Evolution of the lymphoid system. II. Evidence for immunoglobulin determinants on all rainbow trout lymphocytes and demonstration of mixed leukocyte reaction

Resistant and susceptible rainbow trout (Oncorhynchus mykiss) lines show distinctive immune response to Lactococcus garvieae

Lactococcosis is one of the main bacterial diseases affecting rainbow trout (Oncorhynchus mykiss), with significant economic and sanitary repercussion. Vaccination and antibiotic treatments are commonly used to prevent and control the infection outbreaks; however, these strategies have some drawbacks including limited coverage, handling costs, induction of antibiotic resistance and chemical residues in the environment. Selective breeding programs represent a promising complementary approach for increasing fish disease resistance in commercial farms and some immunological parameters may be tentatively used as indirect indicators for this purpose. The present study investigated for the first time some innate and adaptive immune responses in two groups of rainbow trout derived from selected lines (susceptible and resistant) showing a different "in field" phenotypical resistance to Yersinia ruckeri, Flavobacterium branchiophilum, F. psychrophilum, and Ichthyophthirius multifiliis, after an immersion-dilution based exposure to Lactococcus garvieae carried out in controlled experimental conditions. Twenty-six resistant and twenty-six susceptible female rainbow trout (mean body weight 80 g, 9 months aged, F5 generation) were obtained from an intensive farm considered L. garvieae free and were exposed to the pathogen. Moreover, 10 resistant and 10 susceptible fish were used as uninfected controls. After 5 days, blood and tissue samples were collected for immunological analyses. A significantly higher serum and mucus lysozyme activity was recorded in resistant rainbow trout compared to susceptible fish (P ≤ 0.05), both before and after exposure to L. garvieae. Similarly, respiratory burst activity of head kidney leukocytes resulted more intense in resistant fish (P ≤ 0.05), suggesting that phagocytes could more quickly activate their microbicidal mechanisms to counteract the bacterial spread. Resistant group displayed also an up-regulation of immunoglobulins M (IgM), major histocompatibility complex II (MHC-II) and interleukin 8 (IL-8) gene expression (P ≤ 0.05) and a significantly higher blood lymphocytes count (P ≤ 0.05), highlighting their potential better ability to trigger the recruitment of defensive cells and the initiation of specific immune processes such as antigen presentation to CD4⁺ T lymphocytes and IgM synthesis. The results herein presented might be useful for the identification of immunological markers to be used as indirect indicators in rainbow trout selective breeding programs.

Medicinal plant extracts modulate respiratory burst and proliferation activity of rainbow trout (Oncorhynchus mykiss) leukocytes

The present study was designed to investigate the immunomodulatory effects of Aloe vera, Curcuma longa, Echinacea purpurea, Lavandula officinalis, Origanum vulgare, Panax ginseng, and Rheum officinale extracts on leukocytes purified from rainbow trout (Oncorhynchus mykiss) head kidney. The cells were cultured in a medium containing increasing doses of extracts; afterwards, they were tested for reactive oxygen species production after stimulation with phorbol myristate acetate (PMA) and proliferation in the presence or absence of phytohemagglutinin from Phaseolus vulgaris (PHA-P). After a 2-h exposure, the extracts of L. officinalis, O. vulgare, and R. officinale strongly reduced the oxidative burst activity of PMA-stimulated leukocytes, in a dose-dependent manner (P ≤ 0.05). A. vera, C. longa, E. purpurea, and P. ginseng extracts reduced this response with lower efficacy and especially at lower concentrations. On the contrary, the highest concentration of ginseng extract stimulated the respiratory burst of leukocytes compared to untreated control cells. After a 72-h exposure, the extracts of L. officinalis, R. officinale, C. longa, E. purpurea, and P. ginseng had a clear dose-dependent stimulatory effect on leukocyte proliferation (P ≤ 0.05). The results suggest that these medicinal plants can be considered as reliable sources of new antioxidants or immunostimulants to be used in aquaculture.

A CD4 homologue in sea bass (Dicentrarchus labrax): molecular characterization and structural analysis

CD4 is a transmembrane glycoprotein fundamental for cell-mediated immunity. Its action as a T cell co-receptor increases the avidity of association between a T cell and an antigen-presenting cell by interacting with portions of the complex between MHC class II and TR molecules. In this paper we report the cDNA cloning, expression and structural analysis of a CD4 homologue from sea bass (Dicentrarchus labrax). The sea bass CD4 cDNA consists of 2071 bp that translates in one reading frame to give the entire molecule containing 480 amino acids. The analysis of the sequence shows the presence of four putative Ig-like domains and that some fundamental structural features, like a disulphide bond in domain D2 and the CXC signalling motif in the cytoplasmic tail, are conserved from sea bass to mammals. Real-time PCR analysis showed that very high levels of CD4 mRNA transcripts are present in thymus, followed by gut and gills. In vitro stimulation of head kidney leukocytes with LPS and PHA-L gave an increase of CD4 mRNA levels after 4h and a decrease after 24h. Homology modelling has been applied to create a 3D model of sea bass CD4 and to investigate its interaction with sea bass MHC-II. The analysis of the 3D complex between sea bass CD4 and sea bass MHC-II suggests that the absence of a disulfide bond in the CD4 D1 domain could make this molecule more flexible, inducing a different conformation and affecting the binding and the way of interaction between CD4 and MHC-II. Our results will add new insights into the sea bass T cell immune responses and will help in the identification of T cell subsets in teleost fishes to better understand the evolution of cell-mediated immunity from fish to mammals.

Immunocytochemical detection of Ig-positive cells in blood, lymphoid organs and the gut associated lymphoid tissue of the turbot (Scophthalmus maximus)

The present study was designed to search for the sites of the B-cell lineage in the different lymphoid organs of turbot (Scophthalmus maximus) by immunoperoxidase staining with a rabbit polyclonal antiserum against deglycosylated turbot IgM (TUDG-6). A turbot immunoglobulin (Ig) fraction, isolated by protein A, was checked for purity by gel filtration and SDS-PAGE under reducing conditions. The turbot IgM was deglycosylated and used to raise an antiserum. The antiserum titre was evaluated in ELISA. It was then used to analyse turbot peripheral blood leucocytes for membrane and cytoplasmic Ig and for immunohistochemistry with turbot lymphoid tissues. Very low numbers of Ig+ cells were found in thymus sections. In sections of spleen, Ig+ cells were observed in white pulp, around ellipsoids but were mostly concentrated and associated with melanomacrophage centers (MMCs). The lymphoid Ig+ cells in the kidney tended to be dispersed among haematopoietic and granulopoietic cell populations and were in intimate association with the MMCs and blood vessels. This association between MMCs and Ig+ cells in the spleen and the kidney, is discussed with respect to the role played by these organs in the immune system of fish. Last, the lymphoid population in the gut associated lymphoid tissue (GALT) of turbot was characterised with respect to staining for Ig. Immunoreactive cells were rarely detected in the epithelial layer although many lymphocytes were present, but they were frequently observed in the lamina propria, presumably as part of the GALT and involved in mucosal immune responses.

Cellular requirements for mixed leucocyte reactivity in the cyclostome, Eptatretus stoutii

Unfractionated peripheral blood leucocytes from allogeneic hagfish proliferated vigorously in one-way mixed leucocyte reaction (MLR) at 20-22 degrees. Maximum responsiveness was observed after 5 days of co-culture of responder and irradiated stimulator cells. Separation of leucocytes on the basis of laser scatter parameters yielded a small leucocyte population which responded but did not stimulate in MLR and a large leucocyte population capable of stimulating but not responding. Adherent cell depletion of the total leucocyte population did not affect the ability to respond in MLR but did ablate the stimulatory capacity of hagfish leucocytes. Together with previous phenotypic studies, these results confirm the presence of lymphocyte and macrophage/accessory cell populations in hagfish peripheral leucocytes.

Evolution of antibody structure and effector functions: comparative hemolytic activities of monomeric and tetrameric IgM from rainbow trout, Salmo gairdnerii

Monomeric and tetrameric IgM anti-haptin antibodies isolated from the sera of rainbow trout (S. gairdnerii) by immunoaffinity chromatography were compared both immunochemically and with regard to their functional abilities to lyse haptenated trout erythrocytes in the presence of trout complement. The two populations had similar binding affinities for hapten and apparently identical L chains, but differed in H chain peptide maps and immunoreactivity with rabbit anti-trout H chain serum. These differences could not be attributed to J-chain. The abilities of the two antibody subpopulations to activate C to lyse haptenated trout erythrocytes also differed dramatically. Such functional differences are not simply explained by the greater avidity of the tetrameric form since preliminary studies show that the monomeric form of trout IgM activates C via an alternative pathway mechanism while the tetrameric form activates both classical and alternative pathway mechanisms. Results suggest divergent evolution of antibody structures involved in the familiar effector functions (C activation, transport, etc.).

Nonspecific cytotoxic cells in fish (Ictalurus punctatus). I. Optimum requirements for target cell lysis

Nonspecific cytotoxic cells (NCC) obtained from the head (anterior) kidney of fish (Ictalurus punctatus) lyse human transformed B-cell targets. Lysis depended on direct cell-cell contact. Fish size, age, environmental holding temperatures, and lytic reaction conditions such as osmolality and optimum effector:target cell ratios were optimized. Experiments to characterize optimum kinetics demonstrated highly efficient killing after two hours incubation. This rapid cytolysis was further studied by determining NCC activity against appropriately labeled target cells after 30, 60, 90 and 120 minutes of cocultivation. At 160:1 (E:T) greater than 40% of the 5 hour percent specific release value was produced after 30 minutes. After 90 minutes, more than 90% of total percent specific release was observed. At least one mechanism of regulation of NCC killing was described. In the presence of normal (homologous or heterologous) catfish serum (CFS), essentially no NCC activity was observed. This suppression was reversible by preincubation in 10% fetal bovine serum (FBS). NCC "activation" by preincubation in 10% FBS was time-dependent (at least four hours was required to generate significant lysis). NCC activation could be reversed by treating potentially lytic cells with supernatants containing dissociated CFS. In addition, reversible activation could be demonstrated by treating potentially lytic effector cells with CFS to produce suppression. Regulation occurred at the effector cell level because treated target cells did not suppress NCC activity. These data demonstrate a population of nonspecific effector cytolytic cells that potentially represent a phylogenetic precursor to mammalian natural killer cells.

Surface immunoglobulin on granular and agranular leukocytes in the thymus and spleen of the snapping turtle, Chelydra serpentina

Spleen and thymus suspensions from the turtle, Chelydra serpentina were examined by indirect immunofluorescence and found to contain both agranulocytes and granulocytes positive for surface Ig. Among the splenic agranulocytes , 50% of the lymphocyte and monocyte population was positive for surface Ig. In the thymus only 7% of the thymocytes were positive. Ninety-two percent of the granulocytes, composed primarily of basophils and eosinophils, were positive for surface Ig in both the thymus and spleen. The presence of surface immunoglobulins on turtle splenic leukocytes was confirmed by sheep red blood cell (SRBC) immunization studies. Basophils, lymphocytes and monocytes from immunized turtles formed SRBC rosettes, while eosinophils from immunized turtles were found to specifically phagocytose SRBC. Splenic leukocytes from control turtles did not phagocytose or rosette with SRBC. This study demonstrates that indirect immunofluorescent techniques can be used to identify surface immunoglobulin on turtle granulocytes as well as agranulocytes .

The mixed leukocyte reaction (MLR) in carp: bidirectional and undirectional MLR responses

Two-way and one-way mixed leukocyte reaction (MLR) was demonstrated in peripheral blood leukocytes (PBL) of carp. Primary two-way MLR in randomly selected donor pairs were highly variable. Weak primary responses could be strongly augmented by mutual in vivo priming of the reacting donors. One-way MLR was performed using irradiated (16,000 R) allogeneic PBL as stimulators. Reciprocal responses of randomly paired donors were usually unequal, suggesting the usefulness of this method for genetic analysis of MLR-recognized histocompatibility antigens in carp. Kinetics of the primary and secondary two-way MLR were studied, as well as the kinetics of primary one-way MLR against pooled allogenic stimulator cells.

Heterogeneity of mitogen-responsive lymphocytes in carp (Cyprinus carpio)

Peripheral blood leukocytes (PBL) of carp respond in vitro to a variety of phytomitogens, shown to be T-cell specific or B-cell specific in mammalian systems. Some basic differences have been observed in the proliferative response of carp PBL to PHA (phytohemagglutinin), ConA ( concanvalin A) and LPS (lipopolysaccharide): (1) The response to PHA and ConA was found to be highly dependent upon the continuous presence of mitogen in the medium, in contrast to LPS, where after the initial stimulation, cells could continue to proliferate for several days without mitogen. (2) Lymphoblasts grown in long term culture with either PHA or Con A could be transferred into medium containing the other mitogen without impairing cell proliferation, but cell growth was reduced to background level following transfer into LPS-containing medium. LPS grown cells continue to proliferate independently of the mitogen content of the medium. (3) Co-stimulation with LPS+PHA or LPS+ConA results in a synergistic response, while co-stimulation with PHA+ConA results in inhibition of DNA synthesis. (4) Several morphological differences have been observed between cells proliferating in the presence of PHA and those proliferating in the presence of LPS. It is suggested that while the PHA and ConA responsive cells may belong to the same lymphocyte subpopulation, they are distinct from the LPS-responsive subpopulation.

Lymphocyte heterogeneity in the trout, Salmo gairdneri, defined with monoclonal antibodies to IgM

A monoclonal antibody to trout serum IgM was tested by immunofluorescence analysis with lymphocytes from thymus, spleen and head kidney. By visual examination, the antibody reacted with only a subpopulation of lymphocytes. The mean values +/- SE for positive cells were 5.2 +/- 2.3% in the thymus, 30.3 +/- 7.9% in the spleen and 12.4 +/- 3.0% in the head kidney. Flow cytofluorometric analysis revealed evidence of heterogeneity by size among the membrane IgM-positive cells of the head kidney and spleen. Depletion of head kidney cells positive for surface IgM by an immune affinity adherence technique of panning, using monoclonal anti-IgM, significantly reduced the mitogenic response to lipopolysaccharide but not to concanavalin A. It is suggested that this information supports the existence of distinct subpopulations of fish lymphocytes that may be homologous in certain respects to mammalian T and B type cells.

Rainbow trout lymphoid organs: cellular effects of corticosteroids and anti-thymocyte serum

Cellular populations were studied in lymphoid organs of rainbow trout. Kidney appeared to be the most important organ by the number of cells harvested. A high percentage of peroxidase positive cells was measured in kidney, spleen and blood. The use of cortisone and rabbit anti-thymocyte serum showed differences in ratio of sensitive cells from different organs. In fish treated with steroids, cell depletion was verified in each organ but histologic lesions were observed only in thymus. Rabbit anti-thymocyte serum was cytotoxic for the quasi-totality of thymocytes while it destroyed only a part of kidney, spleen and blood cells. No correlations could be made between steroid sensitive cells and anti-thymocyte serum sensitive cells.