Full length article Influence of moderate hypoxia on vaccine efficacy against Vibrio anguillarum in Oreochromis niloticus (Nile tilapia) Sanchala Gallage a , Takayuki Katagiri a , Makoto Endo a , Kunihiko Futami a , Masato Endo b , Masashi Maita a, * a Laboratory of Fish Health Management, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato, Tokyo 108-8477, Japan b Laboratory of Aquaculture, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato, Tokyo 108-8477, Japan article info Article history: Received 2 December 2015 Received in revised form 18 February 2016 Accepted 18 February 2016 Available online 23 February 2016 Keywords: Cichlidae Dissolved oxygen Vaccination Antibody titer Bacterium abstract Hypoxia is known as a potential immunomodulator in fish. This study therefore assesses the impact of chronic, moderate hypoxia on vaccine efficacy in Oreochromis niloticus. Serum antibody titer was used as a surrogate marker to detect vaccine efficacy. The fish were acclimatized to either moderate hypoxia (55 ± 5% DO) or normoxia (85 ± 5%DO) and immunized with formalin inactivated Vibrio anguillarum. Significantly, a higher antibody titer was found in normoxic fish than in moderate hypoxia. The normoxic group titer peaked at 14th dpv (days post vaccination) while the moderate hypoxic group peaked at 21st or 28th dpv. The absolute blood lymphocyte counts and serum bactericidal activities against V. anguillarum were significantly higher in normoxic fish. Serum killing of V. anguillarum appeared to be mainly via antibody-dependent classical complement pathway. Furthermore, the first week following vaccination appears critical for antibody production. This view was further supported by results obtained from gene expression assay, where the transcription level of all the detected immune related genes (IgM, IL-1 b, TCR-b, MHC-II b), except B cell activating factor, were significantly suppressed following exposure to moderate hypoxia. The overall results highlight that even though moderate hypoxia is not easily detectable in Oreochromis niloticus, it negatively affects antibody production by suppressing and delaying antibody response, ultimately affecting vaccine efficacy. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Intensive fish culture operations often result in enhanced out- breaks of infectious diseases. Huge economic losses from mortality or reduced product quality of infected fish affect sustainability of the industry worldwide [1]. To overcome this challenge, preventive and control measures are practiced and vaccination has proven the most promising and environmentally-friendly management strat- egy for many existing infectious diseases. Vaccines induce specific immune system in fish to produce specific antibodies, memory B and T cell components against target pathogens or pathogen- associated molecules, and thus offer long-lasting protection [2e4]. As in mammals, vaccine effectiveness in fish is regulated by several factors, including vaccine-derived factors, administration route, endogenous factors within the fish and exogenous factors, such as nutrition, marginal or stressful environmental conditions [5e7]. Optimum environmental factors in aquaculture systems play an important role in health of cultured organisms. Dissolved oxygen (DO) is one of the most critical aquatic environmental factors, since water contains relatively less oxygen compared to the atmosphere [8]. Hypoxia is the condition where oxygen concentration in water is measurably low [9] and it seems to be frequent episode in aquaculture systems. There are many factors promoting hypoxia: decomposition, accumulation of organic matter through faeces and unconsumed food and high stocking density all result in increased biological oxygen demand within the system. Despite the fact that physiological threshold oxygen level varies considerably among fish species [10], if hypoxia is strong enough to alter homeostasis, energy reserves are diverted to maintain homeostasis. Ultimately, growth, reproduction, im- munity and other energy demanding activities may be adversely affected [11e 13]. * Corresponding author. E-mail addresses: sanchalashyama@gmail.com (S. Gallage), takakata@kaiyodai. ac.jp (T. Katagiri), fishendo@kaiyodai.ac.jp (M. Endo), futami@kaiyodai.ac.jp (K. Futami), asteroid@kaiyodai.ac.jp (M. Endo), mmaita@kaiyodai.ac.jp (M. Maita). Contents lists available at ScienceDirect Fish & Shellfish Immunology journal homepage: www.elsevier.com/locate/fsi http://dx.doi.org/10.1016/j.fsi.2016.02.024 1050-4648/© 2016 Elsevier Ltd. All rights reserved. Fish & Shellfish Immunology 51 (2016) 271e281