Antigenic variations in the CD4 induced sites of the CCR5-tropic, pathogenic SHIVsf162p3 gp120 variants

In vivo passage of non-pathogenic, CCR5-tropic simian/human immunodeficiency virus (SHIV) - SHIVsf162 resulted in a pathogenic isolate, SHIVsf162p3. In an attempt to characterize envelope (Env)-mediated properties that may contribute to its pathogenicity, major (P3 major) and minor (P3 minor) Env gp120 variants were cloned from the plasma of a SHIVsf162p3-infected animal, and expressed in the context of luciferase reporter viruses. Entry mediated by these envelopes and susceptibility to neutralization by CD4 induced-site (CD4i) antibodies (MAbs) was analyzed in comparison to parental SF162. Sequence analysis revealed that the P3 major and minor variant Envs contained 14 and 17 amino acid changes, respectively, compared with SF162. The rank order of entry mediated by the three envelopes was P3 major > SF162 > P3 minor, whereas the reverse order was observed for susceptibility to neutralization by CD4i MAbs. Since CD4i epitopes overlap the coreceptor (CoR) binding site, these findings suggest that the amino acid changes accumulated upon in vivo passage of SHIVsf162 result in Env gp120 structural rearrangements that modulate the exposure and/or conformation of the CoR binding site. This, in turn, led to increased entry and infectivity of the P3 major variant and may be responsible, in part, for the enhanced pathogenicity of SHIVsf162p3.

Antibody protects macaques against vaginal challenge with a pathogenic R5 simian/human immunodeficiency virus at serum levels giving complete neutralization in vitro

A major unknown in human immunodeficiency virus (HIV-1) vaccine design is the efficacy of antibodies in preventing mucosal transmission of R5 viruses. These viruses, which use CCR5 as a coreceptor, appear to have a selective advantage in transmission of HIV-1 in humans. Hence R5 viruses predominate during primary infection and persist throughout the course of disease in most infected people. Vaginal challenge of macaques with chimeric simian/human immunodeficiency viruses (SHIV) is perhaps one of the best available animal models for human HIV-1 infection. Passive transfer studies are widely used to establish the conditions for antibody protection against viral challenge. Here we show that passive intravenous transfer of the human neutralizing monoclonal antibody b12 provides dose-dependent protection to macaques vaginally challenged with the R5 virus SHIV(162P4). Four of four monkeys given 25 mg of b12 per kg of body weight 6 h prior to challenge showed no evidence of viral infection (sterile protection). Two of four monkeys given 5 mg of b12/kg were similarly protected, whereas the other two showed significantly reduced and delayed plasma viremia compared to control animals. In contrast, all four monkeys treated with a dose of 1 mg/kg became infected with viremia levels close to those for control animals. Antibody b12 serum concentrations at the time of virus challenge corresponded to approximately 400 (25 mg/kg), 80 (5 mg/kg), and 16 (1 mg/kg) times the in vitro (90%) neutralization titers. Therefore, complete protection against mucosal challenge with an R5 SHIV required essentially complete neutralization of the infecting virus. This suggests that a vaccine based on antibody alone would need to sustain serum neutralizing antibody titers (90%) of the order of 1:400 to achieve sterile protection but that lower titers, around 1:100, could provide a significant benefit. The significance of such substerilizing neutralizing antibody titers in the context of a potent cellular immune response is an important area for further study.

A method of video-assisted thoracoscopic surgery for collection of thymic biopsies in rhesus monkeys (Macaca mulatta)

To support an infectious disease study, we developed a surgical procedure to collect serial thymic biopsies in rhesus monkeys (Macaca mulatta). In many instances in which thymic tissue is required from living animals, open surgical approaches (thoracotomies) are used, which result in greater postoperative pain and longer recovery periods than those associated with thoracoscopic procedures. Our intent was to develop a surgical procedure that allowed serial biopsy of the thymus with minimal surgical morbidity. We modified a previously published experimental method of thoracoscopic total thymectomy in the dog to collect thymic biopsies in M. mulatta. Of the 15 animals evaluated, 8 underwent two biopsy procedures separated by a 5- to 6-week interoperative interval. The other seven animals underwent a single biopsy procedure. Thymic tissue was collected successfully during all procedures, with an average surgical time of 15 min. No significant intra- or postoperative complications were noted, and the animals recoveries from the surgical procedures were uneventful. Minimally invasive thoracoscopic surgical techniques can be used successfully to collect thymic tissue from adult and juvenile rhesus monkeys with minimal surgical morbidity.

Selection for neutralization resistance of the simian/human immunodeficiency virus SHIVSF33A variant in vivo by virtue of sequence changes in the extracellular envelope glycoprotein that modify N-linked glycosylation

We previously reported on the in vivo adaptation of an infectious molecular simian/human immunodeficiency virus (SHIV) clone, SHIVSF33, into a pathogenic biologic viral variant, designated SHIVSF33A. In the present study, we show that SHIVSF33A is resistant to neutralization by human immunodeficiency virus (HIV) and SHIV antisera. Multiple amino acid substitutions accumulated over time throughout the env gene of SHIVSF33A; some of them coincided with the acquisition of the neutralization resistance of the virus. Of interest are changes that resulted in the removal, repositioning, and addition of potential glycosylation sites within the V1, V2, and V3 regions of envelope gp120. To determine whether potential glycosylation changes within these principal neutralization domains of HIV type 1 formed the basis for the resistance to serum neutralization of SHIVSF33A, mutant viruses were generated on the backbone of parental SHIVSF33 and tested for their neutralization sensitivity. The mutations generated did not alter the in vitro replication kinetics or cytopathicity of the mutant viruses in T-cell lines. However, the removal of a potential glycosylation site in the V1 domain or the creation of such a site in the V3 domain did allow the virus to escape serum neutralization antibodies that recognized parental SHIVSF33. The combination of the V1 and V3 mutations conferred an additive effect on neutralization resistance over that of the single mutations. Taken together, these data suggest that (i) SHIV variants with changes in the Env SU can be selected in vivo primarily by virtue of their ability to escape neutralizing antibody recognition and (ii) carbohydrates play an important role in conferring neutralization escape, possibly by altering the structure of envelope gp120 or by shielding principal neutralization sites.