Soluble CD4 enhances the efficacy of immunotoxins directed against gp41 of the human immunodeficiency virus

Conjugation of anti-HIV gp41 monoclonal antibody to a drug capable of targeting resting lymphocytes produces an effective cytotoxic anti-HIV immunoconjugate

HIV-infected cells persisting in the face of suppressive antiretroviral therapy are the barrier to curing infection. Cytotoxic immunoconjugates targeted to HIV antigens on the cell surface may clear these cells. We showed efficacy in mouse and macaque models using immunotoxins, but immunogenicity blunted the effect. As an alternative, we propose antibody drug conjugates (ADCs), as used in cancer immunotherapy. In cancer, the target is a dividing cell, whereas it may not be in HIV. We screened cytotoxic drugs on human primary cells and cell lines. An anthracycline derivative, PNU-159682 (PNU), was highly cytotoxic to both proliferating and resting cells. Human anti-gp41 mAb 7B2 was conjugated to ricin A chain or PNU. The conjugates were tested in vitro for cytotoxic efficacy and anti-viral effect, and in vivo for tolerability. The specificity of killing for both conjugates was demonstrated on Env+ and Env- cells. The toxin conjugate was more potent and killed more rapidly, but 7B2-PNU was effective at levels achievable in patients. The ricin conjugate was well tolerated in mice; 7B2-PNU was toxic when administered intraperitoneally but was tolerated intravenously. We have produced an ADC with potential to target the persistent HIV reservoir in both dividing and non-dividing cells while avoiding immunogenicity. Cytotoxic anti-HIV immunoconjugates may have greatest utility as part of an "activate and purge" regimen, involving viral activation in the reservoir. This is a unique comparison of an immunotoxin and ADC targeted by the same antibody and tested in the same systems.IMPORTANCEHIV infection can be controlled with anti-retroviral therapy, but it cannot be cured. Despite years of therapy that suppresses HIV, patients again become viremic shortly after discontinuing treatment. A long-lived population of memory T cells retain the genes encoding HIV, and these cells secrete infectious HIV when no longer suppressed by therapy. This is the persistent reservoir of HIV infection. The therapies described here use anti-HIV antibodies conjugated to poisons to kill the cells in this reservoir. These poisons may be of several types, including protein toxins (immunotoxins) or anti-cancer drugs (antibody drug conjugates, ADCs). We have previously shown that an anti-HIV immunotoxin had therapeutic effects in animal models, but it elicited an anti-drug immune response. Here, we have prepared an anti-HIV ADC, which would be less likely to provoke an immune response, and show its potential for use in eliminating the persistent reservoir of HIV infection.

Soluble CD4 and low molecular weight CD4-mimetic compounds sensitize cells to be killed by anti-HIV cytotoxic immunoconjugates

IMPORTANCE

HIV infection can be effectively treated to prevent the development of AIDS, but it cannot be cured. We have attached poisons to anti-HIV antibodies to kill the infected cells that persist even after years of effective antiviral therapy. Here we show that the killing of infected cells can be markedly enhanced by the addition of soluble forms of the HIV receptor CD4 or by mimics of CD4.

Identification of Anti-gp41 Monoclonal Antibodies That Effectively Target Cytotoxic Immunoconjugates to Cells Infected with Human Immunodeficiency Virus, Type 1

We are developing cytotoxic immunoconjugates (CICs) targeting the envelope protein (Env) of the Human Immunodeficiency Virus, type 1 (HIV) to purge the persistent reservoirs of viral infection. We have previously studied the ability of multiple monoclonal antibodies (mAbs) to deliver CICs to an HIV-infected cell. We have found that CICs targeted to the membrane-spanning gp41 domain of Env are most efficacious, in part because their killing is enhanced in the presence of soluble CD4. The ability of a mAb to deliver a CIC does not correlate with its ability to neutralize nor mediate Ab-dependent cellular cytotoxicity. In the current study, we seek to define the most effective anti-gp41 mAbs for delivering CICs to HIV-infected cells. To do this, we have evaluated a panel of human anti-gp41 mAbs for their ability to bind and kill two different Env-expressing cell lines: persistently infected H9/NL4-3 and constitutively transfected HEK293/92UG. We measured the binding and cytotoxicity of each mAb in the presence and absence of soluble CD4. We found that mAbs to the immunodominant helix-loop-helix region (ID-loop) of gp41 are most effective, whereas neutralizing mAbs to the fusion peptide, gp120/gp41 interface, and the membrane proximal external region (MPER) are relatively ineffective at delivering CICs. There was only a weak correlation between antigen exposure and killing activity. The results show that the ability to deliver an effective IC and neutralization are distinct functions of mAbs.

Distinct Metabolic States Are Observed in Hypoglycemia Induced in Mice by Ricin Toxin or by Fasting

Hypoglycemia may be induced by a variety of physiologic and pathologic stimuli and can result in life-threatening consequences if untreated. However, hypoglycemia may also play a role in the purported health benefits of intermittent fasting and caloric restriction. Previously, we demonstrated that systemic administration of ricin toxin induced fatal hypoglycemia in mice. Here, we examine the metabolic landscape of the hypoglycemic state induced in the liver of mice by two different stimuli: systemic ricin administration and fasting. Each stimulus produced the same decrease in blood glucose and weight loss. The polar metabolome was studied using ¹H NMR, quantifying 59 specific metabolites, and untargeted LC-MS on approximately 5000 features. Results were analyzed by multivariate analyses, using both principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA), to identify global metabolic patterns, and by univariate analyses (ANOVA) to assess individual metabolites. The results demonstrated that while there were some similarities in the responses to the two stimuli including decreased glucose, ADP, and glutathione, they elicited distinct metabolic states. The metabolite showing the greatest difference was O-phosphocholine, elevated in ricin-treated animals and known to be affected by the pro-inflammatory cytokine TNF-α. Another difference was the alternative fuel source utilized, with fasting-induced hypoglycemia primarily ketotic, while the response to ricin-induced hypoglycemia involves protein and amino acid catabolism.

Bispecific Anti-HIV Immunoadhesins That Bind Gp120 and Gp41 Have Broad and Potent HIV-Neutralizing Activity

We have constructed bispecific immunoglobulin-like immunoadhesins that bind to both the HIV-envelope glycoproteins: gp120 and gp41. These immunoadhesins have N terminal domains of human CD4 engrafted onto the N-terminus of the heavy chain of human anti-gp41 mAb 7B2. Binding of these constructs to recombinant Env and their antiviral activities were compared to that of the parental mAbs and CD4, as well as to control mAbs. The CD4/7B2 constructs bind to both gp41 and gp140, as well as to native Env expressed on the surface of infected cells. These constructs deliver cytotoxic immunoconjugates to HIV-infected cells, but not as well as a mixture of 7B2 and sCD4, and opsonize for antibody-mediated phagocytosis. Most surprisingly, given that 7B2 neutralizes weakly, if at all, is that the chimeric CD4/7B2 immunoadhesins exhibit broad and potent neutralization of HIV, comparable to that of well-known neutralizing mAbs. These data add to the growing evidence that enhanced neutralizing activity can be obtained with bifunctional mAbs/immunoadhesins. The enhanced neutralization activity of the CD4/7B2 chimeras may result from cross-linking of the two Env subunits with subsequent inhibition of the pre-fusion conformational events that are necessary for entry.

Antibody Conjugates for Targeted Therapy Against HIV-1 as an Emerging Tool for HIV-1 Cure

Although advances in antiretroviral therapy (ART) have significantly improved the life expectancy of people living with HIV-1 (PLWH) by suppressing HIV-1 replication, a cure for HIV/AIDS remains elusive. Recent findings of the emergence of drug resistance against various ART have resulted in an increased number of treatment failures, thus the development of novel strategies for HIV-1 cure is of immediate need. Antibody-based therapy is a well-established tool in the treatment of various diseases and the engineering of new antibody derivatives is expanding the realms of its application. An antibody-based carrier of anti-HIV-1 molecules, or antibody conjugates (ACs), could address the limitations of current HIV-1 ART by decreasing possible off-target effects, reduce toxicity, increasing the therapeutic index, and lowering production costs. Broadly neutralizing antibodies (bNAbs) with exceptional breadth and potency against HIV-1 are currently being explored to prevent or treat HIV-1 infection in the clinic. Moreover, bNAbs can be engineered to deliver cytotoxic or immune regulating molecules as ACs, further increasing its therapeutic potential for HIV-1 cure. ACs are currently an important component of anticancer treatment with several FDA-approved constructs, however, to date, no ACs are approved to treat viral infections. This review aims to outline the development of AC for HIV-1 cure, examine the variety of carriers and payloads used, and discuss the potential of ACs in the current HIV-1 cure landscape.

Photoinduced Photosensitizer-Antibody Conjugates Kill HIV Env-Expressing Cells, Also Inactivating HIV

HIV-infected cells persist for decades in patients administered with antiretroviral therapy (ART). Meanwhile, an alarming surge in drug-resistant HIV viruses has been occurring. Addressing these issues, we propose the application of photoimmunotherapy (PIT) against not only HIV Env-expressing cells but also HIV. Previously, we showed that a human anti-gp41 antibody (7B2) conjugated to cationic or anionic photosensitizers (PSs) could specifically target and kill the HIV Env-expressing cells. Here, our photolysis studies revealed that the binding of photoimmunoconjugates (PICs) on the membrane of HIV Env-expressing cells is sufficient to induce necrotic cell death due to physical damage to the membrane by singlet oxygen, which is independent of the type of PSs. This finding persuaded us to study the virus photoinactivation of PICs using two HIV-1 strains, X4 HIV-1 NL4-3 and JR-CSF virus. We observed that the PICs could destroy the viral strains, probably via physical damage on the HIV envelope. In conclusion, we report the application of PIT as a possible dual-tool for HIV immunotherapy and ART by killing HIV-expressing cells and cell-free HIV, respectively.

Antiviral Activity of Ribosome-Inactivating Proteins

Ribosome-inactivating proteins (RIPs) are rRNA N-glycosylases from plants (EC 3.2.2.22) that inactivate ribosomes thus inhibiting protein synthesis. The antiviral properties of RIPs have been investigated for more than four decades. However, interest in these proteins is rising due to the emergence of infectious diseases caused by new viruses and the difficulty in treating viral infections. On the other hand, there is a growing need to control crop diseases without resorting to the use of phytosanitary products which are very harmful to the environment and in this respect, RIPs have been shown as a promising tool that can be used to obtain transgenic plants resistant to viruses. The way in which RIPs exert their antiviral effect continues to be the subject of intense research and several mechanisms of action have been proposed. The purpose of this review is to examine the research studies that deal with this matter, placing special emphasis on the most recent findings.

Photoimmunotherapy Using Cationic and Anionic Photosensitizer-Antibody Conjugates against HIV Env-Expressing Cells

Different therapeutic strategies have been investigated to target and eliminate HIV-1-infected cells by using armed antibodies specific to viral proteins, with varying degrees of success. Herein, we propose a new strategy by combining photodynamic therapy (PDT) with HIV Env-targeted immunotherapy, and refer to it as HIV photoimmunotherapy (PIT). A human anti-gp41 antibody (7B2) was conjugated to two photosensitizers (PSs) with different charges through different linking strategies; "Click" conjugation by using an azide-bearing porphyrin attached via a disulfide bridge linker with a drug-to-antibody ratio (DAR) of exactly 4, and "Lysine" conjugation by using phthalocyanine IRDye 700DX dye with average DARs of 2.1, 3.0 and 4.4. These photo-immunoconjugates (PICs) were compared via biochemical and immunological characterizations regarding the dosimetry, solubility, and cell targeting. Photo-induced cytotoxicity of the PICs were compared using assays for apoptosis, reactive oxygen species (ROS), photo-cytotoxicity, and confocal microscopy. Targeted phototoxicity seems to be primarily dependent on the binding of PS-antibody to the HIV antigen on the cell membrane, whilst being independent of the PS type. This is the first report of the application of PIT for HIV immunotherapy by killing HIV Env-expressing cells.

Increasing the Clinical Potential and Applications of Anti-HIV Antibodies

Preclinical and early human clinical studies of broadly neutralizing antibodies (bNAbs) to prevent and treat HIV infection support the clinical utility and potential of bNAbs for prevention, postexposure prophylaxis, and treatment of acute and chronic infection. Observed and potential limitations of bNAbs from these recent studies include the selection of resistant viral populations, immunogenicity resulting in the development of antidrug (Ab) responses, and the potentially toxic elimination of reservoir cells in regeneration-limited tissues. Here, we review opportunities to improve the clinical utility of HIV Abs to address these challenges and further accomplish functional targets for anti-HIV Ab therapy at various stages of exposure/infection. Before exposure, bNAbs' ability to serve as prophylaxis by neutralization may be improved by increasing serum half-life to necessitate less frequent administration, delivering genes for durable in vivo expression, and targeting bNAbs to sites of exposure. After exposure and/or in the setting of acute infection, bNAb use to prevent/reduce viral reservoir establishment and spread may be enhanced by increasing the potency with which autologous adaptive immune responses are stimulated, clearing acutely infected cells, and preventing cell-cell transmission of virus. In the setting of chronic infection, bNAbs may better mediate viral remission or "cure" in combination with antiretroviral therapy and/or latency reversing agents, by targeting additional markers of tissue reservoirs or infected cell types, or by serving as targeting moieties in engineered cell therapy. While the clinical use of HIV Abs has never been closer, remaining studies to precisely define, model, and understand the complex roles and dynamics of HIV Abs and viral evolution in the context of the human immune system and anatomical compartmentalization will be critical to both optimize their clinical use in combination with existing agents and define further strategies with which to enhance their clinical safety and efficacy.

Selective cytotoxicity of a novel immunotoxin based on pulchellin A chain for cells expressing HIV envelope

Immunotoxins (ITs), which consist of antibodies conjugated to toxins, have been proposed as a treatment for cancer and chronic infections. To develop and improve the ITs, different toxins such as ricin, have been used, aiming for higher efficacy against target cells. The toxin pulchellin, isolated from the Abrus pulchellus plant, has similar structure and function as ricin. Here we have compared two plant toxins, recombinant A chains from ricin (RAC) and pulchellin (PAC) toxins, for their ability to kill HIV Env-expressing cells. In this study, RAC and PAC were produced in E. coli, and chromatographically purified, then chemically conjugated to two different anti-HIV monoclonal antibodies (MAbs), anti-gp120 MAb 924 or anti-gp41 MAb 7B2. These conjugates were characterized biochemically and immunologically. Cell internalization was studied by flow cytometry and confocal microscopy. Results showed that PAC can function within an effective IT. The ITs demonstrated specific binding against native antigens on persistently HIV-infected cells and recombinant antigens on Env-transfected cells. PAC cytotoxicity appears somewhat less than RAC, the standard for comparison. This is the first report that PAC may have utility for the design and construction of therapeutic ITs, highlighting the potential role for specific cell targeting.

Identification of Human Anti-HIV gp160 Monoclonal Antibodies That Make Effective Immunotoxins

The envelope (Env) glycoprotein of HIV is the only intact viral protein expressed on the surface of both virions and infected cells. Env is the target of neutralizing antibodies (Abs) and has been the subject of intense study in efforts to produce HIV vaccines. Therapeutic anti-Env Abs can also exert antiviral effects via Fc-mediated effector mechanisms or as cytotoxic immunoconjugates, such as immunotoxins (ITs). In the course of screening monoclonal antibodies (MAbs) for their ability to deliver cytotoxic agents to infected or Env-transfected cells, we noted disparities in their functional activities. Different MAbs showed diverse functions that did not correlate with each other. For example, MAbs against the external loop region of gp41 made the most effective ITs against infected cells but did not neutralize virus and bound only moderately to the same cells that they killed so effectively when they were used in ITs. There were also differences in IT-mediated killing among transfected and infected cell lines that were unrelated to the binding of the MAb to the target cells. Our studies of a well-characterized antigen demonstrate that MAbs against different epitopes have different functional activities and that the binding of one MAb can influence the interaction of other MAbs that bind elsewhere on the antigen. These results have implications for the use of MAbs and ITs to kill HIV-infected cells and eradicate persistent reservoirs of HIV infection.

IMPORTANCE

There is increased interest in using antibodies to treat and cure HIV infection. Antibodies can neutralize free virus and kill cells already carrying the virus. The virus envelope (Env) is the only HIV protein expressed on the surfaces of virions and infected cells. In this study, we examined a panel of human anti-Env antibodies for their ability to deliver cell-killing toxins to HIV-infected cells and to perform other antiviral functions. The ability of an antibody to make an effective immunotoxin could not be predicted from its other functional characteristics, such as its neutralizing activity. Anti-HIV immunotoxins could be used to eliminate virus reservoirs that persist despite effective antiretroviral therapy.

Design and In Vivo Characterization of Immunoconjugates Targeting HIV gp160

The envelope (Env) glycoprotein of HIV is expressed on the surface of productively infected cells and can be used as a target for cytotoxic immunoconjugates (ICs), in which cell-killing moieties, including toxins, drugs, or radionuclides, are chemically or genetically linked to monoclonal antibodies (MAbs) or other targeting ligands. Such ICs could be used to eliminate persistent reservoirs of HIV infection. We have found that MAbs which bind to the external loop of gp41, e.g., MAb 7B2, make highly effective ICs, particularly when used in combination with soluble CD4. We evaluated the toxicity, immunogenicity, and efficacy of the ICs targeted with 7B2 in mice and in simian-human immunodeficiency virus-infected macaques. In the macaques, we tested immunotoxins (ITs), consisting of protein toxins bound to the targeting agent. ITs were well tolerated and initially efficacious but were ultimately limited by their immunogenicity. In an effort to decrease immunogenicity, we tested different toxic moieties, including recombinant toxins, cytotoxic drugs, and tubulin inhibitors. ICs containing deglycosylated ricin A chain prepared from ricin toxin extracted from castor beans were the most effective in killing HIV-infected cells. Having identified immunogenicity as a major concern, we show that conjugation of IT to polyethylene glycol limits immunogenicity. These studies demonstrate that cytotoxic ICs can target virus-infected cells in vivo but also highlight potential problems to be addressed.

IMPORTANCE

It is not yet possible to cure HIV infection. Even after years of fully effective antiviral therapy, a persistent reservoir of virus-infected cells remains. Here we propose that a targeted conjugate consisting of an anti-HIV antibody bound to a toxic moiety could function to kill the HIV-infected cells that constitute this reservoir. We tested this approach in HIV-infected cells grown in the lab and in animal infections. Our studies demonstrated that these immunoconjugates are effective both in vitro and in test animals. In particular, ITs constructed with the deglycosylated A chain prepared from native ricin were the most effective in killing cells, but their utility was blunted because they provoked immune reactions that interfered with the therapeutic effects. We then demonstrated that coating of the ITs with polyethylene glycol minimized the immunogenicity, as has been demonstrated with other protein therapies.

The future of antiviral immunotoxins

There is a constant need for new therapeutic interventions in a wide range of infectious diseases. Over the past few years, the immunotoxins have entered the stage as promising antiviral treatments. Immunotoxins have been extensively explored in cancer treatment and have achieved FDA approval in several cases. Indeed, the design of new anticancer immunotoxins is a rapidly developing field. However, at present, several immunotoxins have been developed targeting a variety of different viruses with high specificity and efficacy. Rather than blocking a viral or cellular pathway needed for virus replication and dissemination, immunotoxins exert their effect by killing and eradicating the pool of infected cells. By targeting a virus-encoded target molecule, it is possible to obtain superior selectivity and drastically limit the side effects, which is an immunotoxin-related challenge that has hindered the success of immunotoxins in cancer treatment. Therefore, it seems beneficial to use immunotoxins for the treatment of virus infections. One recent example showed that targeting of virus-encoded 7 transmembrane (7TM) receptors by immunotoxins could be a future strategy for designing ultraspecific antiviral treatment, ensuring efficient internalization and hence efficient eradication of the pool of infected cells, both in vitro and in vivo. In this review, we provide an overview of the mechanisms of action of immunotoxins and highlight the advantages of immunotoxins as future anti-viral therapies.

Targeted cytotoxic therapy kills persisting HIV infected cells during ART

Antiretroviral therapy (ART) can reduce HIV levels in plasma to undetectable levels, but rather little is known about the effects of ART outside of the peripheral blood regarding persistent virus production in tissue reservoirs. Understanding the dynamics of ART-induced reductions in viral RNA (vRNA) levels throughout the body is important for the development of strategies to eradicate infectious HIV from patients. Essential to a successful eradication therapy is a component capable of killing persisting HIV infected cells during ART. Therefore, we determined the in vivo efficacy of a targeted cytotoxic therapy to kill infected cells that persist despite long-term ART. For this purpose, we first characterized the impact of ART on HIV RNA levels in multiple organs of bone marrow-liver-thymus (BLT) humanized mice and found that antiretroviral drug penetration and activity was sufficient to reduce, but not eliminate, HIV production in each tissue tested. For targeted cytotoxic killing of these persistent vRNA(+) cells, we treated BLT mice undergoing ART with an HIV-specific immunotoxin. We found that compared to ART alone, this agent profoundly depleted productively infected cells systemically. These results offer proof-of-concept that targeted cytotoxic therapies can be effective components of HIV eradication strategies.

Anti-HIV double variable domain immunoglobulins binding both gp41 and gp120 for targeted delivery of immunoconjugates

Background

Anti-HIV immunoconjugates targeted to the HIV envelope protein may be used to eradicate the latent reservoir of HIV infection using activate-and-purge protocols. Previous studies have identified the two target epitopes most effective for the delivery of cytotoxic immunoconjugates the CD4-binding site of gp120, and the hairpin loop of gp41. Here we construct and test tetravalent double variable domain immunoglobulin molecules (DVD-Igs) that bind to both epitopes.

Methods

Synthetic genes that encode DVD-Igs utilizing V-domains derived from human anti-gp120 and anti-gp41 Abs were designed and expressed in 293F cells. A series of constructs tested different inter-V-linker domains and orientations of the two V domains. Antibodies were tested for binding to recombinant Ag and native Env expressed on infected cells, for neutralization of infectious HIV, and for their ability to deliver cytotoxic immunoconjugates to infected cells.

Findings

The outer V-domain was the major determinant of binding and functional activity of the DVD-Ig. Function of the inner V-domain and bifunctional binding required at least 15 AA in the inter-V-domain linker. A molecular model showing the spatial orientation of the two epitopes is consistent with this observation. Linkers that incorporated helical domains (A[EAAAK]_nA) resulted in more effective DVD-Igs than those based solely on flexible domains ([GGGGS]_n). In general, the DVD-Igs outperformed the less effective parental antibody and equaled the activity of the more effective. The ability of the DVD-Igs to deliver cytotoxic immunoconjugates in the absence of soluble CD4 was improved over that of either parent.

Conclusions

DVD-Igs can be designed that bind to both gp120 and gp41 on the HIV envelope. DVD-Igs are effective in delivering cytotoxic immunoconjugates. The optimal design of these DVD-Igs, in which both domains are fully functional, has not yet been achieved.

Antibodies for HIV treatment and prevention: window of opportunity?

Monoclonal antibodies are routinely used as therapeutics in a number of disease settings and have thus also been explored as potential treatment for human immunodeficiency virus (HIV)-1 infection. Antibodies targeting viral antigens, and those directed to the cellular receptors, have been considered for use in prevention and therapy. For virus-targeted antibodies, attention has focused primarily on their neutralizing activity, but such antibodies also have the potential to exert antiviral effects via effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), opsonization, or complement activation. Anti-cell antibodies act through occlusion or down-modulation of the viral receptors with notable impact in vivo, as recent trials have shown. This review summarizes the diverse specificities and modes of action of therapeutic antibodies against HIV-1 infection. Successes, challenges, and future opportunities of harnessing antibodies for therapy of HIV-1 infection are discussed.

A modified SCID mouse model of HIV infection with utility for testing anti-HIV therapies

Using human tumor cells we have developed a mouse model of active HIV infection that may be used for testing antiviral agents, although it does not reflect the pathogenesis of human infection. Irradiated SCID/NOD mice are injected with a tumor of human CD4+ lymphoma cells susceptible to infection and at a separate site, tumor cells persistently infected with either primary or T cell line-adapted strains of HIV. The spread of infection from the infected to the susceptible tumor is monitored as plasma p24 and the presence of HIV-infected cells in the spleen. We have used this model to examine the relative efficacy of neutralizing anti-HIV antibodies to halt the spread of infection. We have found that the tetrameric CD4-antibody fusion protein, CD4-IgG2, is highly effective compared to an anti-V3 loop antibody. This animal model, while not replicating the human disease, allows for the simultaneous testing of efficacy, toxicity, and pharmacokinetics of potential new antiviral therapies. The model can easily be powered to enable comparisons between different therapeutic agents and dosing regimens.

In vivo efficacy of anti-glycoprotein 41, but not anti-glycoprotein 120, immunotoxins in a mouse model of HIV infection

Immunotoxins (ITs) targeting the HIV envelope protein are among the most efficacious antiviral therapies when tested in vitro. Yet a first-generation IT targeted to gp120, CD4-PE40 (chimeric immunotoxin using CD4 and the translocation and enzymatic domains of Pseudomonas exotoxin A), showed limited promise in initial clinical testing, highlighting the need for improved ITs. We have used a new mouse model of HIV infection to test the comparative efficacy of anti-HIV ITs targeted to gp120 or to gp41. Irradiated SCID/nonobese diabetic mice are injected with a tumor of human CD4(+) cells susceptible to infection and at a separate site persistently HIV-infected cells. The spread of infection from infected to susceptible tumor is monitored by plasma p24 and the presence of HIV-infected cells in the spleen. Anti-gp41 ITs in combination with tetrameric CD4-human Ig fusion protein have pronounced anti-HIV effects. Little if any anti-HIV efficacy was found with either CD4-PE40 or an Ab-targeted anti-gp120 IT. These data support continued exploration of the utility of ITs for HIV infection, particularly the use of anti-gp41 ITs in combination with soluble CD4 derivatives.

Rationale for the use of immunotoxins in the treatment of HIV-infected humans

The first step in the replication of human immunodeficiency virus (HIV) is selective binding of the envelope glycoprotein (gp120) to CD4 receptors on T cells or macrophages. After penetration in these cells, the genome of the virus is integrated in the human genome. HIV-infection causes depletion of CD4-positive cells resulting in a severe immunosuppression. It is believed that eliminating HIV-infected cells is crucial in limiting further reduction of CD4-positive cells and thus, preventing disease progression. The most commonly used drugs, such as zidovudine (AZT), appeared to be not completely effective. Therefore many investigators are searching for alternative treatment modalities. The use of immunotoxins (ITs) to eliminate HIV-infected cells is discussed. ITs are chimeric molecules in which cell-binding ligands are coupled to toxins and can specifically eliminate undesired cells. The cell-binding carriers of anti-HIV ITs have been directed against different regions of the HIV envelope glycoprotein (gp120 and gp41) and surface antigens (e.g CD4, CD25). The ITs have been composed of different ribosome-inactivating proteins (RIPs) like pokeweed antiviral protein (PAP), Pseudomonas exotoxin (PE), Diphtheria toxin (DT), or ricin. In in vitro studies, several of these ITs have been shown to be effective and specific in killing acute and persistently HIV-infected cells. The ITs were effective at concentrations (ID50 range from 10(-9) M to 10(-12) M) that were not toxic to uninfected cells or cells without the antigen. The IT CD4(178)PE40, a fusion protein directed against the CD4 binding site of gp120, has been investigated in two in vivo trials. The results were disappointing considering the antiviral activity in vitro. This was thought to be due to the rapid clearance of the IT and the differential resistance of clinical HIV isolates. Use of a panel of ITs is likely to be more effective because multiple approaches cover the intrinsic variability of HIV and the presence of IT-resistant or latently infected cells, as well as the blocking presence of neutralizing anti-HIV antibodies and the immunogenicity of most ITs. It may be possible to control the virus completely with a panel of ITs in combination with other antiviral or immunosuppressive agents such as RT inhibitors (e.g AZT), interferon alpha, or cyclosporine. More research will be necessary to develop such a combined therapy.

Immunologic cross-reaction between HIV type 1 p17 and Mycoplasma hyorhinis variable lipoprotein

Monoclonal antibodies directed against the HIV-1 matrix protein p17 that react with a component present on the surface of HIV-1-infected cells have previously been described. In this study we show that one of these monoclonal antibodies binds to persistently HIV-1-infected cell lines that are coinfected with Mycoplasma hyorhinis, but not to cell lines that are uninfected with mycoplasma. Mycoplasma-infected cells secrete HIV-1 at a higher rate, have a slight increase in cell surface expression of gp120 and gp41, and are less sensitive to immunotoxins than uninfected cells. The anti-p17 antibody binds to a protein of M. hyorhinis grown in cell-free culture. The variable expression and size of the protein among strains is typical of the variable lipoprotein (Vlp) system of M. hyorhinis. Confirmation of the reactivity of the antibody with a Vlp was provided by demonstrating its specific binding to recombinant VlpF expressed in E. coli, and to a synthetic peptide representing the carboxy-terminal region of VlpF, but not to other recombinant Vlp products or peptides. This is a true cross-reaction because the antibody also binds to recombinant p17 expressed in E. coli and the binding is inhibited by the VlpF peptide. These analyses highlight the potential of mycoplasma contamination of tissue culture cell lines to cause anomalous results. With regard to HIV-1, mycoplasma infection of cells results in increased rates of virus secretion, and introduces a potential confounding immunologic cross-reaction as well. The existence of a cell surface form of p17 is unlikely.

Therapeutic potential of anti-HIV immunotoxins

In vitro analyses have shown anti-HIV immunotoxins to be among the most effective AIDS antivirals tested. Because HIV has been continually selected by antibody, immunotoxins targeted to constant domains of viral antigens may not elicit drug-resistant mutants. A clinical trial with CD4-PE40, a possibly flawed immunotoxin with nonspecific toxicity and short serum half-life, has reduced interest in this form of therapy. It is proposed that the use of an immunotoxin directed against gp41 in combination with a CD4-Ig chimera is more likely to have a therapeutic effect than CD4-PE40. Clinical trials are also underway utilizing an immunotoxin that eliminates activated T-cells, an important cellular locus of HIV-replication.

In vitro effects of anti-HIV immunotoxins directed against multiple epitopes on HIV type 1 envelope glycoprotein 160

We have used a panel of anti-gp160 MAbs to construct anti-HIV immunotoxins by coupling antibodies to ricin A chain (RAC). The ability of the immunotoxins to kill HIV-1-infected cells and halt the spread of infection was tested in tissue culture on persistently and acutely infected cell lines and primary lymphocyte cultures stimulated with phytohemagglutinin (PHA blasts). Laboratory strains and clinical isolates of HIV both were tested. The constitution and antigen-binding capacity of the immunotoxins were confirmed by ELISA and indirect immunofluorescence. Immunotoxins that bind epitopes exposed on the cell surface effectively killed persistently infected cells, although killing was not directly proportional to binding of immunotoxin to cell. The activity of anti-gp41, but not anti-gp120, immunotoxins was markedly enhanced in the presence of soluble CD4 or peptides corresponding to the CDR3 region of CD4. CD4-mediated enhancement of anti-gp41 immunotoxin activity was observed for laboratory strains neutralized by sCD4 and for clinical isolates that were resistant to neutralization by sCD4. Immunotoxin action was potentiated by brefeldin A, bafilomycin A1, cortisone, and an amphipathic fusion peptide, but not by cytochalasin D, nocodazol, monodansyl cadaverine, or trans-retinoic acid. Anti-HIV immunotoxins are useful tool with which to study the functional expression of gp120/gp41 antigens on the surface of HIV-infected cells, as well as potential AIDS therapeutics. Because these studies relate to the accessibility of viral antigens to antibody-mediated attack, these studies also have relevance for vaccine development.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.

Processing of the envelope glycoprotein gp160 in immunotoxin-resistant cell lines chronically infected with human immunodeficiency virus type 1

We describe the isolation and characterization of variant cell lines which are chronically infected with the human immunodeficiency virus (HIV) and resistant to the action of immunotoxins directed against the HIV envelope protein. These variants all produce normal levels of HIV proteins, budding virions, and the envelope protein precursor gp160. Two of the variants, 10E and 11E, contain a mutation within the env gene which results in the production of a truncated precursor and altered processing and transport of the protein to the cell surface. Variants B9 and G4 are defective in gp160 cleavage and do not efficiently transport the envelope protein to the cell surface. There are no mutations in the expressed viruses of B9 and G4. These cell lines express higher levels of CD4 protein and mRNA than H9/NL4-3. Thus, 10E, 11E, B9, and G4 have escaped immunotoxin action by downmodulating the envelope protein from their cell surfaces. None of these variants produce infectious HIV. Two other immunotoxin-resistant variants, E9-3 and 41-17, produce normal levels of gp160, efficiently transport the cleaved and processed subunits to the cell surface, and secrete infectious HIV. These studies identify alterations in gp160 processing that underscore the importance of the relationship between HIV and the cell that it infects.

Direct interaction of complement factor H with the C1 domain of HIV type 1 glycoprotein 120

A protein that binds specifically to Env 105-119 (HEDIISLWDQSLKPC) was found in pools of normal human plasma when this peptide was used in affinity chromatography procedures. These samples represented the negative control in experiments aimed at the purification of putative human antibodies to the Env 105-119 region from AIDS sera. In this article we describe the biochemical characterization of this protein, which turned out to be complement factor H (CFH). We propose a functional role for this protein in the complex, early steps of CD4-dependent HIV infection.

Unique insertion sequence and pattern of CD4 expression in variants selected with immunotoxins from human immunodeficiency virus type 1-infected T cells

To study the variability of human immunodeficiency virus type 1 (HIV-1), we used immunotoxins to select for variants within a population of H9 cells persistently infected with a molecular clone of HIV-1 designated NL4-3. Chimeric immunotoxin CD4-PE40 (a chimeric fusion protein consisting of the amino-terminal two domains of CD4 and the carboxy-terminal domains of Pseudomonas exotoxin A) was used to select for cells lacking cell surface expression of HIV Env (envelope proteins gp160, gp120, and gp41). The cells described here (A1, A7, C9, and E9) fail to express HIV proteins because they have markedly diminished transcription of the integrated provirus (A1, A7, and E9) or no HIV provirus (C9). Analysis demonstrated that two different cloned variants, A1 and E9, contain the complementary sequence of tRNA(3Lys) (45 bp) inserted 3' to the primer-binding site, following by a 169-bp deletion through the start of the gag gene. No HIV mRNA was detected by Northern (RNA) blotting, but PCR demonstrated the presence of the viral message. These variants were found very infrequently in the unselected H9/NL4-3 cell population, and they contained proviruses distinct from that found in the dominant population. In addition, all of these variants had similar patterns of CD4 surface expression that allowed them to escape reinfection within the tissue culture. The data are discussed with regard to mechanisms and errors of HIV reverse transcription, as well as the evolution of mutants within a population of persistently infected cells.