thymocytes. Uniform distribution of DC and thymocytes membrane antigens was confirmed with a backscattered SEM study. This morphological and immunolabeling TEM and SEM analysis demonstrates that human thymic DC may form associations with CD4+CD8+CD3weak thymocytes and raises questions about their physiological relationship.

Human thymic dendritic cells. Characterization, isolation and functional assays

The phenotypic analysis of human thymic dentritic cells (DC) in culture and in purified suspensions has been studied with light and electron microscopic (EM) immunolabelling techniques. Using a series of monoclonal antibodies (mAb) and a protein A-gold technique, we demonstrated that DR- and T6-positive cultured DC strongly bind the 9.3F10 mAb, an anti-DR-related antibody produced against human blood DC, and weakly express the T4 antigen, a membrane marker shared by Langerhan's cells (LC). On the other hand, thymic-cultured DC are negative for the other T-cell and monocyte-macrophage antigens. These results support the hypothesis that human thymic DC may be related to blood DC and epidermal LC. Moreover DC, unlike thymic macrophages, do not phagocytose latex particles, opsonized sheep red blood cells (SRBC) or Candida albicans. An efficient two-step technique of isolation, using a Percoll density gradient followed by an indirect panning technique, yields a purified (70-80%) thymic DC population, OKIa1-, 9.3F10- and OKT6-positive and esterase-negative. Immunolabelling and electron microscopy confirm that these isolated DC present similar phenotypic and ultrastructural features to human thymic DC in situ and in culture. Purified DC, used as stimulator cells in mixed leucocyte reaction (MLR), induce stronger proliferative responses than peripheral blood monocytes used as a control; blocking assays with OKIa1 mAb plus complement greatly reduced this stimulatory potency. These functional assays demonstrate that we obtained a purified typical DC population that can be used in immunological functional studies to elucidate the specific role of DC in human thymus.

Demonstration of OKT6 antigen on human thymic dendritic cells in culture

In situ, two types of dendritic cells (DCs) have been characterized in more detail: the interdigitating DCs of peripheral lymphoid tissues and the epidermal Langerhans cells. In order to characterize human thymic DCs, normal human thymus specimens were obtained from children undergoing cardiovascular surgery. In culture, DCs could be easily differentiated from macrophages and epithelial cells by their long fine processes, irregular nucleus, and dark, membrane-bound granules. By immunoelectron microscopy, using the protein A-gold and the avidin-biotinperoxidase complex techniques, cultured DCs were strongly labeled with anti-Ia and anti-OKT6 monoclonal antibodies. These results demonstrate that the phenotype of thymic DCs is similar to that of epidermal Langerhans cells and suggest that they may belong to the same cellular lineage.