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J Virol, April 1998, p. 3427-3431, Vol. 72, No. 4
Department of Surgery, Duke University
Medical Center, Durham, North Carolina1;
Division of Viral Pathogenesis,
Received 27 August 1997/Accepted 6 December 1997
The magnitude and breadth of neutralizing antibodies raised in
response to infection with chimeric simian-human immunodeficiency virus
(SHIV) in rhesus macaques were evaluated. Infection with either
SHIV-HXB2, SHIV-89.6, or SHIV-89.6PD raised high-titer neutralizing
antibodies to the homologous SHIV (SHIV-89.6P in the case of
SHIV-89.6PD-infected animals) and significant titers of neutralizing
antibodies to human immunodeficiency virus type 1 (HIV-1) strains MN
and SF-2. With few exceptions, however, titers of neutralizing
antibodies to heterologous SHIV were low or undetectable. The
antibodies occasionally neutralized heterologous primary isolates of
HIV-1; these antibodies required >40 weeks of infection to reach
detectable levels. Notable was the potent neutralization of the HIV-1
89.6 primary isolate by serum samples from SHIV-89.6-infected macaques.
These results demonstrate that SHIV-HXB2, SHIV-89.6, and SHIV-89.6P
possess highly divergent, strain-specific neutralization epitopes. The
results also provide insights into the requirements for raising
neutralizing antibodies to primary isolates of HIV-1.
An important goal in the
development of a successful vaccine for human immunodeficiency virus
type 1 (HIV-1) is to generate an effective neutralizing antibody
response. The surface gp120 and, to a lesser extent, transmembrane gp41
envelope glycoproteins of the virus are major targets for neutralizing
antibodies (for a review, see reference 3) and have
been the basis for several candidate vaccines. To date, most HIV-1
envelope vaccines that have advanced to clinical trials in humans are
derived from T-cell line-adapted (TCLA) strains of virus and are
administered as molecularly cloned monomeric subunits (43).
Some of these vaccines have induced significant levels of neutralizing
antibodies against the vaccine strain of virus and, to a lesser extent,
against heterologous TCLA strains (1, 12, 13, 23, 32, 38,
45). However, with the exception of a rare clade B variant that
is highly sensitive to neutralization (isolate BZ167) (46),
sera from vaccinated volunteers have failed to neutralize low-passaged
field strains (i.e., primary isolates) of the virus (14, 23,
24). Neutralizing antibodies raised by these vaccines have been
shown to recognize primarily linear epitopes (41), including
those in the V3 loop of gp120 (25).
It seems prudent to develop an HIV-1 vaccine that will induce
antibodies capable of neutralizing a broad spectrum of primary isolates. The fact that primary isolates are only occasionally neutralized by sera from infected individuals (4, 17, 26, 29, 33,
42) is an indication that this will not be an easy task. One
approach in preclinical development has been to use oligomeric forms of
the viral envelope glycoproteins as immunogens (2, 9, 36).
This approach gains support from the observation that native gp120 and
gp41 exist as oligomeric complexes on virus particles (8, 21,
44) and that virus neutralization is associated with antibodies
that bind these complexes efficiently (10, 30, 37, 40).
Although the neutralization epitopes on primary isolates are ill
defined, the broad and potent neutralization of TCLA variants and
primary isolates by human monoclonal antibodies b12, 2G12, and 2F5 has
revealed the presence of several neutralization epitopes that are
highly conserved (for a review, see reference 3).
These epitopes must be poorly immunogenic, however, since primary
isolates are not broadly neutralized by sera from most HIV-1-infected
individuals. The fact that primary isolates are neutralized
sporadically by these sera (17, 26, 29, 33) suggests the
presence of additional neutralization epitopes that are strain specific
and are both antigenic and immunogenic. Oligomeric gp120 may differ
antigenically from the monomer (2, 9, 36), but the effect
this has on the immunogenicity of primary isolate neutralization
epitopes remains uncertain.
The exploration of new vaccine approaches for raising neutralizing
antibodies to HIV-1 would benefit greatly from an appropriate animal
model. The recent development of chimeric simian-human immunodeficiency
virus (SHIV), in which the env, tat, and
rev of molecularly cloned SIVmac239 are replaced with the
corresponding regions of HIV-1 (15, 16, 19, 20, 22, 34, 35,
39), affords novel opportunities to dissect the molecular
determinants of HIV-1 pathogenesis and to assess HIV-1 envelope
glycoprotein vaccines in a highly relevant animal model. The magnitude
and strain specificity of neutralizing antibody responses in
SHIV-infected macaques were evaluated in this study to gain a better
understanding of how the SHIV model may be best utilized to assess
HIV-1 vaccine efficacy and associated immunologic correlates and to
provide insights into the design of a vaccine that will raise
antibodies capable of neutralizing primary isolates of HIV-1.
Serum samples were obtained from rhesus macaques (Macaca
mulatta) at various time points after intravenous inoculation with one of five SHIV variants. The first variant, SHIV-HXB2, contained the
envelope glycoproteins of HIV-1 strain IIIB (19, 20). A
second variant, SHIV-89.6, contained the envelope glycoproteins of a
primary isolate that is dualtropic for T cells and macrophages and can
utilize both CXCR4 and CCR5 as coreceptors for virus entry (5-7,
34). The third and fourth variants were generated by serial
passage of SHIV-89.6 in rhesus macaques to generate SHIV-89.6' (second
passage) and SHIV-89.6PD (fourth passage) (16, 34). The
latter virus was obtained from plasma and was isolated in CEMx174
cells. SHIV-89.6P, which was used only in neutralization assays, was
obtained directly from blood and lymphoid tissue mononuclear cells from
the same monkey after the fourth passage of SHIV-89.6 and, therefore,
is closely related to SHIV-89.6PD (16, 34). The fifth SHIV
used for infection of macaques is a molecular clone of SHIV-89.6P,
designated SHIV-KB9 (16).
SHIV-HXB2 and SHIV-89.6 produce transient viremia but have caused no
immune suppression or disease in animals observed for up to 2 to 4 years of infection (19, 20, 35; unpublished observations). In contrast, rapid immune suppression and death from
AIDS are induced by infection with either SHIV-89.6PD (unpublished observations) or SHIV-89.6P (34). Four of four macaques
infected with SHIV-89.6P progressed very rapidly and failed to
seroconvert as shown by Western blot and virus neutralization assays,
and, therefore, their sera were not included in the study. Animals were
housed at either the New England Regional Primate Research Center, the
GTC Mason primate facility, or the National Institutes of Health
(NIH)-National Center for Research Resources primate housing facilities
and were maintained in accordance with guidelines set forth in
Guide for the Care and Use of Laboratory Animals (31).
Sera from SHIV-infected macaques were assessed for their ability to
neutralize multiple SHIV variants and heterologous TCLA strains of
HIV-1 (MN and SF-2) in either MT-2 or CEMx174 cells as described
previously (27, 33). Seed stocks of MN and SF-2 were
obtained from the NIH AIDS Research and Reference Reagent Program; the
derivation of these viruses has been reported elsewhere (11,
18). Neutralizing antibodies were further assessed with a panel
of six heterologous primary isolates consisting of three isolates with
a syncytium-inducing phenotype (V89872, V67970, and W179273) and three
others having a non-syncytium-inducing phenotype (P59423, W25798, and
W79290) (26). These primary isolates are occasionally
neutralized by sera from HIV-1-infected individuals (26,
33), and none of them would be considered unusually sensitive or
resistant to neutralization compared with the majority of primary isolates evaluated by others. The fact that these isolates are neutralized sporadically by sera from HIV-1-infected individuals (26, 33) is evidence that they possess a variety of
antigenically distinct neutralization determinants that are useful when
assessing the breadth of neutralizing antibodies in serum samples. Some assessments of neutralizing antibodies were also made with the uncloned
HIV-1 89.6 primary isolate and its molecularly cloned counterpart,
HIV-1 89.6mc (6). All primary isolates were obtained by peripheral blood mononuclear cell (PBMC) coculture and were of low
passage number in PBMCs exclusively (one or two passages of the
original coculture supernatant). Antibody-mediated neutralization of
primary isolates was assessed by a reduction in p24 synthesis in human
PBMCs as described previously (26, 33).
As shown in Table 1, infection with
SHIV-HXB2 raised high-titer neutralizing antibodies to SHIV-HXB2 and
HIV-1 strains MN and SF-2 but little or no detectable neutralizing
antibodies to SHIV-89.6. These results are in agreement with those
obtained in a previous study (20). Animals infected with
either SHIV-89.6 or SHIV-89.6PD developed high-titer neutralizing
antibodies to the homologous SHIV and significant titers of
neutralizing antibodies to MN and SF-2, although the titers to
SHIV-HXB2 were low or undetectable. Similar strain-specific
neutralization of SHIV-HXB2 and SHIV-89.6 has been observed for sera
from macaques immunized with IIIB and 89.6 envelope glycoprotein
subunit vaccines (unpublished observations). Finally, infection with
the intermediate animal-passaged SHIV-89.6' raised neutralizing
antibodies to SHIV-89.6, SHIV-89.6P, MN, and SF-2 but no detectable
neutralizing antibodies to SHIV-HXB2.
The above results indicate that the neutralization determinants of
SHIV-HXB2 differ from those of SHIV-89.6 and SHIV-89.6P. Poor
neutralization of SHIV-89.6P by sera from SHIV-89.6-infected animals
indicates that these two viruses differ in their neutralization determinants also. Failure to detect neutralizing antibodies to SHIV-89.6 in serum samples obtained after 11 weeks of infection with
SHIV-89.6PD (Table 1) supports this conclusion. A notable exception was
the detection of neutralizing antibodies to SHIV-89.6 after 38 weeks of
infection with SHIV-89.6PD, making it possible that the animal-passaged
stock of SHIV-89.6PD contained a mixture of genetic quasispecies,
including a minor population of SHIV-89.6 that raised these antibodies.
To minimize the possible effects of genetic quasispecies, we evaluated
sera from two animals infected for up to 41 weeks with molecularly
cloned SHIV-KB9. High titers of neutralizing antibodies to SHIV-KB9 and
SHIV-89.6P were detected by 8 and 17 weeks of infection, respectively
(Table 2). No neutralizing antibodies to
SHIV-89.6 were detected for up to 29 weeks of infection, although
low-titer neutralizing antibodies to this virus were detected after 41 weeks of infection (Table 2). Thus, the cross-reactive neutralizing
antibodies to SHIV-89.6 generated by SHIV-89.6PD infection could have
been due to long-term exposure to antigen rather than to a mixture of
genetic quasispecies in the viral stock. Nonetheless, multiple animal
passages clearly resulted in a loss of strain-specific neutralization
epitopes on SHIV-89.6 and the acquisition of new and different
neutralization epitopes on SHIV-89.6P. Associated with these changes
are 12 amino acid substitutions found throughout the gp120 and gp41 of
the KB9 molecular clone of SHIV-89.6P, including the loss of two
potential N-glycosylation sites (16). Similar changes might
occur when other SHIVs are passaged multiple times in animals.
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Neutralizing Antibodies in Sera from Macaques
Infected with Chimeric Simian-Human Immunodeficiency Virus Containing
the Envelope Glycoproteins of either a Laboratory-Adapted Variant
or a Primary Isolate of Human Immunodeficiency Virus Type
1
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ABSTRACT
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TABLE 1.
Breadth of neutralizing antibody responses in
SHIV-infected macaques
TABLE 2.
Neutralizing antibodies in sera from macaques infected
with SHIV-KB9
The demonstration of highly divergent, strain-specific neutralization epitopes on SHIV-HXB2, SHIV-89.6, and SHIV-89.6P (including molecularly cloned SHIV-KB9) has important implications for the choice of challenge virus when assessing HIV-1 envelope glycoprotein vaccines in monkeys. For example, it might be essential to match the challenge virus to the vaccine strain in order to achieve maximum benefit from neutralizing antibodies raised by experimental HXB2, 89.6, and 89.6P vaccines. These results also provide a foundation for studies in which animals may be challenged with a SHIV that is heterologous to the vaccine strain as far as in vitro neutralization determinants are concerned. This will be particularly useful when assessing correlates of immunity and breadth of efficacy in the SHIV model. Finally, these SHIV variants may be used in addition to other strains of HIV-1 to assess the breadth of neutralizing antibodies raised by experimental vaccines.
Neutralizing antibody responses in SHIV-infected macaques have additional relevance to HIV-1 vaccine development that was worth investigating. For example, infection with highly attenuated SHIV-HXB2 and SHIV-89.6 is analogous to immunizing with a mixture of different forms of molecularly cloned viral envelope glycoproteins from a single strain of virus, with the understanding that the glycoproteins may undergo sequence variation during infection. The different forms of envelope glycoproteins would include native oligomers on intact virus particles in addition to monomeric gp120 and gp41 and fragments thereof released into circulation by infected cells or when infected cells are lysed by cytotoxic T lymphocytes. It was therefore of interest to determine whether SHIV infection raised antibodies that are capable of neutralizing primary isolates. This became more of an interest once it was determined that some of these antibodies had potent neutralizing activity against heterologous TCLA variants of HIV-1. Similar assessments in humans have shown that antibodies raised in response to HIV-1 infection neutralize primary isolates occasionally but not with nearly the frequency or titer by which they neutralize TCLA variants (3, 4, 17, 26, 29, 33, 42). Assessments in the SHIV macaque model are of particular interest because the infecting strain of virus is known and, in some cases, is molecularly cloned and highly attenuated.
Antibodies raised in response to chronic SHIV-HXB2 infection occasionally neutralized one or more primary isolates, but only after the animals had been infected for more than 40 weeks (Table 1). No neutralization of heterologous primary isolates was seen for serum samples from macaques infected with either SHIV-89.6, SHIV-89.6', or SHIV-89.6PD (Table 1), although sera from SHIV-89.6-infected macaques were able to neutralize the parental, uncloned HIV-1 89.6 primary isolate (Table 3). The last of these serum samples had similar neutralization titers against HIV-1 89.6, molecularly cloned HIV-1 89.6mc, and SHIV-89.6 (Table 3), indicating that the molecular and biological manipulations used in the construction of HIV-1 89.6mc and SHIV-89.6 produced little if any changes in the neutralization determinants of the parental virus. Due to the small number of SHIV-89.6-infected animals evaluated and to possible immunosuppression in animals infected with SHIV-89.6' and SHIV-89.6PD, our results do not necessarily imply that infection with SHIV-HXB2 is more likely to raise neutralizing antibodies to primary isolates than is infection with other SHIV variants.
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Differences in either the amount of antigen or the ratio of oligomeric to monomeric envelope glycoproteins produced during infection might explain why only SHIV-HXB2 raised antibodies that occasionally neutralized primary isolates. SHIV-89.6 has been shown to replicate to higher levels than SHIV-HXB2 in rhesus macaques (35), which means there should have been no relative shortage of antigen in SHIV-89.6-infected animals. Measurements of oligomeric and monomeric envelope glycoprotein in infected animals are technically difficult and were not attempted here. It is also possible that sequence variation in the SHIV-HXB2 env gave rise to a wide range of neutralization epitopes during infection. In a previous study, 5 to 14 amino acid changes were shown to be present in molecularly cloned env from two monkeys infected for 55 to 66 weeks with SHIV-HXB2, and one of these clones was less sensitive to neutralization than the parental HXB2 clone (20). Neutralizing antibodies to primary isolates raised by SHIV-HXB2 in our studies were detected after 76 to 124 weeks of infection but not after 21 to 40 weeks of infection (Table 1). This requirement for long-term infection is consistent with sequence variation being a critical component of the ability to raise antibodies that neutralize primary isolates. Less variation in env might have occurred during infection with the SHIV-89.6 series of variants, possibly owing to differences in their coreceptor usage and cellular tropism compared to that of HXB2 (5-7, 28) or to the overall host response to infection. Finally, we cannot exclude the possibility that the antibody response in SHIV-infected macaques requires a long time to mature (e.g., antibody affinity), independently of the source and sequence variation of env, before the antibodies are capable of neutralizing primary isolates.
The limited ability for SHIV infection to raise antibodies that neutralize heterologous primary isolates suggests that the use of oligomeric rather than monomeric envelope glycoproteins as immunogens is unlikely to solve the problem of raising broadly cross-reactive neutralizing antibodies to primary isolates when the immunogens are derived from a single strain of virus. In this regard, the ability of sera from SHIV-89.6-infected macaques to neutralize HIV-1 89.6 indicates that vaccines derived from molecularly cloned envelope glycoproteins will raise antibodies that are at least capable of neutralizing the homologous vaccine strain of virus when it is a primary isolate, as has been the case for TCLA strains. Whether this will require the structure of the immunogen to be monomeric or oligomeric remains to be determined. Efforts to improve the breadth of neutralization might require either a polyvalent vaccine composed of envelope glycoproteins from multiple primary isolates or a vaccine in which the structure of the envelope glycoproteins is modified so as to be capable of raising antibodies to conserved neutralization epitopes that are otherwise poorly immunogenic. These approaches will require greater knowledge of the immunogenicity of primary isolate envelope glycoproteins and of the structures on these proteins that are targets for neutralizing antibodies.
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ACKNOWLEDGMENTS |
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We thank Miroslawa Bilska and Alicia Gaitin for assistance in growing virus stocks and in performing virus neutralization assays and Yichen Lu for providing SHIV-89.6PD.
This work was supported by grants from the NIH (AI-35166, 6S-1649, AI-15114, AI-35502, AI-20729, CA-50139, AI-33832, RR-07000, RR-00163, RR-00168, AI-28662, and AI-28691) and the Department of the Army (DAMD17-93-V-3004). Additional support was provided by the G. Harold and Leila Mathers Foundation, Friends 10, and a gift from the late William F. McCarty-Cooper.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Surgery, Duke University Medical Center, Box 2926, Durham, NC 27710. Phone: (919) 684-5278. Fax: (919) 684-4288. E-mail: monte005{at}mc.duke.edu.
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