Short-term exposure of Chinook salmon (Oncoryhnchus tshawytscha) to o,p-DDE or DMSO during early life-history stages causes long-term humoral immunosuppression

We evaluated the effect of short-term exposures to a xenobiotic chemical during early life-history stages on the long-term immune competence of chinook salmon (Oncoryhnchus tshawytscha). Immersion of chinook salmon eggs in a nominal concentration of o,p-dichlorodiphenyldichloroethylene (o,p-DDE; 10 ppm) for 1 hr at fertilization followed by immersion in the same dose for 2 hr at hatch resulted in a significant reduction in the ability of splenic leukocytes from fish 1 year after treatment to undergo blastogenesis upon in vitro stimulation with lipopolysaccharide. We also observed that the vehicle, dimethyl sulfoxide (DMSO), caused a significant reduction in the ability of the splenic leukocytes to express surface immunoglobin M (SIgM) at this time. The concentration of o,p-DDE in a pooled sample of whole fry from this treatment was 0.53 microg/g lipid 1 month after first feeding but was undetectable in all other treatments. Mortality rate, time to hatch, fish length, and weight were unaffected by treatment with o,p-DDE. Similarly, sex ratios, gonadal development, and concentrations of plasma estradiol and 11-ketotestosterone were not affected by the treatment. In addition, we found no evidence that plasma lysozyme concentrations or the mitogenic responses of splenic leukocytes to concanavalin A or polyinosinic-polycytidylic acid were influenced by the treatment. In this experiment, a brief period of exposure to o,p-DDE or DMSO during early development was able to induce long-term effects on humoral immune competence of chinook salmon. Such immunosuppression may increase susceptibility to disease, which may in turn be critical to regulating the population.

In vitro detection of functional humoral immunocompetence in juvenile chinook salmon (Oncorhynchus tshawytscha) using flow cytometry

A flow cytometric (FCM) assay for detection of immunomodulatory effects of environmental factors on the humoral response of chinook salmon (Oncorhynchus tshawytscha) is described and validated. This technique combines exposure of whole animals or leucocyte cultures to immunomodulatory agents/conditions with in vitro mitogenic activation of B-lymphocytes. The proportion of leucocytes undergoing blastogenesis following in vitro stimulation with lipopolysaccharide (LPS) is quantified by FCM analysis of forward and side scatter properties. In addition, binding of a fluorescein isothiocyanate labelled anti-rainbow trout immunoglobulin M monoclonal antibody (anti-RBT SIgM-FITC), quantified by FCM analysis, is used to determine the ability of the lymphoblasts to express surface immunoglobulin M (SIgM). Through a series of calibration steps, it was confirmed that anti-RBT IgM-FITC was specific for B-lymphocyte SIgM in chinook salmon. Binding of anti-RBT IgM-FITC to chinook salmon SIgM positive leucocytes was effectively blocked with salmon serum and an isotype control was established. B-lymphocytes were partially removed from a population of leucocytes through adherence to a nylon wool column, which then demonstrated a consequent reduction in anti-RBT IgM-FITC binding. Using anti-RBT IgM-FITC as a marker, the distribution of resting lymphocytes expressing SIgM in lymphoid tissues of juvenile chinook salmon was described. The mean percentage of SIgM positive cells in spleen, pronephros and blood were found to be 62.1 (+/-2.82), 34.8 (+/-1.86) and 56.7% (+/-4.7) of all viable leucocytes, respectively. In a time-course experiment for optimal in vitro activation of leucocytes for this assay, blastogenesis and up-regulation of SIgM expression of splenic leucocytes were observed through FCM by 4 days post in vitro stimulation with LPS, continued through 7 days, but was no longer visible by 10 days post stimulation. Using this assay, reduced expression of SIgM in splenic and pronephric B-lymphocytes was detected following in vitro exposure to physiologically relevant stress concentrations of cortisol in conjunction with mitogenic stimulation. This technique will be a useful addition to the assays already available in the rapidly growing field of fish immunology.