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In mammals, IL-21 is a common γ chain cytokine produced by activated CD4+ T cells and NKT cells that acts on multiple lineages of cells. Although IL-21 has also been discovered in birds, amphibians, and fish, to date, no functional studies have been reported for any nonmammalian IL-21 molecule. We have sequenced an IL-21 gene (tIL-21) in rainbow trout, which has a six-exon/five-intron structure, is expressed in immune tissues, and is induced by bacterial and viral infection and the T cell stimulant PHA. In contrast to mammals, calcium ionophore and PMA act synergistically to induce tIL-21. Recombinant tIL-21 (rtIL-21) induced a rapid and long-lasting (4–72 h) induction of expression of IFN-γ, IL-10, and IL-22, signature cytokines for Th1-, Th2-, and Th17-type responses, respectively, in head kidney leukocytes. However, rtIL-21 had little effects on the expression of other cytokines studied. rtIL-21 maintained the expression of CD8α, CD8β, and IgM at a late stage of stimulation when their expression was significantly decreased in controls and increased the expression of the Th cell markers CD4, T-bet, and GATA3. Intraperitoneal injection of rtIL-21 confirmed the in vitro bioactivity and increased the expression of IFN-γ, IL-10, IL-21, IL-22, CD8, and IgM. Inhibition experiments revealed that the activation of JAK/STAT3, Akt1/2, and PI3K pathways were responsible for rtIL-21 action. This study helps to clarify the role of IL-21 in lower vertebrates for the first time, to our knowledge, and suggests IL-21 is a likely key regulator of T and B cell function in fish.

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We identified viral haemorrhagic septicaemia virus (VHSV) isolates classified within Genotype Ib which are genetically similar (> 99.4% glycoprotein amino acid identity) yet, based on their isolation history, were suspected to differ in virulence in juvenile rainbow trout. The virulence of an isolate recovered in 2000 from a viral haemorrhagic septicaemia disease episode in a marine rainbow trout farm in Sweden (SE-SVA-1033) was evaluated in juvenile rainbow trout via intraperitoneal injection and immersion challenge alongside 3 isolates recovered from wild-caught marine fish (DK-4p37, DK-5e59 and UKMLA98/6HE1) suspected of being of low pathogenicity to trout. Mortality data revealed that isolate SE-SVA-1033 caused VHSV-specific mortality in both intraperitoneal and immersion challenges (75.0 and 15.4%, respectively). The remaining Genotype Ib isolates caused significantly lower mortalities using the same experimental infection routes (< 35.0 and < 2.0%, respectively). Having identified VHSV isolates with clear differences in their pathogenicity, coding and inter-genic non-coding regions of 2 isolates (SE-SVA-1033 and DK-4p37) were determined and compared in order to identify potential markers responsible for the observed differences in virulence. Only 4 predicted amino acid substitutions were identified across the genome sequenced; these occurred in the N (R46G), G (S113G), NV (L12F) and L (S56A) proteins. These findings form the basis for further studies aimed at determining the biological significance of these mutations and suggest that small changes at the molecular level can cause significant changes in the virulence properties of VHSV isolates. KEY WORDS: Rainbow trout · VHSV · Pathogenicity · Sequencing · Virulence determinantsResale or republication not permitted without written consent of the publisher

Multilamellar liposomes (lipid bilayer vesicles) composed of phosphatidylcholine, cholesterol, and phosphatidylserine (molar ratio, 6:3:1) were produced and then made to entrap an aqueous solution of cephalothin. Resident murine peritoneal macrophages were shown to be capable of interiorizing the liposome-antibiotic complex; this event resulted in a relatively high intracellular concentration of cephalothin. In macrophages infected in vitro with Salmonella typhimurium, intracellular killing of the bacteria was maximal at 60 min of incubation; at this time, 60% of the interiorized organisms had been killed. Treatment of infected macrophages with liposome-encapsulated cephalothin enhanced the intraphagocytic killing of S typhimurium over that by macrophages treated with free cephalothin. These results demonstrate the superiority of liposome-encapsulated antibiotics to free antibiotics in effecting the elimination of a facultative intracellular bacterium from its intracellular site. This type of complex may find application in the treatment of diseases caused by this group of microorganisms.