An oral delivery system for controlling white spot syndrome virus infection in shrimp using transgenic microalgae

Abstract White spot disease (WSD) is a longstanding and serious viral disease of various shrimp species that has caused high mortality rates for many decades. Currently, there is no practical method to control this disease. Therefore, we have explored the development of a novel vaccine-based method to control this disease using transgenic algae. During infection by white spot syndrome virus (WSSV), the interaction between viral envelope proteins and cell surface protein receptors on target cells is the key step of viral entry and replication. Hence, transgenic lines of the green microalga Chlamydomonas reinhardtii harboring a WSSV VP28 viral envelope protein were created as an oral delivery system for vaccinating shrimp. Two type of transplastomic lines containing wild-type and codon optimized gene sequences for VP28 were evaluated for recombinant protein levels. Only the codon optimized line gave rise to detectable VP28 in western blot analysis, which demonstrated that optimization for chloroplast codon bias improved the efficiency of expression and that the gene design produced a favorable RNA secondary structure with suitable free energy for translation. In addition, bile salt and acid tolerance tests demonstrated this transgenic Chlamydomonas can tolerate mildly acidic (pH 4.0) conditions and 0.30% bile salts. These features indicated that algal cells are suitable for delivering viral antigen through a shrimp's digestive system. In WSSV infection experiments, the highest survival rate (87%) was recorded in shrimps fed with the codon optimized VP28 line mixed into their feed indicating that this line could be employed in the control of WSSV spread in shrimp populations. This algal strategy offers an alternative new, efficient, fast and less labor-intensive method for the control of other diseases in aquatic animals through oral delivery.