Efforts to broaden HIV-1-specific immunity by boosting with heterologous peptides or envelope protein and the influence of prior exposure to virus.

In two previous studies, we have demonstrated the successful protection of human immunodeficiency virus type 1 (HIV-1)-vaccinated rhesus macaques from challenge with SHIV(SF13) with envelop immunogens derived from the closely related HIV-1(SF2) strain. Here we report on two follow-up studies in which we aimed to broaden immunity in order to elicit protection from a more diverse heterologous challenge with SHIV(SF33). In the first study, animals were boosted once with HIV-1(SF33) V2 and V3 peptides that were cross-linked to influenza immune-stimulating complexes (ISCOMs). In the second study, monkeys were boosted twice at 12-week intervals, using a heterologous recombinant gp120 derived from HIV-1(SF33) that was either incorporated into ISCOMs or mixed with the MF59 adjuvant. In both studies, the animals were challenged with 50 monkey infectious doses of SHIV(SF33) 4 weeks after the final boost. All controls became readily infected with the heterologous challenge virus SHIV(SF33). Neither boosting with heterologous SF33 peptides or gp120 afforded protection from infection to SF2-vaccinated animals that had previously resisted SHIV(SF13) challenge. These results demonstrate the importance of developing vaccine strategies that are capable of generating broad immune responses early in the immunization protocol. Furthermore, these findings may illustrate the potential pitfalls of early antigenic sin.

An anti-HIV strategy combining chemotherapy and therapeutic vaccination.

Combination chemotherapy using potent anti-retroviral agents has led to significant advances in the clinical management of human immunodeficiency virus (HIV) disease. However, the emergence of multiple drug-resistant mutants, the high need for compliance to adhere to demanding drug-dosing schemes, and the remaining toxic side-effects of drugs make the perspective of life-long treatment unattractive and possibly unrealistic. Therefore, means must be sought to shorten the time span during which treatment is necessary. Such means could be to stimulate an efficient immune response during the period of low virus load and restored CD4 + cell levels, which might be capable of keeping the virus under long-lasting control after treatment is stopped. Here we tested this concept of combined chemotherapy/ therapeutic vaccination in a non-human primate model. Rhesus macaques chronically infected with the chimeric simian/human immunodeficiency virus (SHIV) containing the HIV type 1 (HIV-1) HXBc2 gene for reverse transcriptase (RT) in the genomic background of simian immunodeficiency virus (SIV)(mac239) (RT-SHIV) were treated with (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA), a potent anti-HIV drug. When virus load had decreased significantly, we immunized with SIV genes env, gag/pol, rev, tat, and nef inserted in two different expression vector systems. Four weeks after the second immunization, drug treatment was stopped. Animals were monitored to determine if virus load stayed low or if it increased again to the original levels and if CD4+ T-cell levels remained stable. Humoral and cellular immune responses were also measured. This combined chemotherapy/ therapeutic vaccination regimen induced a significant reduction in the steady-state level of viremia in one out of two chronically infected rhesus macaques. Chemotherapeutic treatment alone did not achieve reduction of viremia in two chronically infected animals. The nature of the immune responses assumed to have been induced by vaccination in one out of the two monkeys remains to be elucidated.

The human immunodeficiency virus type 1 nef gene can to a large extent replace simian immunodeficiency virus nef in vivo.

The nef gene of the pathogenic simian immunodeficiency virus (SIV) 239 clone was replaced with primary human immunodeficiency virus type 1 (HIV-1) nef alleles to investigate whether HIV-1 Nef can substitute for SIV Nef in vivo. Initially, two rhesus macaques were infected with the chimeric viruses (Nef-SHIVs). Most of the nef alleles obtained from both animals predicted intact open reading frames. Furthermore, forms containing upstream nucleotide substitutions that enhanced expression of the inserted gene became predominant. One animal maintained high viral loads and slowly progressed to immunodeficiency. nef long terminal repeat sequences amplified from this animal were used to generate a second generation of Nef-SHIVs. Two macaques, which were subsequently infected with a mixture of cloned chimeric viruses, showed high viral loads and progressed to fatal immunodeficiency. Five macaques received a single molecular clone, named SHIV-40K6. The SHIV-40K6 nef allele was active in CD4 and class I major histocompatibility complex downregulation and enhanced viral infectivity and replication. Notably, all of the macaques inoculated with SHIV-40K6 showed high levels of viral replication early in infection. During later stages, however, the course of infection was variable. Three animals maintained high viral loads and developed immunodeficiency. Of the remaining two macaques, which showed decreasing viral loads after the acute phase of infection, only one efficiently controlled viral replication and remained asymptomatic during 1.5 years of follow-up. The other animal showed an increasing viral load and developed signs of progressive infection during later stages. Our data demonstrate that HIV-1 nef can, to a large extent, functionally replace SIVmac nef in vivo.