Bispecific Antibodies Targeting Different Epitopes on the HIV-1 Envelope Exhibit Broad and Potent Neutralization.

UNLABELLED: The potency and breadth of the recently isolated neutralizing human monoclonal antibodies to HIV-1 have stimulated interest in their use to prevent or to treat HIV-1 infection. Due to the antigenically diverse nature of the HIV-1 envelope (Env), no single antibody is highly active against all viral strains. While the physical combination of two broadly neutralizing antibodies (bNAbs) can improve coverage against the majority of viruses, the clinical-grade manufacturing and testing of two independent antibody products are time and resource intensive. In this study, we constructed bispecific immunoglobulins (IgGs) composed of independent antigen-binding fragments with a common Fc region. We developed four different bispecific IgG variants that included antibodies targeting four major sites of HIV-1 neutralization. We show that these bispecific IgGs display features of both antibody specificities and, in some cases, display improved coverage over the individual parental antibodies. All four bispecific IgGs neutralized 94% to 97% of antigenically diverse viruses in a panel of 206 HIV-1 strains. Among the bispecific IgGs tested, VRC07 × PG9-16 displayed the most favorable neutralization profile. It was superior in breadth to either of the individual antibodies, neutralizing 97% of viruses with a median 50% inhibitory concentration (IC50) of 0.055 µg/ml. This bispecific IgG also demonstrated in vivo pharmacokinetic parameters comparable to those of the parental bNAbs when administered to rhesus macaques. These results suggest that IgG-based bispecific antibodies are promising candidates for the prevention and treatment of HIV-1 infection in humans. IMPORTANCE: To prevent or treat HIV-1 infection, antibodies must potently neutralize nearly all strains of HIV-1. Thus, the physical combination of two or more antibodies may be needed to broaden neutralization coverage and diminish the possibility of viral resistance. A bispecific antibody that has two different antibody binding arms could potentially display neutralization characteristics better than those of any single parental antibody. Here we show that bispecific antibodies contain the binding specificities of the two parental antibodies and that a single bispecific antibody can neutralize 97% of viral strains with a high overall potency. These findings support the use of bispecific antibodies for the prevention or treatment of HIV-1 infection.

Inhibition of the alloantibody response by CD95 ligand.

We investigated the effect of Fas/APO1-ligand (CD95L) gene transfer on allogeneic immune responses in vivo. A colon carcinoma cell line from BALB/c mice, CT26, was stably transfected with a vector encoding mouse CD95L and was inoculated into C57BL/6 mice. CD95L expression markedly reduced allogeneic cytotoxic T lymphocyte and helper T lymphocyte activity directed toward CT26. Strikingly, expression of CD95L on these allogeneic tumors completely inhibited the generation of alloantibodies of both IgM and IgG subclasses. Thus, CD95L inhibited alloantibody production and conferred localized immune suppression through this mechanism. These results provide insight into the role of CD95L in regulating the alloantibody response and the generation of local immune responses.

The inhibition of pro-apoptotic ICE-like proteases enhances HIV replication.

Accelerated programmed cell death, or apoptosis, contributes to the CD4+ T-cell depletion characteristic of infection by human immunodeficiency virus (HIV). It has therefore been proposed that limiting apoptosis may represent a therapeutic modality for HIV infection. We found, however, that T leukemia cells or peripheral blood mononuclear cells (PBMCs) exposed to HIV-1 underwent enhanced viral replication in the presence of the cell death inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-AVD-fmk). Furthermore, z-VAD-fmk, which targets the pro-apoptotic interleukin-1 beta-converting enzyme (ICE)-like proteases, stimulated endogenous virus production in activated PBMCs derived from HIV-1-infected asymptomatic individuals. These findings suggest that programmed cell death may serve as a beneficial host mechanism to limit HIV spread and that strategies to inhibit it may have deleterious consequences for the infected host.

The p21 cyclin-dependent kinase inhibitor suppresses tumorigenicity in vivo.

The p21 gene encodes a cyclin-dependent kinase inhibitor that affects cell-cycle progression, but the potential of this gene product to serve as a tumour suppressor in vivo has not been established. In this report, we show that the growth of malignant cells in vitro and in vivo is inhibited by expression of p21. Expression of p21 resulted in an accumulation of cells in G0/G1, altered morphology, and cell differentiation, but apoptosis was not induced. Introduction of p21 with adenoviral vectors into malignant cells completely suppressed their growth in vivo and also reduced the growth of established pre-existing tumours. Gene transfer of p21 may provide a molecular genetic approach to arresting cancer cell growth by committing malignant cells irreversibly to a pathway of terminal differentiation.

Identification of the Ebola virus glycoprotein as the main viral determinant of vascular cell cytotoxicity and injury.

Here we defined the main viral determinant of Ebola virus pathogenicity; synthesis of the virion glycoprotein (GP) of Ebola virus Zaire induced cytotoxic effects in human endothelial cells in vitro and in vivo. This effect mapped to a serine-threonine-rich, mucin-like domain of this type I transmembrane glycoprotein, one of seven gene products of the virus. Gene transfer of GP into explanted human or porcine blood vessels caused massive endothelial cell loss within 48 hours that led to a substantial increase in vascular permeability. Deletion of the mucin-like region of GP abolished these effects without affecting protein expression or function. GP derived from the Reston strain of virus, which causes disease in nonhuman primates but not in man, did not disrupt the vasculature of human blood vessels. In contrast, the Zaire GP induced endothelial cell disruption and cytotoxicity in both nonhuman primate and human blood vessels, and the mucin domain was required for this effect. These findings indicate that GP, through its mucin domain, is the viral determinant of Ebola pathogenicity and likely contributes to hemorrhage during infection.

Immunization for Ebola virus infection.

Infection by Ebola virus causes rapidly progressive, often fatal, symptoms of fever, hemorrhage and hypotension. Previous attempts to elicit protective immunity for this disease have not met with success. We report here that protection against the lethal effects of Ebola virus can be achieved in an animal model by immunizing with plasmids encoding viral proteins. We analyzed immune responses to the viral nucleoprotein (NP) and the secreted or transmembrane forms of the glycoprotein (sGP or GP) and their ability to protect against infection in a guinea pig infection model analogous to the human disease. Protection was achieved and correlated with antibody titer and antigen-specific T-cell responses to sGP or GP. Immunity to Ebola virus can therefore be developed through genetic vaccination and may facilitate efforts to limit the spread of this disease.

Heme oxygenase-1 protects against vascular constriction and proliferation.

Heme oxygenase (HO-1, encoded by Hmox1) is an inducible protein activated in systemic inflammatory conditions by oxidant stress. Vascular injury is characterized by a local reparative process with inflammatory components, indicating a potential protective role for HO-1 in arterial wound repair. Here we report that HO-1 directly reduces vasoconstriction and inhibits cell proliferation during vascular injury. Expression of HO-1 in arteries stimulated vascular relaxation, mediated by guanylate cyclase and cGMP, independent of nitric oxide. The unexpected effects of HO-1 on vascular smooth muscle cell growth were mediated by cell-cycle arrest involving p21Cip1. HO-1 reduced the proliferative response to vascular injury in vivo; expression of HO-1 in pig arteries inhibited lesion formation and Hmox1-/- mice produced hyperplastic arteries compared with controls. Induction of the HO-1 pathway moderates the severity of vascular injury by at least two adaptive mechanisms independent of nitric oxide, and is a potential therapeutic target for diseases of the vasculature.

A cloned cell line mediating natural killer cell function inhibits immunoglobulin secretion.

We previously described a cloned cell line that combines information for a unique display of cell surface antigens and specialized function similar to activated natural killer (NK) cells. In addition to conventional cellular targets such as the YAC-1 and MBL-2 lymphomas, this cloned line also lysed lipopolysaccharide-activated B lymphocytes. To determine whether some NK cells can inhibit B cell function, we tested the ability of NK-like clones to suppress Ig secretion in vitro and in vivo. These cloned cells suppressed Ig secretion when they constituted as few as 0.2% of the total cell population and inhibition did not require identity at the H-2 locus. We suggest that some NK cells might recognize non-major histocompatibility complex gene products on activated B lymphocytes and lyse these cells, and this might represent a fundamental cell-cell interaction that regulates antibody secretion by activated B cells.

Multiple activities of a cloned cell line mediating natural killer cell function.

A special class of immunologic cells can lyse or damage a variety of target cells, notably malignant cells in vitro. These cells have been called natural killer (NK) cells because lysis does not require deliberate immunization by tumor cells. Although these cells can be distinguished from conventional T cells, B cells, and phagocytic cells, they have been difficult to define. We describe a representative cloned cell line that was obtained by cloning Ig -Ly-5+ cells from spleen. This clone, Cl.Ly-1-2-NK-1+/11, displays Thy-1, Ly-5, Qat-4, Qat-5 and NK-1 cell surface antigens and lyses the NK-sensitive YAC-1 lymphoma cells, but does not lyse RL-12 cells, an NK-resistant lymphoma. In addition, this clone lysed the P815 mastocytoma, EL4 lymphoma, and lipopolysaccharide-activated B lymphocyte targets. This cloned population therefore combined information for a unique display of cell surface antigens and specialized function similar to "activated" NK cells. Because this cloned population forms conjugates with susceptible but not resistant target cells, it may prove useful to identify the structure of cell surface molecules that recognize foreign cells. Finally, cells of this clone also specificity lysed target cells coated by antibodies to determinants on the target cell surface, demonstrating that a single cloned cell population can mediate two specialized immunologic functions: antibody-dependent cellular cytotoxicity and NK cell lysis.

Antigen-specific T lymphocyte clones. II. Purification and biological characterization of an antigen-specific suppressive protein synthesized by cloned T cells.

We have generated an antigen-specific T suppressor clone that synthesizes 70,000-mol wt peptides that have antigen-specific-binding activity. Although these data also indicated that antigen-binding peptides completely inhibited the in vitro primary response to a complex antigen, suppression might reflect the combined biologic activities of many different 70-mol wt polypeptides or polypeptides associated with the 70,000-mol wt material by noncovalent interactions. The protein responsible for antigen-specific suppression was therefore purified to virtual homogeneity after sequential separation of internally labeled supernate peptides on Sephacryl S-200 and DEAE-cellulose columns followed by isoeleetrofocusing. The resulting protein is greater than 95 percent homogeneous according to sodium dodeeyl sulfate-polyacrylamide electrophoresis and represents two peptides having two very close but distinguishable isoelectric point values of approximately 5.0. The purified molecules are retained by columns coated with lentil lectin or antigen but not by columns coated with antisera specific for immunoglobulins, the I region of the major histocompatibility complex or Ly-1 or Ly-2 antigens. Less than 50 pg of the purified glycoprotein specifically and completely suppresses production of anti-sheep erythrocyte plaque-forming cell by mixtures of 10(6) Ly-1 cells and B cells and this is a result of inactivation of Ly-l-mediated helper function. Specific inactivation of T (Th) cells by the 70,000-mol wt molecule is rapid, specific, and requires the presence of antigen. The mechanism of specific suppression of Th function may depend upon two functionally distinct regions of the 70,000-mol wt molecule: one that binds antigen and a second that mediates suppression.

Antigen-specific T lymphocyte clones. I. Characterization of a T lymphocyte clone expressing antigen-specific suppressive activity.

We have generated continuously propagatable T lymphocyte clones to study antigen-specific T cell functions. All Ly-2+ clones mediate suppressive activity and secrete a characteristic pattern of polypeptides that differs from Ly-2- T cell clones. Cells of one clone, Cl.Ly23/4, specifically bind glycophorin from sheep erythrocytes (SRBC). After incubation with [35S]methionine, supernate material from this clone also contains biosynthetically labeled 70,000-mol wt proteins that specifically bind to SRBC and this binding is inhibited by glycophorin from sheep but not other erythrocytes. These antigen-binding 70,000-mol wt peptides specifically and completely suppress primary anti-SRBC responses generated by mixtures of primed Ly-1+2- cells and B cells. Suppression by these antigen-binding peptides reflects direct inhibition of T-helper activity.

Stable expression of transdominant Rev protein in human T cells inhibits human immunodeficiency virus replication.

The human immunodeficiency virus (HIV) Rev protein is essential for viral structural protein expression (Gag, Pol, and Env) and, hence, for viral replication. In transient transfection assays, mutant forms of Rev have been identified that inhibit wild-type Rev activity and therefore suppress viral replication. To determine whether such transdominant Rev proteins could provide long-term protection against HIV infection without affecting T cell function, T leukemia cell lines were stably transduced with a retroviral vector encoding a transdominant mutant of the Rev protein, M10. While all the M10-expressing cell lines remained infectable by HIV-1, these same cells failed to support a productive replication cycle when infected with a cloned isolate of HIV-1. In addition, two out of three M10-expressing CEM clones were also resistant to highly productive infection by a heterogeneous HIV-1 pool. Expression of M10 did not affect induction of HIV transcription mediated by the kappa B regulatory element or Tat. Importantly, constitutive expression of Rev M10 did not alter the secretion of interleukin 2 in response to mitogen stimulation of EL-4 and Jurkat cells. The inhibition of HIV infection in cells stably expressing a transdominant Rev protein, in the absence of any deleterious effect on T cell function, suggests that such a strategy could provide a therapeutic effect in the T lymphocytes of acquired immunodeficiency syndrome patients.

Cloned mouse cells with natural killer function and cloned suppressor T cells express ultrastructural and biochemical features not shared by cloned inducer T cells.

We have examined the morphology, cytochemistry, and biochemistry of mouse leukocyte subsets by analyzing cloned leukocyte populations specialized to perform different immunologic functions. Cloned cells expressing high-affinity plasma membrane receptors for IgE and mediating natural killer (NK) lysis and cloned antigen-specific suppressor T cells contained prominent osmiophilic cytoplasmic granules similar by ultrastructure to those of mouse basophils. Both clones also incorporated 35SO4 into granule-associated sulfated glycosaminoglycans, expressed a characteristic ultrastructural pattern of nonspecific esterase activity, incorporated exogenous [3H]5-hydroxytryptamine, and contained cytoplasmic deposits of particulate glycogen. By contrast, cloned inducer T cells lacked cytoplasmic granules and glycogen, incorporated neither 35SO4 nor [3H]5-hydroxytryptamine, and differed from the other clones in pattern of nonspecific esterase activity. These findings establish that certain cloned cells with NK activity and cloned suppressor T cells express morphologic and biochemical characteristics heretofore associated with basophilic granulocytes. However, these clones differ in surface glycoprotein expression and immunologic function, and the full extent of the similarities and differences among these populations and basophils remains to be determined.

Mast cell clones: a model for the analysis of cellular maturation.

Cloned mouse mast cells resemble, by ultrastructure, immature mast cells observed in vivo. These mast cell clones can be grown in the absence of any other cells, facilitating direct investigations of their biochemistry and function. We find that cloned mast cells express plasma membrane receptors (Fc epsilon R) that bind mouse IgE with an equilibrium constant (KA) similar to that of normal mouse peritoneal mast cells. In addition, cloned mast cells do not display detectable la antigens and cannot enhance lg secretion when added to lymphocyte cultures or mediate natural killer lysis. In the presence of 1 mM sodium butyrate, cloned mast cells stop dividing and acquire abundant electron-dense cytoplasmic granules similar to those of mature mast cells. Their histamine content increases concomitant with cytoplasmic granule maturation and may exceed that of untreated mast cells by 50-fold. Unlike peritoneal mast cells, cloned mast cells incorporate 35SO4 into chondroitin sulfates rather than heparin. These findings demonstrate that, unlike fully differentiated mouse peritoneal mast cells, cloned immature mouse mast cells contain no heparin and low levels of histamine. In addition, they establish that high-affinity Fc epsilon R are expressed early in mast cell maturation, well before completion of cytoplasmic granule synthesis and mediator storage.

Regulation of the proinflammatory effects of Fas ligand (CD95L).

Fas ligand (CD95L) inhibits T cell function in immune-privileged organs such as the eye and testis, yet in most tissues CD95L expression induces potent inflammatory responses. With a stably transfected colon carcinoma cell line, CT26-CD95L, the molecular basis for these divergent responses was defined. When injected subcutaneously, rejection of CT26-CD95L was caused by neutrophils activated by CD95L. CT26-CD95L survived in the intraocular space because of the presence of transforming growth factor-beta (TGF-beta), which inhibited neutrophil activation. Providing TGF-beta to subcutaneous sites protected against tumor rejection. Thus, these cytokines together generate a microenvironment that promotes immunologic tolerance, which may aid in the amelioration of allograft rejection.

Site-specific gene expression in vivo by direct gene transfer into the arterial wall.

A recombinant beta-galactosidase gene has been expressed in a specific arterial segment in vivo by direct infection with a murine amphotropic retroviral vector or by DNA transfection with the use of liposomes. Several cell types in the vessel wall were transduced, including endothelial and vascular smooth muscle cells. After retroviral infection, a recombinant reporter gene was expressed for at least 5 months, and no helper virus was detected. Recombinant gene expression achieved by direct retroviral infection or liposome-mediated DNA transfection was limited to the site of infection and was absent from liver, lung, kidney, and spleen. These results demonstrate that site-specific gene expression can be achieved by direct gene transfer in vivo and could be applied to the treatment of such human diseases as atherosclerosis or cancer.

Recombinant gene expression in vivo within endothelial cells of the arterial wall.

A technique for the transfer of endothelial cells and expression of recombinant genes in vivo could allow the introduction of proteins of therapeutic value in the management of cardiovascular diseases. Porcine endothelial cells expressing recombinant beta-galactosidase from a murine amphotropic retroviral vector were introduced with a catheter into denuded iliofemoral arteries of syngeneic animals. Arterial segments explanted 2 to 4 weeks later contained endothelial cells expressing beta-galactosidase, an indication that they were successfully implanted on the vessel wall.

Alternative mechanisms for activation of human immunodeficiency virus enhancer in T cells.

The expression of human immunodeficiency virus (HIV) after T cell activation is regulated by NF-kappa B, an inducible DNA-binding protein that stimulates transcription. Proteins encoded by a variety of DNA viruses are also able to activate expression from the HIV enhancer. To determine how this activation occurs, specific genes from herpes simplex virus type 1 and adenovirus that activate HIV in T lymphoma cells have been identified. The cis-acting regulatory sequences in the HIV enhancer that mediate their effect have also been characterized. The relevant genes are those for ICP0-an immediate-early product of herpes simplex virus type 1-and the form of E1A encoded by the 13S messenger RNA of adenovirus. Activation of HIV by adenovirus E1A was found to depend on the TATA box, whereas herpesvirus ICP0 did not work through a single defined cis-acting element. These findings suggest multiple pathways that can be used to bypass normal cellular activation of HIV, and they raise the possibility that infection by herpes simplex virus or adenovirus may directly contribute to the activation of HIV in acquired immunodeficiency syndrome by mechanisms independent of antigenic stimulation in T cells.

Regulation of gene expression with double-stranded phosphorothioate oligonucleotides.

Alteration of gene transcription by inhibition of specific transcriptional regulatory proteins is necessary for determining how these factors participate in cellular differentiation. The functions of these proteins can be antagonized by several methods, each with specific limitations. Inhibition of sequence-specific DNA-binding proteins was achieved with double-stranded (ds) phosphorothioate oligonucleotides that contained octamer or kappa B consensus sequences. The phosphorothioate oligonucleotides specifically bound either octamer transcription factor or nuclear factor (NF)-kappa B. The modified oligonucleotides accumulated in cells more effectively than standard ds oligonucleotides and modulated gene expression in a specific manner. Octamer-dependent activation of a reporter plasmid or NF-kappa B-dependent activation of the human immunodeficiency virus (HIV) enhancer was inhibited when the appropriate phosphorothioate oligonucleotide was added to a transiently transfected B cell line. Addition of phosphorothioate oligonucleotides that contained the octamer consensus to Jurkat T leukemia cells inhibited interleukin-2 (IL-2) secretion to a degree similar to that observed with a mutated octamer site in the IL-2 enhancer. The ds phosphorothioate oligonucleotides probably compete for binding of specific transcription factors and may provide anti-viral, immunosuppressive, or other therapeutic effects.

Gene therapy for vascular smooth muscle cell proliferation after arterial injury.

Accumulation of vascular smooth muscle cells as a consequence of arterial injury is a major feature of vascular proliferative disorders. Molecular approaches to the inhibition of smooth muscle cell proliferation in these settings could potentially limit intimal expansion. This problem was approached by introducing adenoviral vectors encoding the herpesvirus thymidine kinase (tk) into porcine arteries that had been injured by a balloon on a catheter. These smooth muscle cells were shown to be infectable with adenoviral vectors, and introduction of the tk gene rendered them sensitive to the nucleoside analog ganciclovir. When this vector was introduced into porcine arteries immediately after a balloon injury, intimal hyperplasia decreased after a course of ganciclovir treatment. No major local or systemic toxicities were observed. These data suggest that transient expression of an enzyme that catalyzes the formation of a cytotoxic drug locally may limit smooth muscle cell proliferation in response to balloon injury.