The acute HIV infection: implications for intervention, prevention and development of an effective AIDS vaccine

Neutralizing antibody affords comparable protection against vaginal and rectal simian/human immunodeficiency virus challenge in macaques

OBJECTIVE

Passive administration of broadly neutralizing antibodies has been shown to protect against both vaginal and rectal challenge in the simian/human immunodeficiency virus (SHIV)/macaque model of HIV transmission. However, the relative efficacy of antibody against the two modes of exposure is unknown and, given differences in the composition and immunology of the two tissue compartments, this is an important gap in knowledge. To investigate the significance of the challenge route for antibody-mediated protection, we performed a comparative protection study in macaques using the highly potent human monoclonal antibody, PGT126.

DESIGN

Animals were administered PGT126 at three different doses before challenged either vaginally or rectally with a single dose of SHIVSF163P3.

METHODS

Viral loads, PGT126 serum concentrations, and serum neutralizing titers were monitored.

RESULTS

In vaginally challenged animals, sterilizing immunity was achieved in all animals administered 10 mg/kg, in two of five animals administered 2 mg/kg and in one of five animals administered 0.4 mg/kg PGT126. Comparable protection was observed for the corresponding groups challenged rectally as sterilizing immunity was achieved in three of four animals administered 10 mg/kg, in two of four animals administered 2 mg/kg and in none of four animals administered 0.4 mg/kg PGT126. Serological analysis showed similar serum concentrations of PGT126 and serum neutralization titers in animals administered the same antibody dose.

CONCLUSION

Our data suggest that broadly neutralizing antibody-mediated protection is not strongly dependent on the mucosal route of challenge, which indicates that a vaccine aimed to induce a neutralizing antibody response would have broadly similar efficacy against both primary transmission routes for HIV.

Lack of viral control and development of combination antiretroviral therapy escape mutations in macaques after bone marrow transplantation

OBJECTIVE

We have previously demonstrated robust control of simian/human immunodeficiency virus (SHIV1157-ipd3N4) viremia following administration of combination antiretroviral therapy (cART) in pigtailed macaques. Here, we sought to determine the safety of hematopoietic stem cell transplantation (HSCT) in cART-suppressed and unsuppressed animals.

DESIGN

We compared disease progression in animals challenged with SHIV 100 days post-transplant, to controls that underwent transplant following SHIV challenge and stable cART-dependent viral suppression.

METHODS

SHIV viral load, cART levels, and anti-SHIV antibodies were measured longitudinally from plasma/serum from each animal. Flow cytometry was used to assess T-cell subset frequencies in peripheral blood and the gastrointestinal tract. Deep sequencing was used to identify cART resistance mutations.

RESULTS

In control animals, virus challenge induced transient peak viremia, viral set point, and durable suppression by cART. Subsequent HSCT was not associated with adverse events in these animals. Post-transplant animals were challenged during acute recovery following HSCT, and displayed sustained peak viremia and cART resistance. Although post-transplant animals had comparable plasma levels of antiretroviral drugs and showed no evidence of enhanced infection of myeloid subsets in the periphery, they exhibited a drastic reduction in virus-specific antibody production and decreased T-cell counts.

CONCLUSIONS

These results suggest that virus challenge prior to complete transplant recovery impairs viral control and may promote drug resistance. These findings may also have implications for scheduled treatment interruption studies in patients on cART during post-HSCT recovery: premature scheduled treatment interruption could similarly result in lack of viral control and cART resistance.

Contribution of HIV-1 genomes that do not integrate to the basic reproductive ratio of the virus

Recent experimental data indicate that HIV-1 DNA that fails to integrate (from now on called uDNA) can by itself successfully produce infectious offspring virions in resting T cells that become activated after infection. This scenario is likely important at the initial stages of the infection. We use mathematical models to calculate the relative contribution of unintegrated and integrated viral DNA to the basic reproductive ratio of the virus, R0, and the models are parameterized with preliminary data. This is done in the context of both free virus spread and transmission of the virus through virological synapses. For free virus transmission, we find that under preliminary parameter estimates, uDNA might contribute about 20% to the total R0. This requires that a single copy of uDNA can successfully replicate. If the presence of more than one uDNA copy is required for replication, uDNA does not contribute to R0. For synaptic transmission, uDNA can contribute to R0 regardless of the number of uDNA copies required for replication. The larger the number of viruses that are successfully transmitted per synapse, however, the lower the contribution of uDNA to R0 because this increases the chances that at least one virus integrates. Using available parameter values, uDNA can maximally contribute 20% to R0 in this case. We argue that the contribution of uDNA to virus reproduction might also be important for continued low level replication of HIV-1 in the presence of integrase inhibitor therapy. Assuming a 20% contribution of uDNA to the overall R0, our calculations suggest that R0=1.6 in the absence of virus integration. While these are rough estimates based on preliminary data that are currently available, this analysis provides a framework for future experimental work which should directly measure key parameters.

An HIV-1 replication pathway utilizing reverse transcription products that fail to integrate

Integration is a central event in the replication of retroviruses, yet ≥ 90% of HIV-1 reverse transcripts fail to integrate, resulting in accumulation of unintegrated viral DNA in cells. However, understanding what role, if any, unintegrated viral DNA plays in the natural history of HIV-1 has remained elusive. Unintegrated HIV-1 DNA is reported to possess a limited capacity for gene expression restricted to early gene products and is considered a replicative dead end. Although the majority of peripheral blood CD4(+) T cells are refractory to infection, nonactivated CD4 T cells present in lymphoid and mucosal tissues are major targets for infection. Treatment with cytokine interleukin-2 (IL-2), IL-4, IL-7, or IL-15 renders CD4(+) T cells permissive to HIV-1 infection in the absence of cell activation and proliferation and provides a useful model for infection of resting CD4(+) T cells. We found that infection of cytokine-treated resting CD4(+) T cells in the presence of raltegravir or with integrase active-site mutant HIV-1 yielded de novo virus production following subsequent T cell activation. Infection with integration-competent HIV-1 naturally generated a population of cells generating virus from unintegrated DNA. Latent infection persisted for several weeks and could be activated to virus production by a combination of a histone deacetylase inhibitor and a protein kinase C activator or by T cell activation. HIV-1 Vpr was essential for unintegrated HIV-1 gene expression and de novo virus production in this system. Bypassing integration by this mechanism may allow the preservation of genetic information that otherwise would be lost.

Using nonhuman primates to model HIV transmission

PURPOSE OF REVIEW

One of the major obstacles in fully understanding HIV transmission comes from the impracticality of studying transmission in humans. Because of this encumbrance, the early phases of HIV transmission and systemic dissemination are poorly understood. In order to fully comprehend these critical steps in HIV infection, animal models must be devised to accurately reflect HIV's mode of action. This review seeks to highlight the essential nature of modelling HIV transmission in nonhuman primates (NHPs).

RECENT FINDINGS

Recently, it was discovered that HIV infection is established in newly infected recipients by a single or few transmitted/founder variants. This has reshaped how animal modelling is conducted with researchers currently recapitulating a physiologically relevant, low-titre infection. Pertinent animal models have been established for the most common routes of infection, including rectal, vaginal and penile transmission; models for intravenous and oral transmission are still in developmental stages.

SUMMARY

These limited dose models now accurately reflect HIV transmission in humans and provide a realistic experimental platform for vaccine development and other intervention strategies that can be used to inform vaccine development in humans. Using information obtained in NHP and human trials, it is conceivable to envision effective prevention modalities in the near future.