The appearance of escape variants in vivo does not account for the failure of recombinant envelope vaccines to protect against simian immunodeficiency virus

The presence or evolution of immune escape variants has been proposed to account for the failure of recombinant envelope vaccines to protect macaques against challenge with simian immunodeficiency virus (SIVmac). To address this issue, two groups of three cynomolgus macaques were immunized with recombinant SIV Env vaccines using two different vaccine schedules. One group of macaques received four injections of recombinant SIV gp120 in SAF-1 containing threonyl muramyl dipeptide as adjuvant. A second group were primed twice with recombinant vaccinia virus expressing SIV gp160 and then boosted twice with recombinant SIV gp120. Both vaccine schedules elicited neutralizing antibodies to Env. However, on the day of challenge, titres of anti-Env antibodies measured by ELISA were higher in macaques primed with recombinant vaccinia virus. Following intravenous challenge with 10 monkey infectious doses of the SIVmac J5M challenge stock, five of the six immunized macaques and all four naive controls became infected. The virus burdens in PBMC of macaques that were primed with recombinant vaccinia virus were lower than those of naive controls, as determined by virus titration and quantitative DNA PCR. Sequence analysis was performed on SIV env amplified from the blood of immunized and naive infected macaques. No variation of SIV env sequence was observed, even in macaques with a reduced virus load, suggesting that the appearance of immune escape variants does not account for the incomplete protection observed. In addition, this study indicates that the measurement of serum neutralizing antibodies may not provide a useful correlate for protection elicited by recombinant envelope vaccines.

Monoclonal antibodies recognize at least five epitopes on the SIV Nef protein and identify an in vitro-induced mutation

Eleven monoclonal antibodies (MAbs) to SIV Nef were produced and characterized. Five antibody-binding sites on SIV Nef were identified on the basis of the reactivity of the antibodies with recombinant proteins. Two of the five epitopes were defined using overlapping peptides. A further three epitopes could not be defined with peptides but all antibodies reacted in Western blot, suggesting that the epitopes were at least partially conformation dependent. Antibodies in two of the five epitope groups were further differentiated by competition analysis. The panel of MAbs described is able to distinguish between a number of recombinant Nef proteins currently under investigation in vivo in macaques. Two of the MAbs described are able to distinguish between the Nef protein from pathogenic (J5) and attenuated (C8) strains of SIV, thus providing useful tools for studying the relevance of the Nef protein in the pathogenesis of SIV infection. In FACScan analysis two of the MAbs, KK70 and KK75, were used to identify an in vitro-induced mutation in J5 Nef grown in C8166 cells. Sequence analysis of the phenotypic variants identified a mutation of the tryptophan (TGG) at amino acid 214 to a stop codon (TGA), thus truncating the Nef protein. The functional significance of this observation remains unclear but highlights the need to interpret data with caution if virus has been cultured in vitro even for a short period of time.

Live attenuated SIV--a model of a vaccine for AIDS

The experimental infection of macaques with simian immunodeficiency virus (SIV) has provided strong evidence that it may be possible to develop a vaccine against AIDS. Live attenuated SIV vaccines have been found to confer the most potent protection against challenge with a variety of pathogenic viruses. This article summarizes the work performed at NIBSC to characterize the protection conferred by live attenuated SIV and to identify mechanisms of vaccine protection. The results of these experiments are discussed in conjunction with observations from related studies made by other groups.

Candidate vaccines protect macaques against primate immunodeficiency viruses

The preclinical evaluation of the efficacy of potential vaccines against AIDS requires challenge models. The experimental infection of macaques with simian immunodeficiency virus, human immunodeficiency virus type 2 (HIV-2) or chimeric viruses have proven to be most valuable. The progress made using simian models to assess the efficacy and identify the correlates or mechanism of protection by whole inactivated virus, live attenuated virus or recombinant sub-unit vaccines is reviewed. It is possible to conclude from these studies that an effective AIDS vaccine is feasible. Furthermore, it is likely that these different vaccine approaches protect through distinct mechanisms. These results provide a scientific basis for the development and selection of suitable candidate human AIDS vaccines for testing in humans.

Mechanisms of protection induced by attenuated simian immunodeficiency virus. II. Lymphocyte depletion does not abrogate protection

To determine the role that cellular immune responses play in the protection conferred by vaccination with attenuated SIVmac32H (pC8), we have attempted to deplete macaques of their CD8+ cells prior to challenge with wild-type SIVmac32H (pJ5). In two of four pC8-infected macaques, N109 and N112, a transient partial depletion of CD8+ cells by antibody treatment was achieved. On the day of challenge peripheral CD2+CD4-CD8+ cell counts were reduced by 92 and 95%, respectively, in animals N109 and N112 and their lymph nodes revealed a 46 and 58% reduction, respectively, in CD2+CD4-CD8+ cells. Two other pC8-immunized macaques, N110 and N111, treated in the same way, did not show significant depletion of CD8+ cells. None of these four pC8-immunized animals became infected when challenged with 50 MID50 of pJ5. Treatment of a further four pC8-infected and protected macaques and two naive control animals with Campath-1H antibody successfully depleted peripheral CD3+ cell counts by >99% in all treated animals. Campath-1H depletion resulted in enhanced, longer lasting lymphoid depletion. Yet subsequent challenge with 20 MID50 of pJ5 still failed to infect the pC8-immunized animals. All eight of the naive controls, including two Campath-1H-treated animals, became infected following challenge. In summary, partial depletion of circulating CD8+ cells or total lymphocytes prior to challenge failed to abrogate the protection conferred by vaccination with pC8.

A clinically relevant HIV-1 subunit vaccine protects rhesus macaques from in vivo passaged simian-human immunodeficiency virus infection

OBJECTIVES

To investigate whether immunization with recombinant HIV-1 envelope protein derived from a clinical isolate could protect macaques from infection with an in vivo passaged chimeric simian-human immunodeficiency virus (SHIV).

DESIGN AND METHODS

A total of 16 animals were studied from which three groups of four animals were immunized with vaccine formulations of the CC-chemokine receptor-5-binding recombinant gp120 of HIV-1W6.1D. Four weeks after the last immunization, all 16 animals were intravenously challenged with in vivo passaged SHIV derived from the same HIV-1 group B clinical isolate (W6.1D) as the vaccines.

RESULTS

Vaccine protection from infection was demonstrated in 10 out of 12 macaques immunized with recombinant gp120. Complete protection from infection was achieved with all of the animals that received the SBAS2-W6.1D formulation, a potent inducer of both T-cell and humoral immune responses. Partial protection was achieved with SBAS1-W6.1D, a formulation based on immunomodulators known to induce T-cell responses in humans. In vaccinated animals that were infected, virus load was reduced and infection was delayed.

CONCLUSIONS

In a relatively large number of primates, vaccine efficacy was demonstrated with a clinically relevant HIV-1 vaccine. These results reveal that it is possible to induce sterilizing immunity sufficient to protect from infection with SHIV which was passaged multiple times in vivo. Our findings have implications for current HIV-1 clinical vaccine trials and ongoing efforts to develop safe prophylactic AIDS vaccines.

Evaluation of a candidate human immunodeficiency virus type 1 (HIV-1) vaccine in macaques: effect of vaccination with HIV-1 gp120 on subsequent challenge with heterologous simian immunodeficiency virus-HIV-1 chimeric virus

Human immunodeficiency virus type 1 (HIV-1) envelope vaccines can now be evaluated for efficacy in macaques by challenging with chimeric viruses in which the env, tat and rev genes of simian immunodeficiency virus (SIV) have been replaced by those of HIV-1. Most experiments have so far been conducted using gp120 molecules derived from T-cell-adapted LAI or MN strains of HIV-1, which predominantly use the CXCR-4 co-receptor. These vaccines protect against infection by apathogenic chimeric virus carrying the same envelope sequences. In the experiment described here, four macaques were vaccinated with W61D gp120 derived from a low passage Dutch isolate and capable of inhibiting the binding of MIP1beta to the co-receptor CCR-5. This vaccine was potent, inducing high titres of binding and neutralizing antibodies against the homologous HIV-1 and tenfold lower titres against a heterologous challenge virus (SHIV(SF33)) in which the env, tat and rev genes of SIV had been replaced by those of a San Francisco isolate, HIV-1(SF33). Despite strong immune responses to the vaccine there was no evidence that it protected against challenge with this chimeric virus. The antigenic divergence between vaccine and challenge virus or the increased virulence of the challenge virus may be responsible for the inability of this vaccine to protect against infection by SHIV(SF33).

Comparison of in vitro and in vivo infectivity of different clade B HIV-1 envelope chimeric simian/human immunodeficiency viruses in Macaca mulatta

The use of HIV-1 env/SIVmac chimeric viruses expressing divergent HIV-1 envelopes of clinical isolates, facilitates homologous and heterologous evaluation of various recombinant HIV-1 envelope vaccine candidates in lower primates. In this study we compare the in vitro and in vivo infectivity, via intravenous (IV) and intravaginal (IVAG) routes of infection, of stocks of chimeric viruses expressing env from four different clade B HIV-1 isolates. The TCID50/ml was 7.1 x 10(4), 1.0 x 10(4), 6.3 x 10(4), and 1.2 x 10(3) for SHIVsf13, SHIVHan2, SHIVNM-3rn, and SHIVW6.1D, respectively, with a MID50/ml upon IV inoculation of 3.2 x 10(3), 3.2 x 10(4), 3.2 x 10(4), and 3.2 x 10(3), respectively. The same SHIVsf13 stock was infectious after IVAG administration, requiring a 300-fold higher virus dose. Plasma antigenemia and cell-associated viremia were generally highest at weeks 2 or 4 after infection and decreased to subdetectable levels after 8-12 weeks. All infected animals tested developed anti-HIV-1 gp120 antibodies. Inoculated virus dose showed no (linear) quantitative correlation with cellular virus load, duration of viremia, plasma antigenemia, and anti-gp120 antibody titers. No significant changes in peripheral blood CD4 cell levels were observed and none of the animals has shown evidence of disease progression to date (i.e., 13 months postinfection). Four in vivo passages of cell-associated SHIVW6.1D did not result in increased virulence. Vaccine development studies in macaques monkeys have become feasible with the use of various clade B HIV-1 env SHIV chimeras.

Mechanisms of protection induced by attenuated simian immunodeficiency virus. I. Protection cannot be transferred with immune serum

To evaluate its role in protection, immune serum was collected from four macaques which were chronically infected with live attenuated simian immunodeficiency virus (SIVmacC8) and had resisted challenge with wild-type SIVmacJ5. The immune serum was transferred to two naive cynomolgus macaques by intraperitoneal injection (11 ml/kg). Four control macaques received an intraperitoneal injection of normal saline. One day later, all macaques were challenged with 10 MID50 of the J5M challenge stock of SIV. After challenge, all macaques became infected as determined by virus co-culture and diagnostic PCR. Virus loads in PBMC at 2 weeks post-challenge were indistinguishable between the two groups of macaques. Thus, the failure of passive immunization to transfer protection indicates that serum components alone are not sufficient to mediate the potent protection obtained using live attenuated vaccines. This is the first time that serum has been transferred from animals known to be protected against superinfection.

Evasion of cytotoxic T lymphocyte (CTL) responses by nef-dependent induction of Fas ligand (CD95L) expression on simian immunodeficiency virus-infected cells

Inoculation of macaques with live attenuated SIV strains has been shown to protect against subsequent challenge with wild-type SIV. The protective mechanism(s) remain obscure. To study the effect in more detail, we have investigated the role of virus-specific CTL responses in macaques infected with an attenuated SIV strain (pC8), which has a four-amino acid deletion in the nef gene, as compared with the wild-type SIVmac32H clone (pJ5). Cynomolgus macaques infected with pC8 were protected against subsequent challenge with pJ5 and did not develop any AIDS-like symptoms in the 12 months after infection. The pC8-induced protection was associated with high levels of virus-specific CTL responses to a variety of viral antigens. In contrast, pJ5-infected macaques had little, if any, detectable CTL response to the viral proteins after three months. The latter group of macaques also showed increased Fas expression and apoptotic cell death in both the CD4(+) and CD8(+) populations. In vitro, pJ5 but not pC8 leads to an increase in FasL expression on infected cells. Thus the expression of FasL may protect infected cells from CTL attack, killing viral-specific CTLs in the process, and providing a route for escaping the immune response, leading to the increased pathogenicity of pJ5. pC8, on the other hand does not induce FasL expression, allowing the development of a protective CTL response. Furthermore, interruption of the Fas-FasL interaction allows the regeneration of viral-specific CTL responses in pJ5-infected animals. This observation suggests an additional therapeutic approach to the treatment of AIDS.

The construction and evaluation of SIV/HIV chimeras that express the envelope of European HIV type 1 isolates

The molecular construction of SIV/HIV-1 chimeric viruses (or SHIVs), provides a means of infecting macaques with immunodeficiency viruses that express the envelope protein of HIV-1. However, to date, most SHIVs produced express the envelope of isolates of HIV-1 that have been passaged repeatedly in T cell lines. We have taken SHIV-4 and replaced an NheI-AvrII fragment that encompasses the gp120 region and the extracellular portion of gp41 with the equivalent region of two European isolates of HIV-1 (ACH320.3.1 and HIV-1Han-2). Neither of these viruses had been passaged in T cell lines for prolonged periods prior to molecular cloning. Virus stocks were prepared of both SHIV constructs. In vitro, the relative ability of each clone to replicate in four T cell lines mirrored closely the pattern observed with the parental virus donating the envelope sequences. In vivo, only one of the chimeric viruses was infectious in cynomolgus macaques and its recovery was transient. The factors that affect the replication of SHIVs in vitro and in vivo are discussed.

Mechanisms of protection induced by attenuated simian immunodeficiency virus. IV. Protection against challenge with virus grown in autologous simian cells

Attenuated simian immunodeficiency virus (SIV) induces potent protection against infection with wild-type virus, but the mechanism of this immunity remains obscure. Allogeneic antibodies, which arise within animals as a result of SIV infection, might protect against challenge with exogenous SIV grown in allogeneic cells. To test this hypothesis, eight macaques were infected with attenuated SIV and subsequently challenged with wild-type SIV grown in autologous cells or heterologous cells. The results clearly demonstrated that animals infected with attenuated SIV are protected against wild-type SIV grown in autologous or heterologous cells. Thus, the hypothesis that live attenuated SIV protects by the induction of allogeneic antibodies is not tenable.

Similar patterns of simian immunodeficiency virus env sequences are found in the blood and lymphoid tissues of chronically infected macaques

Two cynomolgus macaques were infected with a genetically complex challenge stock of simian immunodeficiency virus (SIVmac251-32H). One animal developed SIV-induced disease and was sacrificed at 16 months postinfection. The second remained healthy until it too was sacrificed at 20 months postinfection. The polymerase chain reaction (PCR) was used to amplify env gp120-coding sequences from provirus present in samples of blood, spleen, and inguinal lymph node taken from both animals on the day of sacrifice. The proviral burden present in each of the tissue samples was also determined using a quantitative PCR assay. The proviral burdens in the blood, spleen, and inguinal lymph node of the healthy animal (I225) were similar. This was not the case for animal I227, in which the burden in the inguinal lymph node was much higher than for blood or spleen. Phenogram analysis of the hypervariable V1 region of env revealed that the diversity of nucleotide sequences recovered from each tissue of both macaques were similar and overlapping. Some selected amino acid differences were observed that were specific for a tissue or one of the macaques. However, the results do not suggest that the overall evolution of env in provirus populations recovered from lymphoid tissues is distinct from that recovered from the blood.

Early suppression of SIV replication by CD8+ nef-specific cytotoxic T cells in vaccinated macaques

In order to develop a successful subunit vaccine against infection with the human immunodeficiency virus (HIV), protective immune effector functions must be identified. Until now, there has been only indirect evidence that HIV-specific cytotoxic T lymphocytes (CTLs) fulfill this role. Using the macaque simian immunodeficiency virus (SIV) model, the protective potential of nef-specific CTLs, stimulated by vaccination, was examined in animals challenged with a high intravenous dose of the pathogenic simian immunodeficiency virus, SIVmac251(32H)(pJ5). An inverse correlation was found between the vaccine-induced nef-specific CTL precursor frequency and virus load measured after challenge. In addition, the early decline in viraemia, observed in both vaccinated and unvaccinated control animals was associated with the development of virus-specific CTL activity and not with the presence of virus-specific neutralizing antibodies. The results imply that vaccines that stimulate strong CTL responses could protect against HIV infection.

Protection by attenuated simian immunodeficiency virus in macaques against challenge with virus-infected cells

A vaccine against AIDS will probably have to protect against challenge both by viable virus-infected cells and by cell-free virus. Eight cynomolgus macaques infected with attenuated simian immunodeficiency virus (SIV) were challenged (four each) with cell-free and cell-associated SIV. All were protected, whereas eight controls were all infected after challenge. These findings show that live-attenuated vaccine can confer protection against SIV in macaques. Extrapolation to human beings will require extensive evaluation of the safety of attenuated retroviruses. Alternatively, the mechanism of this potent protection must be understood and reproduced by less hazardous means.

Immunization with class I human histocompatibility leukocyte antigen can protect macaques against challenge infection with SIVmac-32H

OBJECTIVE

To evaluate the efficacy of immunopurified class I human histocompatibility leukocyte antigen (HLA) to protect against SIV infection.

METHODS

HLA class I antigens were immunopurified from a human B-lymphoblastoid cell line. Groups of four macaques were vaccinated subcutaneously with four doses of the immunogen in adjuvant, or with adjuvant alone and subsequently challenged intravenously with 10 median monkey infectious doses of cell-free SIVmac-32H. Infection was determined by polymerase chain reaction for SIVmac proviral DNA and by virus isolation. Antigen-specific humoral and cellular immune responses were monitored.

RESULTS

Macaques immunized with the HLA molecules produced anti-HLA class I antibodies that inhibited SIV replication in vitro and downregulated autologous T-cell proliferation against irradiated C8166 cells. They were partially protected (two out of four) from virus infection for at least 33 weeks when challenged with SIV grown in human cells. All four control animals were infected.

CONCLUSIONS

This demonstration of partial protection, together with our previous work reporting that vaccination with allogenic cynomolgus lymphocytes can protect against challenge infection with SIV grown in simian cells, suggests that allogenic immune response induced before or during establishment of HIV infection may have important implications for AIDS disease progression.

The development of PCR based assays for the detection and differentiation of simian immunodeficiency virus in vivo

Polymerase chain reaction based assays, which amplify a region of the gag gene, have been developed for the direct detection of simian immunodeficiency virus (SIV) DNA sequences in the blood of experimentally infected cynomolgus macaques. In macaques infected with a characterised virus pool (11/88 pool SIVmac 32H), an assay employing a single round of amplification was found to be highly sensitive and specific. However, in animals infected with the SIV molecular clones J5 and C8 (Rud et al., J. Gen. Virol. 75, 529-543), it was necessary to use two rounds of amplification and nested primer pairs in order to achieve sensitivity > 90%. In order to differentiate macaques infected with either of the two genetically distinct SIV clones, J5 or C8, a third PCR based assay has been developed, which amplifies a 492 bp region of the nef gene. Sequence differences between the nef genes of the two molecular clones enabled the PCR product amplified from each virus to be distinguished by restriction analysis. These sensitive and specific assays complement virological detection of SIV and enable superinfection studies to be evaluated; a prerequisite for the testing of live attenuated immunodeficiency virus vaccines.

Absence of lentiviral and human T cell leukemia viral sequences in patients with rheumatoid arthritis

OBJECTIVE

The etiology of rheumatoid arthritis (RA) is unknown, and the possibility that an infectious agent is involved has not been excluded. Lentiviruses can cause chronic arthritis in humans and in animals and have been suggested as candidate agents in RA. We therefore tested for the presence of lentiviruses and also for human T cell leukemia virus type I (HTLV-I)/HTLV-II in cells from patients with RA.

METHODS

We used the polymerase chain reaction with degenerate primers designed to recognize highly conserved nucleotide sequences from 5 different pathogenic lentiviruses. This method allowed the detection of at least 1 infected cell/20,000 uninfected cells in control experiments.

RESULTS

Testing of synovial cells and blood cells from patients with early RA and patients with established RA did not yield any specific viral product.

CONCLUSION

Our results do not support the presence of lentiviruses or HTLV-like sequences in RA.

Complex splicing of simian immunodeficiency virus (mac 251-32H) rev gene transcript from infected macaques

Sequences of the simian immunodeficiency virus (SIV) rev mRNA transcripts were characterized from peripheral blood lymphocytes (PBLs) of two macaques experimentally infected with SIVmac251 (32H reisolate). This analysis has demonstrated that the complex splicing patterns observed for the rev mRNA transcripts originally identified in vitro is not an artifact of tissue culture, but is also found in vivo.

Purification of crystallizable recombinant SIVmac251-32H proteinase

We have cloned a simian immunodeficiency virus (SIV) proteinase gene directly from proviral DNA of the infectious viral stock SIVmac251-32H (11/88 pool). The deduced amino acid sequence from this proteinase gene is similar to that for the published SIVmac239 molecular clone. SIVmac251-32H proteinase (SIV PR) and its flanking pol sequences were expressed in Escherichia coli as a fusion protein with most of the T7 bacteriophage gene 10 protein. The expressed protein formed cytoplasmic inclusion bodies which were solubilized in 8 M urea, and the recombinant SIV PR was refolded, yielding active, self-processed enzyme. The SIV PR was purified to homogeneity using a single pepstatin A affinity chromatography step, and had a specific peptidolytic activity of 20 mumol/min/mg. Enzymatic characteristics similar to those previously documented for other immunodeficiency virus proteinases (EC 3.4.23) were observed. These include an acidic pH optimum (pH 5.3), sensitivity to sodium chloride concentration, and complete inhibition by pepstatin A. In addition to these properties we have observed quantitative crystallization from low protein concentrations. We describe the first crystal habit for the proteinase from the HIV-2/SIV class of immunodeficiency virus, which is distinctly different from that for HIV-1 proteinase crystals.

Sequence variation in the env gene of simian immunodeficiency virus recovered from immunized macaques is predominantly in the V1 region

Three cynomolgus macaques were immunized with recombinant envelope protein preparations derived from simian immunodeficiency virus (SIV). Although humoral and cellular responses were elicited by the immunization regime, all macaques became infected upon challenge with 10 MID50 of the 11/88 virus challenge stock of SIVmac251-32H. The polymerase chain reaction was used to amplify proviral SIV gp120 sequences present in the blood of both immunized and control macaques at 2 months post-infection. A comparison of the predominant sequences found in the region from V2 to V5 of gp120 failed to differentiate provirus recovered from either immunized or control animals. A detailed investigation of sequences obtained from the hypervariable V1 region identified a mixture of sequences in both immunized and control macaques. Some sequences were identical to those previously detected in the virus challenge stock, whereas others had not been detected previously. Phenogram analysis of the new V1 sequences found in immunized animals revealed that they were quite distinct from those from the virus challenge stock and that they included alterations to potential N-linked glycosylation sites. In contrast, new sequence variants recovered from the control animals were closely related to sequences from the virus challenge stock. The difference in diversity of new V1 sequences recovered from immunized and control macaques was highly significant (P < 0.001). Thus, the presence of pre-existing immune responses to SIV envelope protein is associated with greater genetic change in the V1 region of gp120. These data are discussed in relation to the epitopes of SIV gp120 that may confer protection from in vivo challenge.

Simian immunodeficiency virus (mac 251-32H) transmembrane protein sequence remains conserved throughout the course of infection in macaques

Two cynomolgus macaques were infected with a genetically complex challenge stock of simian immunodeficiency virus (SIVmac251-32H). The polymerase chain reaction (PCR) was used to amplify the env gp41, rev, and nef overlapping coding sequences from provirus present in the blood of both animals at 1, 6, and 15 months post infection (p.i.). The predominant, env sequences found in both animals at the three time points were very similar to that found in the original 11/88 challenge stock. The functionally important hydrophobic fusion and membrane-spanning domains within gp41 remained conserved throughout the course of infection. Nucleotide variation within the region corresponding to the REV response element (RRE) was limited to four positions, none of which were predicted to cause any significant disruption to the secondary structure of the RRE. Very little genetic variation was observed in and around the cluster of potential glycosylation sites of the external portion of gp41. However, the existence of a previously assigned variable region elsewhere in the cytoplasmic domain of gp41 was confirmed. The three gene loci (env, rev, and nef) examined varied independently. All changes in the predominant protein sequences were brought about by single nucleotide substitutions only. After 15 months of infection with SIV, 1 animal was sick from SIV-induced disease whereas the other remained healthy. In-frame stop codons within the transmembrane protein occurred with a much greater frequency in the healthy animal.