Functional activity of an HIV-1 neutralizing IgG human monoclonal antibody: ADCC and complement-mediated lysis

Nanobody-mediated complement activation to kill HIV-infected cells

The complement system which is part of the innate immune response against invading pathogens represents a powerful mechanism for killing of infected cells. Utilizing direct complement recruitment for complement-mediated elimination of HIV-1-infected cells is underexplored. We developed a novel therapeutic modality to direct complement activity to the surface of HIV-1-infected cells. This bispecific complement engager (BiCE) is comprised of a nanobody recruiting the complement-initiating protein C1q, and single-chain variable fragments of broadly neutralizing antibodies (bNAbs) targeting the HIV-1 envelope (Env) protein. Here, we show that two anti-HIV BiCEs targeting the V3 loop and the CD4 binding site, respectively, increase C3 deposition and mediate complement-dependent cytotoxicity (CDC) of HIV-1 Env-expressing Raji cells. Furthermore, anti-HIV BiCEs trigger complement activation on primary CD4 T cells infected with laboratory-adapted HIV-1 strain and facilitates elimination of HIV-1-infected cells over time. In summary, we present a novel approach to direct complement deposition to the surface of HIV-1-infected cells leading to complement-mediated killing of these cells.

Innate Immune Response Against HIV-1

The innate immune system is comprised of both cellular and humoral players that recognise and eradicate invading pathogens. Therefore, the interplay between retroviruses and innate immunity has emerged as an important component of viral pathogenesis. HIV-1 infection in humans that results in hematologic abnormalities and immune suppression is well represented by changes in the CD4/CD8 T cell ratio and consequent cell death causing CD4 lymphopenia. The innate immune responses by mucosal barriers such as complement, DCs, macrophages, and NK cells as well as cytokine/chemokine profiles attain great importance in acute HIV-1 infection, and thus, prevent mucosal capture and transmission of HIV-1. Conversely, HIV-1 has evolved to overcome innate immune responses through RNA-mediated rapid mutations, pathogen-associated molecular patterns (PAMPs) modification, down-regulation of NK cell activity and complement receptors, resulting in increased secretion of inflammatory factors. Consequently, epithelial tissues lining up female reproductive tract express innate immune sensors including anti-microbial peptides responsible for forming primary barriers and have displayed an effective potent anti-HIV activity during phase I/II clinical trials.

In vitro Evidence That Combination Therapy With CD16-Bearing NK-92 Cells and FDA-Approved Alefacept Can Selectively Target the Latent HIV Reservoir in CD4+ CD2hi Memory T Cells

Elimination of the latent HIV reservoir remains the biggest hurdle to achieve HIV cure. In order to specifically eliminate HIV infected cells they must be distinguishable from uninfected cells. CD2 was recently identified as a potential marker enriched in the HIV-1 reservoir on CD4+ T cells, the largest, longest-lived and best-characterized constituent of the HIV reservoir. We previously proposed to repurpose FDA-approved alefacept, a humanized α-CD2 fusion protein, to reduce the HIV reservoir in CD2hi CD4+ memory T cells. Here, we show the first evidence that alefacept can specifically target and reduce CD2hi HIV infected cells in vitro. We explore a variety of natural killer (NK) cells as mediators of antibody-dependent cell-mediated cytotoxicity (ADCC) including primary NK cells, expanded NK cells as well as the CD16 transduced NK-92 cell line which is currently under study in clinical trials as a treatment for cancer. We demonstrate that CD16.NK-92 has a natural preference to kill CD2hi CD45RA- memory T cells, specifically CD45RA- CD27+ central memory/transitional memory (TCM/TM) subset in both healthy and HIV+ patient samples as well as to reduce HIV DNA from HIV+ samples from donors well controlled on antiretroviral therapy. Lastly, alefacept can combine with CD16.NK-92 to decrease HIV DNA in some patient samples and thus may yield value as part of a strategy toward sustained HIV remission.

Absolute Configuration of the Antiviral Agent (−)-cis-5-Fluoro-1-[2-Hydroxymethyl)-1,3-Oxathiolan-5-yl]Cytosine

The structure and absolute configuration of (−)- cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine (FTC), has been determined by X-ray crystallographic analysis. The results confirm that the L-isomer of the nucleoside analogue is the most active enantiomer and that the correct absolute configuration of (−)-FTC is 5-fluoro-(2′R,5′S)-(−)-1-[2-hydroxymethyl)oxathiolan-5-yl]-fluorocytosine. The two molecules in the asymmetric unit show conformations that combine conformational features of two other classes of potent antiviral nucleosides. Both oxathiolane rings have the 3′-sulphur atom in nearly perfect S3′- exo envelope conformations, similar to what is observed for 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxycytidine. One of the two molecules has a glycosylic link conformation in which the base is eclipsed with the C5′-O1′ bond. This mimics the high- anti conformation that has been observed in the structures of several 2′,3′-didehydro-2′,3′-dideoxypyrimidine nucleosides but is inaccessible for saturated pyrimidine nucleosides. However, the observed conformations cannot be superimposed adequately with other active antiviral nucleosides to suggest a common ‘active site’ conformation.

Activation of CD35 and CD55 in HIV associated normal and pre-eclamptic pregnant women

OBJECTIVE

The delicate balance which exists between complement activation and its regulation is altered in HIV infection and pregnancy disorders such as pre-eclampsia. Therefore, the purpose of this study was to investigate the expression of complement regulatory (Creg) proteins (CD35 and CD55) in HIV associated normal and pre-eclamptic pregnancies.

STUDY DESIGN

The total study population (n=100) consisted of normotensive pregnant (n=50) and pre-eclamptic (n=50) women. These groups were equally sub-stratified into HIV infected and uninfected groups (n=25 per group). Standard haematological tests were conducted. Flow cytometric analysis of isolated neutrophils were performed using fluorescein isothiocyanate-conjugated anti-CD35 and phycoerythrin-cyanine 5 conjugated anti-CD55.

RESULTS

HELLP syndrome characteristics of increased lactate dehydrogenase enzymes levels, low platelet counts, cell morphological abnormalities (red cell fragmentation) and anaemia were observed in 40% of the HIV infected pre-eclamptic group. Red cell fragmentation inclusive of burr cells and schistocytes were also noted. Activated partial thromboplastin time and fibrinogen differed significantly between the HIV uninfected pre-eclamptic compared to the HIV infected pre-eclamptic groups (p<0.01). Irrespective of HIV status, the mean fluorescence intensity of CD35 and CD55 were significantly higher in the pre-eclamptic compared to the normotensive pregnant (p=0.0001; p=0.0001 respectively) groups. In the pre-eclamptic groups, the expression of both CD35 and CD55 did not significantly differ between HIV infected and uninfected women (p=0.486; p=0.767 respectively).

CONCLUSIONS

This study demonstrates an up-regulation of complement regulatory proteins, CD35 and CD55 in HIV associated pre-eclamptic compared to normotensive pregnancy. This elevation of the Creg proteins is an adaptive immune response to the high complement-mediated cell lysis that occurs in HIV infection and further aggravated by the complement activated state of pre-eclampsia.

Epitope specificity of human immunodeficiency virus-1 antibody dependent cellular cytotoxicity [ADCC] responses

Antibody dependent cellular cytotoxicity [ADCC] has been suggested to play an important role in control of Human Immunodeficiency Virus-1 [HIV-1] viral load and protection from infection. ADCC antibody responses have been mapped to multiple linear and conformational epitopes within the HIV-1 envelope glycoproteins gp120 and gp41. Many epitopes targeted by antibodies that mediate ADCC overlap with those recognized by antibodies capable of virus neutralization. In addition, recent studies conducted with human monoclonal antibodies derived from HIV-1 infected individuals and HIV-1 vaccine-candidate vaccinees have identified a number of antibodies that lack the ability to capture primary HIV-1 isolates or mediate neutralizing activity, but are able to bind to the surface of infected CD4+ T cells and mediate ADCC. Of note, the conformational changes in the gp120 that may not exclusively relate to binding of the CD4 molecule are important in exposing epitopes recognized by ADCC responses. Here we discuss the HIV-1 envelope epitopes targeted by ADCC antibodies in the context of the potential protective capacities of ADCC.

Complement and HIV-I infection/HIV-associated neurocognitive disorders

The various neurological complications associated with HIV-1 infection, specifically HIV-associated neurocognitive disorders (HAND) persist as a major public health burden worldwide. Despite the widespread use of anti-retroviral therapy, the prevalence of HAND is significantly high. HAND results from the direct effects of an HIV-1 infection as well as secondary effects of HIV-1-induced immune reaction and inflammatory response. Complement, a critical mediator of innate and acquired immunity, plays important roles in defeating many viral infections by the formation of a lytic pore or indirectly by opsonization and recruitment of phagocytes. While the role of complement in the pathogenesis of HIV-1 infection and HAND has been previously recognized for over 15 years, it has been largely underestimated thus far. Complement can be activated through HIV-1 envelope proteins, mannose-binding lectins (MBL), and anti-HIV-1 antibodies. Complement not only fights against HIV-1 infection but also enhances HIV-1 infection. In addition, HIV-1 can hijack complement regulators such as CD59 and CD55 and can utilize these regulators and factor H to escape from complement attack. Normally, complement levels in brain are much lower than plasma levels and there is no or little complement deposition in brain cells. Interestingly, local production and deposition of complement are dramatically increased in HIV-1-infected brain, indicating that complement may contribute to the pathogenesis of HAND. Here, we review the current understanding of the role of complement in HIV-1 infection and HAND, as well as potential therapeutic approaches targeting the complement system for the treatment and eradications of HIV-1 infection.

Synthetic immunotherapy induces HIV virus specific Th1 cytotoxic response and death of an HIV-1 infected human cell line through classic complement activation

Background

This manuscript describes the development of a novel synthetic immunotherapy (HIV-v) composed of four multi-epitope polypeptides targeting conserved regions in the Nef, Rev, Vif and Vpr viral proteins. Immunogenicity and cytotoxicity of HIV-v are discussed.

Methods

Short conserved T-cell multi-epitope regions were identified in silico in the HIV proteome. The immunogenicity of the identified HIV-v polypeptides was assessed in vivo by immunisation of C57BLK6 mice transgenic for HLA-A*0201. Splenocytes from immunised animals were exposed in vitro to soluble HIV-v polypeptides or to syngeneic (T1) or allogeneic (Jurkat) cells transfected with these polypeptides. Specific T-cell reactivity was assessed by cell-based IFN-γ ELISA. Virus specific CD3 + CD8+ IFN-γ+ recall responses were also determined by flow cytometry following in vitro exposure of splenocytes from immunised mice to syngeneic (T1) and allogeneic (H9) cells infected with HIV-1 strain IIIB. HIV-v specific antibodies were quantified by ELISA whilst antibody mediated anti-viral immunotherapeutic effect on T1 cells infected with a laboratory adapted and a primary isolate of the HIV-1 virus was assessed in a LDH-based complement mediated lysis assay.

Results

HIV-v elicited antigen-specific IgG and IFN−γ responses against the synthetic polypeptides in the formulation. HIV-v specific T cells recognised polypeptides presented either as soluble antigen or complexed to HLA-A*0201 following natural processing and presentation by syngeneic human T1 cells. Moreover, the CD3 + CD8+ component of the response recognised syngeneic T1 cells naturally infected with HIV-1 in a virus-specific and MHC restricted-manner. The HIV-v specific IgG response was also able to recognise human T1 cells naturally infected with HIV-1 and induce cell death through classic activation of complement.

Conclusions

HIV-v induces a vaccine-specific type I immune response characterised by activation of effector CD8+ T cell and antibody responses that recognise and kill human cell lines naturally infected with a laboratory adapted and a primary isolate of the HIV-1 virus. The data supports the hypothesis that alternative HIV protein targets can be effectively used to prime both cellular and antibody immune responses of clinical value in the prevention and treatment of HIV infection.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Complement modulates pathogenesis and antibody-dependent neutralization of West Nile virus infection through a C5-independent mechanism

Although the interactions of complement and viruses have been widely studied, the function of C5 and the membrane attack complex in the context of viral infection or antibody-mediated neutralization remains controversial. Using C5-depleted or -deficient human or mouse sera, we show that C5 does not contribute to the antibody-dependent or -independent neutralization of West Nile virus (WNV) in cell culture. Consistent with this, C5 neither contributed to protection against WNV pathogenesis nor augmented the neutralizing efficacy of complement-fixing anti-WNV neutralizing antibodies in mice. Although previous studies established that activation of the classical, lectin, and alternative complement pathways restricts WNV infection, our results show little effect of C5 and by inference the terminal lytic complement components. Overall, these results enhance our mechanistic understanding of how complement controls flavivirus infections.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

Potent human immunodeficiency virus-neutralizing and complement lysis activities of antibodies are not obligatorily linked

To evaluate the contribution of complement-mediated lysis to the in vivo activities of neutralizing antibodies, we analyzed the influence of complement activation on treatment success in a recent passive immunization trial with the neutralizing monoclonal antibodies 2G12, 2F5, and 4E10. Administration of monoclonal antibodies led to an immediate, high activation of the complement system even in the absence of viremia in the 14 participating human immunodeficiency virus-infected individuals. Lysis activity measured in patient plasma increased during passive immunization; however, the increases were modest and only partially attributable to the administration of antibodies. We found that unlike neutralization activity, lysis activity was not associated with treatment success in this trial. Compared to complement lysis mounted by the polyclonal antibody response in vivo, monoclonal antibodies were weak inducers of this activity, suggesting that polyclonal responses are more effective in reaching the required threshold of complement activation. Importantly, strong neutralization activity of the monoclonal antibodies did not predict complement lysis activity against patient and reference viruses, suggesting that these activities are not linked. In summary, our data support the notion that the in vivo activities of 2G12, 2F5, and 4E10 are likely due to direct neutralization or Fc receptor-mediated mechanisms such as phagocytosis and antibody-dependent cellular cytotoxicity.

Humoral immunity to HIV-1: neutralization and beyond

Humoral immunity is considered a key component of effective vaccines against HIV-1. Hence, an enormous effort has been put into investigating the neutralizing antibody response to HIV-1 over the past 20 years which generated key information on epitope specificity, potency, breadth and in vivo activity of the neutralizing antibodies. Less clear is still the role of antibody-mediated effector functions (antibody-dependent cellular cytotoxicity, phagocytosis, complement system) and uncertainty prevails whether Fc-mediated mechanisms are largely beneficial or detrimental for the host. The current knowledge on the manifold functions of the humoral immune response in HIV infection, their underlying mechanisms and potential in vaccine-induced immunity will be discussed in this review.

Complement lysis activity in autologous plasma is associated with lower viral loads during the acute phase of HIV-1 infection

BACKGROUND

To explore the possibility that antibody-mediated complement lysis contributes to viremia control in HIV-1 infection, we measured the activity of patient plasma in mediating complement lysis of autologous primary virus.

METHODS AND FINDINGS

Sera from two groups of patients-25 with acute HIV-1 infection and 31 with chronic infection-were used in this study. We developed a novel real-time PCR-based assay strategy that allows reliable and sensitive quantification of virus lysis by complement. Plasma derived at the time of virus isolation induced complement lysis of the autologous virus isolate in the majority of patients. Overall lysis activity against the autologous virus and the heterologous primary virus strain JR-FL was higher at chronic disease stages than during the acute phase. Most strikingly, we found that plasma virus load levels during the acute but not the chronic infection phase correlated inversely with the autologous complement lysis activity. Antibody reactivity to the envelope (Env) proteins gp120 and gp41 were positively correlated with the lysis activity against JR-FL, indicating that anti-Env responses mediated complement lysis. Neutralization and complement lysis activity against autologous viruses were not associated, suggesting that complement lysis is predominantly caused by non-neutralizing antibodies.

CONCLUSIONS

Collectively our data provide evidence that antibody-mediated complement virion lysis develops rapidly and is effective early in the course of infection; thus it should be considered a parameter that, in concert with other immune functions, steers viremia control in vivo.

Down-regulated expression of complement factor I: a potential suppressive protein for gastric cancer identified by serum proteome analysis

BACKGROUND

Screening tests are needed for gastric cancer. In order to find serologic biomarkers for gastric cancer screening, we used proteomics to search for protein biomarkers that may be detected in serum of gastric cancer patients.

METHODS

Four groups of serum samples (from 20 gastric cancer patients) included pTNM stages I to IV were compared with two control groups of serum samples. The serum samples were first chromatographed using an immunoaffinity column to selectively remove albumin and IgG. Then serum proteins separated by 2-DE and identified by MALDI-TOF/TOF-MS.

RESULTS

There were 3 proteins including complement C4-B precursor, complement factor I (CFI) precursor and haptoglobin precursor were found significantly different between the healthy and gastric cancer patients. Further validation study by Western blot showed that the detection of CFI precursor was under-expressed in gastric cancer sera compared to normal sera (p<0.05), in addition to a declining expression with the advanced pTNM stage from stage I to IV of gastric cancer patients. Area under the receiver operating characteristic (ROC) curve was 0.78 for the expression of CFI precursor (p<0.01).

CONCLUSIONS

2-DE-based serum proteome analysis may be useful in the screening of serum biomarkers for gastric cancer. CFI precursor could aid in the diagnosis and indicate the advancement of gastric cancer.

Involvement of Fc gamma RI (CD64) in the mechanism of HIV-1 inhibition by polyclonal IgG purified from infected patients in cultured monocyte-derived macrophages

The aim of this study was to investigate the mechanism of HIV-1 neutralization using monocyte-derived macrophages (MDM) in comparison to PBMC as target cells. For this purpose, we analyzed neutralizing activities of different human polyclonal IgG samples purified from sera of HIV-1-infected individuals using a single cycle infection assay. We found an increase of the neutralizing titer when macrophages vs PBMC were used as target cells. Moreover, polyclonal IgG from HIV-1-infected patients that are not able to neutralize virus when PBMC are used as target cells strongly inhibit MDM infection. Similar results were obtained with neutralizing mAbs. To explore the participation of FcgammaRs in HIV-1 inhibition, F(ab')(2) and Fab of these Igs were produced. Results indicated that both F(ab')(2) and Fab are less effective to inhibit virus replication in MDM. Moreover, competition experiments with Fc fragments of IgG from healthy donors or with purified monoclonal anti-human FcgammaRs Ab strengthen the participation of the FcgammaRs, and in particular of FcgammaRI (CD64) in HIV-1 inhibition on MDM. Mechanisms by which HIV-specific IgG inhibit virus replication in cultured macrophages are proposed and the benefit of inducing such Abs by vaccination is discussed.

HLA-C and HLA-E reduce antibody-dependent natural killer cell-mediated cytotoxicity of HIV-infected primary T cell blasts

OBJECTIVE

To determine whether the presence of HLA-C and HLA-E on HIV-infected cells modulates autologous natural killer (NK) cells from implementing antibody-dependent cell-mediated cytotoxicity (ADCC) of HIV-infected cells.

DESIGN

The capability of HLA-C and HLA-E to control NK cell killing of HIV-infected autologous T cells coated with anti-gp120 monoclonal antibody was determined by blocking the interaction between the inhibitory receptors on NK cells and the MHC class I molecules on infected cells.

METHODS

Phytohemagglutinin-treated CD4 T cells were infected in vitro with HIV-1. Infected cells were separated from uninfected cells by removal of CD4 T cells. Infected cells were labeled with chromium-51, treated with a cocktail of four different monoclonal antibodies against HIV gp120, and co-cultured with freshly isolated autologous NK cells that were incubated with or without anti-CD159a, anti-CD158a, and CD158b, or all three antibodies combined. Killing of the HIV-infected cells by NK cells was assessed in a 4 h cytotoxic assay.

RESULTS

When the interaction between NK cell inhibitory receptors (i.e., CD158a, CD158b, and CD159a) and MHC class I molecules (i.e., HLA-C and HLA-E) on HIV-infected autologous T cells was blocked, a drastic increase in killing of anti-gp120-coated HIV-infected cells by NK cells was observed.

CONCLUSION

These studies indicate that the presence of HLA-C and HLA-E molecules on HIV-infected cells may facilitate evasion of NK-mediated killing of antibody-coated HIV-infected cells.

Epitope mapping and characterization of a novel CD4-induced human monoclonal antibody capable of neutralizing primary HIV-1 strains

Human immunodeficiency virus (HIV-1) enters target cells by binding its gp120 exterior envelope glycoprotein to CD4 and one of the chemokine receptors, CCR5 or CXCR4. CD4-induced (CD4i) antibodies bind gp120 more efficiently after CD4 binding and block the interaction with the chemokine receptor. Examples of CD4i antibodies are limited, and the prototypes of the CD4i antibodies exhibit only weak neutralizing activity against primary, clinical HIV-1 isolates. Here we report the identification of a novel antibody, E51, that exhibits CD4-induced binding to gp120 and neutralizes primary HIV-1 more efficiently than the prototypic CD4i antibodies. The E51 antibody blocks the interaction of gp120-CD4 complexes with CCR5 and binds to a highly conserved, basic gp120 element composed of the beta 19-strand and surrounding structures. Thus, on primary HIV-1 isolates, this gp120 region, which has been previously implicated in chemokine receptor binding, is accessible to a subset of CD4i antibodies.

Expression and functional activity of isotype and subclass switched human monoclonal antibody reactive with the base of the V3 loop of HIV-1 gp120

Immunoglobulins undergo isotype switching in response to antigenic stimulation. The C(H) domains, in particular the hinge region, impose structural constraints on the interaction of antibody with antigen, especially multivalent antigens such as HIV. We previously showed that switching the IgG1 anti-HIV human monoclonal antibody (HMAb) F105 to an IgG3 resulted in significantly enhanced neutralization of HIV. To further investigate the influence of isotype, including the functional activity of HMAbs switched to IgA, which may be important in mucosal defenses, isotype switched antibodies have been generated for the anti-V3 loop base IgG2 HMAb F425B4e8. Reactivity of the IgG1 antibody was greater than the parental IgG2 antibody for SF2 infected cells but less for primary isolate virions. In contrast, there was less reactivity of the IgG3 with either infected cells or virions. IgA reacted significantly more with infected cells and virions as compared to the IgG subclasses. In contrast to previous studies whereby IgG3 enhanced neutralization, comparable neutralization of primary isolate virus was observed for IgG subclasses (IgG1, IgG2, IgG3) and IgA. This may reflect differences in the exposure of epitopes recognized by the HMAb with antibody flexibility being important to neutralization by antibodies reactive with obscured epitopes (e.g., CD4 binding site). Further analysis of the in vitro activity of isotype or subclass switched antibodies, IgA in particular, alone and in combination with other HMAbs, will provide important information on the role of IgG subclass and IgA antibodies on protective immunity to HIV.

Hyperglycosylated mutants of human immunodeficiency virus (HIV) type 1 monomeric gp120 as novel antigens for HIV vaccine design

The ability to induce broadly neutralizing antibodies should be a key component of any forthcoming vaccine against human immunodeficiency virus type 1. One potential vaccine candidate, monomeric gp120, has generally failed to elicit such antibodies. We postulated that gp120 might be a better immunogen if it could be engineered to preferentially bind known broadly neutralizing antibodies. In a first study, we found that four alanine substitutions on the perimeter of the so-called Phe-43 cavity of gp120 could reduce binding of weakly neutralizing CD4-binding site antibodies (R. Pantophlet, E. O. Saphire, P. Poignard, P. W. H. I. Parren, I. A. Wilson, and D. R. Burton, J. Virol. 77:642-658, 2003), while slightly enhancing binding of the potent, broadly neutralizing antibody b12. In the present study, we sought to reduce or abolish the binding of a wider range of nonneutralizing antibodies, by incorporating extra N-glycosylation motifs at select positions into the hypervariable loops and the gp120 core. A hyperglycosylated mutant containing seven extra glycosylation sequons (consensus sequences) and the four alanine substitutions described above did not bind an extensive panel of nonneutralizing and weakly neutralizing antibodies, including a polyclonal immunoglobulin preparation (HIVIG) of low neutralizing potency. Binding of b12, at lowered affinity, and of four antibodies to the C1 and C5 regions was maintained. Removal of N- and C-terminal residues in the C1 and C5 regions, respectively, reduced or abolished binding of the four antibodies, but this also adversely affected b12 binding. The hyperglycosylated mutant and its analogues described here are novel antigens that may provide a new approach to eliciting antibodies with b12-like neutralizing properties.

Fine mapping of the interaction of neutralizing and nonneutralizing monoclonal antibodies with the CD4 binding site of human immunodeficiency virus type 1 gp120

Alanine scanning mutagenesis was performed on monomeric gp120 of human immunodeficiency virus type 1 to systematically identify residues important for gp120 recognition by neutralizing and nonneutralizing monoclonal antibodies (MAbs) to the CD4 binding site (CD4bs). Substitutions that affected the binding of broadly neutralizing antibody b12 were compared to substitutions that affected the binding of CD4 and of two nonneutralizing anti-CD4bs antibodies (b3 and b6) with affinities for monomeric gp120 comparable to that of b12. Not surprisingly, the sensitivities to a number of amino acid changes were similar for the MAbs and for CD4. However, in contrast to what was seen for the MAbs, no enhancing mutations were observed for CD4, suggesting that the virus has evolved toward an optimal gp120-CD4 interaction. Although the epitope maps of the MAbs overlapped, a number of key differences between b12 and the other two antibodies were observed. These differences may explain why b12, in contrast to nonneutralizing antibodies, is able to interact not only with monomeric gp120 but also with functional oligomeric gp120 at the virion surface. Neutralization assays performed with pseudovirions bearing envelopes from a selection of alanine mutants mostly showed a reasonable correlation between the effects of the mutations on b12 binding to monomeric gp120 and neutralization efficacy. However, some mutations produced an effect on b12 neutralization counter to that predicted from gp120 binding data. It appears that these mutations have different effects on the b12 epitope on monomeric gp120 and functional oligomeric gp120. To determine whether monomeric gp120 can be engineered to preferentially bind MAb b12, recombinant gp120s were generated containing combinations of alanine substitutions shown to uniquely enhance b12 binding. Whereas b12 binding was maintained or increased, binding by five nonneutralizing anti-CD4bs MAbs (b3, b6, F105, 15e, and F91) was reduced or completely abolished. These reengineered gp120s are prospective immunogens that may prove capable of eliciting broadly neutralizing antibodies.

Neutralizing antibodies against HIV -- back in the major leagues?

The past few months have seen encouraging successes for neutralizing antibodies against HIV; human monoclonal antibodies targeting conserved HIV envelope epitopes potently neutralized primary virus isolates, including strains of different clades. In primates, passive immunization with combinations containing human monoclonal antibodies completely prevented infection, even after mucosal virus challenges. Epitopes recognized by the protective monoclonal antibodies are important determinants for protection and provide a rational basis for AIDS vaccine development.

Passive immunization with human neutralizing monoclonal antibodies: correlates of protective immunity against HIV

Passive immunization with synergistic combinations of human monoclonal antibodies (mAbs) directed against conserved epitopes of the human immunodeficiency virus (HIV) envelope completely protected 13 out of 16 rhesus monkeys challenged intravenously or orally with chimeric simian-human immunodeficiency virus (SHIV) strains; partial protection was seen in another two. A high degree of protection was seen among orally challenged neonates. Thus, we propose that passive immunization with synergistic combinations of neutralizing human mAbs may be effective in preventing maternal HIV transmission when given as post-exposure prophylaxis at birth and as prophylaxis against milk-borne transmission. Because we only used mAbs with well-defined epitope specificities, our studies also yield key information for designing AIDS vaccines: the correlates of immune protection. Vaccine strategies that can evoke antibody responses to epitopes recognized by the mAbs used in our primate studies could be important components of successful AIDS vaccines.

Postnatal passive immunization of neonatal macaques with a triple combination of human monoclonal antibodies against oral simian-human immunodeficiency virus challenge

To develop prophylaxis against mother-to-child human immunodeficiency virus (HIV) transmission, we established a simian-human immunodeficiency virus (SHIV) infection model in neonatal macaques that mimics intrapartum mucosal virus exposure (T. W. Baba et al., AIDS Res. Hum. Retroviruses 10:351-357, 1994). Using this model, neonates were protected from mucosal SHIV-vpu(+) challenge by pre- and postnatal treatment with a combination of three human neutralizing monoclonal antibodies (MAbs), F105, 2G12, and 2F5 (Baba et al., Nat. Med. 6:200-206, 2000). In the present study, we used this MAb combination only postnatally, thereby significantly reducing the quantity of antibodies necessary and rendering their potential use in humans more practical. We protected two neonates with this regimen against oral SHIV-vpu(+) challenge, while four untreated control animals became persistently infected. Thus, synergistic MAbs protect when used as immunoprophylaxis without the prenatal dose. We then determined in vitro the optimal MAb combination against the more pathogenic SHIV89.6P, a chimeric virus encoding env of the primary HIV89.6. Remarkably, the most potent combination included IgG1b12, which alone does not neutralize SHIV89.6P. We administered the combination of MAbs IgG1b12, 2F5, and 2G12 postnatally to four neonates. One of the four infants remained uninfected after oral challenge with SHIV89.6P, and two infants had no or a delayed CD4(+) T-cell decline. In contrast, all control animals had dramatic drops in their CD4(+) T cells by 2 weeks postexposure. We conclude that our triple MAb combination partially protected against mucosal challenge with the highly pathogenic SHIV89.6P. Thus, combination immunoprophylaxis with passively administered synergistic human MAbs may play a role in the clinical prevention of mother-to-infant transmission of HIV type 1.

Protection of neonatal macaques against experimental SHIV infection by human neutralizing monoclonal antibodies

Neonatal macaques were completely protected against oral challenge with SHIV-vpu+, a simian-human immunodeficiency virus that encodes the envelope gene of a laboratory-adapted HIV strain, by pre- and post-natal treatment with a triple combination of human neutralizing monoclonal antibodies (mAbs). The mAbs were directed either against the CD4 binding site, a glycosylation-dependent gp120 epitope, or against a linear epitope on gp41. This triple combination was highly synergistic in vitro and neutralized primary HIV completely. Subsequently, oral challenge was performed with pathogenic SHIV89.6P, an animal-passaged variant of a chimeric virus that encodes the envelope gene of the primary, dual-tropic HIV89.6. Only post-natal treatment with a similar triple mAb combination was used. One out of 4 mAb-treated infants was completely protected from infection. In the other 3 treated animals, there was a tendency towards lower peak viral RNA loads compared with untreated controls. Two out of 4 mAb-treated infants maintained normal CD4+ T-cell numbers, in contrast to all controls that had steep declines at 2 weeks post-challenge. We conclude that the triple mAb combination significantly protected the neonates, even against mucosal challenge with pathogenic SHIV89.6P. Passively administered synergistic human mAbs may play a role in preventing mother-infant transmission of HIV, both against intrapartum transmission as well as against infection through breast milk. As passive immunization is a tool to assess correlates of immune protection, we conclude that the epitopes recognized by the mAbs in our combinations are important for AIDS vaccine development. Future passive immunization studies may reveal other important conserved epitopes.

Immunoglobulin G3 from polyclonal human immunodeficiency virus (HIV) immune globulin is more potent than other subclasses in neutralizing HIV type 1

Passive antibody prophylaxis against human immunodeficiency virus type 1 (HIV-1) has been accomplished in primates, suggesting that this strategy may prove useful in humans. While antibody specificity is crucial for neutralization, other antibody characteristics, such as subclass, have not been explored. Our objective was to compare the efficiencies of immunoglobulin G (IgG) subclasses from polyclonal human HIV immune globulin (HIVIG) in the neutralization of HIV-1 strains differing in coreceptor tropism. IgG1, IgG2, and IgG3 were enriched from HIVIG by using protein A-Sepharose. All three subclasses bound major HIV-1 proteins, as shown by Western blot assay and enzyme-linked immunosorbent assay. In HIV-1 fusion assays using X4, R5, or X4R5 envelope-expressing effector cells, IgG3 more efficiently blocked fusion. In neutralization assays with cell-free viruses using X4 (LAI, IIIB), R5 (BaL), and X4R5 (DH123), a similar hierarchy of neutralization was found: IgG3 > IgG1 > IgG2. IgG3 has a longer, more flexible hinge region than the other subclasses. To test whether this is important, IgG1 and IgG3 were digested with pepsin to generate F(ab')(2) fragments or with papain to generate Fab fragments. IgG3 F(ab')(2) fragments were still more efficient in neutralization than F(ab')(2) of IgG1. However, Fab fragments of IgG3 and IgG1 demonstrated equivalent neutralization capacities and the IgG3 advantage was lost. These results suggest that the IgG3 hinge region confers enhanced HIV-neutralizing ability. Enrichment and stabilization of IgG3 may therefore lead to improved HIVIG preparations. The results of this study have implications for the improvement of passive immunization with polyclonal or monoclonal antibodies and suggest that HIV-1 vaccines which induce high-titer IgG3 responses could be advantageous.

Human neutralizing monoclonal antibodies of the IgG1 subtype protect against mucosal simian-human immunodeficiency virus infection

Although maternal human immunodeficiency virus type 1 (HIV-1) transmission occurs during gestation, intrapartum and postpartum (by breast-feeding), 50-70% of all infected children seem to acquire HIV-1 shortly before or during delivery. Epidemiological evidence indicates that mucosal exposure is an important aspect of intrapartum HIV transmission. A simian immunodeficiency virus (SIV) macaque model has been developed that mimics the mucosal exposure that can occur during intrapartum HIV-1 transmission. To develop immunoprophylaxis against intrapartum HIV-1 transmission, we used SHIV-vpu+ (refs. 5,6), a chimeric simian-human virus that encodes the env gene of HIV-IIIB. Several combinations of human monoclonal antibodies against HIV-1 have been identified that neutralize SHIV-vpu+ completely in vitro through synergistic interaction. Here, we treated four pregnant macaques with a triple combination of the human IgG1 monoclonal antibodies F105, 2G12 and 2F5. All four macaques were protected against intravenous SHIV-vpu+ challenge after delivery. The infants received monoclonal antibodies after birth and were challenged orally with SHIV-vpu+ shortly thereafter. We found no evidence of infection in any infant during 6 months of follow-up. This demonstrates that IgG1 monoclonal antibodies protect against mucosal lentivirus challenge in neonates. We conclude that epitopes recognized by the three monoclonal antibodies are important determinants for achieving substantial protection, thus providing a rational basis for AIDS vaccine development.

Longitudinal study of antibody reactivity against HIV-1 envelope and a peptide representing a conserved site on Gp41 in HIV-1-infected patients

This study was designed to distinguish between antibodies in HIV-1-infected patients directed against epitopes accessible on the native HIV-1 envelope (Env) complex and non-native Env epitopes. Peptide p#13 (Env. aa642-673) containing the neutralising 2F5 epitope and recombinant soluble glycoprotein 160 (rsgp160) were used in ELISA to determine the antibody (Ab) reactivity in sera of 116 HIV-1-infected individuals and 18 HIV negative controls. The reactivity of sera classified CDC stage C against p#13 was significantly decreased in comparison to stage A sera, while staying constant against rsgp160. Accordingly, in 6 out of 8 individual patients tested over time the response against p#13 was declining at later time points of infection. The reactivity of patients' sera against p#13 corresponded directly to the recognition of infected T cells and largely also to the CD4 cell count. The causal relationships of these phenomena are not clear. It is conceivable that antibodies against epitopes on HIV are lost or escape mutants arise and consequently control of HIV is lost and virus load increases as it is known for CDC stage C. Alternatively, increasing virus load may affect B cells recognising native Env epitopes and turn antibody production down by some mechanism. In this latter scenario helper T cells might have a critical role.

Phase I study of a human monoclonal antibody directed against the CD4-binding site of HIV type 1 glycoprotein 120

A phase I dose escalation study was conducted with the human monoclonal anti-gp120 antibody F105, to evaluate the safety, pharmacokinetics, and functional activity of F105 in HIV-1-infected individuals. F105 is an IgG1(kappa) antibody reactive with a discontinuous epitope that overlaps the CD4-binding site of gp120. F105 neutralizes laboratory strains of HIV-1 and some primary isolates, and synergizes with other antibodies in neutralizing an expanded spectrum of isolates. Four patients each with CD4 counts between 200 and 500/mm3 received a single dose of F105 at 100 or 500 mg/m2, intravenously. Sustained levels of F105 were obtained in plasma, and there was no evidence of an immune response to F105 as determined by a double-antigen immunoassay. No patient experienced any toxicity. Infused antibody retained full functional activity as detected by the ability of sera to block the binding of labeled F105 to HIV-1-infected cells. Of note, all patients had preexisting antibody to the gp120 CD4-binding site. The ability to culture virus by quantitative microculture remained unchanged by this single dose of antibody. Thus, it can be concluded that F105 is safe and nontoxic as a single injection at the doses tested. Furthermore, the antibody retains full gp120-binding activity. In these patients, with preexisting CD4-binding site antibody, there is no evidence of anti-HIV-1 activity following a single antibody infusion.

Antibody-dependent cellular cytotoxicity directed against cells expressing human immunodeficiency virus type 1 envelope of primary or laboratory-adapted strains by human and chimpanzee monoclonal antibodies of different epitope specificities

The characteristics of antibody-dependent cellular cytotoxicity (ADCC) directed by a panel of human and chimpanzee antienvelope (anti-Env) monoclonal antibodies (MAbs) of different epitope specificities were studied; this was accomplished by using target cells expressing human immunodeficiency virus type 1 (HIV-1) Envs of either primary or laboratory-adapted strains. Human MAbs of similar apparent affinities (1 x 10(9) to 2 x 10(9) liters/mol) against either a "cluster II"-overlapping epitope of gp41 or against the CD4 binding site, V3 loop, or C5 domain of gp120 directed substantial and comparable levels of specific lysis against targets infected with laboratory-adapted strains of HIV-1. As expected, those MAbs specific for relatively conserved regions of Env generally exhibited ADCC activity against a broader range of HIV-1 strains than those directed against variable epitopes. Significant ADCC activities of selected MAbs against primary isolate Env-expressing cells were demonstrated. In addition, a new ADCC epitope in the V2 domain of gp120 was defined. CD56+ cells were demonstrated to be the effector cells in these studies by fluorescence-activated cell sorting followed by ADCC assays. Notably, all anti-Env MAbs tested in this study, including MAbs directed against each of the known neutralization epitope clusters in gp120, directed significant levels of ADCC against targets expressing Env of one or more HIV-1 strains. These results imply that many, if not most, HIV-1-neutralizing human Abs of high affinity (> or = 3 x 10(8) liters/mol in these studies) and of the immunoglobulin G1 (IgG1) subclass (i.e., the predominate IgG subclass) are capable of directing ADCC. Since neutralizing Abs have been associated with long-term survival following HIV-1 infection, this suggests that ADCC activity may be beneficial in vivo.

A novel antibody-dependent cellular cytotoxicity epitope in gp120 is identified by two monoclonal antibodies isolated from a long-term survivor of human immunodeficiency virus type 1 infection

Two monoclonal antibodies (MAbs), 42F and 43F, were isolated some 14 months apart from a single long-term survivor of human immunodeficiency virus type 1 (HIV-1) infection. These MAbs were found to be indistinguishable in terms of their isotypes, specificities, affinities, and biological activities. Both 42F and 43F directed substantial antibody-dependent cellular cytotoxicity (ADCC) against cells infected with four divergent lab-adapted strains of HIV-1, but no neutralizing activity against these strains was detectable. The ability of MAbs 42F and 43F, as well as that of MAbs against two other gp120 epitopes, to direct ADCC against uninfected CD4+ cells to which recombinant gp120SF2 had been adsorbed (i.e., "innocent bystanders") was demonstrated to be less efficient by at least an order of magnitude than their ability to direct ADCC against HIV-1-infected cells. Flow cytometry analyses showed that 42F and 43F also bind to native primary isolate Envs from clades B and E expressed on cell surfaces. By direct binding and competition assays, it was demonstrated that the 42F/43F epitope lies in a domain of gp120 outside the previously described CD4-binding site and V3 loop ADCC epitope clusters. Immunoblot analysis revealed that the 42F/43F epitope is not dependent on disulfide bonds or N-linked glycans in gp120. Epitope mapping of 42F and 43F by binding to linear peptides demonstrated specificity of these MAbs for a sequence of 10 amino acids in the C5 domain comprising residues 491 to 500 (Los Alamos National Laboratory numbering for the HXB2 strain). Thus, 42F and 43F define a new ADCC epitope in gp120. Because of the relative conservation of this epitope and the fact that it appears to have been significantly immunogenic in the individual from which these MAbs were derived, it may prove to be a useful component of HIV vaccines. Furthermore, these MAbs may be used as tools to probe the potential importance of ADCC as an antiviral activity in HIV-1 infection.

Human monoclonal antibody 2G12 defines a distinctive neutralization epitope on the gp120 glycoprotein of human immunodeficiency virus type 1

We have isolated and characterized human monoclonal antibody 2G12 to the gp120 surface glycoprotein of human immunodeficiency virus type 1 (HIV-1). This antibody potently and broadly neutralizes primary and T-cell line-adapted clade B strains of HIV-1 in a peripheral blood mononuclear cell-based assay and inhibits syncytium formation in the AA-2 cell line. Furthermore, 2G12 possesses neutralizing activity against strains from clade A but not from clade E. Complement- and antibody-dependent cellular cytotoxicity-activating functions of 2G12 were also defined. The gp120 epitope recognized by 2G12 was found to be distinctive; binding of 2G12 to LAI recombinant gp120 was abolished by amino acid substitutions removing N-linked carbohydrates in the C2, C3, V4, and C4 regions of gp120. This gp120 mutant recognition pattern has not previously been observed, indicating that the 2G12 epitope is unusual. consistent with this, antibodies able to block 2G12 binding to recombinant gp120 were not detected in significant quantities in 16 HIV-positive human serum samples.

A humanized, bispecific immunoadhesin-antibody that retargets CD3+ effectors to kill HIV-1-infected cells

We have developed a humanized, bispecific immunoadhesin-antibody (BsIAb) that targets and kills HIV-infected cells. Comprised of CD4-IgG and humanized anti-CD3-IgG, this BsIAb is bifunctional. First, in targeting, it exploits the natural affinity of CD4 for gp120 to target the BsIAb to HIV-infected cells, and second, it recruits and activates, through its anti-CD3 moiety, cytotoxic T lymphocytes (CTL) to lyse target cells in a non-MHC restricted manner. To produce purified BsIAb from supernantants of transfected mammalian cells, we designed a three-step recovery scheme based on the structural elements of this heterotrimeric protein. The ability of purified BsIAb to specifically lyse HIV-infected target cells was demonstrated using CTL from two different sources: whole peripheral blood lymphocyte (PBL) fractions and pure CTL preparations. In contrast, a human anti-gp120 antibody mediated lysis of HIV-infected target cells only with PBL fractions and not with purified CTL. Moreover, lysis observed in the presence of the human anti-gp120 antibody was completely blocked in the presence of human serum (which competes for Fc gamma receptor binding), whereas BsIAb-mediated lysis of target cells was not affected. We measured the monovalent affinities of BsIAb for HIV-gp120 on infected cells and for CD3 epsilon on CTL. Relative to the bivalent parent molecules, CD4/gp120 affinity in the BsIAb is unchanged, whereas anti-CD3/CD3 is substantially decreased. We further demonstrated by fluorescence microscopy that physical association of CD3+ cells with gp120-expressing cells occurs only in the presence of BsIAb. Thus, the cytocidal activity of BsIAb in the presence of serum reflects its unique ability to recruit CTL as effector cells and highlights a potentially important advantage of this type of construct over antibodies for HIV-directed therapy.

Antibody-dependent complement-mediated cytotoxicity in sera from patients with HIV-1 infection is controlled by CD55 and CD59

Various immune mechanisms have been reported to contribute to the progressive destruction of Th cells in HIV-1-infected patients. Among these, complement mediated lysis of infected cells has been suggested. An increased sensitivity of lymphocytes from HIV-1-infected patients to lysis by monoclonal antibodies directed to MHC class I antigen and complement has been directly correlated with a decreased expression of the decay accelerating factor (CD55). It also has been reported that the expression of the membrane inhibitor of reactive lysis (CD59) is decreased during HIV-1 infection. We examined the effect of antibodies in the serum of HIV-1-positive individuals and normal human serum (NHS) as source of complement on several HIV-1-infected cell lines differing in their expression of CD55 and CD59. When HIV-1-infected target cells without membrane expression of CD55 and CD59 were used, a highly significant cytotoxic effect was observed in the presence of heat inactivated anti-HIV-1-positive sera and NHS, while heat-inactivated anti-HIV-1-negative sera and NHS were unable to induce cytolysis. Similar results were obtained using purified IgG isolated from HIV-1-positive sera and either NHS or guinea pig serum as source of complement. Lysis of HIV-1-infected cells correlated with expression of viral antigens on the cell surface. HIV-1-infected CD55 and CD59 positive target cells showed specific lysis, when the function of these molecules was abrogated by blocking antibodies to CD55 and CD59. The finding of anti-HIV-1-specific cytotoxic antibodies in sera from HIV-1-infected patients should be considered in the pathogenesis of the HIV-1-infection.

Functional activities of 20 human immunodeficiency virus type 1 (HIV-1)-specific human monoclonal antibodies

Antibodies that are useful in the treatment of HIV infection should result in virus neutralization or lysis of infected cells but should not enhance infection. In this study, the potential clinical use of 20 HIV-1-specific human monoclonal antibodies (HuMAbs) was determined by measuring their enhancing (C-ADE) activities using HIVLAI as the target virus. Two HuMAbs mediated both C-ADE and ADCC, two exclusively neutralized, and five exclusively mediated ADCC. Ten HuMAbs demonstrated no activity in any of the three assays. Three antibodies that neutralized HIVLAI were tested against HIVSF2; all three also neutralized HIVSF2. Four of five HuMAbs mediating ADCC against HIVLAI that were also tested against HIVSF2 had ADCC activity against HIVSF2. These results demonstrate that many HuMAbs have unique functions, allowing the separation of potentially beneficial and harmful activities. Combinations of HuMAbs with ADCC and neutralizing functions may have therapeutic utility.

Increased adhesion as a mechanism of antibody-dependent and antibody-independent complement-mediated enhancement of human immunodeficiency virus infection

Enhancement of human immunodeficiency virus (HIV) infection by complement alone or in conjunction with antibodies was studied experimentally and theoretically. Experimental studies showed that while HIV-positive sera neutralize HIV infection, the addition of fresh complement abrogated neutralization and could even cause enhancement. Enhancement was blocked by anti-complement receptor 2 antibodies, and infection under enhancing conditions could be blocked by soluble CD4. Antibody-dependent complement-mediated enhancement (C'ADE) was dependent on the alternative complement activation pathway, as factor B-deficient serum could enhance only after the addition of factor B. The observed enhancement was also antibody dependent, since the addition of antibodies increased the level of enhancement. Under C'ADE conditions, infection reached a plateau within 5 min and was not caused by activation of cells by factors in the human serum. On the contrary, preincubation of cells with complement decreased the level of enhancement. A theoretical model of HIV infection in vitro which exhibited similar enhancement in an antibody- and complement concentration-dependent way was developed. Model studies indicated that the enhanced infection process could be explained by the fact that virions, because of complement deposition on the surface, bind more efficiently to cells. The model also indicated that the saturation of the enhanced infection process seen after a few minutes could be caused by saturation of the complement receptors. The effect of neutralizing antibodies can thus be overcome by the enhancing effect of complement that facilitates the contact between gp120 and CD4. These studies demonstrate that the main features of the complement-dependent enhancement phenomenon can be understood in terms of a simple mathematical model.

Analysis of envelope glycoprotein-specific antibodies from SIV-infected and gp110-immunized monkeys in ACC and ADCC assays

Sera collected from SIV-infected or recombinant glycoprotein-immunized monkeys were characterized for antibodies participating in antibody-complement-mediated cytolysis (ACC) and antibody-dependent cellular cytolysis (ADCC) in terms of their IgG subclass and epitope specificity. In a competitive inhibition ELISA, gp110-specific antibody reactivity with nondenatured rgp110 was blocked completely by soluble homologous rgp110 and partially inhibited by heterologous rgp110, suggesting cross-reactivity between viral strains. However, only partial inhibition was observed with denatured recombinant gp140 (rgp140) in selected monkeys, indicating that the majority of gp110-specific antibodies recognized conformational epitopes. ACC activity against recombinant vaccinia-infected, envelope-expressing targets was found in sera from both infected and immunized monkeys, whereas ADCC activity was observed only in sera from infected monkeys. ACC was blocked with denatured rgp140 as well as nondenatured rgp110, indicating that ACC-mediating antibodies recognized mainly linear epitopes. In contrast, rgp140 did not compete as effectively as rgp110 in the ADCC assay, indicating that the majority of ADCC antibodies recognized conformational epitopes. Competitive inhibition using three peptide fragments of gp110 indicated that epitopes recognized by ACC antibodies lie within amino acid residues 214-471, a region that spans V3, whereas ADCC-reactive epitopes lie between amino acid residues 52 and 214 at the N-terminal end of gp110. Column chromatography of rhesus IgG resulted in three subclass-enriched fractions, designated IgG-I, IgG-II, and IgG-III. IgG-I, but not IgG-II or IgG-III, from both infected and immunized monkeys mediated ACC, whereas IgG-I and IgG-II from infected monkeys mediated ADCC.(ABSTRACT TRUNCATED AT 250 WORDS)

Uses of flow cytometry in virology

This article reviews some of the published applications of flow cytometry for in vitro and in vivo detection and enumeration of virus-infected cells. Sample preparation, fixation, and permeabilization techniques for a number of virus-cell systems are evaluated. The use of flow cytometry for multiparameter analysis of virus-cell interactions for simian virus 40, herpes simplex viruses, human cytomegalovirus, and human immunodeficiency virus and its use for determining the effect of antiviral compounds on these virus-infected cells are reviewed. This is followed by a brief description of the use of flow cytometry for the analysis of several virus-infected cell systems, including blue tongue virus, hepatitis C virus, avian reticuloendotheliosis virus, African swine fever virus, woodchuck hepatitis virus, bovine viral diarrhea virus, feline leukemia virus, Epstein-Barr virus, Autographa californica nuclear polyhedrosis virus, and Friend murine leukemia virus. Finally, the use of flow cytometry for the rapid diagnosis of human cytomegalovirus and human immunodeficiency virus in peripheral blood cells of acutely infected patients and the use of this technology to monitor patients on antiviral therapy are reviewed. Future prospects for the rapid diagnosis of in vivo viral and bacterial infections by flow cytometry are discussed.

Infectious amplification of wild-type human immunodeficiency virus from patients' lymphocytes and modulation by reverse transcriptase inhibitors in vitro

The relative in vitro potency of nine human immunodeficiency virus (HIV) type 1 reverse transcriptase inhibitors was evaluated in a coculture assay which measures the frequencies of infectious primary cells from HIV-positive patients by the limiting dilution technique and measures their apparent reduction under increasing concentrations of drugs. An advantage of this assay is that it utilizes a variety of wild-type viruses not selected by in vitro propagation. Potency ranking placed the (-)-L-enantiomer of 2',3'-dideoxy-5-fluoro-3'-thiacytidine [(-)-FTC], an oxathiolane pyrimidine nucleoside analog (90% effective concentration = 55 nM), before 2',3'-dideoxycytidine (DDC) (74 nM), (-)-2',3'-dideoxy-3'-thiacytidine (3TC) (300 nM), 3'-azido-3'-deoxythymidine (AZT) (530 nM), TIBO R82913 (670 nM), and 2',3'-dideoxyinosine (DDI) (6,400 nM). HIV from AZT-naive patients' lymphocytes was more sensitive to the inhibitory effect of (-)-FTC, 3TC, or DDC than was highly AZT-resistant HIV obtained from AZT-treated patients' cells, indicating partial cross-resistance between thymidine and cytidine analogs. Combined inhibitory concentrations of AZT with (-)-FTC, 3TC, DDC, and DDI produced synergistic interactions as determined by the multiple-drug effect analysis. Synergistic interactions were demonstrable with AZT plus (-)-FTC or with AZT plus DDC with cells bearing AZT-resistant HIV. The inhibitory concentrations of AZT established by this cell-to-cell virus transmission assay are closer than those determined by the conventional assay system to the extracellular AZT concentrations required in patients' plasma to achieve comparable levels of HIV inhibition in vivo.

Synthesis and virucidal activity of a water-soluble, configurationally stable, derivatized C60 fullerene

The bis(monosuccinimide) derivative of p,p'-bis(2-aminoethyl)diphenyl-C60 (compound 1), prepared by the fulleroid route, is active against human immunodeficiency virus type 1 (HIV-1) and HIV-2 (50% effective concentration [EC50] averaging approximately 6 microM) in acutely or chronically infected human lymphocytes and is active in vitro against 3'-azido-3'-deoxythymidine-resistant HIV-1 (EC50, approximately 3 microM). The virucidal properties of compound 1 were confirmed by virus inactivation assays. Compound 1 was noncytotoxic up to 100 microM in peripheral blood mononuclear cells and H9, Vero, and CEM cells. In cell-free assays, whereas the fullerene showed comparable activity against HIV-1 reverse transcriptase and DNA polymerase alpha (50% inhibitory concentration of approximately 5 microM), it demonstrated selective activity against HIV-1 protease.

Selective inhibition of human immunodeficiency viruses by racemates and enantiomers of cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

2',3'-Dideoxy-5-fluoro-3'-thiacytidine (FTC) has been shown to be a potent and selective compound against human immunodeficiency virus type 1 in acutely infected primary human lymphocytes. FTC is also active against human immunodeficiency virus type 2, simian immunodeficiency virus, and feline immunodeficiency virus in various cell culture systems, including human monocytes. The antiviral activity can be prevented by 2'-deoxycytidine, but not by other natural nucleosides, suggesting that FTC must be phosphorylated to be active and 2'-deoxycytidine kinase is responsible for the phosphorylation. By using chiral columns or enzymatic techniques, the two enantiomers of FTC were separated. The (-)-beta-enantiomer of FTC was about 20-fold more potent than the (+)-beta-enantiomer against human immunodeficiency virus type 1 in peripheral blood mononuclear cells and was also effective in thymidine kinase-deficient CEM cells. Racemic FTC and its enantiomers were nontoxic to human lymphocytes and other cell lines at concentrations of up to 100 microM. Studies with human bone marrow cells indicated that racemic FTC and its (-)-enantiomer had a median inhibitory concentration of > 30 microM. The (+)-enantiomer was significantly more toxic than the (-)-enantiomer to myeloid progenitor cells. The susceptibilities to FTC of pretherapy isolates in comparison with those of posttherapy 3'-azido-3'-deoxythymidine-resistant viruses in human lymphocytes were not substantially different. Similar results were obtained with well-defined 2',3'-dideoxyinosine- and nevirapine-resistant viruses. (-)-FTC-5'-triphosphate competitively inhibited human immunodeficiency virus type 1 reverse transcriptase, with an inhibition constant of 2.9 microM, when a poly(I)n.oligo(dC)19-24 template primer was used. These results suggest that further development of the (-)-Beta-enantiomer of FTC is warranted as an antiviral agent for infections caused by human immunodeficiency viruses.