Modulation of Immune Function Parameters in Fish Caused by Sudden Environmental Changes

Abstract

Three related projects were conducted using multiple tiered bioassays to determine the effects of acute changes in salinity, low oxygen saturation (hypoxia), or triamcinolone on immune function in teleost (bony) fish using the well characterized tilapia fish model.

We hypothesized that sudden changes in the aquatic environment, specifically salinity and dissolved oxygen, would compromise immune function in teleost fish leading to alterations of cytokine expression, namely increase in TGF-β and decrease in IL-1β; suppression of phagocytosis; and adverse changes in plasma chemistry and hematology. Further, we hypothesized that these immune system changes would be similar to those caused by administration of triamcinolone. Our overall hypothesis was that the total immunocompromise caused by rapid environmental changes leads to increased susceptibility to diseases such as epizootic ulcerative syndrome (EUS).

To model the rapid environmental changes often experienced by fish in North Carolina estuaries, tilapia (Oreochromis niloticus) were exposed under controlled laboratory conditions to either acute salinity changes, acute drop in dissolved oxygen, or intraperitoneal injection of triamcinolone (positive control). Tiered bioassays of increasing specificity for immune function were performed on tissue samples: Tier I, histopathology; Tier II, hematology, plasma biochemistry, and plasma cortisol; Tier III, phagocytosis of peripheral blood leukocytes (PBL), and mRNA expression of transforming growth factor-β (TGF-β) & interleukin-1β (IL-1β) splenic mononuclear cells using real time PCR.

While histopathology was a useful tool for assessing overall health of the test fish, no remarkable lesions were found in that could be attributed to any of the three stressors. Triamcinolone administration (i.p., 10mg/kg) for 3 days induced lymphopenia, neutrophilia with overall leukopenia, and monocytosis. Hematocrit and circulatory red blood cell count did not change with triamcinolone administration nor with acute decrease in salinity. Increase in salinity (5-20 ppt) had no effects on any of the aforementioned parameters. Hypoxia induced lymphopenia and neutrophilia without change in monocytes and additionally inhibited values of PO2, SO2, total protein, and lipase in exposed fish. Decrease in salinity (20-5 ppt) did not affect plasma cortisol levels, nor did triamcinolone administration or hypoxia treatment. Plasma protein level was not changed in response to decrease in salinity or triamcinolone administration. Triamcinolone administration suppressed phagocytosis of peripheral blood leukocytes (PBL) without a significant change in phagocytic index (P.I). Decrease in salinity elevated phagocytosis without change in P.I. in exposed tilapia. Increase in TGF-β transcription and decrease in IL-1β transcription with in vitro LPS stimulation were identified after triamcinolone administration. Acute decrease in salinity upregulated TGF-β transcription, whereas IL-1β transcription without in vitro LPS stimulation was not detectable in exposed fish. During hypoxia it was observed that changes in phagocytosis and TGF-β transcription were correlated negatively and that changes in IL-1β transcription and phagocytosis correlated positively.

Taken together, these controlled laboratory experiments suggest that rapid changes in the aquatic environment can lead to increased disease susceptibility via compromise of the fish immune system. In these studies, this immunocompromise was especially evident in the modulation of the inflammatory mediators TGF-β and IL-1β, as well as in shifts in blood leukocyte distribution. It is likely that such perturbations of teleost immune system parameters have much wider implications, including in North Carolina estuaries.