Low Temperature-Dependent Salmonid Alphavirus Glycoprotein Processing and Recombinant Virus-Like Particle Formation

Pancreas disease (PD) and sleeping disease (SD) are important viral scourges in aquaculture of Atlantic salmon and rainbow trout. The etiological agent of PD and SD is salmonid alphavirus (SAV), an unusual member of the Togaviridae (genus Alphavirus). SAV replicates at lower temperatures in fish. Outbreaks of SAV are associated with large economic losses of ∼17 to 50 million $/year. Current control strategies rely on vaccination with inactivated virus formulations that are cumbersome to obtain and have intrinsic safety risks. In this research we were able to obtain non-infectious virus-like particles (VLPs) of SAV via expression of recombinant baculoviruses encoding SAV capsid protein and two major immunodominant viral glycoproteins, E1 and E2 in Spodoptera frugiperda Sf9 insect cells. However, this was only achieved when a temperature shift from 27°C to lower temperatures was applied. At 27°C, precursor E2 (PE2) was misfolded and not processed by host furin into mature E2. Hence, E2 was detected neither on the surface of infected cells nor as VLPs in the culture fluid. However, when temperatures during protein expression were lowered, PE2 was processed into mature E2 in a temperature-dependent manner and VLPs were abundantly produced. So, temperature shift-down during synthesis is a prerequisite for correct SAV glycoprotein processing and recombinant VLP production

Expression of Sindbis virus 26S cDNA in Spodoptera frugiperda (Sf9) cells, using a baculovirus expression vector.

To study protein processing in an insect Spodoptera frugiperda (fall armyworm; Sf9) cell line, a 26S cDNA encoding the sequence of Sindbis virus structural proteins (capsid protein, of 30 kilodaltons [kDa]; p62 [the precursor of E3 and E2], of 62 kDa; a 6-kDa peptide; and the E1 protein, of 56 kDa) was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) adjacent to the polyhedrin promoter. By immunoblot analysis with antisera directed against whole Sindbis virus and the individual structural proteins (capsid, E2, and E1), we have shown that polypeptides similar in size and antigenicity to those synthesized in Sindbis virus-infected BHK cells are expressed in Sf9 cells infected with the recombinant baculovirus Ac373-SV26. By pulse-chase labeling in the presence or absence of tunicamycin, by endo-beta-N-acetylglucosaminidase H (endo-H) treatment of the recombinant glycoproteins, and by N-terminal sequence analysis of the E1 envelope glycoprotein, we have further shown that the 26S transcription translation unit of Sindbis virus, although normally encoded by nonnuclear RNA, is expressed and proteolytically cleaved similarly, if not identically, in Sf9 cells as compared with BHK cells when a baculovirus expression vector is used

Detection of rubella virus-specific immunoglobulin M antibodies with a baculovirus-expressed E1 protein.

The structural proteins of rubella virus (RV) were expressed in insect cells by using the baculovirus expression vector system. The recombinant E1 envelope glycoprotein was purified by immunoaffinity chromatography and used to detect RV-specific immunoglobulin M antibodies in a time-resolved fluoroimmunoassay. Correlation analysis between the reactivities of antibodies against this recombinant E1 and the reactivities against authentic RV antigen shows that purified E1 can detect RV antibodies of the immunoglobulin M type