AUTHOR=Xue Xi , Caballero-Solares Albert , Hall Jennifer R. , Umasuthan Navaneethaiyer , Kumar Surendra , Jakob Eva , Skugor Stanko , Hawes Christopher , Santander Javier , Taylor Richard G. , Rise Matthew L. 

TITLE=Transcriptome Profiling of Atlantic Salmon (Salmo salar) Parr With Higher and Lower Pathogen Loads Following Piscirickettsia salmonis Infection

JOURNAL=Frontiers in Immunology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2021.789465

DOI=10.3389/fimmu.2021.789465

ISSN=1664-3224

ABSTRACT=<p>Salmonid rickettsial septicemia (SRS), caused by <italic>Piscirickettsia salmonis</italic>, is one of the most devastating diseases of salmonids. However, the transcriptomic responses of Atlantic salmon (<italic>Salmon salar</italic>) in freshwater to an EM-90-like isolate have not been explored. Here, we infected Atlantic salmon parr with an EM-90-like isolate and conducted time-course qPCR analyses of pathogen load and four biomarkers (<italic>campb</italic>, <italic>hampa</italic>, <italic>il8a</italic>, <italic>tlr5a</italic>) of innate immunity on the head kidney samples. Transcript expression of three of these genes (except <italic>hampa</italic>), as well as pathogen level, peaked at 21 days post-injection (DPI). Multivariate analyses of infected individuals at 21 DPI revealed two infection phenotypes [lower (L-SRS) and higher (H-SRS) infection level]. Five fish from each group (Control, L-SRS, and H-SRS) were selected for transcriptome profiling using a 44K salmonid microarray platform. We identified 1,636 and 3,076 differentially expressed probes (DEPs) in the L-SRS and H-SRS groups compared with the control group, respectively (FDR = 1%). Gene ontology term enrichment analyses of SRS-responsive genes revealed the activation of a large number of innate (e.g. “phagocytosis”, “defense response to bacterium”, “inflammatory response”) and adaptive (e.g. “regulation of T cell activation”, “antigen processing and presentation of exogenous antigen”) immune processes, while a small number of general physiological processes (e.g. “apoptotic process”, development and metabolism relevant) was enriched. Transcriptome results were confirmed by qPCR analyses of 42 microarray-identified transcripts. Furthermore, the comparison of individuals with differing levels of infection (H-SRS vs. L-SRS) generated insights into the biological processes possibly involved in disease resistance or susceptibility. This study demonstrated a low mortality (~30%) EM-90-like infection model and broadened the current understanding of molecular pathways underlying <italic>P. salmonis</italic>-triggered responses of Atlantic salmon, identifying biomarkers that may assist to diagnose and combat this pathogen.</p>