A potential aquaculture vaccine vector: Evaluation of a double-gene attenuated Listeria monocytogenes in zebrafish (Danio rerio)

Although engineered Lm has strong adjuvant effects such as the delivery of antigens in antigen-presenting cells (APCs), it has not been evaluated if attenuated Lm has a potential to be used as an aquaculture vaccines vector. In this study, a double-gene attenuated Lm strain (Lm-actA/inlB) was constructed to evaluate the potential that using this attenuated strain to provide a delivery platform for aquatic vaccines. The attenuated Lm strain Lm-actA/inlB showed decreased invasion and apoptosis of HepG2 and Caco-2 host cells in in vitro infection models. Interestingly, the expression levels of the hly and plcB genes were significantly increased in Lm-actA/inlB compared with wild-type EGDe. The higher expression levels of the hly and plcB genes are indicative of greater advantages in terms of antigen presentation. To further explore the innate immune and adaptive immune response elicited by Lm-actA/inlB and its feasibility for use as a non-injection vaccine vector, we evaluated the expression of Toll-like receptor (TLR) signalling pathway-related gene markers, antigen presentation pathway-related gene markers and IgM via Lm-actA/inlB intraperitoneal injection and bath delivery. Transcription analysis indicated that the TLR signalling pathway might play a major role in the innate immune response and the major histocompatibility complex (MHC)-I and MHC-II antigen presentation pathways might also both exist in vertebrate fish. In conclusion, the live-attenuated Lm strain Lm-actA/inlB may be a potential platform for delivering aquatic vaccines against certain fish bacterial diseases commonly seen in the aquatic farm industry.