The third variable domain (V3) of the human immunodeficiency virus type 1 external envelope contains determinants of cell tropism, cytopathicity, and infectivity and elicits antibodies able to block infectivity in vitro and in vivo. Our study encompassed point-mutational analysis of HXB-2 viruses containing patient-derived V3 regions and expressing a non-syncytium-inducing, low-replicating phenotype in T-cell line SupTl. The mutation within V3 of a serine at position 306 into an also naturally occurring arginine (S to R) required an additional, naturally occurring mutation at position 320 (aspartate to glutamine, D to Q) or 324 (aspartate to asparagine, D to N) for ful expression of the syncytium-inducing, high-replicating (SI) phenotype. The naturally occurring mutation of an aspartate into an arginine at position 320 (D to R) was sufficient for production of the SI phenotype. This study proves that introduction of a positively charged amino acid at position 306 or 320, previously shown to be strongly associated with the SI phenotype in field isolates (R. A. M.

Chimeric human immunodeficiency virus type 1 (HIV-1) molecular clones differing only in the envelope V3 region were constructed. The V3 regions were derived from two HIV-1 isolates with a non-syncytium-inducing, non-T-cell-tropic phenotype and from four HIV-1 isolates with a syncytium-inducing, T-cell-tropic phenotype. When assayed in SupT1 cells, the two chimeric viruses with a V3 region derived from the non-syncytium-inducing isolates did not induce syncytia and showed a low level of replication. The four chimeric viruses with a V3 region derived from the syncytium-inducing isolates did induce syncytia and replicated efficiently in SupT1 cells. In A3.01 cells, which do not support syncytium formation, the V3 loop affected replication similarly. Upon prolonged culture in SupT1 cells, the phenotype of a non-syncytium-inducing, low-replicating chimeric HIV-1 converted into a syncytium-inducing, high-replicating phenotype. Mutations within the usually conserved GPGR tip of the loop, which were shown to be responsible for the conversion into the syncytium-inducing, high-replicating phenotype, had occurred. In vitro mutagenesis showed that coupled changes of amino acids at both sides of the tip of the V3 loop were able to convert the viral phenotype from non-syncytium-inducing, low replicating into syncytium inducing, high replicating. Our data show that the V3 loop is involved in both syncytium forming and replicative capacity of HIV-1.

From 1992 onwards, outbreaks of a previously unknown illness have been reported in Asian seabass (Lates calcarifer) kept in maricultures in Southeast Asia. The most striking symptom of this emerging disease is the loss of scales. It was referred to as scale drop syndrome, but the etiology remained enigmatic. By using a next-generation virus discovery technique, VIDISCA-454, sequences of an unknown virus were detected in serum of diseased fish. The near complete genome sequence of the virus was determined, which shows a unique genome organization, and low levels of identity to known members of the Iridoviridae. Based on homology of a series of putatively encoded proteins, the virus is a novel member of the Megalocytivirus genus of the Iridoviridae family. The virus was isolated and propagated in cell culture, where it caused a cytopathogenic effect in infected Asian seabass kidney and brain cells. Electron microscopy revealed icosahedral virions of about 140 nm, characteristic for the Iridoviridae. In vitro cultured virus induced scale drop syndrome in Asian seabass in vivo and the virus could be reisolated from these infected fish. These findings show that the virus is the causative agent for the scale drop syndrome, as each of Koch’s postulates is fulfilled. We have named the virus Scale Drop Disease Virus. Vaccines prepared from BEI- and formalin inactivated virus, as well as from E. coli produced major capsid protein provide efficacious protection against scale drop disease.

No abstract