Productive expression state confers resistance of human immunodeficiency virus (HIV)-2-infected lymphoma cells against superinfection by HIV-1

Evidence for viral virulence as a predominant factor limiting human immunodeficiency virus vaccine efficacy

Current strategies in human immunodeficiency virus type 1 (HIV-1) vaccine development are often based on the production of different vaccine antigens according to particular genetic clades of HIV-1 variants. To determine if virus virulence or genetic distance had a greater impact on HIV-1 vaccine efficacy, we designed a series of heterologous chimeric simian/human immunodeficiency virus (SHIV) challenge experiments in HIV-1 subunit-vaccinated rhesus macaques. Of a total of 22 animals, 10 nonimmunized animals served as controls; the remainder were vaccinated with the CCR5 binding envelope of HIV-1(W6.1D). In the first study, heterologous challenge included two nonpathogenic SHIV chimeras encoding the envelopes of the divergent clade B HIV-1(han2) and HIV-1(sf13) strains. In the second study, all immunized animals were rechallenged with SHIV(89. 6p), a virus closely related to the vaccine strain but highly virulent. Protection from either of the divergent SHIV(sf13) or SHIV(han2) challenges was demonstrated in the majority of the vaccinated animals. In contrast, upon challenge with the more related but virulent SHIV(89.6p), protection was achieved in only one of the previously protected vaccinees. A secondary but beneficial effect of immunization on virus load and CD4(+) T-cell counts was observed despite failure to protect from infection. In addition to revealing different levels of protective immunity, these results suggest the importance of developing vaccine strategies capable of protecting from particularly virulent variants of HIV-1.

In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene

The lack of bovine leukemia virus (BLV) expression is a consistent finding in freshly isolated ovine tumor cells and in the B-cell lines derived from these tumors. In order to gain further insight into the mechanisms of BLV silencing in these tumors, we have used the YR2 B-cell line, which was derived from the leukemic cells of a BLV-infected sheep. This cell line contains a single, monoclonally integrated, silent provirus, which cannot be reactivated either by stimulation in vitro or by in vivo injection of the tumor cells or cloned proviral DNA in sheep. Sequence analysis of the tax gene from the YR2 cell line identified two G-to-A transitions (G7924 to A7924 and G8149 to A8149) that result in E-to-K amino acid changes at positions 228 and 303 in the Tax protein. Following retroviral vector-mediated transfer of a wild-type tax gene into YR2 cells, we showed that BLV mRNA, viral proteins, and virions were produced, demonstrating that the cellular factors required for virus expression were present in the original YR2 cell line. Injection of this transduced YR2 cell line in sheep led to the rescue of replication-competent BLV proviruses. The integrated competent proviruses exhibited unique chimeric tax genes, which arose from homologous recombination between the transduced wild-type tax and the YR2-derived tax sequences. Furthermore, in one of these functional recombinant proviruses, only the A8149-to-G8149 reversion was present, providing clear evidence that the defect underlying the silent phenotype in YR2 cells results from a single C-terminal E303-to-K303 amino acid substitution in the BLV Tax protein. Our observations suggest that a single strategically located mutation in tax provides a mechanism for BLV inactivation in B-cell tumors.

Natural protection against HIV-1 infection provided by HIV-2

Significant differences have been observed in the rates of transmission and disease development in human immunodeficiency virus (HIV) types 1 and 2. Because many HIV-2-infected people remain asymptomatic for prolonged periods, the hypothesis that HIV-2 might protect against subsequent infection by HIV-1 was considered. During a 9-year period in Dakar, Senegal, the seroincidence of both HIV types was measured in a cohort of commercial sex workers. Despite a higher incidence of other sexually transmitted diseases (STDs), HIV-2-infected women had a lower incidence of HIV-1 than did HIV-seronegative women, with a relative risk of 0.32 (P = 0.008). An understanding of the cross-protective mechanisms involved may be directly relevant to HIV-1 vaccine development.

The induction of in vivo superinfection and recombination using feline immunodeficiency virus as the model

This study investigated the hypothesis that under certain conditions, superinfection of cats with feline immunodeficiency virus (FIV), may occur. One FIV isolate (T91) was used to inoculate three FIV and FeLV-free cats. Blood from an FIV-infected cat (N), which contained two variants and differed from T91 by at least 5% in nucleotide sequence in the env gene, was inoculated into a fourth cat. Both T91 and blood from N were inoculated simultaneously into a fifth cat. After 22 weeks, two of the three cats initially infected with T91 were challenged with blood from N. At 30 weeks following initial infection, peripheral blood mononuclear cells were obtained from all cats, DNA was extracted, and a segment of the env gene was PCR amplified, cloned and sequenced. Nucleotide sequence analysis of the cloned PCR product showed that virus strains used in initial infection were recovered from cats not challenged with a second variant. Challenge of cats with the blood of N following initial infection with T91 resulted in superinfection occurring in one cat and recombination occurring in the other. Furthermore, the use of blood as a source of challenge, in cats where superinfection and simultaneous infections were attempted, may have induced the appearance of variants which more closely resembled the most heterologous strain present in the infectious source.