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

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.

Dissecting the humoral immune response to simian immunodeficiency virus: mechanisms of antibody-mediated virus neutralization

The ultimate goal of an AIDS vaccine is to elicit potent cellular and humoral immune responses that will result in broadly enduring protective immunity. During the past several years, we have focused on characterizing the quantitative and qualitative properties of the antibody response, principally working to define the mechanism(s) of antibody-mediated neutralization in vitro. We have utilized a panel of monoclonal antibodies generated from monkeys infected with attenuated SIV for more than 8 mo to dissect the early events of virus infection involved in antibody-mediated neutralization. Presented herein are highlights from our studies that have identified potential mechanisms by which antibodies neutralize SIV in vitro.

Effects of L- and D-REKR amino acid-containing peptides on HIV and SIV envelope glycoprotein precursor maturation and HIV and SIV replication

The aim of the present study was to evaluate the capacity of synthetic l- and d-peptides encompassing the HIV-1(BRU) gp160 REKR cleavage site to interfere with HIV and simian immuno-deficiency virus (SIV) replication and maturation of the envelope glycoprotein (Env) precursors. To facilitate their penetration into cells, a decanoyl (dec) group was added at the N-terminus. The sequences synthesized included dec5d or dec5l (decREKRV), dec9d or dec9l (decRVVQREKRV) and dec14d or dec14l (TKAKRRVVQREKRV). The peptide dec14d was also prepared with a chloromethane (cmk) group as C-terminus. Because l-peptides exhibit significant cytotoxicity starting at 35 microM, further characterization was conducted mostly with d-peptides, which exhibited no cytotoxicity at concentrations higher than 70 microM. The data show that only dec14d and dec14dcmk could inhibit HIV-1(BRU), HIV-2(ROD) and SIV(mac251) replication and their syncytium-inducing capacities. Whereas peptides dec5d and dec9d were inactive, dec14dcmk was at least twice as active as peptide dec14d. At the molecular level, our data show a direct correlation between anti-viral activity and the ability of the peptides to interfere with maturation of the Env precursors. Furthermore, we show that when tested in vitro the dec14d peptide inhibited PC7 with an inhibition constant K(i)=4.6 microM, whereas the peptide dec14l preferentially inhibited furin with a K(i)=28 microM. The fact that PC7 and furin are the major prohormone convertases reported to be expressed in T4 lymphocytes, the principal cell targets of HIV, suggests that they are involved in the maturation of HIV and SIV Env precursors.

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.

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.

Production and characterization of SIV envelope-specific rhesus monoclonal antibodies from a macaque asymptomatically infected with a live SIV vaccine

Five rhesus monoclonal antibodies (RhMAbs) were produced by rhesus EBV transformation of peripheral blood B cells from a rhesus macaque that had been asymptomatically infected with an attenuated, macrophage-tropic SIV strain, 17E-Cl. These MAbs recognized conformation-dependent epitopes on SIV gp120 and could not be mapped using synthetic peptides. All five RhMAbs were able to neutralize the vaccine strain and a heterologous isolate, SIV/DeltaB670. The RhMAbs did not cross-react with HIV-2; by contrast, four human MAbs derived from an HIV-2-infected person were broadly cross-reactive with both SIV and HIV-2 gp120s. Cross-competition analysis indicated that the five RhMAbs could be placed in two groups recognizing two nonoverlapping epitopes; while the HMAbs were placed in two additional competition groups. Binding of the three group I RhMAbs (1.7F, 3.11B, and 1.10A) as well as HMAb 17A was shown to be sensitive to specific amino acid alterations in V4 occurring in natural env variants. The results of this study demonstrate that RhEBV transformation provides a means to probe rhesus antibody responses to SIV infection at the monoclonal level. RhMAbs will allow structural and functional studies of envelope glycoprotein determinants that elicit protective immune responses against SIV.