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We previously showed that envelope (gp160)-based vaccines, used in a live recombinant virus priming and subunit protein boosting regimen, protected macaques against intravenous and intrarectal challenges with the homologous simian immunodeficiency virus SIVmne clone E11S. However, the breadth of protection appears to be limited, since the vaccines were only partially effective against intravenous challenge by the uncloned SIVmne. To examine factors that could affect the breadth and the efficacy of this immunization approach, we studied (i) the effect of priming by recombinant vaccinia virus; (ii) the role of surface antigen gp130; and (iii) the role of core antigens (Gag and Pol) in eliciting protective immunity. Results indicate that (i) priming with recombinant vaccinia virus was more effective than subunit antigen in eliciting protective responses; (ii) while both gp130 and gp160 elicited similar levels of SIV-specific antibodies, gp130 was not as effective as gp160 in protection, indicating a possible role for the transmembrane protein in presenting functionally important epitopes; and (iii) although animals immunized with core antigens failed to generate any neutralizing antibody and were infected upon challenge, their virus load was 50- to 100-fold lower than that of the controls, suggesting the importance of cellular immunity or other core-specific immune responses in controlling acute infection. Complete protection against intravenous infection by the pathogenic uncloned SIVmne was achieved by immunization with both the envelope and the core antigens. These results indicate that immune responses to both antigens may contribute to protection and thus argue for the inclusion of multiple antigens in recombinant vaccine designs.

We previously reported that immunization with recombinant simian immunodeficiency virus SIVmne envelope (gp160) vaccines protected macaques against an intravenous challenge by the cloned homologous virus, E11S. In this study, we confirmed this observation and found that the vaccines were effective not only against virus grown on human T-cell lines but also against virus grown on macaque peripheral blood mononuclear cells (PBMC). The breadth of protection, however, was limited. In three experiments, 3 of 10 animals challenged with the parental uncloned SIVmne were completely protected. Of the remaining animals, three were transiently virus positive and four were persistently positive after challenge, as were 10 nonimmunized control animals. Protection was not correlated with levels of serum-neutralizing antibodies against the homologous SIVmne or a related virus, SIVmac251. To gain further insight into the protective mechanism, we analyzed nucleotide sequences in the envelope region of the uncloned challenge virus and compared them with those present in the PBMC of infected animals. The majority (85%) of the uncloned challenge virus was homologous to the molecular clone from which the vaccines were made (E11S type). The remaining 15% contained conserved changes in the V1 region (variant types). Control animals infected with this uncloned virus had different proportions of the two genotypes, whereas three of four immunized but persistently infected animals had >99% of the variant types early after infection. These results indicate that the protective immunity elicited by recombinant gp160 vaccines is restricted primarily to the homologous virus and suggest the possibility that immune responses directed to the V1 region of the envelope protein play a role in protection.

Infection with attenuated simian immunodeficiency virus (SIV) in rhesus macaques has been shown to raise antibodies capable of neutralizing an animal challenge stock of primary SIVmac251 in CEMx174 cells that correlate with resistance to infection after experimental challenge with this virulent virus (M. S. Wyand, K. H. Manson, M. Garcia-Moll, D. C. Montefiori, and R. C. Desrosiers, J. Virol. 70:3724-3733, 1996). Here we show that these neutralizing antibodies are not detected in human and rhesus peripheral blood mononuclear cells (PBMC). In addition, neutralization of primary SIVmac251 in human and rhesus PBMC was rarely detected with plasma samples from a similar group of animals that had been infected either with SIVmac239Deltanef for 1.5 years or with SIVmac239Delta3 for 3.2 years, although low-level neutralization was detected in CEMx174 cells. Potent neutralization was detected in CEMx174 cells when the latter plasma samples were assessed with laboratory-adapted SIVmac251. In contrast to primary SIVmac251, laboratory-adapted SIVmac251 did not replicate in human and rhesus PBMC despite its ability to utilize CCR5, Bonzo/STRL33, and BOB/gpr15 as coreceptors for virus entry. These results illustrate the importance of virus passage history and the choice of indicator cells for making assessments of neutralizing antibodies to lentiviruses such as SIV. They also demonstrate that primary SIVmac251 is less sensitive to neutralization in human and rhesus PBMC than it is in established cell lines. Results obtained in PBMC did not support a role for neutralizing antibodies as a mechanism of protection in animals immunized with attenuated SIV and challenged with primary SIVmac251.

Using pathogenic simian immunodeficiency virus (SIV) infection of macaques as a model, we explored the limits of the protective immunity elicited by recombinant subunit vaccines and examined factors that affect their efficacy. Envelope gp 160 vaccines, when used in a live recombinant virus-priming and subunit-protein-boosting regimen, protected macaques against a low-dose, intravenous infection by a cloned homologous virus SIVmne E11S. The same regimen was also effective against intrarectal challenge by the same virus and against intravenous challenge by E11S grown on primary macaque peripheral blood mononuclear cells (PBMC). However, only limited protection was observed against uncloned SIVmne. Priming with live recombinant virus was more effective than immunization with subunit gp 160 alone, indicating a potential advantage of native antigen presentation and the possible role of cell-mediated immunity in protection. Whole gp 160 was more effective than the surface antigen (gp 130), even though both antigens elicited similar levels of neutralizing antibodies. Animals immunized with the core (gag-pol) antigens failed to generate any neutralizing antibody and were all infected following challenge. However, their proviral load was 10-100-fold lower than that of the control animals, indicating that immune mechanisms such as cytotoxic T lymphocytes (CTL) may play a role. Finally, animals immunized with both the core and the envelope antigens generated significant protective immunity, even with relatively low neutralizing antibodies. Taken together, these results indicate that multiple mechanisms may contribute to protection. It may therefore be advantageous to incorporate multiple antigens in the design of recombinant subunit vaccines against acquired immunodeficiency syndrome (AIDS).

It has been shown previously that the major neutralizing epitopes in simian immunodeficiency virus (SIV) are discontinuous and conformation dependent and that the V3 loop, in contrast to that of human immunodeficiency virus (HIV) type 1, does not by itself elicit neutralizing antibodies (K. Javaherian et al., Proc. Natl. Acad. Sci. USA 89:1418-1422, 1992). We now present data showing that on the basis of fractionation of infected macaque sera, protease digestion of the envelope, and binding properties of two neutralizing monoclonal antibodies to SIV and SIV-HIV chimeric envelope proteins, changes in V3 can disrupt the conformation-dependent neutralization region. The chimeric protein did not produce significant neutralizing antibodies against either SIV or HIV. We also report that neutralizing antibodies elicited by recombinant SIV envelope proteins of mac251 and B670 isolates cross-neutralize. Finally, we show that deglycosylation of the SIV envelope results in a molecule which binds neither soluble CD4 nor the neutralizing monoclonal antibodies being investigated here and does not elicit sera with a significant neutralizing titer.

We have tested synthetic peptides composed of Th (T1) and V3 loop B cell neutralizing determinants [SP10 MN(A)] of HIVMN gp120 and the fusogenic (F) domain of gp41 as immunogens in rhesus monkeys. After two immunizations with either HIV env peptide T1-SP10 MN(A) or F-T1-SP10 MN(A), rhesus monkey serum neutralization titers against the HIVMN isolate ranged from 1:160 to 1:1400, and in cell-cell syncytium inhibition assay ranged from 1:20 to 1:80. However, in contrast to animals immunized with T1-SP10 MN(A), animals immunized twice with F-T1-SP10 MN(A) had no rise in anti-gp120 and neutralizing antibodies with an additional immunization with F-T1-SP10 MN(A) peptide. One of 4 rhesus monkeys (18987) had anti-HIVMN antibodies that cross-neutralized divergent HIV isolates HIVIIIB and HIVRF. Serum from animal 18987 neutralized 5 of 10 HIV isolates tested, and neutralizing activity against HIVIIIB of 18987 serum was absorbed with the conserved gp120 loop V3 sequence IGPGRAF. Anti-HIV neutralizing antibodies were boosted after a 6-mo rest by 500 micrograms of T1-SP10 MN(A) in 4 of 4 animals previously immunized with T1-SP10 MN(A) and in 2 of 2 animals previously immunized with F-T1-SP10 MN(A). However, immunization after 6-mo rest of animal 18987 with 500 micrograms of T1-SP10 MN(A) peptide, although boosting anti-HIVMN neutralizing antibodies, selectively did not boost cross-neutralizing anti-HIVIIIB antibodies. Thus, synthetic peptides containing T and B cell epitopes of HIV gp120 can induce high levels of anti-HIVMN neutralizing antibodies in primates.

The fusogenic (F) domain of human immunodeficiency virus (HIV) gp41 envelope (env) protein has sequence similarities to many virus and mediates the fusion of HIV-infected cells. During a survey of the immunogenicity of HIV env peptides in chimpanzees, we have observed that HIV peptide immunogenicity was dramatically altered by the NH2-terminal synthesis of the gp41 F domain to an otherwise immunogenic peptide. We compared two hybrid peptide types comprised of T helper (Th) and B cell epitopes of HIV gp120 env protein for their immunogenicity in chimpanzees. The Th-B epitope hybrid peptides contained the HIV gp120 Th cell determinant, T1 (amino acids [aa] 428-440)-synthesized NH2 terminal to gp120 V3 loop peptides, which contain B cell epitopes that induce anti-HIV-neutralizing antibodies (SP10IIIB [aa 303-321] and SP10IIIB [A] [aa 303-327]). The F-Th-B peptide contained the HIV gp41 F domain of HIVIIIB gp41 (aa 519-530)-synthesized NH2 terminal to the Th-B peptide. Whereas Th-B peptides were potent immunogens for chimpanzee antibody and T cell-proliferative responses, the F-Th-B peptide induced lower anti-HIV gp120 T and B cell responses. Moreover, immunization of chimpanzees with F-Th-B peptide but not Th-B peptides induced a significant decrease in peripheral blood T lymphocytes (mean decrease during immunization, 52%; p < 0.02). Chimpanzees previously immunized with F-Th-B peptide did not respond well to immunization with Th-B peptide with T or B cell responses to HIV peptides, demonstrating that the F-Th-B peptide induced immune hyporesponsiveness to Th and B HIV gp120 env determinants. These observations raise the hypothesis that the HIV gp41 env F domain may be a biologically active immunoregulatory peptide in vivo, and by an as yet uncharacterized mechanism, promotes primate immune system hyporesponsiveness to otherwise immunogenic peptides.

Two Macaca fascicularis with preexisting immunity to vaccinia virus were immunized twice with recombinant vaccinia virus expressing SIVmne gp160. Their SIV-specific antibody responses were lower than that of vaccinia-naive animals immunized similarly. Upon repeated boosting with gp160, the SIV-specific antibody titers in vaccinia-primed animals reached similar levels as vaccinia-naive animals and with comparable neutralizing titers. Both animals were protected against repeated intravenous challenge with low-dose SIVmne E11S. These results are significant because SIVmne E11S infection in M. fascicularis is pathogenic and leads to AIDS-like diseases.

More than 200 sera from macaques immunized with several different vaccine preparations were tested in various assays with cells of human and macaque origin. Only in instances where whole inactivated SIV preparations were used for immunization, were reactivities found with normal human cells, and this was the case in every instance. Such sera produced a marked clumping of several normal human cell lines and exhibited strong staining of the cell surface in FACS analysis. In the presence of SIVDeltaB670, these sera also enhanced infectivity and fusion formation. When similar tests were performed with macaque cells as targets, such phenomena were not easily discernible. Likewise, there was no trace of such activities in sera from normal animals, animals chronically infected with SIV, or in those from animals which received recombinant viral subunits as vaccines. Finally, we show that in several instances where whole inactivated virus was used as a vaccine, there is a strong correlation between the titer of anticellular activity with protection.

Vaccines prepared from the envelope glycoprotein, gp120, of the common laboratory isolate of human immunodeficiency virus type 1 (HIV-1) (IIIB/LAV-1) elicit antibodies that neutralize the homologous virus but show little if any cross-neutralizing activity. This may be because the principal neutralizing determinant (PND) of gp120 is highly unusual in the IIIB/LAV-1 strain and is not representative of those found in the majority of field isolates. We have now examined the immunogenicity of recombinant gp120 prepared from the MN strain of HIV-1 (MN-rgp120), whose PND is thought to be representative of approximately 60% of the isolates in North America. Our results show that MN-rgp120 is a potent immunogen and elicits anti-gp120 titers comparable to those found in HIV-1-infected individuals. While both MN-rgp120 and IIIB-rgp120 induced antibodies able to block gp120 binding to CD4, strain-specific and type-common blocking antibodies were detected. Finally, antibodies to MN-rgp120 but not to IIIB-rgp120 were effective in neutralizing a broad range of laboratory and clinical isolates of HIV-1. These studies demonstrate that susceptibility or resistance to neutralization by antibodies to gp120 correlates with the PND sequence and suggest that the problem of antigenic variation may not be insurmountable in the development of an effective AIDS vaccine.

To identify the principal neutralization determinant (PND) of simian immunodeficiency virus (SIV), antisera were generated using recombinant gp110 [the SIV analog of the human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein, gp120], gp140, several large recombinant and proteolytic envelope fragments, and synthetic peptides of the SIVmac251 isolate. When purified under conditions that retain its native structure, gp110 bound CD4 and elicited antisera that neutralized SIVmac251 with high titer. Native gp110 also completely inhibited neutralizing antibody in sera from SIVmac251-infected macaques. In contrast, denatured gp110 and gp140, large envelope fragments, and synthetic peptides (including peptides analogous to the HIV-1 PND) elicited very low or undetectable neutralizing antibody titers and did not inhibit neutralizing antibody in infected macaque sera. Enzymatically deglycosylated gp110 efficiently absorbed neutralizing antibodies from macaque sera, showing that neutralizing antibodies primarily bind the protein backbone. A 45-kDa protease digest product, mapping to the carboxyl-terminal third of gp110, also completely absorbed neutralizing antibodies from infected macaque sera. These results show that the PND(s) of this SIV isolate depends on the native conformation and that linear peptides corresponding to the V3 loop of SIV envelope, in contrast to that of HIV-1, do not elicit neutralizing antibody. This may affect the usefulness of SIVmac for evaluating HIV-1 envelope vaccine approaches that rely on eliciting neutralizing antibody.

Simian immunodeficiency virus (SIV) was used as a model to study the protective efficacy of an immunization regimen currently being evaluated as candidate vaccines against HIV in human subjects. Four Macaca fascicularis were first immunized with recombinant vaccinia virus expressing the envelope glycoprotein gp160 of SIVmne and then boosted with subunit gp160. Both cell-mediated and humoral immune responses against SIV, including neutralizing antibodies, were elicited. The macaques were shown to be protected from a homologous virus infection as determined by serology, lymphocyte cocultivation, polymerase chain reactions and in vivo transmission analyses. Four unimmunized control animals were readily infected. However, viremia in infected control animals could decrease substantially following the initial phase of infection so that persistent infection might not be readily detectable.

Simian immunodeficiency virus (SIV) is a primate lentivirus related to human immunodeficiency viruses and is an etiologic agent for acquired immunodeficiency syndrome (AIDS)-like diseases in macaques. To date, only inactivated whole virus vaccines have been shown to protect macaques against SIV infection. Protective immunity was elicited by recombinant subunit vaccines. Four Macaca fascicularis were immunized with recombinant vaccinia virus expressing SIVmne gp160 and were boosted with gp160 produced in baculovirus-infected cells. All four animals were protected against an intravenous challenge of the homologous virus at one to nine animal-infectious doses. These results indicate that immunization with viral envelope antigens alone is sufficient to elicit protective immunity against a primate immunodeficiency virus. The combination immunization regimen, similar to one now being evaluated in humans as candidate human immunodeficiency virus (HIV)-1 vaccines, appears to be an effective way to elicit such immune responses.

The influence of human anti-human immunodeficiency virus type 1 (HIV-1) antibody on HIV-1 infection of freshly isolated normal human peritoneal macrophages and blood monocytes was examined. Each of 14 HIV antibody-positive human serum samples was found to block the infection of four virus isolates (human T-cell lymphotropic virus type IIIBa-L [HTLV-IIIBa-L], HTLV-IIIB, D.U. 6587-7, and D.U. 7887-8) at serum dilutions ranging from 10(-1) to 10(-2). Three of these isolates (HTLV-IIIBa-L, D.U. 6587-7, and D.U. 7887-8) infected cultures of monocytes and macrophages rapidly and produced high levels of virus reverse transcriptase and p24 antigen. A fourth virus isolate (HTLV-IIIB) infected the monocytes and macrophages more slowly and produced low levels of viral protein. More dilute HIV antibody-positive sera had no significant effect on the overall level or rate of virus infection or expression. Complement did not appear to influence the course of infection by any combination of antisera or virus examined. Successful HIV-1 infection of the peritoneal macrophages and blood monocytes under the conditions tested showed strict dependence on CD4 since a recombinant CD4 polypeptide and an anti-CD4 monoclonal antibody effectively blocked the process.

Sensitive and reproducible assays for SIV infection and syncytium formation have been developed in which high titers of neutralizing and fusion-inhibiting antibodies can be recorded. These assays will contribute toward our understanding of the role of humoral responses in SIV vaccine strategies.

The V3 loop (residues 303-338) of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope protein represents a principal neutralizing determinant for the virus. An HIV-1 proviral clone containing a mutation in the V3 loop was constructed in which the proline residue at position 313 was changed to an alanine (P313-A). This mutation alters the conserved GPGR sequence that is found in the V3 loop sequences of different HIV-1 isolates. The P313-A clone produced virus particles, which were infectious for a number of T-cell lines including MOLT-4, CEM, and SupT1, but demonstrated a relatively low infectivity on the AA5 B-cell line when compared with wild-type viruses, HTLV-IIIB, HXB2/10 (a chimeric molecular clone), and another mutant virus (Q290-T). V3 loop-specific neutralizing polyclonal sera and the 9284 monoclonal antibody, which recognizes the amino side of the V3 loop sequence, effectively blocked infectivity and syncytia formation of all viruses tested. In contrast, the 0.5 beta monoclonal antibody, which is biologically more potent than 9284 and recognizes a different V3 loop determinant, failed to neutralize the P313-A virus. These results suggest that the proline residue in the relatively conserved GPGR "turn" region of the V3 loop is crucial for recognition by the 0.5 beta antibody. The observed variation in sensitivity of the B-cell line to the P313-A virus may reflect the presence of cell-specific factors which could be important in establishing an HIV-1 infection.

An intravenous solution of 99% pure globulin (hyperimmune IgG, HIVIG) was obtained from pooled plasma of selected human immunodeficiency virus (HIV-1)-seropositive asymptomatic donors with greater than 400 CD4+/microliters cells per microliter and a high titer of antibody to HIV-1 p24 protein. HIVIG had high titers of antibody to p24, glycoprotein 41 (gp41), and gp120, group-specific neutralizing activity, and binding to the gp120 hypervariable loop region. It inhibited syncytia formation. At low concentration, it enhanced viral production of HIV-1 in infected peripheral blood monocytes but was inhibitory at higher concentration. HIVIG directed group-specific antibody-dependent cellular cytotoxicity against HIV-infected targets. For a period of 6 to 28 months, plasma donors kept stable antibody titers and had a 1.0% decrease in CD4+ cells per month. One gram per kilogram HIVIG injected in two juvenile chimpanzees was well tolerated and did not transmit HIV, as measured by negative cell culture, IgM immune response to HIV proteins, and polymerase chain reaction. The mean half-life of HIV-1 p24 antibody was 15 days. These preliminary data suggest that HIVIG is a safe product suitable for clinical trial in HIV-1-infected individuals.

The principal neutralizing determinant (PND) of human immunodeficiency virus (HIV)-1 resides within the V3 loop of the envelope protein. Antibodies elicited by peptides of this region were able to neutralize diverse isolates. Serum from one of three animals immunized with the human T cell lymphoma virus (HTLV)-IIIMN PND peptide, RP142, neutralized MN and the sequence-divergent HTLV-IIIB isolate. Serum from one of three animals immunized with a 13-amino acid IIIB PND peptide (RP337) also neutralized both of these isolates. Characterization of these sera revealed that the cross-neutralizing antibodies bound the amino acid sequence GlyProGlyArgAlaPhe (GPGRAF) that is present in both isolates. This sequence is frequently found in the PNDs analyzed in randomly selected HIV-1 isolates. Sera from two rabbits immunized with a peptide containing only the GPGRAF residues neutralized divergent isolates, including IIIB and MN.

In an effort to determine the functional activity of anti-HIV-1 human mAb and to define the epitopes against which they are directed, supernatants from 10 EBV-transformed lymphoblastoid cell lines producing mAb to HIV were tested. Five clones producing mAb to gp41 and five producing mAb to p24 were identified. The anti-HIV-1 human mAb were tested in neutralization and cell fusion assays in the form of cell culture supernatants at concentrations ranging from 1.7 to 22.0 micrograms/ml. None of the human mAb were found either to inhibit HIV-1-(IIIB or RF) associated cell fusion or to neutralize HIV-1 (IIIB) infection of AA5 cells. All human mAb were additionally tested in 6 h 51Cr release assays for their ability to direct HIV-1 specific antibody-dependent cellular cytotoxicity (ADCC). For ADCC assays, PBMC were isolated from healthy seronegative donors and used as effector cells. HIV-1 infected (IIIB, RF, and MN) CEM.NKR cells as well as CEM.NKR cells with purified gp120 adsorbed onto their surface served as targets. None of the anti-p24 mAb mediated ADCC. In contrast, three of the anti-gp41 mAb were able to direct a significant level of ADCC against each of the infected targets, but as expected, failed to lyse gp120 adsorbed cells. To define the specific epitopes against which the anti-gp41 mAb were directed, seven small peptides homologous to regions within the extracellular domain of gp41 were synthesized. Using RIA, two of the mAb could be mapped. The most effective ADCC-directing human mAb bound to a peptide comprising amino acids 644-663, whereas the least effective ADCC directing anti-gp41 human mAb bound to a region within the immunodominant portion of gp41 outlined by amino acids 579-604. Together, these results for the first time assign a functional activity to human mAb directed at specific regions within gp41 by demonstrating that areas within this molecule can serve as targets for ADCC.

Rhesus macaques (Macaca mulatta) immunized with an inactivated whole SIVmac vaccine and muramyl dipeptide (MDP), incomplete Freund's adjuvant (IFA), or aqueous suspension were challenged intravenously with 0.1 TCID50 of cell-free SIVmac. Whereas virus was readily recovered from the peripheral blood lymphocytes of 10 of 10 nonvaccinated controls following this challenge dose, virus was not recovered from the three animals that received the vaccine with MDP nor from one of two animals that received the vaccine with IFA and one of three animals that received the aqueous vaccine. The animals that were protected against challenge were those that had detectable SIV antibody response to the envelop, both the outer glycoprotein (gp120) and the truncated transmembrane glycoprotein (gp31). Protected monkeys tended to have higher titers of syncytial inhibition antibody prior to challenge. An anamnestic response after challenge was observed only in the vaccinated monkeys that became infected. Vaccinated animals that became challenge-infected tended to live longer than infected controls. These results confirm those at two other primate centers and indicate that killed whole SIV vaccines can protect against low challenge doses of SIV and prevent early death in those monkeys that do become infected. The mechanism of this protection remains undetermined. This finding adds optimism to the possibility of an eventual AIDS vaccine.

We have previously described a synthetic peptide (T1-SP10) derived from two noncontiguous regions of HTLVIIIB envelope gp120 (T1, amino acids 428-443; SP10, amino acids 303-321) that induced type-specific anti-HIV neutralizing antibodies and T cell proliferative responses against native HIV gp120 when used as a carrier-free immunogen in goats. In this study, HTLVIIIB T1-SP10 synthetic peptides were used to immunize rhesus monkeys to determine if the peptides were capable of eliciting HIV-specific neutralizing antibody and proliferative responses in primates. Four compounds (alum, polyA:polyU, threonyl-muramyldipeptide (MDP) and IFA) were also compared for efficacy as adjuvants in this system. Rhesus monkeys immunized with T1-SP10 peptides generated high titers of antibodies against the immunogens and also against HTLVIIIB gp120. Sera from all four animals given T1-SP10 in IFA or threonyl-MDP neutralized infection by HTLVIIIB and blocked virus-dependent cell fusion events. A peak neutralization titer of 1:940 was seen in one animal given IFA (19600) and a titer of 1:900 was seen in one of the monkeys (17371) given threonyl-MDP. Proliferative responses of immune rhesus PBMC to T1-SP10 appeared after the first injection. After eight immunizations, two of eight monkeys (one injected with peptides in threonyl-MDP and one given peptides in IFA) had PBMC proliferative responses to native HTLVIIIB gp120. These data demonstrate that the carrier-free T1-SP10 synthetic peptide construct can induce high titers of neutralizing anti-HIV antibody responses and PBMC proliferative responses to HIV in primates.

The principal neutralizing determinant (PND) of human immunodeficiency virus HIV-1 is part of a disulfide bridged loop in the third variable region of the external envelope protein, gp120. Analysis of the amino acid sequences of this domain from 245 different HIV-1 isolates revealed that the PND is less variable than thought originally. Conservation to better than 80 percent of the amino acids in 9 out of 14 positions in the central portion of the PND and the occurrence of particular oligopeptide sequences in a majority of the isolates suggest that there are constraints on PND variability. One constraining influence may be the structural motif (beta strand--type II beta turn--beta strand--alpha helix) predicted for the consensus PND sequence by a neural network approach. Isolates with a PND similar to the commonly investigated human T cell lymphoma virus IIIB (HTLV-IIIB) and LAV-1 (BRU) strains were rare, and only 14 percent of sera from 86 randomly selected HIV-1 seropositive donors contained antibodies that recognized the PND of these virus isolates. In contrast, over 65 percent of these sera reacted with peptides containing more common PND sequences. These results suggest that HIV vaccine immunogens chosen because of their similarity to the consensus PND sequence and structure are likely to induce antibodies that neutralize a majority of HIV-1 isolates.

A synthetic peptide (SP-10-IIIB) with an amino acid sequence [Cys-Thr-Arg-Pro-Asn-Asn-Asn-Thr-Arg-Lys-Ser-Ile-Arg-Ile-Gln-Arg-Gly-Pro -Pro-Gly-(Tyr); amino acids 303-321] from the human immunodeficiency virus (HIV) isolate human T-cell lymphotropic virus type III (HTLV-III) HTLV-IIIB envelope glycoprotein gp120 was coupled to tetanus toxoid and used to raise goat antibodies to HIV gp120. Goat anti-SP-10-IIIB serum bound to the surface of HTLV-IIIB-infected CEM T cells but not to the surface of HTLV-IIIRF-infected or uninfected CEM T cells. Anti-SP-10-IIIB antibodies also selectively bound to gp120 from lysates of HTLV-IIIB cells in immunoblot assays. Twenty-one percent of sera (28 of 175) from patients seropositive for HIV contained antibodies that reacted with SP-10-IIIB in RIA. Human anti-SP-10-IIIB antibodies affinity purified from acquired immunodeficiency syndrome (AIDS) patient serum bound to HTLV-IIIB-infected cells and immunoprecipitated gp120. Goat antibodies to SP-10-IIIB neutralized HTLV-IIIB (80% neutralization titer of 1/600), inhibited HTLV-IIIB-induced syncytium formation, but did not neutralize HIV isolates HTLV-IIIRF or HTLV-IIIMN or inhibit syncytium formation with these isolates. Also, goat antiserum to an homologous synthetic peptide [SP-10-IIIRF(A), (Cys)-Arg-Lys-Ser-Ile-Thr-Lys-Gly-Pro-Gly-Arg-Val-Ile-Tyr] from gp120 of HIV isolate HTLV-IIIRF inhibited syncytium formation by HTLV-IIIRF, but did not inhibit syncytium formation by HTLV-IIIB or by HTLV-IIIMN. Thus, the amino acid sequences of SP-10-IIIB and SP-10-IIIRF(A) define homologous regions of gp120 that are important in type-specific virus neutralization. The identification of these type-specific neutralizing epitopes should facilitate the design of a polyvalent, synthetic vaccine for AIDS.

Cytophilic antibodies which mediate antibody dependent cellular cytotoxicity (ADCC) against envelope antigens of human immunodeficiency virus (HIV) can be found in seropositive individuals. In these experiments, sera from a wide spectrum of HIV infected patients ranging from asymptomatic to overt acquired immunodeficiency syndrome (AIDS) were shown to contain high titers of antibodies that mediate ADCC. Not only did patient antibodies bind to surface expressed viral antigens and mediate ADCC against cells chronically infected with human T-lymphotropic virus type IIIB (HTLV-IIIB), but also against cells infected with the divergent HTLV-IIIRF2 and HTLV-IIIMN viral isolates. Similar results were obtained with target cells bearing purified GP 120 from HTLV-IIIB and HTLV-IIIRF2, indicating that a major portion of the activity was mediated by anti-GP 120 antibodies. Consistent with this was the ability to absorb most of the group-specific ADCC activity from the serum of an HIV infected individual using affinity columns bearing purified HTLV-IIIB GP 120. The finding that human antibodies reactive against the HIV envelope glycoprotein mediate ADCC against cells chronically infected with divergent strains of HIV will have important implications in designing rational approaches to passive and active immunotherapy.

Forty-one patients seropositive for human immunodeficiency virus type 1 (HIV-1) were assessed for cell-mediated cytotoxicity (CMC) against autologous target cells bearing the major envelope glycoprotein of HIV-1, gp120. Effector lymphocytes from over 85% of seropositive patients showed CMC specific for gp120-coated targets, whereas seronegative individuals had no detectable CMC. As a group, symptomless individuals had the highest levels of CMC; patients with AIDS-related complex and AIDS showed progressively diminished reactivity. The gp120-specific CMC was mediated by a population of non-T-cell effectors phenotypically resembling NK/K cells. Cytolysis was not restricted by major histocompatibility complex determinants, as shown by killing of heterologous gp120-adsorbed targets and of HIV-1-infected cell-lines. Gp120-specific CMC was highly augmented in the presence of interleukin 2, so it may be possible to develop therapeutic strategies aimed at destruction of virus-producing cell reservoirs in infected individuals through stimulation of HIV-specific host CMC.

A highly immunogenic epitope from a conserved COOH-terminal region of the human immunodeficiency virus (HIV) gp120 envelope protein has been identified with antisera from HIV-seropositive subjects and a synthetic peptide (SP-22) containing 15 amino acids from this region (Ala-Pro-Thr-Lys-Ala-Lys-Arg-Arg-Val-Val-Gln-Arg-Glu-Lys-Arg). Peptide SP-22 absorbed up to 100% of anti-gp120 antibody reactivity from select HIV+ patient sera in immunoblot assays and up to 79% of serum anti-gp120 antibody reactivity in competition RIA. In RIA, 45% of HIV-seropositive subjects had antibodies that bound to peptide SP-22. Human anti-SP-22 antibodies that bound to and were eluted from an SP-22 affinity column reacted with gp120 in RIA and immunoblot assays but did not neutralize HIV or inhibit HIV-induced syncytium formation in vitro, even though these antibodies comprised 70% of all anti-gp120 antibodies in the test serum. In contrast, the remaining 30% of SP-22 nonreactive anti-gp120 antibodies did not react with gp120 in immunoblot assays but did not react in RIA and neutralized HIV in vitro. Thus, approximately 50% of HIV-seropositive patients make high titers of nonneutralizing antibodies to an immunodominant antigen on gp120 defined by SP-22. Moreover, the COOH terminus of gp120 contains the major antigen or antigens identified by human anti-gp120 antibodies in immunoblot assays.

The principal neutralizing determinant of human immunodeficiency virus type 1 (HIV-1) is located in the external envelope protein, gp120, and has previously been mapped to a 24-amino acid-long sequence (denoted RP135). We show here that deletion of this sequence renders the envelope unable to elicit neutralizing antibodies. In addition, using synthetic peptide fragments of RP135, we have mapped the neutralizing determinant to 8 amino acids and found that a peptide of this size elicits neutralizing antibodies. This sequence contains a central Gly-Pro-Gly that is generally conserved between different HIV-1 isolates and is flanked by amino acids that differ from isolate to isolate. Antibodies elicited by peptides from one isolate do not neutralize two different isolates, and a hybrid peptide, consisting of amino acid sequences from two isolates, elicits neutralizing antibodies to both isolates. By using a mixture of peptides of this domain or a mixture of such hybrid peptides the type-specificity of the neutralizing antibody response to this determinant can perhaps be overcome.

Animals immunized with the human immunodeficiency virus type 1 gp160 glycoprotein or certain recombinant envelope components develop potent virus-neutralizing activity. This activity is principally due to antibodies directed toward a hypervariable region of gp120 between cysteine residues 302 and 337 and is virus isolate specific. These antisera, as well as two neutralizing monoclonal antibodies directed against the same hypervariable sequence, do not appreciably block gp120 from binding CD4. In contrast, serum samples from infected humans possess high titers of antibodies that block gp120-CD4 binding; these titers approximately correlate with the serum neutralization titers. Our results suggest that there are at least two targets on the envelope glycoprotein for virus neutralization. The target responsible for the broader neutralizing activity of human serum may be a conserved region of gp120 involved in CD4 binding. The antibodies directed at the hypervariable region of the envelope inhibit a different step in virus infection which is subsequent to receptor binding. The extent to which these two different epitopes of gp120 may be involved in protection against human immunodeficiency virus infection is discussed.

Four virus-infected avian cell lines have been established in culture. Two of these lines, infected with BAI strain A virus, liberate only small quantities of virus in the culture fluid. The cells retain the ability to induce myeloblastic leukemia when inoculated i.v. into 1- to 2-day-old chicks, but do so less efficiently than freshly obtained myeloblasts. These cells do not appear to be transplantable, since the disease produced is characterized by the presence of myeloblasts that liberate large quantities of virus. The other two cell lines, infected with the MC29 strain of avian leukosis virus, liberate normal levels of infectious virus in the culture fluid. When these cells are inoculated into the wing web of 1- to 2-day-old chicks, tumors develop at the site of inoculation which are detectable as early as 4 to 7 days after challenge. Chromosome studies demonstrate that the four cell lines have karyotypes typical of Gallus domesticus. The myeloblastic cell lines (D.U. 11157 and D.U. 1765) show a reduction in the number of microchromosomes. These cell lines have been carried in continuous culture for various lengths of time, can be frozen, are easily recovered in viable form, and appear to be capable of indefinite growth.

The lymphocyte differentiation antigen CD4 serves as a receptor for human retroviruses associated with acquired immunodeficiency syndrome (AIDS) through its interaction with the major envelope virion glycoprotein, gp120, which is also expressed on the surface of infected cells. In these experiments, purified gp120 was shown to bind to normal human T-lymphocyte populations. The gp120-CD4 complex served as a target antigen for antibody-dependent complement-mediated cytolysis by a goat serum raised against native gp120. However, patient sera that bound to gp120-adsorbed cells failed to direct their destruction in the presence of complement. In contrast, these sera were potent mediators of antibody-dependent cellular cytotoxicity. These studies demonstrate that gp120 situated on the cell surface can serve as an effective target for immune destruction by patient antibodies and effector lymphocytes. The possible contribution of this type of immunity to control of disease progression, on the one hand, and to lymphocyte destruction and immunopathology observed in AIDS, on the other, is discussed.

Two populations of virus having subgroup-specific homogeneity (A and B) were isolated from standard avian myeloblastosis virus stocks by passage in vivo through genetically defined chickens. Each possesses leukemogenic activity in vivo. Other properties and potential usefulness of these agents are discussed.

We have studied the biologic and physical properties of a monoclonal antibody that binds to gp120, the exterior envelope glycoprotein of the human immunodeficiency virus (HIV) strain HTLV-IIIB. Designated 9284, the antibody possesses viral neutralizing activity and inhibits syncytium formation by infected cells. The antibody recognized a region of the polypeptide backbone previously described as an important neutralizing epitope. This region lies 307-330 residues from amino terminus of the glycoprotein. We have compared the biologic and physical properties of this antibody to those of the recently described 0.5 beta monoclonal antibody to gp120. The 0.5 beta antibody was biologically more potent and bound an epitope slightly downstream to that of the 9284 antibody. The antibodies did not differ significantly in their affinity for gp120. In competition studies, the 0.5 beta antibody was displaced by the 9284 antibody, but the binding of the latter was unaffected by 0.5 beta.

The human immunodeficiency virus envelope gene was expressed in insect cells by using a Baculovirus expression vector. The protein has an apparent molecular mass of 160 kDa, appears on the surface of infected insect cells, and does not appear to be cleaved to glycoproteins gp120 and gp41. Goats immunized with the 160-kDa protein have high titers of antibody that neutralizes virus infection as measured by viral gene expression or cell cytolysis. In addition, immune sera can block fusion of human immunodeficiency virus-infected cells in culture. Both neutralization and fusion-blocking activities are bound to and eluted from immobilized gp120.

A total of 18 overlapping peptides were synthesized that covered envelope amino acid sequences (amino acids 288-472 of the III-B isolate) of the human immunodeficiency virus type 1 (HIV-1). Antibodies from human immunodeficiency virus-1-infected individuals bound to three of the peptides tested. Guinea pigs were immunized with each of the overlapping peptides and the resultant sera analyzed for biologic activity. One of the peptides elicited antibodies that had both neutralizing and fusion blocking activities that were type specific. This peptide, designated 1-69, was also one of the peptides reactive with the human immunodeficiency virus type 1-positive human sera.

Interactions between retroviruses associated with acquired immunodeficiency syndrome and their receptors on lymphocytes represent the initial steps in the process of infection and are also involved in multinucleated giant cell formation, which is one form of virus-mediated cytopathology. The exterior envelope glycoprotein of the retrovirus has been identified as gp120, and we demonstrate here that purified gp120 binds directly to cells expressing the CD4 (T4) surface antigen at a site spatially related to that recognized by the OKT4A monoclonal antibody. The gp120 was also able to temporarily interfere with viral infection and to block the process of multinucleated giant cell formation. However, if the carbohydrate chains were removed from gp120 by enzymatic treatment, CD4 binding and blockade of cell fusion was reduced by about a factor of 50. The significance of these results in relation to preventive and interventive approaches for acquired immunodeficiency syndrome is discussed.

An in vitro assay has been developed that mimics the potent in vivo protective capacity of B6 anti-AD755a serum in a passive therapy protocol. In the presence of small volumes of hyperimmune serum or IgG2a antibody, thioglycollate-elicited B6 mouse peritoneal cells (PEC) inhibit the growth of AD755 Cl.10 target cells and other cells provided that they express FLV-related antigens. When hyperimmune serum is replaced by normal serum, or elicited PEC from BALB/c mice are used, growth inhibition of the AD755 Cl.10 target cells is not seen. The latter is in accord with the inability to protect BALB/c mice with the anti-AD755a serum. Tumor cells to which antibody has been pre-bound are not inhibited by thioglycollate-elicited B6 PEC, analogous to the undiminished tumorigenicity of antibody-bound AD755a cells in vivo. These results, along with those presented in the accompanying papers, have led to the construction of a model to explain the specificity and mechanism of the potent in vivo passive protection characteristic of this system. The major features of this model are that the protective IgG2a antibody must first interact with an appropriate host effector cell, presumably of the monocyte/macrophage lineage, and that such an antibody-directed cell then binds to the tumor cell and effects its elimination, possibly by a phagocytic mechanism.

By analogy to other retroviruses, the major envelope glycoprotein - gp120 - of HTLV-III/LAV is a probable target for neutralizing antibody. This antigen has been purified from H9 cells chronically infected with the HTLV-IIIB prototype strain. Several goats immunized with the gp120 produced antibodies that neutralized infection of H9 by the homologous virus isolate. These same sera failed to neutralize the divergent HTLV-IIIRF isolate. Individuals infected with HTLV-III/LAV commonly develop antibodies to gp120 which could be isolated using the gp120 antigen coupled to an immunoadsorbent resin. The antibody fraction that bound tightly to such a resin was found to neutralize the IIIB but not the RF isolate in a fashion similar to that of the goat anti-gp120 sera. However, the nonbinding fraction (effluent) from the resin also contained neutralizing activity which was able to block infection by both virus isolates with similar efficacy. Human antibodies to the other virus envelope gene product, the transmembrane gp41, were also affinity-purified utilizing the recombinant peptide 121, but they failed to influence infection by either virus isolate.

The human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome, infects humans and chimpanzees. To determine the efficacy of immunization for preventing infection, chimpanzees were immunized with gp120 purified from human T-cell lymphotrophic virus type IIIB (HTLV-IIIB)-infected cell membranes and challenged with the homologous virus, HTLV-IIIB. A challenge stock of HTLV-IIIB was prepared by using unconcentrated HTLV-IIIB produced in H9 cells. The titer of the virus from this stock on human and chimpanzee peripheral blood mononuclear cells and in human lymphoid cell lines was determined; a cell culture infectivity of 10(4) was assigned. All chimpanzees inoculated intravenously with 40 cell culture infectious units or more became infected, as demonstrated by virus isolation and seroconversion. One of two chimpanzees inoculated with 4 cell culture infectious units became infected. Chimpanzees immunized with gp120 formulated in alum developed antibodies which precipitated gp120 and neutralized HTLV-IIIB. Peripheral blood mononuclear cells from gp120-vaccinated and HIV-infected animals showed a significantly greater response in proliferation assays with HIV proteins than did peripheral blood mononuclear cells from nonvaccinated and non-HIV-infected chimpanzees. Two of the gp120-alum-immunized chimpanzees were challenged with virus from the HTLV-IIIB stock. One animal received 400 cell culture infectious units, and one received 40 infectious units. Both animals became infected with HIV, indicating that the immune response elicited by immunization with gp120 formulated in alum was not effective in preventing infection with HIV-1.

Attempts were made to augment the antibody-dependent killing of the ascitic AD755a tumor in vivo to protect C57BL/6J mice against the outgrowth of larger tumor burdens. The lethal dose for this tumor is less than 100 cells, and antibodies contained in a hyperimmune antitumor serum (HIS) were found to suppress the outgrowth of a maximum of about 5 X 10(5) cells. Thioglycollate injected ip increased the number of peritoneal macrophages, potential effectors for antibody-dependent cell-mediated cytotoxicity (ADCC), by tenfold to fortyfold and raised the maximum treatable tumor challenge (MTTC) to about 4 X 10(6) cells. By comparison, ip injection of Corynebacterium parvum increased the total peritoneal cell population by only twofold but raised the MTTC to about 20 X 10(6) cells. Neither agent alone had an effect on long-term survival, even at very low tumor inocula (1 X 10(3) cells). The protective HIS is known to contain tumor-binding antibodies in each of the IgG1, IgG2A, and IgG2B isotype fractions. Although the IgG2A fraction is far superior in vivo in the suppression of tumor outgrowth, the IgG2A fraction was also found to be most effective in combination with thioglycollate treatment in agreement with the observed preference of thioglycollate-elicited macrophages for this isotype in in vitro killing assays. In contrast following C. parvum treatment, all three isoptype fractions were equally suppressive to tumor outgrowth. A second major change following C. parvum treatment was that tumor cells precoated in vitro with antibodies were effectively eliminated in vivo. The same antibody-coated cells administered to thioglycollate-treated or unmanipulated animals were uniformly lethal even at much lower tumor doses. Taken together these results suggested a major qualitative change in the antibody-dependent tumor-killing process following C. parvum treatment. This change was most likely due to the C. parvum activation of highly lytic effector cells for ADCC, the identity of which was examined in an accompanying manuscript.