Virus infections induce changes in the expression of host cell genes. A global knowledge of these modifications should help to better understand the virus/host cell interactions. To obtain a more comprehensive view of the rainbow trout response to a viral infection, we used the subtractive suppressive hybridization methodology in the viral hemorrhagic septicemia model of infection. We infected rainbow trout leukocytes with viral hemorrhagic septicemia virus (VHSV), and total RNA from infected and mock-infected cells was compared at 40 h postinfection. Twenty-four virus-induced genes were ultimately retrieved from the subtracted cDNA library, and their differential expression was further confirmed by semiquantitative reverse transcription-PCR and Northern blot analysis. Among these sequences, three were already described as VHSV-induced genes. Eight sequences with known homologs were extended to full-length cDNA using 5 and 3 rapid amplification of cDNA ends, and they were subsequently divided into three functional subsets. Four genes were homologous to mammalian interferon responsive genes, three were similar to chemo-attractant molecules (CXC chemokine, galectin), and two had nucleic acid binding domains. All of the virus-induced genes were also induced by rainbow trout interferon, indicating that the interferon pathway is the predominant component of the anti-VHSV response. They were also expressed in vivo in experimentally infected fish, indicating their biological relevance in natural infection.

Virulence mechanisms utilized by the salrnonld fish pathogen Renibactedum salmoninarum are poorly understood. One potential virulence factor is p57 (also designated MSA for major soluble antigen), an abundant 57 kDa soluble protein that is predominately localized on the bacterial cell surface with significant levels released into the extracellular milieu. Previous studies of an attenuated strain, MT 239, indicated that it differs from virulent strains in the amount of surface-associated p57. In this report, we show overall expression of p57 in R. salmoninarum MT 239 is considerably reduced as compared to a virulent strain, ATCC 33209. The amount of cell-associated p57 is decreased while the level of p57 in the culture supernatant is nearly equivalent between the strains. To determine if the lowered amount of cell-associated p57 was due to a sequence defect in p57, a genetic comparison was performed. Two copies of the gene encoding p57 (msal and msa2) were found in 33209 and MT 239, as well as in several other virulent isolates. Both copies from 33209 and MT 239 were cloned and sequenced and found to be identical to each other, and identical between the 2 strains. A comparison of msal and msa2 within each strain showed that their sequences diverge 40 base pairs 5' to the open reading frame, while sequences 3' to the open reading frame are essentially identical for at least 225 base pairs. Northern blot analysis showed no difference in steady state levels of msa mRNA between the 2 strains. These data suggest that while cell-surface localization of p57 may be important for R. salmoninarum virulence, the Wferences in localization and total p57 expression between 33209 and MT 239 are not due to differences in msa sequence or differences in steady state transcript levels.

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