Disturbance of the intestinal mucosal immune system of farmed Atlantic salmon (Salmo salar), in response to long-term hypoxic conditions L. Niklasson a, * , H. Sundh a , F. Fridell b , G.L. Taranger c , K. Sundell a a Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, University of Gothenburg, P.O. Box 463, S-405 30 Gothenburg, Sweden b PHARMAQ AS, P.O. Box 267 Skøyen, Oslo, Norway c Institute of Marine Research, P.O. Box 1870 Nordnes, 5817 Bergen, Norway article info Article history: Received 12 March 2011 Received in revised form 29 August 2011 Accepted 9 September 2011 Available online 21 September 2011 Keywords: Cytokines Neutrophils Chronic intestinal inflammation Gene expression Immunohistochemistry abstract The gastrointestinal (GI) tract has many important biological functions. One is to serve as a barrier between the fish and the external environment. A decreased physical barrier function of the intestine may lead to increased inflow of luminal content and subsequent activation of the intestinal mucosal immune system. This activation is governed by the ability of various compounds to induce cytokine release and immune cell activity, leading to an immune response. In mammals, the impact of stress on the intestinal barrier is well documented and results in increased intestinal permeability and thus increased stimulation of the mucosal immune system. Fish reared in sea cages may at times be exposed to unfavourable environmental conditions leading to chronic stress and disturbed intestinal integrity. This change in permeability may increase the exposure of the mucosal immune system to activating compounds. In the present study, the effect of a prolonged stress on the intestinal mucosal immune system of fish is therefore addressed. Atlantic salmon were exposed to low levels (50%) of dissolved oxygen (DO) for 6e7 weeks in consecutive experiments performed at 8 and 16  C. Immune parameters were assessed in terms of mRNA expression of the key cytokines, interleukin-1b (IL-1b), IL-8, IL-10, interferon-g (IFNg) and transforming growth factor-b (TGFb) as well as the immune regulatory inhibitor of nuclear factor kB (IkB). In the experiment at 8  C also mucosal neutrophil infiltration was monitored. Subjecting the fish to low DO levels at 8  C resulted in an increased mucosal neutrophil infiltration together with a down-regulation of IkB. At the higher temperature, 16  C, low DO levels created decreased expression of the pro-inflammatory cytokine IL-1b in both intestinal regions as well as an increased expression of IL-10 in the proximal intestine. These results suggest that husbandry conditions in sea cages with DO levels as low as 50% clearly affects the intestinal mucosal immune system and results in a chronic inflammation. Moreover, the effects of low DO levels on the immune factors examined were more pronounced in the 16  C experiment suggesting additive effects of high temperatures. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The intestine is an important organ for nutrient absorption, ion and water transport as well as a primary barrier that prevents entrance of harmful agents. The intestinal mucosa is continuously exposed to a broad spectrum of antigens derived from the ingested food, water, the commensal flora as well as pathogens. Therefore, the intestinal barrier function is of outmost importance to maintain health and welfare of the fish. The outer (extrinsic) part of the barrier consists of a mucus layer with antibacterial compounds such as lysozymes, complement factors and antibodies. The intrinsic part, made up by the epithelial cells and the tight junc- tions, constitutes the rest of the physical barrier and is the major regulator of the intestinal permeability. Located in and beneath the physical barrier is the immune barrier which includes the immune cells. The mucosal immune system is activated through receptor recognition mechanisms of the host and through antigen sampling in the epithelium with subsequent antigen presentation to the mucosal acquired immune system [1e3]. In the mammalian intestine, the mucosal immune system is highly organized in specialized regions like the Peyer’s patches and lymphoid follicles [4]. Antigen presenting cells i.e. M-cells and dendritic cells, together with the underlying leucocytes, enables an integration of innate and adaptive mechanisms [5,6]. The mucosal immune system of fish is often described as a more diffuse set of cell clusters scattered * Corresponding author. Tel.: þ46 31 786 36 28; fax: þ46 31 786 38 07. E-mail address: lars.niklasson@zool.gu.se (L. Niklasson). Contents lists available at SciVerse ScienceDirect Fish & Shellfish Immunology journal homepage: www.elsevier.com/locate/fsi 1050-4648/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fsi.2011.09.011 Fish & Shellfish Immunology 31 (2011) 1072e1080