Production and of monoclonal antibodies to simian immunodeficiency virus envelope glycoproteins

Expression of CD40L by the ALVAC-Simian Immunodeficiency Virus Vector Abrogates T Cell Responses in Macaques

Immunization with recombinant ALVAC/gp120 alum vaccine provided modest protection from human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) acquisition in humans and macaques. Vaccine-mediated protection was associated with the elicitation of IgG against the envelope V2 loop and of envelope-specific CD4⁺ T cell responses. We hypothesized that the simultaneous expression of the costimulatory molecule CD40L (CD154) by the ALVAC-HIV vector could increase both protective humoral and cellular responses. We engineered an ALVAC-SIV coexpressing CD40L with SIV_mac251 (ALVAC-SIV/CD40L) gag, pol, and env genes. We compared its immunogenicity in macaques with that of a canonical ALVAC-SIV, with both given as a vector-prime/gp120 in alum boost strategy. The ALVAC-SIV/CD40L was superior to the ALVAC-SIV regimen in inducing binding and tier 1 neutralizing antibodies against the gp120. The increase in humoral responses was associated with the expression of the membrane-bound form of the CD40L by CD4⁺ T cells in lymph nodes. Unexpectedly, the ALVAC-SIV/CD40L vector had a blunting effect on CD4⁺ Th1 helper responses and instead favored the induction of myeloid-derived suppressor cells, the immune-suppressive interleukin-10 (IL-10) cytokine, and the down-modulatory tryptophan catabolism. Ultimately, this strategy failed to protect macaques from SIV acquisition. Taken together, these results underlie the importance of balanced vaccine-induced activating versus suppressive immune responses in affording protection from HIV.IMPORTANCE CD40-CD40 ligand (CD40L) interaction is crucial for inducing effective cytotoxic and humoral responses against pathogens. Because of its immunomodulatory function, CD40L has been used to enhance immune responses to vaccines, including candidate vaccines for HIV. The only successful vaccine ever tested in humans utilized a strategy combining canarypox virus-based vector (ALVAC) together with an envelope protein (gp120) adjuvanted in alum. This strategy showed limited efficacy in preventing HIV-1/SIV acquisition in humans and macaques. In both species, protection was associated with vaccine-induced antibodies against the HIV envelope and CD4⁺ T cell responses, including type 1 antiviral responses. In this study, we tested whether augmenting CD40L expression by coexpressing it with the ALVAC vector could increase the protective immune responses. Although coexpression of CD40L did increase humoral responses, it blunted type 1 CD4⁺ T cell responses against the SIV envelope protein and failed to protect macaques from viral infection.

Analysis of Complement-Mediated Lysis of Simian Immunodeficiency Virus (SIV) and SIV-Infected Cells Reveals Sex Differences in Vaccine-Induced Immune Responses in Rhesus Macaques

An effective human immunodeficiency virus (HIV) vaccine has yet to be developed, and defining immune correlates of protection against HIV infection is of paramount importance to inform future vaccine design. The complement system is a component of innate immunity that can directly lyse pathogens and shape adaptive immunity. To determine if complement lysis of simian immunodeficiency virus (SIV) and/or SIV-infected cells represents a protective immune correlate against SIV infection, sera from previously vaccinated and challenged rhesus macaques were analyzed for the induction of antibody-dependent complement-mediated lysis (ADCML). Importantly, the vaccine regimen, consisting of a replication-competent adenovirus type 5 host-range mutant SIV recombinant prime followed by a monomeric gp120 or oligomeric gp140 boost, resulted in overall delayed SIV acquisition only in females. Here, sera from all vaccinated animals induced ADCML of SIV and SIV-infected cells efficiently, regardless of sex. A modest correlation of SIV lysis with a reduced infection rate in males but not females, together with a reduced peak viremia in all animals boosted with gp140, suggested a potential for influencing protective efficacy. Gag-specific IgG and gp120-specific IgG and IgM correlated with SIV lysis in females, while Env-specific IgM correlated with SIV-infected cell lysis in males, indicating sex differences in vaccine-induced antibody characteristics and function. In fact, gp120/gp140-specific antibody functional correlates between antibody-dependent cellular cytotoxicity, antibody-dependent phagocytosis, and ADCML as well as the gp120-specific IgG glycan profiles and the corresponding ADCML correlations varied depending on the sex of the vaccinees. Overall, these data suggest that sex influences vaccine-induced antibody function, which should be considered in the design of globally effective HIV vaccines in the future.IMPORTANCE An HIV vaccine would thwart the spread of HIV infection and save millions of lives. Unfortunately, the immune responses conferring universal protection from HIV infection are poorly defined. The innate immune system, including the complement system, is an evolutionarily conserved, basic means of protection from infection. Complement can prevent infection by directly lysing incoming pathogens. We found that vaccination against SIV in rhesus macaques induces antibodies that are capable of directing complement lysis of SIV and SIV-infected cells in both sexes. We also found sex differences in vaccine-induced antibody species and their functions. Overall, our data suggest that sex affects vaccine-induced antibody characteristics and function and that males and females might require different immune responses to protect against HIV infection. This information could be used to generate highly effective HIV vaccines for both sexes in the future.

In vivo characterization of macrophage-tropic simian immunodeficiency virus molecular clones in rhesus macaques

Macrophages are a major target of HIV/SIV infection and play an important role in pathogenesis by serving as viral reservoirs in the central nervous system. Previously, a unique early SIVmac251 envelope (Env) variant, deSIV147 was cloned from blood of a rhesus macaque with rapid disease progression and SIV-associated encephalitis. Here, we show that infectious molecular clone deSIV147 caused systemic infection in rhesus macaques following intravenous or intrarectal exposure. Next, we inoculated deSIV147 into macaques depleted of CD4+ T cells and found that animals were SIV-positive, with high plasma and CSF viral loads. These macaques also showed SIVp17-positive macrophages in brain, lymph nodes, colon, lung, and liver. Furthermore, accumulation of perivascular macrophages, multinucleated giant cells, and microgliosis was detected. These findings suggest that the neurotropic deSIV147 clone will be useful to study macrophage infection in HIV/SIV-associated neurocognitive disorders, gain insights into myeloid cell reservoirs in brain and other anatomical sites, as well as test strategies for eradication.

Interferon-Inducible CD169/Siglec1 Attenuates Anti-HIV-1 Effects of Alpha Interferon

A hallmark of human immunodeficiency virus type 1 (HIV-1) infection in vivo is chronic immune activation concomitant with type I interferon (IFN) production. Although type I IFN induces an antiviral state in many cell types, HIV-1 can replicate in vivo via mechanisms that have remained unclear. We have recently identified a type I IFN-inducible protein, CD169, as the HIV-1 attachment factor on dendritic cells (DCs) that can mediate robust infection of CD4⁺ T cells in trans Since CD169 expression on macrophages is also induced by type I IFN, we hypothesized that type I IFN-inducible CD169 could facilitate productive HIV-1 infection in myeloid cells in cis and CD4⁺ T cells in trans and thus offset antiviral effects of type I IFN. In support of this hypothesis, infection of HIV-1 or murine leukemia virus Env (MLV-Env)-pseudotyped HIV-1 particles was enhanced in IFN-α-treated THP-1 monocytoid cells, and this enhancement was primarily dependent on CD169-mediated enhancement at the virus entry step, a phenomenon phenocopied in HIV-1 infections of IFN-α-treated primary monocyte-derived macrophages (MDMs). Furthermore, expression of CD169, a marker of type I IFN-induced immune activation in vivo, was enhanced in lymph nodes from pigtailed macaques infected with simian immunodeficiency virus (SIV) carrying HIV-1 reverse transcriptase (RT-SHIV), compared to uninfected macaques, and interestingly, there was extensive colocalization of p27^gag and CD169, suggesting productive infection of CD169⁺ myeloid cells in vivo While cell-free HIV-1 infection of IFN-α-treated CD4⁺ T cells was robustly decreased, initiation of infection in trans via coculture with CD169⁺ IFN-α-treated DCs restored infection, suggesting that HIV-1 exploits CD169 in cis and in trans to attenuate a type I IFN-induced antiviral state.IMPORTANCE HIV-1 infection in humans causes immune activation characterized by elevated levels of proinflammatory cytokines, including type I interferons (IFN). Although type I IFN induces an antiviral state in many cell types in vitro, HIV-1 can replicate in vivo via mechanisms that have remained unclear. In this study, we tested the hypothesis that CD169, a type I IFN-inducible HIV-1 attachment factor, offsets antiviral effects of type I IFN. Infection of HIV-1 was rescued in IFN-α-treated myeloid cells via upregulation of CD169 and a subsequent increase in CD169-dependent virus entry. Furthermore, extensive colocalization of viral Gag and CD169 was observed in lymph nodes of infected pigtailed macaques, suggesting productive infection of CD169⁺ cells in vivo Treatment of dendritic cell (DC)-T cell cocultures with IFN-α upregulated CD169 expression on DCs and rescued HIV-1 infection of CD4⁺ T cells in trans, suggesting that HIV-1 exploits CD169 to attenuate type I IFN-induced restrictions.

Paradoxical myeloid-derived suppressor cell reduction in the bone marrow of SIV chronically infected macaques

Myeloid derived suppressor cells (MDSCs), which suppress anti-tumor or anti-viral immune responses, are expanded in the peripheral blood and tissues of patients/animals with cancer or viral infectious diseases. We here show that in chronic SIV infection of Indian rhesus macaques, the frequency of MDSCs in the bone marrow (BM) was paradoxically and unexpectedly decreased, but increased in peripheral blood. Reduction of BM MDSCs was found in both CD14⁺MDSC and Lin^-CD15⁺MDSC subsets. The reduction of MDSCs correlated with high plasma viral loads and low CD4⁺ T cell counts, suggesting that depletion of BM MDSCs was associated with SIV/AIDS disease progression. Of note, in SHIV_SF162P4-infected macaques, which naturally control viral replication within a few months of infection, the frequency of MDSCs in the bone marrow was unchanged. To investigate the mechanisms by which BM MDSCs were reduced during chronic SIV infection, we tested several hypotheses: depletion due to viral infection, alterations in MDSC trafficking, and/or poor MDSC replenishment. We found that the possible mobilization of MDSCs from BM to peripheral tissues and the slow self-replenishment of MDSCs in the BM, along with the viral infection-induced depletion, all contribute to the observed BM MDSC reduction. We first demonstrate MDSC SIV infection in vivo. Correlation between BM CD14⁺MDSC reduction and CD8⁺ T cell activation in tissues is consistent with decreased immune suppression by MDSCs. Thus, depletion of BM MDSCs may contribute to the pathologic immune activation during chronic SIV infection and by extension HIV infection.

Both cancer and infectious diseases including HIV/AIDS lead to the accumulation of myeloid-derived suppressor cells (MDSCs), which can effectively suppress anti-tumor and anti-viral T cell responses to dampen protective immunity. Using a macaque model, we found unexpectedly that the MDSCs in bone marrow (BM) decreased after chronic simian immunodeficiency virus (SIV) infection compared with healthy controls. This was in sharp contrast to the general increase of MDSCs observed in BM during cancer and other infectious/inflammatory diseases, and also contrary to the MDSC expansion in HIV/SIV-infected PBMCs. We further demonstrated that the loss of MDSCs in the bone marrow was associated with the progression to AIDS disease. Investigating the mechanisms by which the MDSCs were decreased in the SIV-infected bone marrow, we found that the possible mobilization of MDSCs from bone marrow to peripheral tissues and the slow self-replenishment of MDSCs in the bone marrow, along with the viral infection-induced depletion, all contribute to the observed bone marrow MDSC reduction. Indeed, this is the first demonstration to our knowledge of SIV infection of MDSCs in vivo. Because of the suppressive nature of the MDSCs, the CD8⁺ T cells might not be effective in killing the virally infected MDSCs. It is tempting to speculate that MDSCs may constitute latent reservoirs. Overall, our data showed that MDSCs act as a double-edged sword in HIV/SIV-infection, and the decrease of MDSCs in bone marrow after SIV infection could serve as an indicator of immune regulatory exhaustion and also contribute to the observed immune hyperactivation seen in HIV/AIDS.

DNA Prime-Boost Vaccine Regimen To Increase Breadth, Magnitude, and Cytotoxicity of the Cellular Immune Responses to Subdominant Gag Epitopes of Simian Immunodeficiency Virus and HIV

HIV sequence diversity and the propensity of eliciting immunodominant responses targeting variable regions of the HIV proteome are hurdles in the development of an effective AIDS vaccine. An HIV-derived conserved element (CE) p24^gag plasmid DNA (pDNA) vaccine is able to redirect immunodominant responses to otherwise subdominant and often more vulnerable viral targets. By homology to the HIV immunogen, seven CE were identified in SIV p27^Gag Analysis of 31 rhesus macaques vaccinated with full-length SIV gag pDNA showed inefficient induction (58% response rate) of cellular responses targeting these CE. In contrast, all 14 macaques immunized with SIV p27CE pDNA developed robust T cell responses recognizing CE. Vaccination with p27CE pDNA was also critical for the efficient induction and increased the frequency of Ag-specific T cells with cytotoxic potential (granzyme B⁺ CD107a⁺) targeting subdominant CE epitopes, compared with the responses elicited by the p57^gag pDNA vaccine. Following p27CE pDNA priming, two booster regimens, gag pDNA or codelivery of p27CE+gag pDNA, significantly increased the levels of CE-specific T cells. However, the CE+gag pDNA booster vaccination elicited significantly broader CE epitope recognition, and thus, a more profound alteration of the immunodominance hierarchy. Vaccination with HIV molecules showed that CE+gag pDNA booster regimen further expanded the breadth of HIV CE responses. Hence, SIV/HIV vaccine regimens comprising CE pDNA prime and CE+gag pDNA booster vaccination significantly increased cytotoxic T cell responses to subdominant highly conserved Gag epitopes and maximized response breadth.

Targeted Isolation of Antibodies Directed against Major Sites of SIV Env Vulnerability

The simian immunodeficiency virus (SIV) challenge model of lentiviral infection is often used as a model to human immunodeficiency virus type 1 (HIV-1) for studying vaccine mediated and immune correlates of protection. However, knowledge of the structure of the SIV envelope (Env) glycoprotein is limited, as is knowledge of binding specificity, function and potential efficacy of SIV antibody responses. In this study we describe the use of a competitive probe binding sort strategy as well as scaffolded probes for targeted isolation of SIV Env-specific monoclonal antibodies (mAbs). We isolated nearly 70 SIV-specific mAbs directed against major sites of SIV Env vulnerability analogous to broadly neutralizing antibody (bnAb) targets of HIV-1, namely, the CD4 binding site (CD4bs), CD4-induced (CD4i)-site, peptide epitopes in variable loops 1, 2 and 3 (V1, V2, V3) and potentially glycan targets of SIV Env. The range of SIV mAbs isolated includes those exhibiting varying degrees of neutralization breadth and potency as well as others that demonstrated binding but not neutralization. Several SIV mAbs displayed broad and potent neutralization of a diverse panel of 20 SIV viral isolates with some also neutralizing HIV-2_7312A. This extensive panel of SIV mAbs will facilitate more effective use of the SIV non-human primate (NHP) model for understanding the variables in development of a HIV vaccine or immunotherapy.

An antibody-based approach targeting human immunodeficiency virus (HIV) envelope (Env) protein may eventually prove to be effective in treating or preventing HIV infection. However, before any candidate HIV treatment or vaccine can be tested in humans, it must first be evaluated in nonhuman primates (NHPs)–the closest living relatives to humans. Simian immunodeficiency virus (SIV) is the closest available non-chimeric virus—NHP model for studying and testing HIV vaccines or therapies. The SIV model complements the simian-human immunodeficiency virus (SHIV) model in distinctive ways, although less is known about SIV Env-specific antibody responses in NHPs. There are several sites on HIV Env that are vulnerable to antibody-mediated protection, and here we isolated and analyzed monoclonal antibodies (mAbs) from NHPs targeting analogous sites on SIV Env. In particular, we studied mAbs for their ability to bind the viral Env protein and to block infection of cells by widely divergent strains of SIV. These well-characterized SIV Env-specific antibodies will allow for more thorough NHP pre-clinical testing of various antibody-based SIV/HIV vaccine and immunotherapeutic strategies before proceeding to human clinical trials and may yield unanticipated findings relating to molecular mechanisms underlying the unusual breadth of neutralization observed in HIV-2 infection.

Simian immunodeficiency virus infection evades vaccine-elicited antibody responses to V2 region

OBJECTIVES

An effective AIDS vaccine should elicit protective antibody responses against HIV/simian immunodeficiency virus (SIV) infection. We recently reported that mucosal priming with a replicating modified vaccinia Tiantan virus (MVTTgpe)-based vaccine regimen induces durable protection against pathogenic SIVmac239 infection in rhesus monkeys. Here, we aim to conduct a comprehensive analysis on antigenic determinants recognized by specific antibody responses generated by vaccination and SIVmac239 infection.

METHODS

A novel yeast surface displayed antigen library of entire SIVmac239 envelope (Env) glycoprotein was established and validated to map the major antigenic determinants (MAD) in monkey sera elicited by vaccination and infection. MAD-directed antibody responses were further analyzed for correlation of protection.

RESULTS AND CONCLUSIONS

The yeast surface displayed library allows the mapping of SIV-specific linear and conformational MAD. The MVTTgpe-based regimen induces antibodies targeting mainly to 6 antigenic domains covering the entire gp160. Critically, this regimen induced a uniquely predominant antibody response against a distinct MAD in variable region 2 (V2) as compared with the Ad5gpe-based vaccine and SIVmac239 infection. This MAD was associated with a higher titer of anti-V2 antibody responses, which was inversely correlated with peak and set-point viral loads. Unexpectedly, the pathogenic SIVmac239 challenge evaded the vaccine-elicited anti-V2 antibody response. Instead of recalling B-cell memory responses to the V2 MAD, viral infection directed anti-V1V2 antibodies primarily to V1 region. Moreover, the anti-V1V2 antibody responses diminished significantly in infected macaques after they enter the stage of simian AIDS. Our findings have critical implications to AIDS vaccine efforts with focus on V2 region.

Early biodistribution and persistence of a protective live attenuated SIV vaccine elicits localised innate responses in multiple lymphoid tissues

Vaccination of Mauritian cynomolgus macaques with the attenuated nef-truncated C8 variant of SIVmac251/32H (SIVmacC8) induces early, potent protection against pathogenic, heterologous challenge before the maturation of cognate immunity. To identify processes that contribute to early protection in this model the pathogenesis, anatomical distribution and viral vaccine kinetics were determined in relation to localised innate responses triggered by vaccination. The early biodistribution of SIVmacC8 was defined by rapid, widespread dissemination amongst multiple lymphoid tissues, detectable after 3 days. Cell-associated viral RNA dynamics identified mesenteric lymph nodes (MLN) and spleen, as well as the gut mucosae, as early major contributors of systemic virus burden. Rapid, localised infection was populated by discrete foci of persisting virus-infected cells. Localised productive infection triggered a broad innate response, with type-1 interferon sensitive IRF-7, STAT-1, TRIM5α and ApoBEC3G genes all upregulated during the acute phase but induction did not prevent viral persistence. Profound changes in vaccine-induced cell-surface markers of immune activation were detected on macrophages, B-cells and dendritic cells (DC-SIGN, S-100, CD40, CD11c, CD123 and CD86). Notably, high DC-SIGN and S100 staining for follicular and interdigitating DCs respectively, in MLN and spleen were detected by 3 days, persisting 20 weeks post-vaccination. Although not formally evaluated, the early biodistribution of SIVmacC8 simultaneously targets multiple lymphoid tissues to induce strong innate immune responses coincident at the same sites critical for early protection from wild-type viruses. HIV vaccines which stimulate appropriate innate, as well as adaptive responses, akin to those generated by live attenuated SIV vaccines, may prove the most efficacious.

Conditionally-live attenuated SIV upregulates global T effector memory cell frequency under replication permissive conditions

BACKGROUND

Live attenuated SIV induces potent protection against superinfection with virulent virus; however the mechanism of this vaccine effect is poorly understood. Such knowledge is important for the development of clinically acceptable vaccine modalities against HIV.

RESULTS

Using a novel, doxycycline dependent, replication-competent live-attenuated SIVmac239Δnef (SIV-rtTAΔnef), we show that under replication-permissive conditions SIV-rtTAΔnef is fully viable. Twelve rhesus macaques were infected with a peak plasma vRNA on average two log10 lower than in 6 macaques infected with unconditionally replication-competent SIVΔnef. Consistent with the attenuated phenotype of the viruses the majority of animals displayed low or undetectable levels of viraemia by 42-84 days after infection. Next, comparison of circulating T cells before and after chronic infection with parental SIVΔnef revealed a profound global polarisation toward CD28-CCR7- T-effector memory 2 (TEM2) cells within CD95+CD4+ and CD95+CD8+ populations. Critically, a similar effect was seen in the CD95+ CD4+ population and to somewhat lesser extent in the CD95+ CD8+ population of SIV-rtTAΔnef chronically infected macaques that were maintained on doxycycline, but was not seen in animals from which doxycycline had been withdrawn. The proportions of gut-homing T-central memory (TCM) and TEM defined by the expression of α4β7 and CD95 and differential expression of CD28 were increased in CD4 and CD8 cells under replication competent conditions and gut-homing CD4 TCM were also significantly increased under non-permissive conditions. TEM2 polarisation was seen in the small intestines of animals under replication permissive conditions but the effect was less pronounced than in the circulation. Intracellular cytokine staining of circulating SIV-specific T cells for IL-2, IFN-γ, TNF-α and IL-17 showed that the extent of polyfunctionality in CD4 and CD8 T cells was associated with replication permissivity; however, signature patterns of cytokine combinations were not distinguishable between groups of macaques.

CONCLUSION

Taken together our results show that the global T memory cell compartment is profoundly skewed towards a mature effector phenotype by attenuated SIV. Results with the replication-conditional mutant suggest that maintenance of this effect, that may be important in vaccine design, might require persistence of replicating virus.

An AUG codon upstream of rev and env open reading frames ensures optimal translation of the simian immunodeficiency virus Env protein

The mRNAs encoding the Rev and Env proteins of simian immunodeficiency virus (SIV) are unique because upstream translation start codons are present that may modulate the expression of these viral proteins. We previously reported the regulatory effect of a small upstream open reading frame (ORF) on Rev and Env translation. Here we study this mechanism in further detail by modulating the strength of the translation signals upstream of the open reading frames in subgenomic reporters. Furthermore, the effects of these mutations on SIV gene expression and viral replication are analyzed. An intricate regulatory mechanism is disclosed that allows the virus to express a balanced amount of these two proteins.

Differential infection patterns of CD4+ T cells and lymphoid tissue viral burden distinguish progressive and nonprogressive lentiviral infections

Nonhuman primate natural hosts for simian immunodeficiency viruses (SIV) develop a nonresolving chronic infection but do not develop AIDS. Mechanisms to explain the nonprogressive nature of SIV infection in natural hosts that underlie maintained high levels of plasma viremia without apparent loss of target cells remain unclear. Here we used comprehensive approaches (ie, FACS sorting, quantitative RT-PCR, immunohistochemistry, and in situ hybridization) to study viral infection within subsets of peripheral blood and lymphoid tissue (LT) CD4(+) T cells in cohorts of chronically SIV-infected rhesus macaques (RMs), HIV-infected humans, and SIVsmm-infected sooty mangabeys (SMs). We find: (1) infection frequencies among CD4(+) T cells in chronically SIV-infected RMs are significantly higher than those in SIVsmm-infected SMs; (2) infected cells are found in distinct anatomic LT niches and different CD4(+) T-cell subsets in SIV-infected RMs and SMs, with infection patterns of RMs reflecting HIV infection in humans; (3) T(FH) cells are infected at higher frequencies in RMs and humans than in SMs; and (4) LT viral burden, including follicular dendritic cell deposition of virus, is increased in RMs and humans compared with SMs. These data provide insights into how natural hosts are able to maintain high levels of plasma viremia while avoiding development of immunodeficiency.

Neuropathology of wild-type and nef-attenuated T cell tropic simian immunodeficiency virus (SIVmac32H) and macrophage tropic neurovirulent SIVmac17E-Fr in cynomolgus macaques

The neuropathology of simian immunodeficiency (SIV) infection in cynomolgus macaques (Macaca fascicularis) was investigated following infection with either T cell tropic SIVmacJ5, SIVmacC8 or macrophage tropic SIVmac17E-Fr. Formalin fixed, paraffin embedded brain tissue sections were analysed using a combination of in situ techniques. Macaques infected with either wild-type SIVmacJ5 or neurovirulent SIVmac17E-Fr showed evidence of neuronal dephosphorylation, loss of oligodendrocyte and CCR5 staining, lack of microglial MHC II expression, infiltration by CD4⁺ and CD8⁺ T cells and mild astrocytosis. SIVmacJ5-infected animals exhibited activation of microglia whilst those infected with SIVmac17E-Fr demonstrated a loss of microglia staining. These results are suggestive of impaired central nervous system (CNS) physiology. Furthermore, infiltration by T cells into the brain parenchyma indicated disruption of the blood brain barrier (BBB). Animals infected with the Δnef-attenuated SIVmacC8 showed microglial activation and astrogliosis indicative of an inflammatory response, lack of MHC II and CCR5 staining and infiltration by CD8⁺ T cells. These results demonstrate that the SIV infection of cynomolgus macaque can be used as a model to replicate the range of CNS pathologies observed following HIV infection of humans and to investigate the pathogenesis of HIV associated neuropathology.

Different effects of the TAR structure on HIV-1 and HIV-2 genomic RNA translation

The 5′-untranslated region (5′-UTR) of the genomic RNA of human immunodeficiency viruses type-1 (HIV-1) and type-2 (HIV-2) is composed of highly structured RNA motifs essential for viral replication that are expected to interfere with Gag and Gag-Pol translation. Here, we have analyzed and compared the properties by which the viral 5′-UTR drives translation from the genomic RNA of both human immunodeficiency viruses. Our results showed that translation from the HIV-2 gRNA was very poor compared to that of HIV-1. This was rather due to the intrinsic structural motifs in their respective 5′-UTR without involvement of any viral protein. Further investigation pointed to a different role of TAR RNA, which was much inhibitory for HIV-2 translation. Altogether, these data highlight important structural and functional differences between these two human pathogens.

Isolation of potent neutralizing monoclonal antibodies from an SIV-Infected rhesus macaque by phage display

The humoral immune response is a mechanism that potently suppresses or prevents viral infections. However, genetic diversity and resistance to antibody-mediated neutralization are serious obstacles in controlling HIV-1 infection. In this study, we isolated monoclonal antibodies from an SIV-infected macaque by using the phage display method to characterize antibodies in SIV infection. Variable regions of immunoglobulin genes were amplified by rhesus macaque-specific primers and inserted into the phagemid pComb3X, which produced the Fab fragment. Antibodies against SIV proteins were selected by biopanning using an SIV protein-coated 96-well plate. A total of 20 Fab clones obtained included 14 clones directed to gp41, four clones to gp120, and two clones to p27. The anti-gp120 Fab clones completely neutralized the homologous neutralization-sensitive SIVsmH635FC and the genetically divergent SIVmac316, and showed at least 50% inhibition against the neutralization-resistant strain, SIVsmE543-3. Competition ELISA revealed that these anti-gp120 Fab clones recognize the same epitope on gp120 including the V3 loop. Identification of antibodies with potent neutralizing activity will help to elucidate the mechanisms for inducing broadly neutralizing antibodies.

Fundamental difference in the content of high-mannose carbohydrate in the HIV-1 and HIV-2 lineages

The virus-encoded envelope proteins of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) typically contain 26 to 30 sites for N-linked carbohydrate attachment. N-linked carbohydrate can be of three major types: high mannose, complex, or hybrid. The lectin proteins from Galanthus nivalis (GNA) and Hippeastrum hybrid (HHA), which specifically bind high-mannose carbohydrate, were found to potently inhibit the replication of a pathogenic cloned SIV from rhesus macaques, SIVmac239. Passage of SIVmac239 in the presence of escalating concentrations of GNA and HHA yielded a lectin-resistant virus population that uniformly eliminated three sites (of 26 total) for N-linked carbohydrate attachment (Asn-X-Ser or Asn-X-Thr) in the envelope protein. Two of these sites were in the gp120 surface subunit of the envelope protein (Asn244 and Asn460), and one site was in the envelope gp41 transmembrane protein (Asn625). Maximal resistance to GNA and HHA in a spreading infection was conferred to cloned variants that lacked all three sites in combination. Variant SIV gp120s exhibited dramatically decreased capacity for binding GNA compared to SIVmac239 gp120 in an enzyme-linked immunosorbent assay (ELISA). Purified gp120s from six independent HIV type 1 (HIV-1) isolates and two SIV isolates from chimpanzees (SIVcpz) consistently bound GNA in ELISA at 3- to 10-fold-higher levels than gp120s from five SIV isolates from rhesus macaques or sooty mangabeys (SIVmac/sm) and four HIV-2 isolates. Thus, our data indicate that characteristic high-mannose carbohydrate contents have been retained in the cross-species transmission lineages for SIVcpz-HIV-1 (high), SIVsm-SIVmac (low), and SIVsm-HIV-2 (low).

The efficacy of T cell-mediated immune responses is reduced by the envelope protein of the chimeric HIV-1/SIV-KB9 virus in vivo

Gp120 is a critical component of the envelope of HIV-1. Its role in viral entry is well described. In view of its position on the viral envelope, gp120 is a part of the retrovirus that immune cells encounter first and has the potential to influence antiretroviral immune responses. We propose that high levels of gp120 are present in tissues and may contribute to the failure of the immune system to fully control and ultimately clear the virus. Herein, we show for the first time that lymphoid tissues from acutely HIV-1/SIV (SHIV)-KB9-infected macaques contain deposits of gp120 at concentrations that are high enough to induce suppressive effects on T cells, thus negatively regulating the antiviral CTL response and contributing to virus survival and persistence. We also demonstrate that SHIV-KB9 gp120 influences functional T cell responses during SHIV infection in a manner that suppresses degranulation and cytokine secretion by CTLs. Finally, we show that regulatory T cells accumulate in lymphoid tissues during acute infection and that they respond to gp120 by producing TGFbeta, a known suppressant of cytotoxic T cell activity. These findings have significant implications for our understanding of the contribution of non-entry-related functions of HIV-1 gp120 to the pathogenesis of HIV/AIDS.

Structure and immunogenicity of alternative forms of the simian immunodeficiency virus gag protein expressed using Venezuelan equine encephalitis virus replicon particles

Venezuelan equine encephalitis virus replicon particles (VRP) were engineered to express different forms of SIV Gag to compare expression in vitro, formation of intra- and extracellular structures and induction of humoral and cellular immunity in mice. The three forms examined were full-length myristylated SIV Gag (Gagmyr+), full-length Gag lacking the myristylation signal (Gagmyr-) or a truncated form of Gagmyr- comprising only the matrix and capsid domains (MA/CA). Comparison of VRP-infected primary mouse embryo fibroblasts, mouse L929 cells and primate Vero cells showed comparable expression levels for each protein, as well as extracellular virus-like particles (VRP-Gagmyr+) and distinctive cytoplasmic aggregates (VRP-Gagmyr-) with each cell type. VRP were used to immunize BALB/c mice, and immune responses were compared using an interferon (IFN)-gamma ELISPOT assay and a serum antibody ELISA. Although all three VRP generated similar levels of IFN-gamma-producing cells at 1 week post-boost, at 10 weeks post-boost the MA/CA-VRP-induced response was maintained at a significantly higher level relative to that induced by Gagmyr+-VRP. Antibody responses to MA/CA-VRP and Gagmyr+-VRP were not significantly different.

Restraining the conformation of HIV-1 gp120 by removing a flexible loop

The trimeric HIV/SIV envelope glycoprotein, gp160, is cleaved to noncovalently associated fragments, gp120 and gp41. Binding of gp120 to viral receptors leads to large structural rearrangements in both fragments. The unliganded gp120 core has a disordered beta3-beta5 loop, which reconfigures upon CD4 binding into an ordered, extended strand. Molecular modeling suggests that residues in this loop may contact gp41. We show here that deletions in the beta3-beta5 loop of HIV-1 gp120 weaken the binding of CD4 and prevent formation of the epitope for monoclonal antibody (mAb) 17b (which recognizes the coreceptor site). Formation of an encounter complex with CD4 binding and interactions of gp120 with mAbs b12 and 2G12 are not affected by these deletions. Thus, deleting the beta3-beta5 loop blocks the gp120 conformational change and may offer a strategy for design of restrained immunogens. Moreover, mutations in the SIV beta3-beta5 loop lead to greater spontaneous dissociation of gp120 from cell-associated trimers. We suggest that the CD4-induced rearrangement of this loop releases structural constraints on gp41 and thus potentiates its fusion activity.

HIV-2 genomic RNA contains a novel type of IRES located downstream of its initiation codon

Eukaryotic translation initiation begins with assembly of a 48S ribosomal complex at the 5' cap structure or at an internal ribosomal entry segment (IRES). In both cases, ribosomal positioning at the AUG codon requires a 5' untranslated region upstream from the initiation site. Here, we report that translation of the genomic RNA of human immunodeficiency virus type 2 takes place by attachment of the 48S ribosomal preinitiation complex to the coding region, with no need for an upstream 5' untranslated RNA sequence. This unusual mechanism is mediated by an RNA sequence that has features of an IRES with the unique ability to recruit ribosomes upstream from its core domain. A combination of translation assays and structural studies reveal that sequences located 50 nucleotides downstream of the AUG codon are crucial for IRES activity.

Simian immunodeficiency virus engrafted with human immunodeficiency virus type 1 (HIV-1)-specific epitopes: replication, neutralization, and survey of HIV-1-positive plasma

To date, only a small number of anti-human immunodeficiency virus type 1 (HIV-1) monoclonal antibodies (MAbs) with relatively broad neutralizing activity have been isolated from infected individuals. Adequate techniques for defining how frequently antibodies of these specificities arise in HIV-infected people have been lacking, although it is generally assumed that such antibodies are rare. In order to create an epitope-specific neutralization assay, we introduced well-characterized HIV-1 epitopes into the heterologous context of simian immunodeficiency virus (SIV). Specifically, epitope recognition sequences for the 2F5, 4E10, and 447-52D anti-HIV-1 neutralizing monoclonal antibodies were introduced into the corresponding regions of SIVmac239 by site-directed mutagenesis. Variants with 2F5 or 4E10 recognition sequences in gp41 retained replication competence and were used for neutralization assays. The parental SIVmac239 and the neutralization-sensitive SIVmac316 were not neutralized by the 2F5 and 4E10 MAbs, nor were they neutralized significantly by any of the 96 HIV-1-positive human plasma samples that were tested. The SIV239-2F5 and SIV239-4E10 variants were specifically neutralized by the 2F5 and 4E10 MAbs, respectively, at concentrations within the range of what has been reported previously for HIV-1 primary isolates (J. M. Binley et al., J. Virol. 78:13232-13252, 2004). The SIV239-2F5 and SIV239-4E10 epitope-engrafted variants were used as biological screens for the presence of neutralizing activity of these specificities. None of the 92 HIV-1-positive human plasma samples that were tested exhibited significant neutralization of SIV239-2F5. One plasma sample exhibited >90% neutralization of SIV239-4E10, but this activity was not competed by a 4E10 target peptide and was not present in concentrated immunoglobulin G (IgG) or IgA fractions. We thus confirm by direct analysis that neutralizing activities of the 2F5 and 4E10 specificities are either rare among HIV-1-positive individuals or, if present, represent only a very small fraction of the total neutralizing activity in any given plasma sample. We further conclude that the structures of gp41 from SIVmac239 and HIV-1 are sufficiently similar such that epitopes engrafted into SIVmac239 can be readily recognized by the cognate anti-HIV-1 monoclonal antibodies.

Identification of two N-linked glycosylation sites within the core of the simian immunodeficiency virus glycoprotein whose removal enhances sensitivity to soluble CD4

Using PCR mutagenesis to disrupt the NXT/S N-linked glycosylation motif of the Env protein, we created 27 mutants lacking 1 to 5 of 14 N-linked glycosylation sites within regions of gp120 lying outside of variable loops 1 to 4 within simian immunodeficiency virus strain 239 (SIV239). Of 18 mutants missing N-linked glycosylation sites predicted to lie within 10 A of CD4 contact sites, the infectivity of 12 was sufficient to measure sensitivity to neutralization by soluble CD4 (sCD4), pooled immune sera from SIV239-infected rhesus macaques, and monoclonal antibodies known to neutralize certain derivatives of SIV239. Three of these 12 mutants (g3, lacking the 3rd glycan at position 79; g11, lacking the 11th glycan at position 212; and g3,11, lacking both the 3rd and 11th glycans) were approximately five times more sensitive to neutralization by sCD4 than wild-type (WT) SIV239. However, these same mutants were no more sensitive to neutralization than WT by pooled immune sera. The other 9 of 12 replication-competent mutants in this group were no more sensitive to neutralization than the WT by any of the neutralizing reagents. Six of the nine mutants that did not replicate appreciably had three or more glycosylation sites eliminated; the other three replication-deficient strains involved mutation of site 15. Our results suggest that elimination of glycan attachment sites 3 and 11 enhanced the exposure of contact residues for CD4. Thus, glycans at positions 3 and 11 of SIV239 gp120 may be particularly important for shielding the CD4-binding site from antibody recognition.

Elimination of retroviral infectivity by N-ethylmaleimide with preservation of functional envelope glycoproteins

The zinc finger motifs in retroviral nucleocapsid (NC) proteins are essential for viral replication. Disruption of these Cys-X2-Cys-X4-His-X4-Cys zinc-binding structures eliminates infectivity. To determine if N-ethylmaleimide (NEM) can inactivate human immunodeficiency virus type 1 (HIV-1) or simian immunodeficiency virus (SIV) preparations by alkylating cysteines of NC zinc fingers, we treated infectious virus with NEM and evaluated inactivation of infectivity in cell-based assays. Inactivation was rapid and proportional to the NEM concentration. NEM treatment of HIV-1 or SIV resulted in extensive covalent modification of NC and other internal virion proteins. In contrast, viral envelope glycoproteins, in which the cysteines are disulfide bonded, remained intact and functional, as assayed by high-performance liquid chromatography, fusion-from-without analyses, and dendritic cell capture. Quantitative PCR assays for reverse transcription intermediates showed that NEM and 2,2'-dipyridyl disulfide (aldrithiol-2), a reagent which inactivates retroviruses through oxidation of cysteines in internal virion proteins such as NC, blocked HIV-1 reverse transcription prior to the formation of minus-strand strong-stop products. However, the reverse transcriptase from NEM-treated virions remained active in exogenous template assays, consistent with a role for NC in reverse transcription. Since disruption of NC zinc finger structures by NEM blocks early postentry steps in the retroviral infection cycle, virus preparations with modified NC proteins may be useful as vaccine immunogens and probes of the role of NC in viral replication.

A chimeric protein of simian immunodeficiency virus envelope glycoprotein gp140 and Escherichia coli aspartate transcarbamoylase

The envelope glycoproteins of the human immunodeficiency virus and the related simian immunodeficiency virus (SIV) mediate viral entry into host cells by fusing viral and target cell membranes. We have reported expression, purification, and characterization of gp140 (also called gp160e), the soluble, trimeric ectodomain of the SIV envelope glycoprotein, gp160 (B. Chen et al., J. Biol. Chem. 275:34946-34953, 2000). We have now expressed and purified chimeric proteins of SIV gp140 and its variants with the catalytic subunit (C) of Escherichia coli aspartate transcarbamoylase (ATCase). The fusion proteins (SIV gp140-ATC) bind viral receptor CD4 and a number of monoclonal antibodies specific for SIV gp140. The chimeric molecule also has ATCase activity, which requires trimerization of the ATCase C chains. Thus, the fusion protein is trimeric. When ATCase regulatory subunit dimers (R(2)) are added, the fusion protein assembles into dimers of trimers as expected from the structure of C(6)R(6) ATCase. Negative-stain electron microscopy reveals spikey features of both SIV gp140 and SIV gp140-ATC. The production of the fusion proteins may enhance the possibilities for structure determination of the envelope glycoprotein either by electron cryomicroscopy or X-ray crystallography.

Microarray profiling of antibody responses against simian-human immunodeficiency virus: postchallenge convergence of reactivities independent of host histocompatibility type and vaccine regimen

We developed antigen microarrays to profile the breadth, strength, and kinetics of epitope-specific antiviral antibody responses in vaccine trials with a simian-human immunodeficiency virus (SHIV) model for human immunodeficiency virus (HIV) infection. These arrays contained 430 distinct proteins and overlapping peptides spanning the SHIV proteome. In macaques vaccinated with three different DNA and/or recombinant modified vaccinia virus Ankara (rMVA) vaccines encoding Gag-Pol or Gag-Pol-Env, these arrays distinguished vaccinated from challenged macaques, identified three novel viral epitopes, and predicted survival. Following viral challenge, anti-SHIV antibody responses ultimately converged to target eight immunodominant B-cell regions in Env regardless of vaccine regimen, host histocompatibility type, and divergent T-cell specificities. After challenge, responses to nonimmunodominant epitopes were transient, while responses to dominant epitopes were gained. These data suggest that the functional diversity of anti-SHIV B-cell responses is highly limited in the presence of persisting antigen.

Assorted mutations in the envelope gene of simian immunodeficiency virus lead to loss of neutralization resistance against antibodies representing a broad spectrum of specificities

Simian immunodeficiency virus (SIV) of macaques isolate SIVmac239 is highly resistant to neutralization by polyclonal antisera or monoclonal antibodies, a property that it shares with most primary isolates of human immunodeficiency virus type 1 (HIV-1). This resistance is important for the ability of the virus to persist at high levels in vivo. To explore the physical features of the viral envelope complex that contribute to the neutralization-resistant phenotype, we examined a panel of SIVmac239 derivatives for sensitivity to neutralization by a large collection of monoclonal antibodies (MAbs). These MAbs recognize both linear and conformational epitopes throughout the viral envelope proteins. The variant viruses included three derivatives of SIVmac239 with substitutions in specific N-linked glycosylation sites of gp120 and a fourth variant that lacked the 100 amino acids that encompass the V1 and V2 loops. Also included in this study was SIVmac316, a variant of SIVmac239 with distributed mutations in env that confer significantly increased replicative capacity in tissue macrophages. These viruses were chosen to represent a broad range of neutralization sensitivities based on susceptibility to pooled, SIV-positive plasma. All three of these very different kinds of mutations (amino acid substitutions, elimination of N-glycan attachment sites, and a 100-amino-acid deletion spanning variable loops V1 and V2) dramatically increased sensitivity to neutralization by MAbs from multiple competition groups. Thus, the mutations did not simply expose localized epitopes but rather conferred global increases in neutralization sensitivity. The removal of specific N-glycan attachment sites from V1 and V2 led to increased sensitivity to neutralization by antibodies recognizing epitopes from both within and outside of the V1-V2 sequence. Surprisingly, while most of the mutations that gave rise to increased sensitivity were located in the N-terminal half of gp120 (surface subunit [SU]), the greatest increases in sensitivity were to MAbs recognizing the C-terminal half of gp120 or the ectodomain of gp41 (transmembrane subunit [TM]). This reagent set and information should now be useful for defining the physical, structural, thermodynamic, and kinetic factors that influence relative sensitivity to antibody-mediated neutralization.

The stoichiometry of trimeric SIV glycoprotein interaction with CD4 differs from that of anti-envelope antibody Fab fragments

Human and simian immunodeficiency viruses infect host lymphoid cells by binding CD4 molecules via their gp160 envelope glycoproteins. Biochemical studies on recombinant SIVmac32H (pJ5) envelope ectodomain gp140 precursor protein show that the envelope is a trimer. Using size exclusion chromatography, quantitative amino acid analysis, analytical ultracentrifugation, and CD4-based competition assay, we demonstrate that the stoichiometry of CD4 receptor-oligomeric envelope interaction is 1:1. By contrast, Fab fragments of both neutralizing and non-neutralizing monoclonal antibodies bind at a 3:1 ratio. Thus, despite displaying equivalent CD4 binding sites on each of the three gp140 protomers within an uncleaved trimer, only one site binds the soluble 4-domain human CD4 extracellular segment. The anti-cooperativity and the faster k(off) of gp140 trimer:CD4 versus gp120 monomer:CD4 interaction suggest that CD4-induced conformational change is impeded in the intact envelope. The implications of these findings for immunity against human immunodeficiency virus and simian immunodeficiency virus are discussed.

Characterization of neutralization epitopes of simian immunodeficiency virus (SIV) recognized by rhesus monoclonal antibodies derived from monkeys infected with an attenuated SIV strain

A major limitation in the simian immunodeficiency virus (SIV) system has been the lack of reagents with which to identify the antigenic determinants that are responsible for eliciting neutralizing antibody responses in macaques infected with attenuated SIV. Most of our information on SIV neutralization determinants has come from studies with murine monoclonal antibodies (MAbs) produced in response to purified or recombinant SIV envelope proteins or intact SIV-infected cells for relatively short periods of time. While these studies provide some basic information on the potential immunogenic determinants of SIV envelope proteins, it is unclear whether these murine MAbs identify epitopes relevant to antibody responses elicited in monkeys during infection with either wild-type or attenuated SIV strains. To accomplish maximum biological relevance, we developed a reliable method for the production of rhesus monoclonal antibodies. In the present study, we report on the production and characterization of a unique panel of monoclonal antibodies derived from four individual monkeys inoculated with SIV/17E-CL as an attenuated virus strain at a time when protective immunity from pathogenic challenge was evident. Results from these studies identified at least nine binding domains on the surface envelope glycoprotein; these included linear determinants in the V1, V2, cysteine loop (analogous to the V3 loop in human immunodeficiency virus type 1), and C5 regions, as well as conformational epitopes represented by antibodies that bind the C-terminal half of gp120 and those sensitive to defined mutations in the V4 region. More importantly, three groups of antibodies that recognize closely related, conformational epitopes exhibited potent neutralizing activity against the vaccine strain. Identification of the epitopes recognized by these neutralizing antibodies will provide insight into the antigenic determinants responsible for eliciting neutralizing antibodies in vivo that can be used in the design of effective vaccine strategies.

Specific proliferative T cell responses and antibodies elicited by vaccination with simian immunodeficiency virus Nef do not confer protection against virus challenge

The efficacy of immunizing with a combination of simian immunodeficiency virus (SIV) Nef vaccines was evaluated. Four vaccinates received three intradermal immunizations with recombinant vaccinia virus that expressed SIV Nef, followed by three intramuscular immunizations with rDNA also expressing SIV Nef. Finally, the four vaccinates received two subcutaneous boosts with recombinant SIV Nef protein. This immunization protocol elicited anti-Nef antibodies in all of the vaccinates as well as specific proliferative responses. However, specific cytotoxic T cell responses were not detected before virus challenge. All vaccinates were challenged intravenously with 10 MID(50) of SIVmacJ5 along with four controls. All eight subjects became infected after SIV challenge and there were no group-specific differences in virus load as measured by virus titration and vRNA analysis. The results of this study support indirectly the report from Gallimore and colleagues (Nat Med 1995;1:1667) suggesting that CD8(+) T lymphocyte responses are required for Nef-based vaccines to restrict SIV infection. If Nef-based vaccines are to be beneficial in controlling infection with immunodeficiency viruses, then it will be necessary to develop more effective immunization protocols that elicit potent CD8(+) cell responses reproducibly.

In situ hybridization and immunolabelling study of the early replication of simian immunodeficiency virus (SIVmacJ5) in vivo

The distribution of virus-infected cells in cynomolgus macaques was determined at 4, 7, 14 and 28 days following intravenous challenge with 1000 TCID(50) of the wild-type simian immunodeficiency virus SIVmacJ5 (stock J5C). At each time-point, pairs of macaques were killed humanely and the presence of SIV was determined and quantified in blood, spleen, peripheral and mesenteric lymph nodes, thymus, lung and ileum by virus co-cultivation with C8166 cells, by quantitative DNA PCR or by in situ hybridization (ISH). At day 4 post-infection (p.i.), detection of the virus was sporadic. By day 7 p.i., however, significant SIV loads were detected in the blood and lymphoid tissues by DNA PCR and virus co-cultivation. Large numbers of cells expressing SIV RNA were detected in mesenteric lymph nodes by ISH and significantly fewer (P<0.05) in the spleen. Significant numbers of ISH-positive cells were also observed in sections of ileum. By day 14 p.i., the distribution of SIV was more even in all lymphoid tissues analysed. By day 28, most of the tissues were negative by ISH, but all remained positive by virus isolation and DNA PCR. Immunolabelling of sections of mesenteric lymph node with monoclonal antibodies specific for SIV envelope and Nef largely confirmed the observations from ISH. These results indicate that, even following intravenous challenge, a major site of the initial replication of SIV is gut-associated lymphoid tissue. Vaccines that induce protection at this site may therefore be superior, even against parenteral challenge.

Antibodies that neutralize SIV(mac)251 in T lymphocytes cause interruption of the viral life cycle in macrophages by preventing nuclear import of viral DNA

Previous reports from our lab had shown that sera obtained from SIV(mac)-infected animals neutralized SIV(mac) infectivity in CD4(+) T cells but failed to protect monkey primary macrophages from infection with the virus. However, the antibodies could inhibit completion of the viral life cycle in the macrophages at the postentry stage(s). In this report we examined the mechanisms of the late effect of the antibodies. Using monoclonal antibodies (MAbs), we demonstrated that only antibodies to the SIV envelope protein (KK17 and KK42) but not antibody to the viral core protein (FA2) had the same inhibitory effect as that of the anti-SIV sera. To identify the stage of the viral replication cycle that was inhibited by anti-SIV antibodies in macrophages, we used various PCR techniques to study viral entry/reverse transcription (by amplifying the viral gag gene), viral genome nuclear transport (by amplifying 2-LTR circular forms), viral integration (by Alu-PCR assay), and viral protein expression (by RIPA). We found that in macrophage cultures inoculated with SIV(mac)251 that were preincubated with antienvelope MAbs, viral DNA was detected at 8 h postinoculation but the 2-LTR circular forms and integrated viral DNAs were undetectable, and viral proteins were not expressed in these infected macrophages. These results strongly suggested that anti-SIV antibodies inhibited SIV(mac) replication in macrophages by blocking nuclear transport of viral genomes since viral DNA could not be detected in the nuclei of treated cultures. Furthermore, we showed that although viral replication in macrophages was interrupted by the antibodies, when cocultured with permissive T cells, the viral genomes presented in the cytoplasm of the macrophages could readily transfer to T cells during cell-cell contact. Importantly, this transfer could not be prevented by the antibodies. These results might explain the failure of passive antibody immunization against SIV(mac)251--a critical obstacle in AIDS vaccine development.

Expression, purification, and characterization of gp160e, the soluble, trimeric ectodomain of the simian immunodeficiency virus envelope glycoprotein, gp160

The envelope glycoprotein, gp160, of simian immunodeficiency virus (SIV) shares approximately 25% sequence identity with gp160 from the human immunodeficiency virus, type I, indicating a close structural similarity. As a result of binding to cell surface CD4 and co-receptor (e.g. CCR5 and CXCR4), both SIV and human immunodeficiency virus gp160 mediate viral entry by membrane fusion. We report here the characterization of gp160e, the soluble ectodomain of SIV gp160. The ectodomain has been expressed in both insect cells and Chinese hamster ovary (CHO)-Lec3.2.8.1 cells, deficient in enzymes necessary for synthesizing complex oligosaccharides. Both the primary and a secondary proteolytic cleavage sites between the gp120 and gp41 subunits of gp160 were mutated to prevent cleavage and shedding of gp120. The purified, soluble glycoprotein is shown to be trimeric by chemical cross-linking, gel filtration chromatography, and analytical ultracentrifugation. It forms soluble, tight complexes with soluble CD4 and a number of Fab fragments from neutralizing monoclonal antibodies. Soluble complexes were also produced of enzymatically deglycosylated gp160e and of gp160e variants with deletions in the variable segments.

Characterization and epitope mapping of neutralizing monoclonal antibodies produced by immunization with oligomeric simian immunodeficiency virus envelope protein

In an attempt to generate broadly cross-reactive, neutralizing monoclonal antibodies (MAbs) to simian immunodeficiency virus (SIV), we compared two immunization protocols using different preparations of oligomeric SIV envelope (Env) glycoproteins. In the first protocol, mice were immunized with soluble gp140 (sgp140) from CP-MAC, a laboratory-adapted variant of SIVmacBK28. Hybridomas were screened by enzyme-linked immunosorbent assay, and a panel of 65 MAbs that recognized epitopes throughout the Env protein was generated. In general, these MAbs detected Env by Western blotting, were at least weakly positive in fluorescence-activated cell sorting (FACS) analysis of Env-expressing cells, and preferentially recognized monomeric Env protein. A subset of these antibodies directed toward the V1/V2 loop, the V3 loop, or nonlinear epitopes were capable of neutralizing CP-MAC, a closely related isolate (SIVmac1A11), and/or two more divergent strains (SIVsmDeltaB670 CL3 and SIVsm543-3E). In the second protocol, mice were immunized with unfixed CP-MAC-infected cells and MAbs were screened for the ability to inhibit cell-cell fusion. In contrast to MAbs generated against sgp140, the seven MAbs produced using this protocol did not react with Env by Western blotting and were strongly positive by FACS analysis, and several reacted preferentially with oligomeric Env. All seven MAbs potently neutralized SIVmac1A11, and several neutralized SIVsmDeltaB670 CL3 and/or SIVsm543-3E. MAbs that inhibited gp120 binding to CD4, CCR5, or both were identified in both groups. MAbs to the V3 loop and one MAb reactive with the V1/V2 loop interfered with CCR5 binding, indicating that these regions of Env play similar roles for SIV and human immunodeficiency virus. Remarkably, several of the MAbs generated against infected cells blocked CCR5 binding in a V3-independent manner, suggesting that they may recognize a region analogous to the conserved coreceptor binding site in gp120. Finally, all neutralizing MAbs blocked infection through the alternate coreceptor STRL33 much more efficiently than infection through CCR5, a finding that has important implications for SIV neutralization assays using CCR5-negative human T-cell lines.

Characterization of a macaque recombinant monoclonal antibody that binds to a CD4-induced epitope and neutralizes simian immunodeficiency virus

A potent neutralizing Fab fragment from a long-term survivor of simian immunodeficiency virus (SIVsm) infection was used to construct a recombinant macaque immunoglobulin G1kappa (IgG1kappa) molecule, designated IgG1-201. A Chinese hamster ovary cell line expressing IgG1-201 was derived by stable transfection and optimized for antibody secretion by methotrexate selection and dihydrofolate reductase gene amplification. IgG1-201 effectively neutralized the homologous, molecularly cloned SIVsmH4 virus but had no activity against the heterologous SIVmac251/BK28 virus. The previously characterized, neutralization-resistant SIVsmE543-3 virus was also not neutralized by IgG1-201. Binding to SIVsmH4 gp120 was enhanced in the presence of recombinant soluble CD4, suggesting that IgG1-201 bound a CD4-induced epitope. IgG1-201 immunoprecipitated the SIVsmH4 but not the SIVsmE543-3 envelope despite a close relationship between these two clones. Immunoprecipitation of a panel of SIVsmH4/SIVsmE543-3 chimeric viruses tentatively assigned the neutralization epitope to the third constant domain, immediately C terminal to the V3 loop. These findings suggest the presence of at least one CD4-induced neutralization epitope on SIV, as is the case with human immunodeficiency virus type 1.

Primary human immunodeficiency virus type 2 (HIV-2) isolates infect CD4-negative cells via CCR5 and CXCR4: comparison with HIV-1 and simian immunodeficiency virus and relevance to cell tropism in vivo

Cell surface receptors exploited by human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) for infection are major determinants of tropism. HIV-1 usually requires two receptors to infect cells. Gp120 on HIV-1 virions binds CD4 on the cell surface, triggering conformational rearrangements that create or expose a binding site for a seven-transmembrane (7TM) coreceptor. Although HIV-2 and SIV strains also use CD4, several laboratory-adapted HIV-2 strains infect cells without CD4, via an interaction with the coreceptor CXCR4. Moreover, the envelope glycoproteins of SIV of macaques (SIV(MAC)) can bind to and initiate infection of CD4(-) cells via CCR5. Here, we show that most primary HIV-2 isolates can infect either CCR5(+) or CXCR4(+) cells without CD4. The efficiency of CD4-independent infection by HIV-2 was comparable to that of SIV, but markedly higher than that of HIV-1. CD4-independent HIV-2 strains that could use both CCR5 and CXCR4 to infect CD4(+) cells were only able to use one of these receptors in the absence of CD4. Our observations therefore indicate (i) that HIV-2 and SIV envelope glycoproteins form a distinct conformation that enables contact with a 7TM receptor without CD4, and (ii) the use of CD4 enables a wider range of 7TM receptors to be exploited for infection and may assist adaptation or switching to new coreceptors in vivo. Primary CD4(-) fetal astrocyte cultures expressed CXCR4 and supported replication by the T-cell-line-adapted ROD/B strain. Productive infection by primary X4 strains was only triggered upon treatment of virus with soluble CD4. Thus, many primary HIV-2 strains infect CCR5(+) or CXCR4(+) cell lines without CD4 in vitro. CD4(-) cells that express these coreceptors in vivo, however, may still resist HIV-2 entry due to insufficient coreceptor concentration on the cell surface to trigger fusion or their expression in a conformation nonfunctional as a coreceptor. Our study, however, emphasizes that primary HIV-2 strains carry the potential to infect CD4(-) cells expressing CCR5 or CXCR4 in vivo.

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

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

Simian immunodeficiency virus (SIV) envelope-specific Fabs with high-level homologous neutralizing activity: recovery from a long-term-nonprogressor SIV-infected macaque

An antibody phage display library was constructed from RNA extracted from lymph node cells of a simian immunodeficiency virus (SIV)-infected long-term-nonprogressor macaque. Seven gp120-reactive Fabs were obtained by selection of the library against SIV monomeric gp120. Although each of the Fabs was unique in sequence, there were two distinct groups based on epitope recognition, neutralizing activity in vitro, and molecular analysis. Group 1 Fabs did not neutralize SIV and bound to a linear epitope in the V3 loop of the SIV envelope. In contrast, two of the group 2 Fabs neutralized homologous, neutralization-sensitive SIVsm isolates with high efficiency but failed to neutralize heterologous SIVmac isolates. Based on competition enzyme-linked immunosorbent assays with mouse monoclonal antibodies of known specificity, these Fabs reacted with a conformational epitope that includes domains V3 and V4 of the SIV envelope. These neutralizing and nonneutralizing Fabs provide valuable standardized and renewable reagents for studying the role of antibody in preventing or modifying SIV infection in vivo.

Production and characterization of monoclonal antibodies to simian immunodeficiency virus envelope glycoproteins

Twelve monoclonal antibodies (MAbs), TB1 to TB12, were produced against a soluble vaccinia recombinant envelope glycoprotein (gp140) from simian immunodeficiency virus SIVmac251. These MAbs recognized SIV gp140 with a relatively high affinity (K0.5 from 6.7 x 10(-8) to 4 x 10(-9) M). All the MAbs except TB9, TB11, and TB12 cross-reacted with HIV-2 envelope glycoproteins, but none of the 12 MAbs recognized those from HIV-1. Using a panel of 87 overlapping synthetic peptides containing 20 amino acid residues, with an overlap of 10 amino acids and spanning the entire primary sequence of gp140, 3 linear epitopes were identified. The first mapped with a neutralizing MAb, TB12, which recognized a linear sequence around amino acids 28-31 within the N-terminal end of the external envelope glycoprotein. The two other new nonneutralizing MAbs recognized linear epitopes around amino acid sequence 380-381 by MAbs TB1, TB2, and TB3, and at the transmembrane glycoprotein amino acids 581-600 by MAb TB6. Seven of the 12 MAbs, TB4, TB5, TB7-9, TB10, and TB11, failed to bind the linear synthetic peptides in ELISA. Moreover, among these seven MAbs only MAbs TB4, TB5, TB9, and TB10 failed to recognize SIV envelope glycoproteins in Western blot (WB) or ELISA after reduction of disulfide bridges by dithiothreitol (DTT), suggesting that they are directed against conformational or discontinuous epitopes. It is of interest to note that MAb TB10 can block the binding of gp140 to the CD4 receptor when the MAb is previously incubated with gp140. Consistent with this result, MAb TB10 cannot bind to gp140 that has been previously complexed with the CD4 receptor. All these results suggest that MAb TB10 recognizes a conformational or discontinuous epitope overlapping or close to the CD4-binding site. These properties are probably implicated in the neutralizing activity observed with this MAb.

Location, exposure, and conservation of neutralizing and nonneutralizing epitopes on human immunodeficiency virus type 2 SU glycoprotein

Eleven rat monoclonal antibodies (MAbs) that recognize the SU glycoprotein of human immunodeficiency virus type 2 (HIV-2) ROD were produced and characterized. Binding sites for eight of these MAbs were mapped to epitopes within the Cl, V1/V2, C2, and V3 envelope regions. The three other MAbs defined at least two conformation-dependent, strain-specific epitopes outside Vl/V2, V3, and the CD4-binding site. The MAbs were used to probe the tertiary structure of oligomeric envelope glycoprotein expressed on the surfaces of infected cells. Epitopes at the apices of V2 and V3 were exposed on the native molecule, whereas other epitopes on V1/V2, Cl, and C2 were hidden. The MAbs defined three neutralization targets on exposed domains: two linear epitopes in the V2 and the V3 loops and one conformational epitope outside V1, V2, and V3.

Antigenicity and immunogenicity of recombinant envelope glycoproteins of SIVmac32H with different in vivo passage histories

Shortly after infection of two rhesus monkeys (Macaca mulatta) either with a SIVmac32H challenge stock or with the same virus that had been passaged in another rhesus monkey for 11 months, SIV-envelope genes were cloned from their peripheral blood mononuclear cells and subsequently expressed by recombinant vaccinia viruses. The molecular weights and antigenicities of the thus produced envelope glycoproteins were largely identical to those of the native SIV. The envelope glycoprotein derived from the in vivo passaged virus proved to be poorly recognized by virus neutralizing monoclonal antibodies directed against one of the seven antigenic sites for which monoclonal antibodies were available. Immunization studies in rats showed that this protein was also less efficient in inducing antibodies against this antigenic site, and that it induced significantly lower levels of virus neutralizing antibodies than the other SIV-envelope glycoprotein. The immunogenicity of the SIV-envelope glycoprotein incorporated into immune stimulating complexes (iscoms) was compared to that of the same protein presented with Quil A or MDP-tsl.

Alanine substitution of two arginines in amino terminus of V3 of SIV disrupts CD4 binding whereas a similar replacement of two amino acids, lysine and arginine, in the carboxyl half of V3 prevents binding of a neutralizing monoclonal antibody

A series of amino acid substitutions were carried out in the V3 loop of SIV gp120 to investigate their effects on binding of the envelope to CD4 and neutralizing monoclonal antibodies. Alanine replacement of two adjacent arginines at the amino terminus of V3 resulted in a molecule that bound neither sCD4 nor conformation-dependent neutralizing monoclonal KK5 and KK9. A similar substitution of two amino acids, lysine and arginine, in the carboxyl half of V3 disrupted binding to KK9 without affecting CD4 binding. Removal of V3 from the envelope gave rise to a molecule that was not secreted. These data suggest a close linkage between V3 and CD4 binding domains of gp120, although neutralizing antibodies directed to V3 do not block binding of gp120 to CD4. We propose that differences in the modes of interactions of the V3 disulfide loops with CD4 in SIV and HIV may be responsible for the observed different neutralizing properties of the two V3 loops.

Evaluation of HIV-1/HIV-2 immunoblots for detection of HIV-2 antibodies

OBJECTIVE

To evaluate the sensitivity of commercially available HIV-2 immunoblots and to identify the HIV-2 glycoproteins on Western blots.

METHODS

HIV-2 Western blot (WB) strips commercially available from Diagnostic Biotechnology, Diagnostic Pasteur and Cambridge Biotech and in-house HIV-2 WB strips were investigated by monoclonal HIV-2 gp36 and gp125 antibodies for identification of the glycoproteins. The WB strips and commercially available HIV-1/HIV-2 line immunoassays (LIAs) from Diagnostic Pasteur (PEPTI-LAV 1-2), Diagnostic Biotechnology (version 2.2) and Innogenetics (INNO-LIA HIV-1/HIV-2 ab) were analyzed by seroconversion panels from HIV-2 infected cynomolgus monkeys (Macaca fascicularis) to investigate their sensitivity for detection of HIV-2 antibodies. The LIAs were also investigated by use of 100 HIV-2 antibody positive human sera from Guinea Bissau. The in-house WB strips contained HIV-2/SBL-6669 antigen treated with various concentrations of sodium dodecyl sulphate (SDS, 0-2%) at 37 degrees C or 100 degrees C for various times to obtain gp36 in oligomeric and/or monomeric form.

RESULTS

By use of monoclonal antibodies, WB strips from Diagnostic Biotechnology and Diagnostic Pasteur were shown to contain gp125 as well as monomeric and oligomeric forms of gp36, whereas Cambridge WB strips contained mainly oligomeric gp36 and no detectable gp125. The sensitivity of the WB strips for detection of HIV-2 seroconversion was similar if WB seropositivity was defined as reactivity with p24 and one envelope protein. When the WHO WB criteria were applied requiring reactivity with at least two envelope proteins for positivity, the sensitivity of the WB strips from Diagnostic Biotechnology and Diagnostic Pasteur was retained, whereas the sensitivity of Cambridge Biotech WB strips was reduced. Among 100 HIV-2 antibody positive human sera all were reactive on PEPTI-LAV 1-2 and INNO-LIA HIV-1/HIV-2 ab, but two of the hundred sera failed to react with the HIV-2 synthetic peptide band on Diagnostic Biotechnology version 2.2 WB strips. On in-house WB strips the relation between monomeric and oligomeric gp36 was changed by altering the SDS concentration and the temperature. Thus the monomeric form increased with the SDS concentration and the temperature. The sensitivity for detection of antibodies during seroconversion did not differ between the monomeric and oligomeric forms of gp36.

CONCLUSIONS

The sensitivity for detection of HIV-2 antibodies during seroconversion was independent of the oligomeric or monomeric structure of the transmembrane glycoprotein. One of the three commercial WB kits tested had a lower sensitivity for detection of HIV-2 seroconversion compared with the other two kits when the WHO criteria for WB positivity were used.

Neutralising epitopes of simian immunodeficiency virus envelope glycoprotein

Monoclonal and polyclonal antibodies with weak SIV neutralising activity bind to the V2 and V4 regions of gp120 or bind to the amino acids DWNND in gp41. Antibodies with the most potent neutralising activity recognise conformation-dependent epitopes involving the V3 and V4 regions of gp120. Monoclonal antibodies that map to the V3 region of SIVmac failed to neutralise. However, one antibody to SIV AGM neutralised but only in the presence of soluble CD4.

Neutralizing monoclonal antibody against a external envelope glycoprotein (gp110) of SIVmac251

Three monoclonal antibodies (M318T, M56S and M815) against an external envelope glycoprotein (gp110) of simian immunodeficiency virus (SIV) mac251 were obtained by immunizing BALB/c mice with recombinant gp110 (rgp110). All three monoclonal antibodies reacted with the surface of cells infected with SIVmac251 but not with that of uninfected counterparts. The binding activity of these monoclonal antibodies against native gp110 was confirmed by means of Western blotting. One of them, M318T neutralized SIVmac251 infection both by cell-free and cell-associated viruses. M318T cross-reacted with human immunodeficiency virus type 2 strains (HIV-2 GH1 and ROD isolates) and SIVmac239 isolates. However, the antibody did not cross-neutralize these viral strains. Epitope mapping revealed that the neutralizing epitope recognized by M318T was localized at 8 residues between amino acids 178 and 185 (KRDKTKEY) in gp110, corresponding to the V2 region of human immunodeficiency virus type 1 (HIV-1). Because some antibodies against the V2 region of HIV-1 reportedly neutralize virus infection by interfering with CD4-gp120 interaction, we tested the activity of M318T against the binding of CD4-gp110. However, M318T did not inhibit CD4-gp110 interaction, suggesting the involvement of another unknown mechanism of M318T-mediated neutralization. In analogy with the V2 region of HIV-1, the V2 region of SIV contains a type specific neutralizing epitope recognized by M318T. Although some amino acid sequence in the epitope was conserved for the isolates of SIV and HIV-2 and there was cross-reactivity of the antibody against the strains, neutralization by M318T was associated with a single amino acid (182 T) in the epitope.

Regions required for CD4 binding in the external glycoprotein gp120 of simian immunodeficiency virus

The external domain of the envelope glycoprotein, gp120, of simian immunodeficiency virus (SIV) has been expressed as a mature secreted product using recombinant baculoviruses and the expressed protein, which has an observed molecular mass of 110 kDa, was purified by monoclonal antibody (MAb) affinity chromatography. N-terminal sequence analysis showed a signal sequence cleavage identity similar to that of the gp120s of both human immunodeficiency virus type 1 (HIV-1) and HIV type 2. The expressed molecule bound to soluble CD4 with an affinity that was approximately 10-fold lower than that of gp120 from HIV-1. A screening of the ability of SIV envelope MAbs to inhibit CD4 binding revealed two groups of inhibitory MAbs. One group is dependent on conformation, while the second group maps to a discrete epitope near the amino terminus. The particular role of the V3 loop region of the molecule in CD4 binding was investigated by the construction of an SIV-HIV hybrid in which the V3 loop of SIV was precisely replaced with the equivalent domain from HIV-1 MN. The hybrid glycoprotein bound HIV-1 V3 loop MAbs and not SIV V3 MAbs but continued to bind conformational SIV MAbs and soluble CD4 as well as the parent molecule.

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

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

Identification and characterization of monoclonal antibodies specific for polymorphic antigenic determinants within the V2 region of the human immunodeficiency virus type 1 envelope glycoprotein

We have identified six monoclonal antibodies (MAbs) mapping to both linear and conformation-dependent epitopes within the V2 region of the human immunodeficiency virus type 1 clone HXB10. Three of the MAbs (12b, 66c, and 66a) were able to neutralize the molecular clones HXB10 and HXB2, with titers in the range of 9.5 to 20.0 micrograms/ml. MAbs mapping to the crown of the V2 loop (12b, 60b, and 74) bound poorly to cell surface-expressed oligomeric gp120, suggesting an explanation for the poor or negligible neutralizing activity of MAbs to this region. In contrast, MAbs 12b and 60b demonstrated good reactivity with recombinant gp120 in an enzyme-linked immunosorbent assay format, suggesting differential epitope exposure between the recombinant and native forms of gp120. Cross-competition analysis of these MAbs and additional V1V2 MAbs for gp120 binding enabled us to assign the MAbs to six groups (A to F). Selection of neutralization escape mutants with MAbs 10/76b and 11/68b, belonging to nonoverlapping competition groups, identified amino acid changes at residues 165 (I to T) and 185 (D to N), respectively. Interestingly, these escape variants remained sensitive to neutralization by the nonselecting V2 MAbs. All MAbs demonstrated good recognition of IIIB viral gp120 yet failed to neutralize nonclonal stocks of IIIB. In addition, MAbs 12b and 62c bound MN and RF viral gp120, respectively, yet failed to neutralize the respective isolates. Cloning and expression of a library of gp120 and V1V2 fragments from IIIB-, MN-, and RF-infected H9 cultures identified a number of polymorphic sites, resulting in antigenic variation and subsequent loss of V2 MAb recognition. In contrast, the V3 region from the clones of the same isolates showed no amino acid changes, suggesting that the V2 region is polymorphic in long-term-passaged laboratory isolates and may account for the reduced antibody recognition observed.

B-cell continuous epitopes of the SIVmac-251 envelope protein in experimentally infected macaques

The humoral immune response of 34 macaques experimentally infected with SIVmac-251 was studied using a combination of an epitope library and synthetic peptides. The course of the immune response was checked for up to 9 months postinfection with a panel of clones expressing SIV fragments. A systematic study was performed with synthetic peptides covering the whole transmembrane (TM) and external (SU) envelope proteins. Seven major immunodominant epitopes were characterized. Four are localized in the SU protein: one in the V1 region (111-130), one in the Cys loop of the V3 region (311-330) and two in the C-terminal end (501-520 and 511-530). Three are localized in the TM protein: one in the extracellular domain (601-619), one in the anchor domain (731-750) and one in the intracytoplasmic domain (861-881). Among these epitopes, only one, 601-619, was found to be reactive with all sera and can be defined as the principal immunodominant epitope.

Protection of rhesus macaques from SIV infection by immunization with different experimental SIV vaccines

The immunogenicity and efficacy of an inactivated whole SIVmac (32H) preparation adjuvanted with muramyl dipeptide (SIV-MDP) and a gp120-enriched SIVmac (32H) ISCOM preparation (SIV-ISCOM), were compared by immunizing four rhesus macaques (Macaca mulatta) four times with SIV-MDP and four others in the same way with SIV-ISCOM. Two monkeys immunized with whole inactivated measles virus (MV) adjuvanted with MDP (MV-MDP) and two monkeys immunized with MV-ISCOM served as controls. In the SIV-ISCOM-immunized monkeys higher SIV-specific serum antibody titres were found than in the SIV-MDP-immunized monkeys. In contrast to the MV-immunized monkeys all SIV-MDP- and SIV-ISCOM-immunized monkeys were protected against intravenous challenge 2 weeks after the last immunization with 10 median monkey infectious doses (MID50) of a cell-free SIVmac (32H) challenge stock propagated in the human T-cell line C8166. After 43 weeks the protected monkeys were reboosted and 2 weeks later rechallenged with 10 MID50 of the same virus produced in peripheral blood mononuclear cells (PBMC) from a rhesus macaque. None of these animals proved to be protected against this challenge. In a parallel experiment in which the same numbers of monkeys were immunized in the same way, the animals were challenged intravenously with 10 MID50 of PBMC from an SIVmac (32H)-infected rhesus macaque. Two out of four SIV-MDP- and two out of four SIV-ISCOM-immunized monkeys proved to be protected from SIV infection.