Functional epitope analysis of the human CD4 molecule: antibodies that inhibit human immunodeficiency virus type 1 gene expression bind to the immunoglobulin CDR3-like region of CD4

Inhibition of HIV replication by a CD4-reactive Fab of an IgM clone isolated from a healthy HIV-seronegative individual

HIV replication is restricted by some anti-CD4 mouse mAb in vitro and in vivo. However, a human monoclonal anti-CD4 Ab has not been isolated. We screened EBV-transformed peripheral B cells from 12 adult donors for CD4-reactive Ab production followed by functional reconstitution of Fab genes. Three independent IgM Fab clones reactive specifically to CD4 were isolated from a healthy HIV-seronegative adult (approximately 0.0013% of the peripheral B cells). The germ line combinations for the VH and VL genes were VH3-33/L6, VH3-33/L12, and VH4-4/L12, respectively, accompanied by somatic hypermutations. Genetic analysis revealed a preference for V-gene usage to develop CD4-reactive Ab. Notably, one of the CD4-reactive clones, HO538-213, with an 1 x 10(-8) M dissociation constant (Kd) to recombinant human CD4, limited the replication of R5-tropic and X4-tropic HIV-1 strains at 1-2.5 microg/mL in primary mononuclear cells. This is the first clonal genetic analysis of human monoclonal CD4-reactive Ab. A mAb against CD4 isolated from a healthy individual could be useful in the intervention of HIV/AIDS.

Targeted transduction via CD4 by a lentiviral vector uses a clathrin-mediated entry pathway

We recently developed a novel targeting Sindbis virus envelope pseudotyped lentiviral vector, 2.2ZZ, which acquires specific transduction capacity by antibody conjugation and binding with specific antigens on the surface of targeted cells. Here we characterize the virological properties of this vector by examining its targeting to CD4 antigen. Our results show that entry is dependent on CD4 cell surface density and occurs via the clathrin-mediated endocytic pathway. These findings provide insight into the mechanism of infection by a new viral vector with combined properties of Sindbis virus and lentiviruses and infectivity conferred by monoclonal antibody-ligand interactions.

Recombinant anti-CD4 antibody 13B8.2 blocks membrane-proximal events by excluding the Zap70 molecule and downstream targets SLP-76, PLC gamma 1, and Vav-1 from the CD4-segregated Brij 98 detergent-resistant raft domains

The biological effects of rIgG(1) 13B8.2, directed against the CDR3-like loop on the D1 domain of CD4, are partly due to signals that prevent NF-kappaB nuclear translocation, but the precise mechanisms of action, particularly at the level of membrane proximal signaling, remain obscure. We support the hypothesis that rIgG(1) 13B8.2 acts by interfering with the spatiotemporal distribution of signaling or receptor molecules inside membrane rafts. Upon cross-linking of Jurkat T lymphocytes, rIgG(1) 13B8.2 was found to induce an accumulation/retention of the CD4 molecule inside polyoxyethylene-20 ether Brij 98 detergent-resistant membranes at 37 degrees C, together with recruitment of TCR, CD3zeta, p56 Lck, Lyn, and Syk p70 kinases, linker for activation of T cells, and Csk-binding protein/phosphoprotein associated with glycosphingolipid adaptor proteins, and protein kinase Ctheta, but excluded Zap70 and its downstream targets Src homology 2-domain-containing leukocyte protein of 76 kDa, phospholipase Cgamma1, and p95(vav). Analysis of key upstream events such as Zap70 phosphorylation showed that modulation of Tyr(292) and Tyr(319) phosphorylation occurred concomitantly with 13B8.2-induced Zap70 exclusion from the membrane rafts. 13B8.2-induced differential raft partitioning was epitope, cholesterol, and actin dependent but did not require Ab hyper-cross-linking. Fluorescence confocal imaging confirmed the spatiotemporal segregation of the CD4 complex inside rafts and concomitant Zap70 exclusion, which occurred within 10-30 s following rIgG(1) 13B8.2 ligation, reached a plateau at 1 min, and persisted until the end of the 1-h experiment. The differential spatiotemporal partitioning between the CD4 receptor and the Zap70-signaling kinase inside membrane rafts interrupts the proximal signal cross-talk leading to subsequent NF-kappaB nuclear translocation and explains how baculovirus-expressed CD4-CDR3-like-specific rIgG(1) 13B8.2 acts to induce its biological effects.

Biological activities on T lymphocytes of a baculovirus-expressed chimeric recombinant IgG1 antibody with specificity for the CDR3-like loop on the D1 domain of the CD4 molecule

A baculovirus-expressed chimeric recombinant IgG1 (rIgG1) antibody, with Cgamma1 and Ckappa human constant domains, was derived from the murine monoclonal antibody (mAb) 13B8.2, which is specific for the CDR3-like loop of the CD4 molecule and which inhibits HIV-1 replication. Chimeric rIgG1 antibody 13B8.2 blocked, in a dose-dependent manner, antigen presentation through inhibition of subsequent IL-2 secretion by stimulated T cells. The one-way mixed lymphocyte reaction was abrogated by previous addition of baculovirus-produced rIgG1 13B8.2 in the T-cell culture. Anti-proliferative activity of rIgG1 was demonstrated on CD3-activated CD4+ T lymphocytes from healthy donors, such effect being associated with reduced IL-2 secretion of activated T cells. On the other hand, no proliferation inhibition was observed on CD4+ T lymphocytes activated with phorbol ester plus ionomycin, suggesting that rIgG1 13B8.2 preferentially acts on a proximal TCR-induced signaling pathway. Treatment of DBA1/J human CD4-transgenic mice with 100 microg of recombinant antibody for three consecutive days led to in vivo recovery of rIgG1 antibody 13B8.2 both coated on murine T lymphocytes and free in mouse serum, without CD4 depletion or down-modulation. These findings predict that the baculovirus-expressed chimeric rIgG1 anti-CD4 antibody 13B8.2 is a promising candidate for immunotherapy.

The di-leucine motif in the cytoplasmic tail of CD4 is not required for binding to human immunodeficiency virus type 1 Nef, but is critical for CD4 down-modulation

The human immunodeficiency virus type 1 (HIV-1) nef gene encodes a 205 residue, myristoylated phosphoprotein that has been shown to play a critical role in the replication and pathogenesis of the virus. One of the most studied functions of the Nef protein is the down-modulation of cell surface CD4. Nef has been reported to interact with both the cytoplasmic tail of CD4 and proteins that are components of the endocytic machinery, thereby enhancing the endocytosis of CD4 through clathrin-coated pits. A di-leucine motif in the cytoplasmic tail of CD4 (residues 413/414) was reported to be essential both for Nef mediated down-modulation and for Nef binding. In order to further characterize the involvement of this di-leucine motif in CD4 down-modulation we generated a CD4 mutant in which the leucines were substituted by alanines, termed CD4(LL-AA). We demonstrate here that, contrary to previous data obtained with the cytoplasmic tail of CD4 alone, full-length CD4(LL-AA) bound to Nef both in vivo, in recombinant baculovirus-infected Sf9 cells, and in vitro. In contrast the di-leucine motif was required for both Nef-mediated and phorbol ester-induced CD4 down-modulation, suggesting that the essential requirement for the di-leucine motif in CD4 down-modulation reflects the fact that this motif is needed for the interactions of CD4 with the endocytic machinery, not for the interaction with Nef. We have also exploited the observation that CD4(LL-AA) is refractory to Nef-mediated down-modulation to provide the first experimental evidence for a physical interaction between Nef and CD4 in intact mammalian cells.

The lectin jacalin induces phosphorylation of ERK and JNK in CD4+ T cells

The CD4 molecule plays an essential role in mediating the transduction of intracellular signals by functioning as a coreceptor for the complex T cell receptor/CD3 and also acts as the primary receptor for human immunodeficiency virus (HIV). Several authors have shown evidence that jacalin, a plant lectin, binds to CD4 and inhibits in vitro HIV infection. We analyzed jacalin-induced intracellular signaling events in CD4(+) T cells and have shown that cell activation resulted in tyrosine phosphorylation of intracellular substrates p56(lck), p59(fyn), ZAP-70, p95 (vav), phospholipase C-gamma1, and ras activation, as assessed by conversion of ras guanosine 5'-diphosphate to ras guanosine 5'-triphosphate. We further examined extracellular regulated kinase (ERK) and c-jun NH(2)-terminal kinase (JNK) phosphorylation following stimulation with jacalin. The data indicate that the kinetics of JNK phosphorylation is delayed. Optimum phosphorylation of ERK2 was observed by 10 min, and that of JNK was observed by 30 min. Pretreatment with gp120 followed by stimulation with jacalin resulted in marked inhibition of all of the aforementioned intracellular events. The data presented here provide insight into the intracellular signaling events associated with the CD4 molecule-jacalin-gp120 interactions and HIV-induced CD4(+) T cell anergy. Jacalin may be used as a possible tool for the study of CD4-mediated signal transduction and HIV-impaired CD4(+) T cell activation.

Postattachment neutralization of a primary strain of HIV type 1 in peripheral blood mononuclear cells is mediated by CD4-specific antibodies but not by a glycoprotein 120-specific antibody that gives potent standard neutralization

De novo infecting HIV-1 or virus released from an infected cell in vivo attaches relatively quickly to a target cell, but the rate of fusion-entry of such virus is slow, with 50% entry taking > or =2 hr. It is thus desirable that antibodies stimulated by any vaccine or given in immunotherapy are able to neutralize not only free virus, but also virus attached to the target cell. Here we investigated postattachment neutralization (PAN) of a primary HIV-1 strain (JRCSF) in peripheral blood mononuclear cells and of a T cell line-adapted strain (IIIB) in C8166 T lymphoblastoid cells, using the highly potent gp120-specific human monoclonal b12 monoclonal IgG, and monoclonal antibodies specific for the CD4 primary cell receptor. In addition, we improved the experimental protocols of related studies by using a pulse of antibody, thus avoiding the complication of neutralizing progeny virus. We found that b12 IgG PAN was inefficient, with PAN of IIIB needing a 1000-fold greater concentration of antibody than was required for standard neutralization, and PAN of JRCSF being detected erratically only at 4 degrees C and unphysiologically high concentrations (300 microg/ml). Nonetheless, under identical conditions a 10-microg/ml pulse of the CD4-specific MAb Q4120 gave up to 99% PAN of JRCSF, and more than 95% even when added 3 hr after infection at 37 degrees C. Possible mechanisms by which PAN by CD4- specific antibodies is mediated are discussed. We suggest that such anti-CD4 antibodies should be considered as a component of HIV-1 immunotherapy.

Sendai virus-based production of HIV type 1 subtype B and subtype E envelope glycoprotein 120 antigens and their use for highly sensitive detection of subtype-specific serum antibodies

We previously described a Sendai virus (SeV)-based expression system for the recombinant gp120 of HIV-1 subtype B (rgp120-B), which has permitted the production of antigenetically and functionally authentic gp120 at a concentration as high as 6 microg/ml of culture supernatant (Yu D et al.: Genes Cells 1997;2:457-466). Here the same procedure was successfully applied to the production of HIV-1 subtype E gp120 (rgp120-E). The remarkable production of the proteins by the SeV expression system enabled us to use crude culture supernatants for serological and functional studies of gp120s. The immunological authenticity of rgp120-E was verified by patient sera and anti-V3 loop monoclonal antibodies specific for HIV-1 subtypes B and E. CD4-binding properties were corroborated by FACS analyses. The rgp120s were then used in an enzyme immunoassay (rgp120-EIA) to detect antibodies in the sera of HIV-1-infected individuals, and the performance was assessed in comparison with a conventional V3 loop peptide EIA (V3-EIA). The initial evaluation of a serum panel (n = 164) consisting of 76 subtype E and 88 subtype B sera revealed that the rgp120-EIA was nearly 1000-fold more sensitive than the V3-EIA and was able to detect subtype-specific antibody with 100% sensitivity and with a complete correlation with the genotypes, whereas the V3-EIA failed to detect 9 and 24% of the same subtype E and B sera, respectively. Furthermore, a study employing a panel of 28 international sera with known genotypes (HIV-1 subtypes A through F) confirmed the remarkable specificity of this method. An EIA reactivity higher than 1.0 was an unambiguous predictor of HIV-1 subtype E and B infections. The data imply the presence of strong subtype-specific epitopes for antibody bindings to these rgp120s.

Selective activity of various antiviral compounds against HHV-7 infection

Human herpesvirus virus type 7 (HHV-7) is a T-lymphotropic herpesvirus which uses the CD4 receptor as main receptor to infect its target cells. Measuring the decrease of CD4 expression during HHV-7 infection is a convenient and accurate method to monitor the efficacy of antiviral agents against HHV-7 infection. Different classes of compounds, such as heparin, pentosan polysulfate (PS), dextran sulfate (DS), aurintricarboxylic acid (ATA), phosphonoformic acid (PFA), 9-(2-phosphonylmethoxyethyl)adenine (PMEA), 2-amino-7-[(1,3-dihydroxy-2-propoxy) methyl] purine (S2242), polyvinylalcohol sulfate (PVAS) and the co-polymer of vinylalcohol sulfate with acrylic acid (PAVAS), acyclovir (ACV), ganciclovir (GCV), penciclovir (PCV), brivudin (BVDU), cidofovir (HPMPC), lobucavir, (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine] (H2G), (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) and sorivudine (BVaraU), were evaluated for their anti-HHV-7 activity in the SupT1 T cell line and in purified CD4+ T lymphocytes. Antiviral activity was monitored by inhibition of: (i) CD4 expression down-regulation; (ii) giant cell formation and (iii) apoptosis induction. In general, PS, DS, PVAS, PAVAS, ATA, PFA, PMEA, S2242, lobucavir and HPMPC had comparable anti-HHV-7 activity in the two cell lines, irrespective of the parameters followed to monitor antiviral activity. One of the exceptions was heparin which had an IC50 of 9.6 microg/ml in SupT1 cells and >250 microg/ml in CD4+ T lymphocytes. The compounds PCV, GCV, H2G and PMPA showed some activity in CD4+ T lymphocytes, but not in SupT1 cells. ACV, BVDU and BVaraU did not show activity in either cell system. None of the chemokines tested, such as platelet factor-4 (PF-4), eotaxin, stromal cell-derived factor 1alpha(SDF-1alpha) and RANTES, had detectable activity against HHV-7. In contrast, the HIV-1 envelope glycoprotein, gp120, and the two anti-CD4 mAbs, 13B8-2 and OKT4, were clearly active against HHV-7 infection.

HIV-1 reactivation in resting peripheral blood mononuclear cells of infected adults upon in vitro CD4 cross-linking by ligands of the CDR2-loop in extracellular domain 1

HIV-1 infects resting peripheral blood mononuclear cells (PBMCs) but remains inactive state until subsequent cell activation. We have demonstrated that the cross-linking of cell surface CD4 by gp120-anti-gp120 immune complexes or heat-inactivated HIV-1 (iHIV-1) is sufficient to trigger activation signals leading to virus reactivation (9). In this study, we demonstrate that NF-kappaB nuclear translocation and stimulation of virus production by iHIV-1 were strictly linked to the concentrations of viral proteins used as exogenous stimuli. Moreover, we further investigated the physiologic relevance of these observations. When submitted to an in vitro CD4 cross-linking by iHIV-1, PBMCs from HIV-1-infected patients were found to produce virus. This viral reactivation was associated with increased NF-kappaB nuclear translocation in patients' PBMCs. Additionally, virus reactivation in resting PBMCs infected in vitro with HIV-1 was found to be specifically induced by ligands of the CDR2-loop in domain 1 (D1) of CD4 (virus envelope and anti-CD4 monoclonal antibodies). In contrast, virus reactivation was not observed following CD4 oligomerization by antibodies that bind other epitopes in D1, including the D1/CDR3-loop. Finally, soluble CD4 (sCD4) prevented virus reactivation by D1/CDR2-loop ligands. Our results indicate that the signaling events initiated in PBMCs by oligomerization of CD4 at the D1/CDR2-loop can trigger HIV-1 upregulation in infected individuals.

Synthetic peptides derived from the variable regions of an anti-CD4 monoclonal antibody bind to CD4 and inhibit HIV-1 promoter activation in virus-infected cells

The monoclonal antibody (mAb) ST40, specific for the immunoglobulin complementarity-determining region (CDR) 3-like loop in domain 1 of the CD4 molecule, inhibits human immunodeficiency virus type 1 (HIV-1) promoter activity and viral transcription in HIV-infected cells. To design synthetic peptides from the ST40 paratope that could mimic these biological properties, a set of 220 overlapping 12-mer peptides frameshifted by one residue, corresponding to the deduced ST40 amino acid sequence, was synthesized by the Spot method and tested for binding to recombinant soluble CD4 antigen. Several peptides that included in their sequences amino acids from the CDRs of the antibody and framework residues flanking the CDRs were found to bind soluble CD4. Eleven paratope-derived peptides (termed CM1-CM11) were synthesized in a cyclic and soluble form. All the synthetic peptides showed CD4 binding capacity with affinities ranging from 1.6 to 86.4 nM. Moreover, peptides CM2, CM6, CM7, CM9, and CM11 were able to bind a cyclic peptide corresponding to the CDR3-like loop in domain 1 of CD4 (amino acids 81-92 of CD4). Peptide CM9 from the light chain variable region of mAb ST40 and, to a lesser extent, peptides CM2 and CM11 were able to inhibit HIV-1 promoter long terminal repeat-driven beta-galactosidase gene expression in the HeLa P4 HIV-1 long terminal repeat beta-galactosidase indicator cell line infected with HIV-1. The binding of mAb ST40 to CD4 was also efficiently displaced by peptides CM2, CM9, and CM11. Our results indicate that the information gained from a systematic exploration of the antigen binding capacity of synthetic peptides from immunoglobulin variable sequences can lead to the identification of bioactive paratope-derived peptides of potential pharmacological interest.

Molecular and cellular analysis of human immunodeficiency virus-induced apoptosis in lymphoblastoid T-cell-line-expressing wild-type and mutated CD4 receptors

We have previously shown that the presence of the CD4 cytoplasmic tail is critical for human immunodeficiency virus (HIV)-induced apoptosis (J. Corbeil, M. Tremblay, and D. D. Richman, J. Exp. Med. 183:39-48, 1996). We have pursued our investigation of the role of the CD4 transduction pathway in HIV-induced apoptosis. To do this, wild-type and mutant forms of the CD4 cytoplasmic tail were stably expressed in the lymphoblastoid T-cell line A2.01. Apoptosis was prevented when CD4 truncated at residue 402 was expressed; however, cells expressing mutated receptors that do not associate with p56(lck) (mutated at the dicysteine motif and truncated at residue 418) but which conserved proximal domains of the cytoplasmic tail underwent apoptosis like wild-type CD4. The differences between wild-type and mutated receptors in the induction of apoptosis were not related to levels of p56(lck) or NF-kappaB activation. Initial signaling through the CD4 receptor played a major role in the sensitization of HIV-infected T cells to undergo apoptosis. Incubation of HIV-infected cells with monoclonal antibody (MAb) 13B8-2, which binds to CD4 in a region critical for dimerization of the receptor, prevented apoptosis without inhibiting HIV replication. Moreover, the apoptotic process was not related to Fas-Fas ligand interaction; however, an antagonistic anti-Fas MAb (ZB-4) enhanced apoptosis in HIV-infected cells without inducing apoptosis in uninfected cells. These observations demonstrate that CD4 signaling mediates HIV-induced apoptosis by a mechanism independent of Fas-Fas ligand interaction, does not require p56(lck) signaling, and may involve a critical region for CD4 dimerization.

An anti-CD4 (CDR3-loop) monoclonal antibody inhibits human immunodeficiency virus type 1 envelope glycoprotein-induced apoptosis

Inhibition of human immunodeficiency virus type 1 (HIV-1)-inducing programmed cell death (PCD) by anti-CD4 monoclonal antibodies (mAbs) was investigated using DNA intercalant YOPRO-1 assay. We found that 13B8.2, an mAb that binds the CDR3-like loop in domain 1 (D1) of CD4, protected infected CEM cell cultures against HIV-1-induced PCD. Protection was not observed using another anti-CD4 mAb (BL4) that binds D1-D2, suggesting that the mechanism involved in cell protection against HIV-1-induced PCD requires engagement of precise CD4 epitopes. Because 13B8.2 is known to inhibit syncytia formation and virus transcription, this mAb could inhibit HIV-1-induced PCD by (1) inhibiting virus gene expression, (2) preventing viral envelope-CD4 interaction, and/or (3) interfering with apoptotic signals. Our data indicated that the absence of enhanced PCD in infected cell cultures treated with 13B8.2 mAb probably was the result of inhibition of HIV-1 replication and virus spread. Moreover, 13B8.2 mAb was found to inhibit PCD mediated by membrane-expressed HIV-1 envelope glycoproteins. Finally, we found that 13B8.2 mAb displayed no protective interference with apoptotic signal induced by Fas, dexamethasone, and serum withdrawal.

A novel complex of proteins binds the HIV-1 promoter upon virus interaction with CD4

HIV-1Lai13EM is a mutant isolate which is less sensitive than the parental HIV-1Lai strain to an in vitro treatment with 13B8-2 anti-CD4 monoclonal antibody (mAb) that generally inhibits transcription of HIV-1 and HIV-2. In contrast to other clade B viruses, this isolate carries a point mutation G > A at position -188 of the viral promoter. The fact that HIV-1NDK, a clade D virus insensitive to 13B8-2 mAb, also carries an A nucleotide at this position has brought our attention to the sequence surrounding position -188. Here we analyzed whether a DNA-binding molecule interacts with this region. Electrophoretic mobility shift assays performed with the -201/-175 HIV-1Lai wild-type sequence or the sequence containing a point mutation G > A at position -188 demonstrated their ability to bind a heterotrimeric complex induced in CEM cells by stimulation with heat-inactivated HIV-1.

The Ick protein tyrosine kinase is not involved in antibody-mediated CD4 (CDR3-loop) signal transduction that inhibits HIV-1 transcription

Monoclonal antibodies (mAb) that bind to the immunoglobulin CDR3-like region in the D1 domain of the CD4 molecule can inhibit the HIV-1 life cycle in CD4-positive T cells and lymphoblastoid cell lines at the stage of transcription. This antiviral effect requires the integrity of the cytoplasmic tail of CD4 which is known to act as a signal transduction region through its association with the protein tyrosine kinase (PTK) p56lck. In this study, we investigated the putative role of this PTK in transducing inhibitory signals that act on HIV-1 replication after triggering by anti-CDR3-like region antibody treatment of infected T cell lines. CEM (CD4+/p56lck + inducible), MT2 (CD4+/p56lck - repressed), HSB-2 (CD4-/p56lck + constitutively), HSB-2 WTCD4 (CD4+/p56lck + constitutively), HSB-2 CD4.402 (CD4+ truncated form which lacks the cytoplasmic domain/p56lck + constitutively), and HSB-2 CD4mut (CD4+ unable to bind lck/p56lck + constitutively) were exposed to HIV-1 and cultured in medium supplemented with an anti-CDR3-like region-specific antibody or a control anti-CD4 mAb which does not inhibit HIV-1 transcription. We found that CDR3-loop-mediated inhibitory signals are efficiently transduced in CD4-positive cells which demonstrate a constitutive activation of p56lck or in CD4-positive cells lacking p56lck expression. Moreover, inhibitory signals were transduced in HSB-2 CD4mut cells expressing a cell surface CD4 with a double cysteine mutation in its cytoplasmic tail that renders the molecule unable to bind p56lck, but not HSB-2 CD4.402 cells expressing a truncated form of CD4 which lacks the cytoplasmic domain. These results indicate that the p56lck plays no direct role in this process and suggests the existence of another signaling partner for CD4.

Inhibition of HIV-1 replication by a monoclonal antibody directed toward the complementarity determining region 3-like domain of CD4 in CD45 expressing and CD45-deficient cells

Monoclonal antibodies directed toward the complementarity determining region (CDR)3-like loop of the aminoterminal domain of CD4 have been shown to inhibit the replication of human immunodeficiency virus (HIV) in CD4 positive T cells. The mechanism of action of these antibodies is not yet elucidated, although several observations suggest that they inhibit viral transcription by signal transduction through the CD4 molecule, potentially implicating the activation of a protein tyrosine kinase (PTK) cascade. Since CD45 is the major protein tyrosine phosphatase associated to the plasma membrane in T cells, and has been shown to regulate the activity of several PTK, we postulated that CD45 may be necessary for the inhibitory action of the CDR3-like specific anti-CD4 antibodies. Therefore we tested the effect of one such anti-CD4 monoclonal antibody, 13B8.2, in repressing HIV replication in CD45 positive cell lines and CD45 deficient variants. Our data show that cells respond to 13B8.2 postinfection treatment regardless of CD45 expression, indicating that neither CD45 nor PTK regulated by CD45 are implicated in the mechanism of action of this antibody.

In vitro selection of HIV-1 resistant to an anti-CD4 monoclonal antibody that inhibits virus transcription

Phase I studies using monoclonal antibodies (mAbs) that bind to the Ig-CDR3-like loop in domain 1 of CD4 (e.g., 13B8-2 mAb) have already been documented for HIV-1-infected patients. In vitro, such mAbs do not inhibit virus to cell fusion but are able to inhibit virus envelope-mediated syncytia formation. Moreover, these mAbs inhibit Tat-induced activation of HIV-1 promoter and HIV-1 transcription in infected CD4+ cells. Here, we report the selection of escaped mutant virus or viruses derived from HIV-1Lai capable of replicating in vitro in the presence of concentrations of 13B8-2 mAb, that usually inhibit HIV-1Lai particle production. The escaped mutant virus or viruses, termed HIV-1Lai13EM, kept the major enzymatic restriction sites found in HIV-1Lai and remained sensitive to anti-CD4 mAb-, soluble CD4-, and recombinant gp120-mediated inhibition of syncytia formation. Possible genetic changes affecting the tat gene or the 5' long terminal repeat (LTR) were investigated. Partial sequence analysis of HIV-1Lai13EM and a control HIV-1Lai grown for 85 days in CEM cells, demonstrated that the first tat exon of these two viruses encoded identical proteins. Although a point mutation G>A was frequently encountered (6 of 13 sequences) in the LTRs of HIV-1Lai13EM at position -188 within the negative regulatory element (NRE), this mutation did not confer the escape mutant phenotype. Our study indicates that the mutant phenotype probably requires genetic changes in a region or regions outside the LTRs.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

Upregulation of human immunodeficiency virus (HIV) replication by CD4 cross-linking in peripheral blood mononuclear cells of HIV-infected adults

This study was conducted with peripheral blood mononuclear cells from 67 human immunodeficiency virus (HIV)-infected adults. It supports the hypothesis that cross-linking of CD4 molecules by HIV gp120 can result in HIV upregulation and spread of infection. Underlying mechanisms include activation of latent infection by factors in addition to, or other than, tumor necrosis factor alpha.

Blocking of feline immunodeficiency virus infection by a monoclonal antibody to CD9 is via inhibition of virus release rather than interference with receptor binding

A monoclonal antibody, MAb vpg15, inhibits feline immunodeficiency virus (FIV) infection in tissue culture. The antibody is directed to a determinant of the feline cell surface marker, CD9, implying that CD9 may serve as a viral receptor or coreceptor in this system. In cells expressing CD9, MAb vpg15 markedly delayed acute virus infection in terms of reverse transcriptase activity detected in cell culture supernatants. This effect was evident if the antibody was added before, immediately after, or 24 h after virus infection. Binding experiments showed that MAb vpg15 did not block virus binding to the cells. PCR analyses at various intervals postinfection also indicated that MAb vpg15 did not block virus uptake, reverse transcription of viral RNA, or integration into host cell DNA. Multiply spliced mRNAs were detected up to 24 h postinfection in both control and MAb vpg15-treated cells. However, viral mRNAs were markedly diminished in MAb vpg15-treated cells after this time, consistent with a failure of the FIV infection to spread in the cell culture. Treatment of chronically infected cells with MAb vpg15 also caused a sharp diminution in viral particle production, while viral mRNA levels were the same in both untreated and MAb-treated infected cells. Analyses of intracellular and extracellular levels of virus-associated antigens showed an enhanced accumulation of intracellular p24. These findings are consistent with the interpretation that MAb vpg15 acts at a posttranscriptional stage by interfering with the assembly and/or release of virus from the cell.

Transduction of activation signal that follows HIV-1 binding to CD4 and CD4 dimerization involves the immunoglobulin CDR3-like region in domain 1 of CD4

The role of CD4 during the human immunodeficiency virus type 1 (HIV-1) life cycle in T cells is not restricted to binding functions. HIV-1 binding to CD4 also triggers signals that lead to nuclear translocation of NF-kappaB and are important to the productive infection process. In addition to its cytoplasmic tail, in the ectodomain, the immunoglobulin (Ig) CDR3-like region of CD4 domain 1 seemed to play a role in this cascade of signals. We demonstrate in this work that the structural integrity of the CDR3-like loop is required for signal transduction. Substitutions of negatively charged residues by positively charged residues within the CDR3-like loop either inhibited NF-kappaB translocation after HIV-1 and gp120-anti-gp120 immune complexes binding to E91K,E92K mutants or induced its constitutive activation for E87K,D88K mutants. Moreover, A2.01-3B cells expressing the E91K,E92K mutant exhibited a lower HIV-1Lai replication. These cells, however, expressed p56(lck), demonstrated NF-kappaB translocation upon PMA stimulation, bound HIV-1Lai envelope glycoprotein with high affinity, and contained HIV-1 DNA 24 h after exposure to virus. E91K, E92K, and E87K,D88K mutant CD4 molecules were unable to bind a CD4 synthetic aromatically modified exocyclic, CDR3.AME-(82-89), that mimics the CDR3-like loop structure and binds to native cell surface CD4. This result together with molecular modeling studies indicates that the CDR3.AME-(82-89) analog binds to the CDR3-like loop of CD4 and strongly suggests that this region represents a site for CD4 dimerization. The negative charges on the CDR3-like loop thus appear critical for CD4-mediated signal transduction most likely related to CD4 dimer formation.

Binding of HIV-1 to its receptor induces tyrosine phosphorylation of several CD4-associated proteins, including the phosphatidylinositol 3-kinase

Cell surface CD4 molecules are known to be important in several physiological responses of T lymphocytes. The use of human immunodeficiency virus (HIV) particles or purified gp120 molecules as CD4 cross-linking agents has been shown to result in a cascade of intracellular biochemical events. In addition, we and other have provided evidence suggesting that virus-mediated CD4 multimerization can lead to modulation of HIV-1 long terminal repeat-dependent activity and virus production. We were thus interested in measuring the effect of HIV-1 particles on intracellular tyrosine-phosphorylation levels, mostly of CD4-associated proteins. Using the T cell line CEM-T4, we observed that HIV-1 induces an increase in tyrosine phosphorylation of four major proteins physically complexed to the CD4 molecule. Immunoblot analysis permitted the identification of two of these proteins, p56lck and phosphatidylinositol 3-kinase (PI 3-kinase) p85 alpha. No concomitant variation in the level of these two CD4-associated proteins was observed after HIV-1-induced CD4 cross-linking. To our knowledge, this is the first report linking HIV-1-mediated CD4 multimerization to an increase in tyrosine phosphorylation of the PI 3-kinase complex. The four CD4-associated molecules described in this report are most likely implicated in virus-induced CD4-linked signaling events.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.

Inhibition of human immunodeficiency virus type 1 production in infected peripheral blood mononuclear cells by human leukocyte antigen class I-specific antibodies: evidence for a novel antiviral mechanism

A well-characterized mechanism by which anti-HLA class I monoclonal antibodies (MAb) inhibit human immunodeficiency virus type 1 (HIV-1) propagation in in vitro cell cultures is the neutralization of the virus through interactions with HLA molecules associated with the virion envelope. Yet, the possibility that another mechanism of inhibition might affect a postbinding stage of the virus life cycle has been strongly suggested by our previous investigations. To demonstrate that the interaction of MAb B1-1G6 with the light chain of cell surface-expressed HLA class I molecules inhibits a postbinding step of the HIV-1 life cycle, peripheral blood mononuclear cells (PBMCs) were exposed to viruses grown in HLA class I-negative, CD4-positive cells (these viruses, which did not carry HLA class I molecules, cannot be neutralized by anti-HLA MAb during the first round of infection), and PCR was used at various times postexposure to search for the different forms of HIV-1 DNA and RNA in virus-exposed PBMCs cultured in either the presence or [correction of] absence of MAb B1-1G6. Although viral DNA was found in MAb B1-1G6-treated cells, spliced HIV-1 mRNA could not be detected in those cells. In contrast, HIV-1 gene expression was found in HIV-1-infected PBMCs treated with B9-12-1, another HLA class I-specific MAb which prevents infection of cells by cell-free viruses but which fails to inhibit cell-to-cell transmission of HIV-1. These results highlight a second antiviral mechanism by which anti-HLA MAb inhibit in vitro HIV-1 propagation.

Selective effects of electrostatic changes in the CD4 CDR-3-like loop on infection by different human immunodeficiency virus type 1 isolates

The role of the CDR-3-like loop of the first domain of the CD4 molecule in infection by the human immunodeficiency virus type 1 (HIV-1) is controversial. In an attempt to determine whether the strong negative charge in the CDR-3-like loop influences HIV-1 infection we have substituted by mutagenesis negative for positively charged residues at position 87/88 and 91/92. These mutations were shown to have no obvious effect on CD4 conformation outside of the CDR-3-like loop. Infection of cells expressing the E87K/D88K substitution mutant resulted in a selective reduction in infectivity for certain HIV-1 viruses compared to cells expressing wile-type CD4. Viruses Hx10, HxB2, and MN were 4- to 13-fold less efficient at infecting the E87K/D88K mutant, whereas SF2, RF, and NDK yielded an efficiency of infection similar to, or slightly greater than, that of the wild type. To investigate the step at which infectivity was selectively reduced, we compared early events in the life cycles of Hx10 and SF2 viruses using PCR entry and gp120-binding assays. Both gp120 binding and virus entry were reduced for Hx10 on the mutant CD4-expressing cells as compared to wild-type CD4-expressing cells, whereas no difference was seen in either assay with SF2. Although relatively small in magnitude, the contribution of the CDR-3-like loop to the overall CD4-gp120 interaction may serve to modify the binding and entry of certain virus isolates.

Envelope glycoproteins of human immunodeficiency virus type 1: profound influences on immune functions

Infection by human immunodeficiency virus type 1 (HIV-1) leads to progressive destruction of the CD4+ T-cell subset, resulting in immune deficiency and AIDS. The specific binding of the viral external envelope glycoprotein of HIV-1, gp120, to the CD4 molecules initiates viral entry. In the past few years, several studies have indicated that the interaction of HIV-1 envelope glycoprotein with cells and molecules of the immune system leads to pleiotropic biological effects on immune functions, which include effects on differentiation of CD34+ lymphoid progenitor cells and thymocytes, aberrant activation and cytokine secretion patterns of mature T cells, induction of apoptosis, B-cell hyperactivity, inhibition of T-cell dependent B-cell differentiation, modulation of macrophage functions, interactions with components of complement, and effects on neuronal cells. The amino acid sequence homologies of the envelope glycoproteins with several cellular proteins have suggested that molecular mimicry may play a role in the pathogenesis of the disease. This review summarizes work done by several investigators demonstrating the profound biological effects of envelope glycoproteins of HIV-1 on immune system cells. Extensive studies have also been done on interactions of the viral envelope proteins with components of the immune system which may be important for eliciting a "protective immune response." Understanding the influences of HIV-1 envelope glycoproteins on the immune system may provide valuable insights into HIV-1 disease pathogenesis and carries implications for the trials of HIV-1 envelope protein vaccines and immunotherapeutics.

Repression of human immunodeficiency virus type 1 long terminal repeat-driven gene expression by binding of the virus to its primary cellular receptor, the CD4 molecule

We have previously postulated that the binding of the human immunodeficiency virus type 1 (HIV-1) to cell surface CD4 induces signal transduction pathways that down-modulate production of progeny virions in acutely infected T cells (M. Tremblay, S. Meloche, S. Gratton, M. A. Wainberg, and R.-P. Sékaly, EMBO J. 13:774-783, 1994). To evaluate the possibility that CD4 cross-linking might indeed affect viral gene expression, we have introduced a molecular construct made of the luciferase reporter gene placed under the control of the regulatory elements of HIV-1 in several CD4-positive T-cell lines. We found that cross-linking of CD4 with defective HIV-1 particles and heat-inactivated viruses inhibits long terminal repeat-dependent luciferase expression. Experiments revealed that the gp120-CD4 interaction was necessary to repress HIV-1 long terminal repeat-dependent luciferase activity. The cytoplasmic domain of CD4 was also found to be required for this effect to occur. The virus-mediated signal transduction was shown to be mediated via p56lck-dependent and -independent pathways. These results indicate that the earliest event in the HIV-1 replicative cycle, namely, the binding of the virus to its cellular receptor, can lead to signal transduction culminating in down-modulation of viral gene expression. Thus we propose that defective viruses could regulate the pathogenesis of HIV disease as they constitute the vast majority of circulating HIV-1 particles.

Structural and Functional Properties of HIV-1(GER) TAR Sequences

Sequencing of HIV-1(GER) long terminal repeat (LTR) has demonstrated, for the first time in an HIV-1 primary isolate, a TAR duplication referred to as TAR1 (nucleotides +1 through +68) and TAR2 (nucleotides +69 through +136). This TAR duplication is stable during replication of HIV-1(GER) isolate in CEM cells. Analysis of LTR-CAT reporter constructs demonstrated that under Tat transactivation the HIV-1(GER)/LTR (containing TAR1 and TAR2) was expressed at a higher level than a similar construct (HIV-1(GER)DeltaTAR) containing a single TAR sequence. Among the two transcription initiation sites found in the HIV-1(GER)/LTR, only the most 5' start site was shown to be functionally active. The predicted secondary structure of the 5'-end mRNAs of HIV-1(GER) suggests it may fold into a double TAR hairpin which resembles that of HIV-2. Finally, HIV-1(GER) Tat protein shows primary sequence similarity with Tat proteins from other isolates of HIV-1 and is apparently unrelated to HIV-2 Tat proteins. This work provides the first evidence of a TAR sequence duplication in HIV-1 which increases the efficiency of transactivation by Tat. Copyright 1996 S. Karger AG, Basel

The cytoplasmic tail of CD4 is required for inhibition of human immunodeficiency virus type 1 replication by antibodies that bind to the immunoglobulin CDR3-like region in domain 1 of CD4

Monoclonal antibodies (MAb) directed against the immunoglobulin complementary determining region 3 (CDR3)-like region of the CD4 molecule inhibit human immunodeficiency virus type 1 (HIV-1) transcription. We report here data showing that the cytoplasmic tail of CD4 is required for such inhibition to be achieved. To this aim, we studied the effect of MAb 13B8-2 treatment on (i) HIV-1 production in A2.01 cells, which express different forms of the CD4 gene, (ii) Tat-induced HIV-1 promoter activation, and (iii) mitogen-activated protein kinase (MAPK) activation, which is induced in CD4-positive cells by HIV-1 cross-linking of CD4. Inhibition of HIV production by 13B8-2 MAb treatment was consistently observed in cells expressing wild-type CD4 and cells expressing a hybrid CD4-CD8 molecule (amino acids 1 to 177 of CD4 fused to the hinge, transmembrane, and cytoplasmic domains of CD8). However, no delay in HIV-1 production was observed in cells expressing a truncated CD4 which lacks the cytoplasmic domain (CD4.401). Chloramphenicol acetyltransferase assays demonstrated that Tat-dependent activation of the HIV-1 long terminal repeat promoter was inhibited by MAb 13B8-2 in A2.01/CD4 and A2.01/CD4-CD8 but not in A2.01/CD4.401 cells. Finally, we found that MAb 13B8-2 treatment inhibited the activation of MAPK induced in A2.01/CD4 and A2.01/CD4-CD8 following cross-linking of CD4 by HIV-1.