Semipermissive replication of Tipula iridescent virus in Aedes albopictus C6/36 cells

First complete and productive cell culture model for members of the genus Iridovirus

Chilo iridescent virus (CIV; the type strain of the genus Iridovirus) replicates productively in larvae of the boll weevil, Anthonomus grandis. This study focuses on characterizing productive infections of a boll weevil cell line, BRL-AG-3A (AG3A), starting with CIV reared in the waxworm, Galleria mellonella. We show that CIV can be continually and productively passaged to high titer in AG3A cells. The replication of larval-derived CIV in AG3A was analyzed by observing viral DNA replication and restriction endonuclease digestion profiles, morphogenesis, and infectivity using TCID(50) assays with AG3A as an indicator cell line. The data showed that virus passaged in the AG3A host is stable. AG3A cells are more efficient than previously utilized CF-124T cells from Choristoneura fumiferana. This system constitutes a superior model for cellular and molecular studies on CIV; it represents the first complete, productive cell culture model for the replication of CIV or any member of the genus Iridovirus.

Characterization of iridovirus IV1 polypeptides: mapping by surface labelling

A comprehensive index of IV1(Tipula iridescent virus, or TIV)-associated polypeptides has been established using enrichments of "empty" and "filled" virions that are considered to be maturation-phase-related. The mapping strategy which involved one- and two-dimensional polyacrylamide gel electrophoresis, silver staining, disulphide bond reduction and 125I iodination of putative surface proteins revealed 103 and 116 polypeptides for empty and filled capsids, respectively. These estimates could be reduced to < or = 70 by reclassing multicharged polypeptides of approximately identical masses as single entitles. At least 10 polypeptides from empty and filled virions were involved in intermolecular sulphhydryl linkages and another 11 species were identified as putative outer shell (surface) polypeptides. These data provide useful criteria for iridovirus classification and identification of candidate polypeptides involved in capsid formation and maturation.

Plaque assay and replication of Tipula iridescent virus in Spodoptera frugiperda ovarian cells

A plaque assay was developed for the study of Tipula iridescent virus (TIV) replication using a cell line derived from the fall army worm Spodoptera frugiperda (Sf9). Infection and plaque formation were monitored with time by phase contrast microscopy, video and fluorescent light microscopy. Structure of virions, viroplasmic centres and organelles of infected cells were examined by transmission electron microscopy (TEM). After 4 h postinfection, plaques were visibly detected within the cell monolayer by the presence of localized cell damage and production of numerous vesicular-like cytoplasmic structures. Quantitation of virions present per A260 unit of TIV preparation was determined by TEM. The number of visible plaques corresponded to virus concentration and 1 A260 produced approximately 10(5) plaques. DNA hybridization analysis revealed no gross differences in genomic DNA from TIV propagated in either Sf9 cells or wax moth Galleria mellonella larvae. These findings indicate that Sf9 is permissive for replication of TIV and superior by some parameters to other cell lines currently in use for the study of host cell/TIV interactions.

Molecular cloning and characterization of a late Tipula iridescent virus gene

Virions of the cytoplasmic, icosahedral insect virus, Tipula iridescent virus (TIV), contain two major DNA components (L, greater than 176 kb; and S1, 10.8 kb) and 25-30 proteins. We characterized a gene (L96) whose 3.6-kb transcript is expressed late in the course of TIV infection of cultured of Estigmene acrea (salt marsh caterpillar, permissive host) and Aedes albopictus (mosquito, semipermissive host) cells. The L96 gene has an open reading frame of 867 codons, predicting a protein of 96 kDa with a pI of 10.9. The C terminus of the L96 protein is rich in hydrophobic amino acids and contains a small region of homology spanning a proteolytic cleavage site within two mammalian viral (GAG) polyproteins. Additional identity with H5 lysine-rich histones in the same region and with other DNA-binding proteins suggests that this protein may be involved in TIV structure. The lengths of the 5'- and 3'-untranslated regions of the L96 transcript were determined to be 21 nucleotides (nt) and 700 nt, respectively. Comparison of the TIV L96- and capsid-encoding genes, both of which are expressed late in infection, revealed that their 5' and 3' regions are generally rich in A and T residues, and that their 3' ends encode at least one eukaryotic polyadenylation signal (AATAAA).