Short Communication Efficacy of α-enolase-based DNA vaccine against pathogenic Streptococcus iniae in Nile tilapia (Oreochromis niloticus) Pattanapon Kayansamruaj a , Ha Thanh Dong a , Nopadon Pirarat b , Dachrit Nilubol a , Channarong Rodkhum a, ⁎ a Department of Veterinary Microbiology, Faculty of Veterinary Science, Chulalongkorn University, Bangkok, Thailand b Wildlife, Exotic and Aquatic Pathology - Special Task Force for Activating Research, Department of Pathology, Faculty of Veterinary Science, Chulalongkorn University, Bangkok, Thailand abstract article info Article history: Received 4 September 2016 Received in revised form 29 September 2016 Accepted 3 October 2016 Available online 03 October 2016 Streptococcus iniae (SI) is an important pathogenic bacterium causing severe mortality in farmed fish worldwide. In the current study, the α-enolase-based DNA vaccine was constructed and examined for its effectiveness against SI infection in Nile tilapia (Oreochromis niloticus). The juvenile tilapia were immunized intramuscularly with DNA vaccine, pEno, and kept for 30 days prior to the intraperitoneal challenge with 2.7 × 10 7 CFU of path- ogenic SI. At two weeks post challenge, the pEno group yielded the highest survival rate at 72.5%, whereas mock vaccination and negative control groups gained only 40 and 25%, respectively. The protection of vaccine tended to be related to the expression of immune-relevant genes (IL-1β, TNF-α, COX-2, IL-12β and IL-13Rα1) at 7 day post- vaccination (dpv) and the anti-SI serum antibody level at 30 dpv (before in vivo challenge). This study indicated that pEno was able to elicit immune responses and conferred protection against streptococcosis associated with SI infection in Nile tilapia. © 2016 Elsevier B.V. All rights reserved. Keywords: α-Enolase DNA vaccine Immune gene expression Nile tilapia Streptococcus iniae 1. Introduction Streptococcus iniae (SI) is a Gram-positive, cocci bacterium, which is known to be the causative agent of warm-water streptococcosis in ma- rine and freshwater aquaculture. The pathogen has been linked to nu- merous mass mortalities in farmed fish, causing tremendous economic loss globally (Agnew and Barnes, 2007). To date, the inactivated vaccine has been commercialized in several countries and proven to be able to mitigate the losses from SI-associated streptococcosis (Agnew and Barnes, 2007; Bachrach et al., 2001; Hastein et al., 2005). However, im- plication of commercially inactivated vaccine in fish farms was still un- able to provide complete protection as disease occasionally emerged even in the vaccinated population (Bachrach et al., 2001; Eyngor et al., 2008). For this reason, several attempts have been made recently by many research groups in order to develop alternative forms of vaccine which can confer competent and sustainable protection against SI infec- tion. These candidate vaccines are, for instance, inactivated bacterin plus extracellular products combination, attenuated modified live vac- cine, recombinant subunit vaccine and DNA vaccines (Buchanan et al., 2005; Klesius et al., 2000; Locke et al., 2008; Membrebe et al., 2016; Sun et al., 2012a). DNA vaccine is well-known for its advantages over conventional vaccines as it is capable of inducing two arms of immune responses, i.e. humoral and cellular, requires no-adjuvant and provides longer pro- tection (Holvold et al., 2014; Lorenzen and LaPatra, 2005; Tonheim et al., 2008). To develop the efficacious DNA vaccine, the selection of the transgene is, undeniably, one of the major steps in vaccine design which substantially influences protective immunity of the recipient (Ingolotti et al., 2010). Recently, streptococcal α-enolase, a surface-lo- calized plasminogen-binding protein, has been raised as a potential vac- cine target due to its high immunogenicity and the highly conserved sequence among streptococcal species (Kim et al., 2007; LaFrentz et al., 2011; Membrebe et al., 2016; Wang et al., 2015). However, the immunoprotective effect of α-enolase in the form of DNA vaccine has not yet been reported. Therefore, the aim of this study is to examine the potential of DNA vaccines targeting α-enolase against SI infection using Nile tilapia as animal model. 2. Materials and methods 2.1. Bacterial strain Streptococcus iniae SI2J was kindly provided by the Laboratory of Ge- nome Science, Tokyo University of Marine Science and Technology, Japan. This bacterial isolate was confirmed as a highly pathogenic strain in our previous study (Kayansamruaj et al., 2015). Bacteria were Aquaculture 468 (2017) 102–106 ⁎ Corresponding author at: Department of Veterinary Microbiology, Faculty of Veterinary Science, Chulalongkorn University, Bangkok 10330, Thailand. E-mail address: Channarong.R@chula.ac.th (C. Rodkhum). http://dx.doi.org/10.1016/j.aquaculture.2016.10.001 0044-8486/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture