Review Adjuvants and immunostimulants in fish vaccines: Current knowledge and future perspectives Carolina Tafalla a, ** , Jarl Bøgwald b , Roy A. Dalmo b, * a Centro de Investigación en Sanidad Animal (CISA-INIA), Carretera de Algete a El Casar km. 8.1, Valdeolmos, 28130 Madrid, Spain b Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway article info Article history: Received 26 October 2012 Received in revised form 7 February 2013 Accepted 26 February 2013 Available online xxx Keywords: Fish vaccinology Adjuvants Immunostimulants abstract Vaccination is the most adequate method to control infectious diseases that threaten the aquaculture industry worldwide. Unfortunately, vaccines are usually not able to confer protection on their own; especially those vaccines based on recombinant antigens or inactivated pathogens. Therefore, the use of adjuvants or immunostimulants is often necessary to increase the vaccine efficacy. Traditional adjuvants such as mineral oils are routinely used in different commercial bacterial vaccines available for fish; however, important side effects may occur with this type of adjuvants. A search for alternative molecules or certain combinations of them as adjuvants is desirable in order to increase animal welfare without reducing protection levels. Especially, combinations that may target specific cell responses and thus a specific pathogen, with no or minor side effects, should be explored. Despite this, the oil adjuvants currently used are quite friendlier with respect to side effects compared with the oil adjuvants previously used. The great lack of fish antiviral vaccines also evidences the importance of identifying optimal combinations of a vaccination strategy with the use of a targeting adjuvant, especially for the promising fish antiviral DNA vaccines. In this review, we summarise previous studies performed with both tradi- tional adjuvants as well as the most promising new generation adjuvants such as ligands for Toll re- ceptors or different cytokines, focussing mostly on their protective efficacies, and also on what is known concerning their effects on the fish immune system when delivered in vivo. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Disease prevention by vaccination is, on economic, environ- mental and ethical grounds the most appropriate method for pathogen control currently available to the aquaculture sector. Traditionally, vaccines comprise either live-attenuated, replicating pathogens or non-replicating, inactivated pathogens or their sub- units. In many countries, live vaccines are not approved for use in aquaculture for safety reasons, while inactivated vaccines based on either killed pathogens or isolated non-replicating pathogen subunits, are in many cases, weakly immunogenic. Thus, adjuvants or immunopotentiators, are highly required for the elicitation of immune responses that may be 100% protective against certain pathogens. During the past, fish vaccines were made by a trial-and-error approach (conventional vaccine design) including pathogen identification, pathogen cultivation, and vaccine formulation con- taining whole cell preparation and oils. Through this strategy, vaccines based on whole inactivated extracellular bacterial patho- gens were quite efficient; resulting in important reductions in mortalities and antibiotic usage in the aquaculture industry [1]. However, many of the economically disastrous diseases of today are due to intracellular pathogens, and for this type of pathogens the production of effective vaccines has not been an easy task. In this sense, the most promising future vaccines that induce protection against viruses are DNA vaccines. Intramuscular injection of a DNA plasmid encoding an immunogenic antigen has proved very effective in fish, in comparison to the results obtained in other animal models such as mammals [2]. Because the antigen is pro- duced by the fish cells, it is exposed on the cell surface both directly or processed in the context of both major histocompatibility com- plex (MHC) class I and MHC class II, thus effectively triggering both humoral and cellular immune responses. Although DNA vaccines offer a number of advantages over conventional vaccines, there are still many aspects that may be optimised with adjuvant help such as alternative routes of immunisation that allow mass-vaccination. Therefore, fish vaccine approaches must be subjected to rational * Corresponding author. Tel.: þ47 77644482; fax: þ47 77646020. ** Corresponding author. Tel.: þ34 91 6202300; fax: þ34 91 6202247. E-mail addresses: tafalla@inia.es (C. Tafalla), roy.dalmo@uit.no (R.A. Dalmo). Contents lists available at SciVerse ScienceDirect Fish & Shellfish Immunology journal homepage: www.elsevier.com/locate/fsi 1050-4648/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fsi.2013.02.029 Fish & Shellfish Immunology xxx (2013) 1e11 Please cite this article in press as: Tafalla C, et al., Adjuvants and immunostimulants in fish vaccines: Current knowledge and future perspectives, Fish & Shellfish Immunology (2013), http://dx.doi.org/10.1016/j.fsi.2013.02.029