Changes in Peripheral Blood Leukocyte Percentages and Function of Neutrophils in Stressed Channel Catfish

Abstract

Channel catfish Ictalurus punctatus were subjected to a simulated handling (including bleeding) and transport stress. Using flow cytometry in conjunction with monoclonal antibodies to channel catfish B and T lymphocytes (minimal cross-reaction with thrombocytes) and neutrophils coupled with routine hematological methods, we demonstrated significant decreases in the percentage of B lymphocytes and increases in T lymphocytes and neutrophils of transported fish, yet there was no change in neutrophil phagocytic function. These data suggest that the length of the stressing event or the duration of exposure of the neutrophils to cortisol could determine if immune responses become suppressed. These data prompted us to expose channel catfish in vivo, and isolated neutrophils in vitro, to cortisol at various concentrations and for various time periods. Exposure of neutrophils to cortisol concentrations of 100 μg/dL for 2 h did not suppress phagocytosis; however, significant decreases in the percent phagocytosis and bacterial killing and increases in total number of neutrophils isolated were obtained after repeated in vivo administration of cortisol. The results of these experiments indicate (1) that bleeding, handling, and transport of channel catfish induce a characteristic relative lymphopenia and neutrophilia, (2) that cortisol does not act alone to induce suppression of phagocytic function (based on the in vitro administration of cortisol), and (3) that high physiological concentration of cortisol in vivo can initiate phagocytic suppression. In general, the intraperitoneal cortisol injection experiments suggested that stress on channel catfish that produces a concomitant high serum concentration of cortisol can suppress neutrophil phagocytic function, perhaps providing an avenue for the onset of infection.