Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins

The cytoplasmic domains of the transmembrane (TM) envelope proteins (TM-CDs) of most retroviruses have a Tyr-based motif, YXXO, in their membrane-proximal regions. This signal is involved in the trafficking and endocytosis of membrane receptors via clathrin-associated AP-1 and AP-2 adaptor complexes. We have used CD8-TM-CD chimeras to investigate the role of the Tyr-based motif of human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV), and human T-leukemia virus type 1 (HTLV-1) TM-CDs in the cell surface expression of the envelope glycoprotein. Flow cytometry and confocal microscopy studies showed that this motif is a major determinant of the cell surface expression of the CD8-HTLV chimera. The YXXO motif also plays a key role in subcellular distribution of the envelope of lentiviruses HIV-1 and SIV. However, these viruses, which encode TM proteins with a long cytoplasmic domain, have additional determinants distal to the YXXO motif that participate in regulating cell surface expression. We have also used the yeast two-hybrid system and in vitro binding assays to demonstrate that all three retroviral YXXO motifs interact with the micro1 and micro2 subunits of AP complexes and that the C-terminal regions of HIV-1 and SIV TM proteins interact with the beta2 adaptin subunit. The TM-CDs of HTLV-1, HIV-1, and SIV also interact with the whole AP complexes. These results clearly demonstrate that the cell surface expression of retroviral envelope glycoproteins is governed by interactions with adaptor complexes. The YXXO-based signal is the major determinant of this interaction for the HTLV-1 TM, which contains a short cytoplasmic domain, whereas the lentiviruses HIV-1 and SIV have additional determinants distal to this signal that are also involved.

Different evolution of simian immunodeficiency virus in a natural host and a new host

To address the mechanisms of host-virus adaptation and pathogenesis of lentiviral infections, we compared the evolution of the same isolate of simian immunodeficiency virus (SIVsmm9) in two different situations: nonpathogenic infection of its natural host, the sooty mangabey, and AIDS-inducing infection of a new host, the rhesus macaque. Samples were obtained at 6, 12, and 23 or 30 months postinfection from three animals of each species. Sequences were derived from the V1 and V2 domains of the surface glycoprotein. In the macaques, we observed specific variations absent from all mangabey samples, indicating that different host species select different virus variants. In the macaques, we also observed a different shape in the phylogenetic tree, a lower divergence of sibling sequences, and a lower synonymous/nonsynonymous change ratio than in the mangabeys. This suggests that the viral population is larger and submitted to weaker selection pressures when host-virus adaptation is achieved, such as in the mangabey.

Apparent enhancement of perinatal transmission of human immunodeficiency virus type 1 by high maternal anti-gp160 antibody titer

The presence of antibodies able to enhance infection in vitro in sera from human immunodeficiency virus (HIV)-1-infected patients raises the possibility that antibodies exert a deleterious activity during natural infection. The anti-HIV-1 humoral response and plasma HIV-1 RNA were measured in a cohort of 98 infected mothers, included in the French Prospective Study on Pediatric HIV Infection, 49 of whom transmitted HIV to their children. Transmission from mother to child was associated with antibody responses to the envelope gp160 (P = .009 for serum dilution of 1/400) and to a highly conserved domain of the transmembrane glycoprotein (P = .055 for serum dilution of 1/400) and with plasma HIV-1 RNA levels (P < .0001). Multivariate logistic regression indicated that a high anti-gp160 response and a high plasma virus load are independent risk factors for perinatal transmission of HIV-1 (odds ratio, 3.4; 95% confidence interval, 1.1-9.9 for anti-gp160; odds ratio, 2.8; 95% confidence interval, 1.6-5.0 for virus load).

Features of the SIVmac transmembrane glycoprotein cytoplasmic domain that are important for Env functions

The cytoplasmic domain (CD) of the SIVmac transmembrane protein (TM) can affect viral infectivity by modulating several Env functions, notably fusogenic capacity and incorporation into virions. In addition, envelopes with a truncated CD are counterselected in primary cells in culture and in vivo in rhesus macaques, suggesting a role for this domain in viral persistence. Here, we have used mutagenesis to examine specific features of the SIVmac TM CD, including the conserved C-terminal alpha helix and the overall length of the CD. Several mutations dramatically reduced and/or delayed virus infectivity in lymphoid cell lines. Detailed analysis of mutants revealed defects in envelope stability, fusogenic capacity, and virion incorporation. The primary defect associated with an envelope containing a 64-residue CD was rapid degradation. A mutant Env lacking the C-terminal alpha helix but encoding an exceptionally long CD (373 residues) was highly fusogenic but inefficiently incorporated into virions. A third mutant, containing amino acid substitutions designed to alter the charge density of the C-terminal helix, retained cytopathic properties and showed enhanced fusogenic capacity but replicated with delayed kinetics. Taken together, these results demonstrate that CD sequence variation entails functional "tradeoffs" that can involve optimization of certain Env functions at the expense of others.

Delayed infection after immunization with a peptide from the transmembrane glycoprotein of the feline immunodeficiency virus

Recent advances in the quantitative assessment of viral burden, by permitting the extension of criteria applied to assess the efficacy of vaccines from all-or-none protection to diminution of the viral burden, may allow the identification of original immunogens of value in combined vaccines. Peptides corresponding to three domains of the envelope glycoproteins of feline immunodeficiency virus that are recognized during natural infection were used to immunize cats. After challenge with a primary isolate of feline immunodeficiency virus, the development of acute infection was monitored by quantitative assessment of the viral burden in plasma and tissues by competitive reverse transcription-PCR, by measurement of the humoral response developed to viral components, and by lymphocyte subset analysis. Whereas immunization with two peptides derived from the surface glycoprotein had no effect on the early course of infection, immunization with a peptide derived from the transmembrane glycoprotein delayed infection, as reflected by a diminished viral burden in the early phase of primary infection and delayed seroconversion. This peptide, located in the membrane-proximal region of the extracellular domain, has homology to an epitope of human immunodeficiency virus type 1 recognized by a broadly neutralizing monoclonal antibody. These results suggest that lentivirus transmembrane glycoproteins share a determinant in the juxtamembrane ectodomain which could be of importance in the design of vaccines against AIDS.

Enhancement of feline immunodeficiency virus (FIV) infection after DNA vaccination with the FIV envelope

Despite intensive experimentation to develop effective and safe vaccines against the human immunodeficiency viruses and other pathogenic lentiviruses, it remains unclear whether an immune response that does not afford protection may, on the contrary, produce adverse effects. In the present study, the effect of genetic immunization with the env gene was examined in a natural animal model of lentivirus pathogenesis, infection of cats by the feline immunodeficiency virus (FIV). Three groups of seven cats were immunized by intramuscular transfer of plasmid DNAs expressing either the wild-type envelope or two envelopes bearing mutations in the principal immunodominant domain of the transmembrane glycoprotein. Upon homologous challenge, determination of plasma virus load showed that the acute phase of viral infection occurred earlier in the three groups of cats immunized with FIV envelopes than in the control cats. Genetic immunization, however, elicited low or undetectable levels of antibodies directed against envelope glycoproteins. These results suggest that immunization with the FIV env gene may result in enhancement of infection and that mechanisms unrelated to enhancing antibodies underlay the observed acceleration.

Retention of viral infectivity after extensive mutation of the highly conserved immunodominant domain of the feline immunodeficiency virus envelope

In lentiviruses, including human immunodeficiency virus and feline immunodeficiency virus (FIV), the principal immunodominant domain (PID) of the transmembrane glycoprotein elicits a strong humoral response in infected hosts. The PID is marked by the presence of two cysteines that delimit a sequence, composed of five to seven amino acids in different lentiviruses, which is highly conserved among isolates of the same lentiviral species. While the conservation of the sequence suggests the presence of functional constraints, the conservation of the immunodominance among divergent lentiviruses raises the hypothesis of a selective advantage for the infecting virus conferred by the host humoral response against this domain. We and others have previously shown that an appropriate structure of the PID is required for the production of a functional envelope. In the present work, we analyzed virological functions and immune reactivity of the envelope after random mutagenesis of the PID of FIV. We obtained nine mutant envelopes which were correctly processed and retained fusogenic ability. Mutation of the two C-terminal residues of the PID sequence between the cysteines in a molecular clone of FIV abolished infectivity. In contrast, three molecular clones containing extensive mutations in the four N-terminal amino acids were infectious. However, the mutations affected PID reactivity with sera from infected cats. Our results suggest that functional constraints, although existent, are not sufficient to account for PID sequence conservation. Such conservation may also result from positive selection by anti-PID antibodies which enhance infection.

The evolutionary rate of nonpathogenic simian immunodeficiency virus (SIVagm) is in agreement with a rapid and continuous replication in vivo

African green monkeys (AGMs) are divided into four species (Cercopithecus aethiops, C. pygerythrus, C. sabaeus, C. tantalus), each harboring a species-specific simian immunodeficiency virus (SIVagm). Little is known about the host and/or viral factors that are responsible for the apathogenicity of SIVagm infections in their natural hosts. In order to analyze the specific selective pressures exerted by the host on the virus in vivo, we compared the genetic evolution of SIVagm.tan in its natural host (C. tantalus) and in a foreign host species (Erythrocebus patas), in which we could obtain a reproducible and persistent infection by SIVagm.tan. As in AGMs, patas monkeys do not develop any disease following SIVagm infection. Our longitudinal study in env (V3-C3-V4-C4-V5) of SIVagm.tan from three AGMs and three patas monkeys revealed a high ratio of synonymous to nonsynonymous mutation frequencies (1.5-6.2). These data indicate that the selective pressures for stability exerted by AGMs and patas monkeys on SIVagm override positive selection for change reported in pathogenic HIV-1 infections. The rapid accumulation of mutations observed in AGMs and patas monkeys (0.4-7.2 x 10(-2) nucleotide substitutions per site per year) suggests a continuous replication of SIVagm viruses in vivo. We thus propose that nonpathogenic SIVagm infections are the result of a long-term selection of SIVagm variants whose dissemination can be controlled in the host, rather than being explained by a low ability of the virus to replicate in vivo.

Conversion of thymidylate synthase into an HIV protease substrate

Thymidylate synthase (TS) is an essential enzyme of DNA metabolism. We have carried out an extensive insertional mutagenesis of the Escherichia coli TS gene (thyA) using three different methods. Insertion of exogenous sequences at unique restriction sites or at random positions produced defective mutants, whereas comparison of TS sequences from different species allowed us to identify six zones permissive for insertions of exogenous sequences. The insertion of Human immunodeficiency virus type 1 (HIV-1) protease substrate sequences into the permissive sites converted TS to an HIV-1 protease substrate, and the in vivo cleavage of these insertions by the cloned HIV-1 protease conferred a thymidylate synthase-deficient phenotype in some of our E. coli mutant strains. In agreement with crystallographic data, these results show that the permissive sites are located in regions of the TS protein not essential for enzyme activity and accessible to cleavage by HIV protease. These results also show that it is possible to control a growth phenotype in E. coli through the protease-mediated destruction of an essential metabolic enzyme. Because both wild type and thymidylate synthase-deficient phenotypes are selectable on the appropriate growth medium, these thyA mutants could be used for genetic selections of protease inhibitors and analysis of protease specificities.

TM domain swapping of murine leukemia virus and human T-cell leukemia virus envelopes confers different infectious abilities despite similar incorporation into virions

We investigated the influence of transmembrane protein (TM) domains on incorporation of retroviral envelopes into virions and on infectivity. We introduced complete, truncated, or chimeric Friend murine leukemia virus (F-MuLV) and human T-cell leukemia virus type 1 (HTLV-1) envelopes into an MuLV particle-producing complementation cell line. As shown previously for HTLV-1 envelopes containing extracellular domains of F-MuLV TM (C. Denesvre, P. Sonigo, A. Corbin, H. Ellerbrok, and M. Sitbon, J. Virol. 69:4149-4157, 1995), reverse chimeric F-MuLV envelopes containing the extracellular domain of HTLV-1 TM were not processed. In contrast, a chimeric MuLV envelope containing the entire HTLV membrane-spanning and cytoplasmic domains (FHTMi) was efficiently processed, fusogenic as tested in a cell-to-cell assay, and efficiently incorporated into MuLV particles. However, these MuLV particles bearing FHTMi envelope proteins could not infect mouse or rat cells which are susceptible to wild-type F-MuLV. Therefore, envelopes which are readily fusogenic in cell-to-cell assays and also efficiently incorporated into virions may not necessarily confer virus-to-cell fusogenicity. HTLV envelopes, whether parental, chimeric (containing the MuLV cytoplasmic tail) or with a truncated cytoplasmic domain, were incorporated into MuLV particles with equal efficiencies, indicating that the cytoplasmic tails of these envelopes did not determine their incorporation into virions. In contrast to FHTMi envelope, HTLV-1 envelopes with F-MuLV membrane-spanning and cytoplasmic domains, as well as wild-type HTLV-1 envelopes, conferred virion infectivity. These results help to define requirements for envelope incorporation into retroviral particles and their cell-free infectivity.

Neutralization sensitivity and accessibility of continuous B cell epitopes of the feline immunodeficiency virus envelope

Antibodies elicited during natural infection of domestic cats by the feline immunodeficiency virus (FIV) recognize continuous epitopes in nine domains of the virus envelope glycoproteins. Whereas antibodies directed against the V3 envelope region can neutralize laboratory-adapted virus, neutralization of FIV has been shown to depend upon cellular substrate, and virus adaptation to laboratory cell lines may alter sensitivity to neutralizing antibodies. We therefore undertook a systematic analysis of the continuous B cell epitopes of the envelope of a primary FIV isolate, Wo. The capacity of feline antisera elicited against nine envelope domains to neutralize primary and laboratory-adapted virus was evaluated in feline peripheral blood mononuclear cells (PBMC). The laboratory-adapted strain Petaluma was used to compare neutralization in PBMC and Crandell feline kidney cells (CrFK). Antibodies specific for the V3 region neutralized both primary and laboratory-adapted virus whether residual infectivity was measured in CrFK or in feline PBMC. However, a large discrepancy in the efficiency of neutralization was observed in these ex vivo models of infection, perhaps reflecting diversity in the interaction between virus and different cellular targets. We also examined the accessibility of epitopes on the functional oligomeric envelope complex of FIV. Most of the epitopes were poorly exposed on native envelope glycoproteins at the surface of live infected cells. The most accessible domain was the only domain sensitive to neutralizing antibodies. These results suggest that inaccessibility on oligomeric envelope glycoproteins may frequently underlie the insensitivity of diverse lentivirus B cell epitopes to neutralization.

Prenatal MRI diagnosis of fetal cerebral tuberous sclerosis

Tuberous sclerosis (TS) is an autosomal dominant phakomatosis. A high percentage of spontaneous mutations leads to the diagnosis of new cases in normal families. This diagnosis is suspected at antenatal ultrasound on the discovery of multiple cardiac tumors. Antenatal cerebral ultrasound shows a normal appearance in affected fetuses. Eight fetuses with multiple cardiac tumors were studied with antenatal MRI with, in five cases, an abnormal appearance showing hyperintense subependymal and cortical nodules on T1-weighted images. Among the three remaining patients MRI was non-contributive in one due to movement artefact, one had abnormal postnatal MRI consistent with TS and one had a normal postnatal and clinical examination. We conclude that MRI is a valuable tool in making the diagnosis of TS in fetuses with multiple cardiac tumors.

Prenatal MR diagnosis of a thick corpus callosum

Fetal sonography revealed cerebral, facial, and genitourinary abnormalities, prompting MR at 33 weeks' gestational age. Cerebral MR confirmed a thickened corpus callosum and showed open sylvian fissures, abnormal gyri in the frontal lobes, and presumed neuronal heterotopias. An abortion was performed at 34 weeks' gestational age, and pathologic findings corresponded well to the MR manifestations.

Highly attenuated SIVmac142 is immunogenic but does not protect against SIVmac251 challenge

We report here the use of the highly attenuated SIVmac142 clone, unable to establish permanent infection of rhesus macaques, in a vaccine trial. Four rhesus macaques were immunized over a long time period with HUT-78 cells infected with wild-type SIVmac142 or, in order to reinforce the safety use of the vaccine, a deleted mutant with similar in vitro infectivity. The first two injections were done with living cells and the remaining boosts with cells emulsified in muramyl dipeptide adjuvant. Three control macaques were injected with uninfected HUT-78 cells. Over 3 years, we have been unable except once to detect viral infections by three methods. However, antibodies directed against the viral Gag proteins and envelope glycoproteins were detected by immunoblots and/or in vitro neutralization assays. All macaques were challenged intravenously with a low dose (10 animal infectious doses) of a highly pathogenic biological clone of SIVmac251 grown on macaque PBMCs. All seven animals became persistently viremic following challenge. The cell-associated viral loads of the vaccinated monkeys were not reduced relative to those of unvaccinated controls during the first weeks postchallenge even if vaccinated monkeys did not present a transient CD4 decrease. Thus, our data reinforced the notion that the efficacy of live attenuated SIV requires the establishment of persistent infection.

Comparison of serological tests for the diagnosis of feline immunodeficiency virus infection of cats

A new enzyme-linked-immunosorbent assay (ELISA) for the detection of antibodies to feline immunodeficiency virus was compared with previously described ELISAs. Serum samples from 184 infected or uninfected cats were tested using a whole virus lysate kit and ELISAs based on recognition of one of two synthetic peptides (P237 and P253) localized in the transmembrane domain of the viral envelope. The whole virus lysate commercial kit led to the detection of 6% false positive and 4.3% false negative sera. The ELISA based on peptide P253 gave no false positive result and failed to detect only one serum that was subsequently shown to be positive by radio-immunoprecipitation assay. A sandwich-ELISA test using Galanthus nivalis agglutinin, a lectin that specifically binds terminal mannose groups of the envelope proteins was used as a confirmatory test for equivocal results with peptide ELISA and gave similar results. This study indicates that recognition of P253 could serve as a sensitive and specific test for the diagnosis of seropositivity to feline immunodeficiency virus, and moreover that the Galanthus nivalis ELISA could be useful in equivocal cases as a confirmatory test.

Influence of transmembrane domains on the fusogenic abilities of human and murine leukemia retrovirus envelopes

The envelopes of two highly divergent oncoviruses, human T-cell leukemia virus type 1 (HTLV-1) and Friend murine leukemia virus (F-MuLV), have distinct patterns of cellular receptor recognition, fusion, and syncytium formation. To analyze the influence of the transmembrane envelope subunit (TM) on fusogenic properties, we substituted either the entire TM or distinct domains from F-MuLV for the corresponding domains in the HTLV-1 envelope. Parental, chimeric, and truncated envelopes cloned into a eukaryotic expression vector were monitored for fusogenic potential in human, rat, and murine indicator cell lines by using a quantitative assay. This highly sensitive assay allowed us to assess the fusogenic properties and syncytium-forming abilities of the HTLV-1 envelope in murine NIH 3T3 cells. All chimeric envelopes containing extracellular sequences of the F-MuLV TM were blocked in their maturation process. Although deletions of the HTLV-1 cytoplasmic domain, alone and in combination with the membrane-spanning domain, did not prevent envelope cell surface expression, they impaired and suppressed fusogenic properties, respectively. In contrast, envelopes carrying substitutions of membrane-spanning and cytoplasmic domains were highly fusogenic. Our results indicate that these two domains in F-MuLV and HTLV-1 constitute structural entities with similar fusogenic properties. However, in the absence of a cytoplasmic domain, the F-MuLV membrane-spanning domain appeared to confer weaker fusogenic properties than the HTLV-1 membrane-spanning domain.

Erratic G-->A hypermutation within a complete caprine arthritis-encephalitis virus (CAEV) provirus

The complete nucleotide sequence of an integrated provirus of caprine arthritis-encephalitis virus (CAEV) has been determined. The provirus was defective due to extensive G-->A hypermutation. Rather than being a smooth phenomenon distributed throughout the genome it was highly erratic with hypermutated and normal regions being juxtaposed, probably reflecting local fluctuations in the intracellular dCTP pool during reverse transcription of the CAEV genome. The pattern of sequence variation within the surface glycoproteins differs subtly from that of the primate lentiviruses.

Continuous secretion of human soluble CD4 in mice transplanted with genetically modified cells

Somatic transgenesis can be used to confer endogenous production of proteins with therapeutic properties. One such product, recombinant soluble human CD4 (sCD4), has been shown to be an efficient inhibitor of human immunodeficiency virus 1 (HIV-1) in vitro, but its too short half-life in vivo has impaired long-term clinical trials in AIDS patients. Using a retroviral vector, we introduced the cDNA of sCD4 into primary mouse fibroblasts. The cells were enclosed in a lattice of collagen and synthetic fibers coated with basic fibroblast growth factor, and implanted in the peritoneal cavity of syngeneic mice. Implantation of such sCD4-secreting organoids into cyclosporin A-treated C3H mice elicited a strong antibody response against sCD4. Implantation of sCD4-secreting organoids into immunotolerant mice (transgenic for transmembrane human CD4) resulted in continuous sCD4 production, detected during 60 days in animal sera. The serum levels obtained were significant, but too limited as yet for anti-HIV purposes. Nevertheless, this model may be of interest in various fields, as it provides the first demonstration that one potentially therapeutic protein, despite its half-life of a few hours, could remain present in vivo 2 months after a single somatic transgenesis.

Structural analysis of the principal immunodominant domain of the feline immunodeficiency virus transmembrane glycoprotein

In the transmembrane envelope glycoprotein (TM) of lentiviruses, including human immunodeficiency virus type 1 (HIV-1) and feline immunodeficiency virus (FIV), two cysteine residues, conserved in most retroviruses, are thought to form a loop containing five to seven amino acids. These elements make up a B-cell epitope recognized by nearly 100% of sera from infected patients or animals, designated the principal immunodominant domain (PID). The PID amino acid sequences are highly conserved between isolates of the same lentivirus but are unrelated, except for the two cysteines, when divergent lentiviruses are compared. The aim of this study was to analyze the relationship between amino acid sequence in the PID and envelope function. We introduced two kinds of mutations in the PID of FIV: mutations which impeded the formation of a loop and mutations which substituted the sequence of FIV with the corresponding sequences from other lentiviruses, HIV-1, visna virus, and equine infectious anemia virus. We analyzed antibody recognition, processing, and fusogenic properties of the modified envelopes, using two methods of Env expression: a cell-free expression system and transfection of a feline fibroblast cell line with gag-pol-deleted FIV proviruses. Most mutations in the PID of FIV severely affected envelope processing and abolished syncytium formation. Only the chimeric envelope containing the HIV-1 PID sequence was correctly processed and maintained the capacity to induce syncytium formation, although less efficiently than the wild-type envelope. We computed three-dimensional structural models of the PID, which were consistent with mutagenesis data and confirmed the similarity of FIV and HIV-1 PID structures, despite their divergence in amino acid sequence. Considering these results, we discussed the respective importance of selection exerted by functional requirements or host antibodies to explain the observed variations of the PIDs in lentiviruses.

Differences in feline immunodeficiency virus host cell range correlate with envelope fusogenic properties

Feline immunodeficiency virus (FIV) establishes persistent infections in cats inducing an acquired immunodeficiency syndrome. Differences in cell tropism have been observed among isolates of FIV (T. R. Phillips et al., J. Virol. 64, 4605-4613, 1990). The progeny of the infectious molecular clone of FIV p34TF10 was able to productively infect a feline fibroblast cell line, Crandell feline kidney cell, (CrFK), while the progeny of the molecular clone pPPR was not. However, pPPR, after transfection of CrFK cells, did produce virions which were able to productively infect feline lymphocytes. To analyze the mechanisms responsible for such differences in tropism and particularly the role of the envelope glycoproteins (Env), Env expression vectors were constructed by deletion of gag and pol genes from 34TF10 and PPR proviral clones. Env expression and function were studied by using a syncytium-formation assay and a quantitative ELISA. After transfection of CrFK, both 34TF10 and PPR Env precursors were correctly processed and Env surface glycoprotein, gp100, was released in culture supernatants. However, the Env of 34TF10 caused a dramatic syncytial effect in CrFK cells, while PPR Env did not induce any syncytium formation. The Env of 34TF10 placed under the control of the long terminal repeat of PPR maintained its ability to induce CrFK fusion. These results suggest that the inability of FIV PPR to infect CrFK fibroblasts is related to a restriction of virus entry mediated by the viral envelope.

Heterologous HIV-2 challenge of rhesus monkeys immunized with recombinant vaccinia viruses and purified recombinant HIV-2 proteins

In an attempt to analyse the role of anti-envelope immunity in the protection of rhesus monkeys against an HIV-2 intravenous challenge, rhesus macaques were immunized twice with recombinant HIV-2 ROD vaccinia viruses (10(8) p.f.u. each) at days 0 and 30, followed by booster injections of purified HIV-2 proteins at months 8, 9, 15 and 27. One group of five macaques was immunized with the Gag, Pol, Vif and Nef antigens, whereas a second group received the same antigens with the addition of HIV-2 Env protein. Eight months after the last boost, the animals were challenged by intravenous injection of 100 AID50 of a monkey PBMC-grown stock of HIV-2 SBL. None of the animals was protected in spite of high humoral immune responses on day of challenge as determined by ELISA and Western Blot assays.