Immunostimulants connecting innate and adaptive immunity in Atlantic salmon (Salmo salar)

The use of immunostimulants in fish farming has received increased attention due to the discovery of Toll-like receptors (TLRs) and pattern recognition receptors (PRRs). These receptors have been found to bind to highly conserved motifs on the surface of pathogens, known as pathogen associated molecular patterns. Most of the studied PAMPs activate antigen presenting cells (APCs) together with naïve T-cells, and may induce TH1 and TH2 responses with production of signature molecules such as IFN-γ and IL-4 respectively (Aderem and Ulevitch, 2000). In this study, different immunostimulants acting as ligands for TLRs were administered to Atlantic salmon by intraperitoneal injection. At different time points (1-14 days) post injection, samples of spleen, head kidney and liver were obtained from the different treatment groups. Gene transcripts for the TH1 signature genes IFN-γ and TNF-α, TH2 transcription factor GATA-3 and regulatory IL-10 as well as the surface receptors CD8 and CD4, were subjected to analysis by Q-RT-PCR. All the applied immunostimulants induced expression of TNF-α in both the spleen and kidney. Induction of IFN-γ gene transcription was performed by β-glucan, LPS, pDNA and imiquimod. Only β-glucan induced GATA-3 expression in both the spleen and head kidney. Interleukin-10 gene expression was induced by all immunostimulants except LPS. Transcription of CD8 and CD4 was induced by LPS and pDNA. β-glucan also induced CD8 gene expression. Adjustment of the expression levels according to their respective control group values showed that upregulation of TH1 expression was induced by β-glucan, imiquimod and pDNA + LPS. Upregulation of GATA-3 gene transcription was induced by β-glucan and imiquimod, while pDNA, imiquimod and pDNA + LPS up-regulated transcription of IL-10. These results may pinpoint the use of selected immunostimulants in order to drive the adaptive immune response into either TH1, TH2, or both.