Comparative immunological development and responses in lower vertebrates: Sturgeon

Sturgeon are harvested worldwide for their meat and eggs, sold as caviar and are among the most economically important fish species worldwide. A generally low physiological response to external stimuli/stressors has been documented in these phylogenetically primitive species.;Considering how little is known about sturgeon immune responses, it is important to study how they respond to such stimuli/stressors. The culture of sturgeon larvae is associated with a high mortality rate during and after yolk sac absorption and onset of feeding. Therefore, it is relevant to study the immune competence of larvae and juvenile sturgeon and determine when their immune organs first appear and how they develop. The meningeal myeloid tissue, the spleen and the thymus were characterized morphologically by light microscopy (LM) and transmission electron microscopy (TEM) in Atlantic sturgeon ( Acipenser oxyrinchus oxyrinchus) during the first 5 months of life (until 2895 growing degree days, °C.day (dd)). In sturgeon, the spleen was first visible approximately 400 dd after the onset of feeding (at 541 dd) and the meningeal myeloid tissue and the thymus at 768 dd after the onset of feeding; these first appeared between 541 dd and 768 dd, respectively. Heterophil and eosinophil percentages were significantly greater in the meningeal myeloid tissue of larger fish (950 and 2895 dd, respectively) when compared with the smaller fish. The percentages of reticular cells were significantly higher in the smallest fish than in the larger fish and the undifferentiated cells were higher in the youngest animals compared to the older age groups. Likewise, the splenic heterophil percentages were significantly higher in the oldest fish (2895 dd) compared to the youngest fish (768 dd). The splenic undifferentiated cell percentages were significantly higher in the younger fish compared to the oldest. This emphasizes less developed immune organs in the smaller animals. Lymphocyte percentages did not change over time in these tissues. In the thymus, lymphocytes were the predominant cell type and only the undifferentiated cell percentages were significantly different and highest in the oldest group, possibly reflecting a more proliferative thymus in these animals.;In order to characterize changes in cell populations in relevant immune organs, a temperature trial was done in juvenile shortnose sturgeon ( Acipenser brevirostrum), kept at 11°C or at 20°C. Relevant immune organs such as the meningeal myeloid tissue, the spleen, the thymus and the skin were studied. A higher temperature had a significant positive influence on the immune cell production in the meningeal myeloid tissue (higher lymphocyte, eosinophil and heterophil percentages). Likewise, temperature resulted in an initial but transient increase in splenic white pulp percentage. A similar effect was seen for splenic expression of interferon regulatory factors 1 and 2 (IRF-1 and IRF-2) and no changes occurred in matrix metalloproteinase 9 (MMP-9) expression for both spleen and skin. IRF-1 and IRF-2 expression in the skin were significantly higher expression in the fish kept at 11°C at 3 and 6 weeks post temperature change. There were no differences in thymus size between the different groups. Further research is required to better characterize the nature of these immune responses in these organs.;These findings suggest that an effective immune response may not occur in early life stages prior and during the onset of feeding but further research is needed to further assess this. Rearing sturgeon juveniles at higher temperatures (up to 20°C) may be beneficial for sturgeon aquaculture, since it maximizes growth rates and positively influences immune cell percentages in relevant immune organs.