Monoclonal Antibodies to Spirosin of Yersinia enterocolitica and Analysis of the Localization of Spirosome by Use of Them

Two hybridomas producing monoclonal antibodies (MAbs) were prepared by fusing myeloma cells (Sp2/0-Ag14) with mouse spleen cells immunized with purified spirosin from Yersinia enterocolitica SYT-11–72 (YE72). The antibodies produced by them were designated MAbs-S5 and S27. They were IgG_2a and IgG₁, respectively, both with χ light chains. MAbs-S5 and S27 reacted specifically with spirosin from YE72. On Western blotting after limited proteolysis with Staphylococcus aureus V8 protease, YE72 spirosin revealed peptide fragments of 35 and 37 kDa reacting markedly with MAb-S5, which suggested the presence of an antigenic determinant on these fragments. By cellular fractionation of YE72 and subsequent EIA and Western blot analysis, spirosome was shown to be present in the cytoplasm of YE72.     

Chromatographic methods for large-scale preparation of immunoglobulin G2a monoclonal antibodies Lym-1 and TNT-1 F(ab')2 fragments

Lym-1 and TNT-1 are two murine immunoglobulin G_2a monoclonal antibodies (MAbs) which have been used for clinical trials in cancer patients. This paper describes methods for large-scale preparation of F(ab')₂ fragments from 50 mg to 4 g of MAbs Lym-1 and TNT-1. Digestion of MAbs with pepsin was optimized and performed at pH 3.8, a pepsin/antibody ratio of 1:250, and 3–4 h of incubation at 37°C. The F(ab')₂ fragments were purified by tandem column procedures using fast protein liquid chromatography. Quality control analyses of the products included protein purity, isoelectric point, immunoreactivity, and endotoxin level. The results revealed that the chromatographic procedures are practical, simple, and effective, and can be used to produce gram quantities of clinical-grade F(ab')₂ fragments for the diagnosis of cancer in patients.     

Hybrid immunoglobulin isotypes of identical specificity produced by genetic recombination in Escherichia coli and expression in lymphoid cells

We have produced a series of hybrid IgG₁-IgG_2a mouse immunoglobulins with identical light chains (L) and variable regions to facilitate the identification of structural features associated with functional differnces between immunoglobulin isotypes. Hybrid heavy chain (H) constant region gene segments were generated by genetic recombination in Escherichia coli between plasmids carrying mouse γ¹ and γ^2a gene segments. Crossovers occured through out these segments although the frequency was highest in regions of high nucleotide sequence homology. Eleven variant immunoglobulins produced by transfected hybridoma cell lines are assembled into H₂L₂ tetramers and properly glycosylated. In addition, all 11 immunoglbulins have identical antigen combining sites specific for the fluorescent hapten ε-dansyll-L-lysine. Protein A binding was used as probe of the structural integrity of the Fc portion of the variant antibodies. Differeneces in protein A binding between IgG₁ and IgG_2a appear to be due to amino acid differances at postions 252 (Thr→Met) and 254 (Thr→Ser) of the heavy chain (EU numbering).     

Age-related prevalence of antibodies to infective larvae ofWuchereria bancrofti in normal individuals from a filaria-endemic region

Antibody isotypic levels (IgM, IgE and IgG subclasses) to infective larvae (L₃) ofWuchereria bancrofti were measured in 104 normal individuals from a filaria-endemic region in Orissa. The titres of antibodies were considerably higher in adults (n = 25, 25.1± 3.8 year) than in children (n = 52, 7.1 ± 2.1 year). Young children (n = 14) less than four years were seronegative to all isotypes other than IgM, the sero-conversion to which was achieved in the children (n=15) by the age of 7.5±1.2 years. The prevalence of other isotypes increased with age and reached a maximum in early adulthood (18.6 ±1.6 years), which persisted in older adults (> 30 years). However, the increase in IgG₃ prevalence with age was less marked. IgG₂ was detected only after 10 years of age. Compared to the high prevalence (100%) of IgM, IgE, IgG₁, and IgG₂, in adults. IgG₃ and IgG₄ prevalences were low, 35% and 28% respectively. IgA level to L₃ antigen was found to be extremely low even in adults. These data indicate that the prevalence of L₃ antibodies was different for different isotypes and the acquisition of antibody response essentially followed an age dependent pattern.     

Galactose-Specific Fimbrial Adhesin of Enteroaggregative <Emphasis Type="Italic">Escherichia coli</Emphasis>: A Possible Aggregative Factor

A galactose-specific adhesin was isolated from the fimbriae of an enteroaggregative Escherichia coli (EAEC) strain. The adhesin was found to be a high molecular weight aggregate of the 18-kDa monomer. The dimeric (36 kDa) and tetrameric (76 kDa) forms appeared in sodium dodecyl sulphate polyacrylamide gel electrophoresis when a higher concentration of the adhesin was used. The IgG_AD (IgG against adhesin) obtained from the immune sera raised in rabbits against purified adhesin could detect all three forms of the adhesin even from the crude fimbrial preparation. The IgG_AD failed to recognize the adhesin in the presence of galactose, thereby suggesting the antibody-binding site and the sugar-binding site on the adhesin might be same or overlapping. Furthermore, the IgG_AD could localize the adhesin exclusively on the fimbriae as observed in immunogold electron microscopy. The aggregative adherence of the bacteria to HEp-2 cells was reduced to 70% in the presence of the IgG_AD. A glycoprotein (34 kDa) present in the membrane fraction of HEp-2 cells interacted with the purified adhesin in a galactose-specific manner. The IgG_AD could recognize the adhesin from the crude fimbrial preparation of 9 out of 10 clinical isolates of EAEC strains but failed to identify any protein from the crude fimbrial preparation of Salmonella typhimurium (fim +ve as well as fim −ve strain), Vibrio cholerae (WO7) or Escherichia coli DH5α.     

Aglycosylated antibodies and antibody fragments produced in a scalable in vitro transcription-translation system

We describe protein synthesis, folding and assembly of antibody fragments and full-length aglycosylated antibodies using an Escherichia coli-based open cell-free synthesis (OCFS) system. We use DNA template design and high throughput screening at microliter scale to rapidly optimize production of single-chain Fv (scFv) and Fab antibody fragments that bind to human IL-23 and IL-13α1R, respectively. In addition we demonstrate production of aglycosylated immunoglobulin G (IgG₁) trastuzumab. These antibodies are produced rapidly over several hours in batch mode in standard bioreactors with linear scalable yields of hundreds of milligrams/L over a 1 million-fold change in scales up to pilot scale production. We demonstrate protein expression optimization of translation initiation region (TIR) libraries from gene synthesized linear DNA templates, optimization of the temporal assembly of a Fab from independent heavy chain and light chain plasmids and optimized expression of fully assembled trastuzumab that is equivalent to mammalian expressed material in biophysical and affinity based assays. These results illustrate how the open nature of the cell-free system can be used as a seamless antibody engineering platform from discovery to preclinical development of aglycosylated monoclonal antibodies and antibody fragments as potential therapeutics.     

Detection of immunoglobulin heavy chain IgG3 polymorphism in wild mice with xenogeneic monoclonal antibodies

We have generated four xenogeneic rat antimouse IgG₃ monoclonal antibodies recognizing at least three different antigenic determinants (epitopes) on BALB/c IgG₃ molecules. These antibodies were used in solid-phase blocking radioimmunoassays for detection of the epitopes in sera of 40 inbred strains and 134 wild mice. These antibodies detect genetic polymorphism of IgG₃ isotype among wild mice even though there is no polymorphism found among 40 inbred strains tested (except X-chromosome-linked immunodeficient CBA/N strain which lacks IgG₃ molecules). An IgG₃ variant was also isolated from hybridomas derived from Mus spretus.Abbreviations Igh-C immunoglobulin heavy chain constant region - PVC polyvinyl chloride - RIA radioimmune assay - ELISA enzyme linked immunosorbent assay     

Monoclonal antibodies for intravesical radioimmunotherapy of human bladder cancer

Local administration of radioimmunoconjugates may allow successful tumor therapy. Bladder cancer appears well suited to this approach, because of its superficial and multifocal nature, and because it will allow direct intravesical administration of conjugates. Implantation of human bladder cancer cell lines in the bladder wall of nude rats results in tumor formation, providing an excellent model to test this. We have developed two murine monoclonal antibodies (MAbs), BLCA-8, IgG₃, and BLCA-38, IgG₁, both of which react with malignant cells and shed into voided urine of patients with transitional cell carcinoma (TCC) of the bladder, but not with normal bladder urothelial cells. Radioimmunoconjugates produced with¹³¹Iodine (¹³¹I) or¹²⁵I have been used for biodistribution studies following administration directly into the bladder. Radioiodinated intact MAbs or Fabs administered intravesically into nontumor-bearing rats did not leak into the systemic circulation and were stable in urine for up to 100h. Biodistribution studies carried out following intraperitoneal or intravesical administration of radioimmunoconjugates to tumor-bearing nude rats indicate good tumor uptake of both MAbs. Together with immunoreactivity assays, these studies demonstrate that¹³¹I-labeled MAbs have considerable potential for intravesical radioimmunotherapy of human bladder tumors, and further studies are under way.     

Modulation of the immune response by passive antibodies. III. Effects of IgG1 and IgG2 anti-carrier antibodies.

The effects of IgG₁ and IgG₂ anti-carrier antibodies were studied on cellular and humoral reactions induced by immunization with a hapten-carrier complex. IgG₁ was shown to depress both delayed hypersensitivity reactions (DHR) to the carrier and anaphylaxis to the hapten whereas IgG₂ had no activity. A mixture of IgG₁ and IgG₂ depressed only DHR to the carrier. The modulating effects of passive anti-carrier antibodies were shown to depend on their immunoglobulin class and the concentration used.     

Genetic characterization of mouse immunoglobulin allotypic determinants (allotopes) defined by monoclonal antibodies

We have generated a new series of monoclonal antibodies recognizing allotypic determinants on mouse IgG₁, IgG_2a, and IgG_2b. In this communication we describe their reactivities with immunoglobulins of the inbred mouse strains. Comparison with serology charts indicates that many of these monoclonal antibodies detect allotypic specificities previously defined by conventional antisera; others define previously undescribed specificities. Strain and isotype distribution allows us to assign five new allotypic specificities to Igh-1 and three new specificities to Igh-3. In addition, on the basis of reactivity with the monoclonal antibodies, we have defined a new Igh haplotype in SWR/J mice, Igh ^p.Abbreviations used in this paper Igh immunoglobulin heavy chain - SDS sodium dodecyl sulfate     

Bacterial expression and refolding of single-chain Fv fragments with C-terminal cysteines

Two antibody single-chain Fv (scFv) fragments carrying five C-terminal histidine residues were expressed inEscherichia coli as periplasmic inclusion bodies. Their variable heavy (V_H) and light (V_L) domains are derived from the mouse monoclonal antibody 215 (MAb215), specific for the largest subunit of RNA polymerase II ofDrosophila melanogaster and rat MAb Yol1/34, specific for pig brain α-tubulin. ScFv-215 contains an additional cysteine residue near to its C-terminus. After solubilization of inclusion bodies followed by immobilized metal affinity chromatography (IMAC) in 6M urea and a renaturation procedure, scFv monomers, noncovalent dimers, and aggregated antibody fragments were separated by size exclusion chromatography. In addition, a fraction of disulfide-bonded scFv-215 homodimers (scFv′)₂ was also isolated. The various antibody forms appear to be in equilibrium after renaturation since first peak composed mainly of aggregates could be resolved into a similar pattern of aggregates, dimers, and monomers after repeating the denaturation/renaturation procedure. All fractions of the recombinant scFv-215 demonstrated high antigen-binding activity and specificity as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis. Affinity measurements carried out by competitive immunoassays showed that covalently linked (scFv′)₂ have binding constants quite close to those of the parental MAbs and fourfold higher than scFv′ monomers. ScFv derivatives, specifically biotinylated through the free sulfhydryl group, recognize the corresponding antigen in ELISA and Western blot analysis, thus demonstrating the possibility of using chemically modified scFv antibodies for immunodetection.     

Improved assembly of bispecific antibodies from knob and hole half‐antibodies

A process was developed for large‐scale assembly of IgG₁ and IgG₄ bispecific antibodies from knob and hole half‐antibodies. We optimized assembly conditions such as pH, temperature, stabilizers, and reducing agent. We also identified and exploited structural changes unique to knob and hole half‐antibodies with the result of improving assembly outcome, specifically storing half‐antibodies at higher pH will condition them to assemble more rapidly and produce fewer high molecular‐weight species (HMWS). Application of heat to the assemblies resulted in an acceleration of assembly rate, with optimal formation of bispecific achieved at 37°C. IgG₄ half‐antibodies were unusually sensitive to temperature‐dependent formation of HMWS in pre‐assembly conditioning as well as during assembly. We selected l ‐histidine and Polyvinylpyrrolidone (PVP) as stabilizers to prevent HMWS formation in IgG₄, and achieved rapid and high‐efficiency assemblies. Using optimized assembly conditions, we developed and scaled up a method for assembling bispecific antibody with 90% assembly efficiency over 6 h with minimal impact to product quality, generating a pool with bispecific antibody for downstream processing. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1315–1322, 2015     

Tumor localization of Lewis lung carcinoma with radiolabeled monoclonal antibodies

Summary Mouse 6D6 IgG_2a and 5B5 IgM monoclonal antibodies which specifically bind murine lung carcinoma cells (3LL cells) were injected to healthy and tumor-bearing mice. In vivo localization was analyzed by counting the tissue radioactivity and by external gamma ray scintigraphy at various times after IV injection of ¹²⁵I- or ¹³¹I-labeled antibodies. The clearance of the two monoclonal antibodies was not modified by the presence of the tumor, and the 6D6 IgG_2a was cleared at a rate slower than the 5B5 IgM. Both antibodies gave a high specific uptake at the tumor level; the tumor-to-healthy tissue ratios were higher with the 6D6 IgG_2a than the 5B5 IgM; unspecific mouse immunoglobulins (IgG₂) did not localize in the tumor. The amount of 6D6 IgG_2a antibody still associated with the tumor after 2 days following IV injection was 10 times higher than that of 5B5 IgM, and was still high enough to localize the tumor after 5 days.Imaging experiments confirmed the ability of 6D6 IgG_2a to detect the presence of tumor cells. The targeting kinetics determined by computer analysis of camera images indicated a rapid targeting of antibodies in tumor with a maximal concentration after 4–6 h; after 48 h the background was quite low and the 6D6 IgG_2a appeared to be specifically localized in the tumor.     

Detection of disease specific sialoglycoconjugate specific antibodies in bronchoalveolar lavage fluid of non-small cell lung cancer patients

In this study, we report the presence of significantly higher level of GM3 specific IgG antibodies (IgG_TL) in the bronchoalveolar lavage fluid obtained from tumor bearing lung of non-small cell lung cancer (NSCLC) patients as compared to other non-neoplastic controls. The antibodies were isolated using DEAE-cellulose anion exchange chromatography and molecular weight of the subunits of IgG_TL was confirmed in SDS-PAGE. IgG_TL revealed high specificity to GM3 and the IgG distribution was confined to IgG1. Furthermore, IgG_TL showed strong reactivity with NSCLC cell lines as well as the tissue biopsies and cells obtained from fine needle aspirations of NSCLC patients. A 66 kDa membrane glycoprotein of NSCLC cell lines was found to interact specifically with IgG_TL, the intensity of which was drastically reduced in presence of GM3. Further, binding of Maackia amurensis agglutinin [specific for NeuAcα(2→3)Gal unit , the same disaccharide unit also known to be present in GM3] to the 66 kDa band confirmed it to be a sialoglycoprotein in nature. IgG_TL could not show any reactivity to alkaline borohydrate treated or periodate oxidised membrane fractions, suggesting the probable involvement of the carbohydrate moiety of the 66 kDa glycoprotein in the interaction with IgG_TL. Thus, the 66 kDa sialoglycoprotein seems to be the NSCLC specific sialoglycoconjugate. Taken together, IgG_TL antibodies may have the potential to serve as a unique probe for detail investigation of NSCLC specific cell surface sialoglycoconjugate. Further, due to high specificity of IgG_TL to GM3, it may be possible to develop a simple alternative diagnostic approach (GM3-ELISA) for NSCLC.     

Selective toxicity of neocarzinostatin-monoclonal antibody conjugates to the antigen-bearing human melanoma cell line in vitro

Summary Monoclonal antibodies (IgG₁) against high molecular weight antigen A-1-43 on human melanoma cell line A-375 were successfully linked to the anti-tumour protein neocarzinostatin (NCS) using the heterobifunctional reagent N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP). The conjugate retained both the reactivity of the antibody and the toxicity of the drug. The antigen-bearing cell line A-375, antigen-lacking cell line MeWo and normal skin fibroblasts were exposed to NCS-monoclonal antibody conjugates. As negative control, cells were also treated with free NCS and NCS coupled to normal mouse IgG₁ antibodies. Inhibition of ³H-thymidine uptake after treatment was used to measure the biological activity of the cytotoxic drug complex or substance, respectively.Comparing the inhibition dose for 50% uptake (ID₅₀) it was found that the monoclonal antibody-drug complex is about 100 times more toxic for the antigen-bearing cell line than free NCS or normal mouse IgG₁-NCS. This high toxicity is due to a local increase of drug concentration on these cells. With the two cell lines lacking the appropriate antigen no significant differences in the ID₅₀ values were observed. A selectivity factor of 40–50 was obtained by comparing the cytotoxic effect of the monoclonal antibody-NCS conjugate upon the antigen-bearing as opposed to the antigen-lacking cell type. These data demonstrate, that the toxicity of NCS can be directed by monoclonal antibodies to human tumour cells carrying the corresponding surface antigen.     

Synergistic capture of Clostridium botulinum type A neurotoxin by scFv antibodies to novel epitopes

A non‐immune library of human single chain fragment variable (scFv) antibodies displayed on Saccharomyces cerevisiae was screened for binding to the Clostridium botulinum neurotoxin serotype A binding domain [BoNT/A (H_c)] with the goal of identifying scFv to novel epitopes. To do this, an antibody‐mediated labeling strategy was used in which antigen‐binding yeast clones were selected after labeling with previously characterized monoclonal antibodies (MAbs) specific to the H_c. Twenty unique scFv clones were isolated that bound H_c. Of these, 3 also bound to full‐length BoNT/A toxin complex with affinities ranging from 5 to 48 nM. Epitope binning showed that the three unique clones recognized at least two epitopes distinct from one another as well as from the detection MAbs. After production in E. coli, scFv were coupled to magnetic particles and tested for their ability to capture BoNT/A holotoxin using an Endopep‐MS assay. In this assay, toxin captured by scFv coated magnetic particles was detected by incubation of the complex with a peptide containing a BoNT/A‐specific cleavage sequence. Mass spectrometry was used to detect the ratio of intact peptide to cleavage products as evidence for toxin capture. When tested individually, each of the scFv showed a weak positive Endopep‐MS result. However, when the particles were coated with all three scFv simultaneously, they exhibited significantly higher Endopep‐MS activity, consistent with synergistic binding. These results demonstrate novel approaches toward the isolation and characterization of scFv antibodies specific to unlabeled antigens. They also provide evidence that distinct scFv antibodies can work synergistically to increase the efficiency of antigen capture onto a solid support. Biotechnol. Bioeng. 2011;108: 2456–2467. © 2011 Wiley Periodicals, Inc.     

Immunoglobulin isotype distribution of malaria-specific antibodies produced during infection with Plasmodium chabaudi adami and Plasmodium yoelii

The antibody response of mice to Plasmodium chabaudi adami and Plasmodium yoelii has been compared using a solid phase isotype-specific radioimmunoassay and sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Serological cross-reactivity between these parasites was substantial. Studies using a radioimmunoassay detecting all classes of malaria-specific antibody demonstrated that during the early part of infection it was not possible to distinguish between homologous and heterologous reactions. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that 50% or more of the protein antigens detected were apparently shared by both parasites although the intensity of bands was always greater with homologous reactions. However, the distribution of isotypes in the antibody (Ab) response differed in the two infections. P. chabaudi infections were characterized by a predominant and persistent IgM response, moderate IgG₂ and IgG₃ and little significant IgG₁ response during a primary infection. By contrast, IgM antibodies were transient in P. yoelii infection, IgG₂ was the predominant isotype, and both IgG₁ and IgG₃ antibodies were present during a primary infection. These differences in isotypes were also detected when sera were tested on the heterologous antigen extracts suggesting that antigens shared by P. chabaudi and P. yoelii do not necessarily induce similar antibody responses in the two infections.     

Recombinant human monoclonal antibodies to human cytomegalovirus glycoprotein B neutralize virus in a complement-dependent manner

Human antibodies specific for HCMV are currently considered as potential anti-HCMV therapeutic agents. In this study, we used a combinatorial human antibody library to isolate and characterize complete human monoclonal antibodies that effectively neutralize HCMV in a complement-dependent manner. One hundred and six clones were isolated in two independent screens using HCMV virions and recombinant glycoprotein B, gB654, as antigens. All of the clones recognized the same molecule gB and were classified into 14 groups based on the amino acid sequence of the V_H region. Seven representative clones from these 14 groups had a strong gB654 binding affinity by surface plasmon resonance (SPR). A pairwise binding competition analysis suggested that there were three groups based on differences in the gB recognition sites. Although Fab fragments of the seven groups showed strong affinity for gB, none of the Fab fragments neutralized HCMV infectivity in vitro. In contrast, complete human IgG₁ antibodies of at least three groups neutralized HCMV in a complement-dependent manner. These data suggest that potent therapeutic antibodies can be obtained from a human antibody library, including most of the functional antibodies that mediate humoral immunity to the selected pathogen.     

Bacterial and plant enterotoxin B subunit-autoantigen fusion proteins suppress diabetes insulitis

Several bacterial and plant enterotoxin B subunit-islet autoantigen fusion proteins were compared for their ability to serve as islet autoantigen carriers and adjuvants for reduction of pancreatic islet inflammation associated with type 1 diabetes. The cholera toxin B subunit (CTB), the heat-labile toxin B subunit from enterotoxigenic Escherichia coli (LTB), the Shigella toxin B subunit (STB), and the plant toxin ricin B subunit (RTB) were genetically linked to the islet autoantigens proinsulin (INS) and glutamic acid decarboxylase (GAD). The adjuvant-autoantigen gene fusions were transferred to a bacterial expression vector and the corresponding fusion proteins synthesized in E. coli. The purified adjuvant-autoantigen proteins were fed to 5-wk-old nonobese diabetic (NOD) mice once a week for 4 wk. Histological examination of pancreatic islets isolated from inoculated mice showed significant levels of insulitis reduction in comparison with uninoculated mice. The ratio of serum anti-INS and anti-GAD IgG_2c to IgG₁ antibody isotype titers increased in all ligand-autoantigen inoculated animal groups, suggesting an increase in effector Th2 lymphocytes in B subunit-mediated insulitis suppression. The results of these experiments indicate that bacterial and plant enterotoxin B subunit ligand-autoantigens enhance insulitis reduction in NOD mice. This research prompts further exploration of a multiadjuvant/autoantigen co-delivery strategy that may facilitate type 1 diabetes prevention and suppression in animals and humans.     

Epitope Mapping of Broadly Neutralizing HIV-2 Human Monoclonal Antibodies

Recent studies have shown that natural infection by HIV-2 leads to the elicitation of high titers of broadly neutralizing antibodies (NAbs) against primary HIV-2 strains (T. I. de Silva, et al., J. Virol. 86:930–946, 2012; R. Kong, et al., J. Virol. 86:947–960, 2012; G. Ozkaya Sahin, et al., J. Virol. 86:961–971, 2012). Here, we describe the envelope (Env) binding and neutralization properties of 15 anti-HIV-2 human monoclonal antibodies (MAbs), 14 of which were newly generated from 9 chronically infected subjects. All 15 MAbs bound specifically to HIV-2 gp120 monomers and neutralized heterologous primary virus strains HIV-2_7312A and HIV-2_ST. Ten of 15 MAbs neutralized a third heterologous primary virus strain, HIV-2_UC1. The median 50% inhibitory concentrations (IC₅₀s) for these MAbs were surprisingly low, ranging from 0.007 to 0.028 μg/ml. Competitive Env binding studies revealed three MAb competition groups: CG-I, CG-II, and CG-III. Using peptide scanning, site-directed mutagenesis, chimeric Env constructions, and single-cycle virus neutralization assays, we mapped the epitope of CG-I antibodies to a linear region in variable loop 3 (V3), the epitope of CG-II antibodies to a conformational region centered on the carboxy terminus of V4, and the epitope(s) of CG-III antibodies to conformational regions associated with CD4- and coreceptor-binding sites. HIV-2 Env is thus highly immunogenic in vivo and elicits antibodies having diverse epitope specificities, high potency, and wide breadth. In contrast to the HIV-1 Env trimer, which is generally well shielded from antibody binding and neutralization, HIV-2 is surprisingly vulnerable to broadly reactive NAbs. The availability of 15 human MAbs targeting diverse HIV-2 Env epitopes can facilitate comparative studies of HIV/SIV Env structure, function, antigenicity, and immunogenicity.