Expression of human immunodeficiency virus type 1 (HIV-1) gag antigens on the surface of a cell line persistently infected with HIV-1 that highly expresses HIV-1 antigens

Cell surface nucleocapsid protein expression: A betacoronavirus immunomodulatory strategy

We recently reported that SARS-CoV-2 nucleocapsid (N) protein is abundantly expressed on the surface of both infected and neighboring uninfected cells, where it enables activation of Fc receptor-bearing immune cells with anti-N antibodies (Abs) and inhibits leukocyte chemotaxis by binding chemokines (CHKs). Here, we extend these findings to N from the common cold human coronavirus (HCoV)-OC43, which is also robustly expressed on the surface of infected and noninfected cells by binding heparan sulfate/heparin (HS/H). HCoV-OC43 N binds with high affinity to the same set of 11 human CHKs as SARS-CoV-2 N, but also to a nonoverlapping set of six cytokines. As with SARS-CoV-2 N, HCoV-OC43 N inhibits CXCL12β-mediated leukocyte migration in chemotaxis assays, as do all highly pathogenic and common cold HCoV N proteins. Together, our findings indicate that cell surface HCoV N plays important evolutionarily conserved roles in manipulating host innate immunity and as a target for adaptive immunity.

Immunogenicity and In Vivo Protective Effects of Recombinant Nucleocapsid-Based SARS-CoV-2 Vaccine Convacell®

The vast majority of SARS-CoV-2 vaccines which are licensed or under development focus on the spike (S) protein and its receptor binding domain (RBD). However, the S protein shows considerable sequence variations among variants of concern. The aim of this study was to develop and characterize a SARS-CoV-2 vaccine targeting the highly conserved nucleocapsid (N) protein. Recombinant N protein was expressed in Escherichia coli, purified to homogeneity by chromatography and characterized by SDS-PAGE, immunoblotting, mass spectrometry, dynamic light scattering and differential scanning calorimetry. The vaccine, formulated as a squalane-based emulsion, was used to immunize Balb/c mice and NOD SCID gamma (NSG) mice engrafted with human PBMCs, rabbits and marmoset monkeys. Safety and immunogenicity of the vaccine was assessed via ELISA, cytokine titer assays and CFSE dilution assays. The protective effect of the vaccine was studied in SARS-CoV-2-infected Syrian hamsters. Immunization induced sustainable N-specific IgG responses and an N-specific mixed Th1/Th2 cytokine response. In marmoset monkeys, an N-specific CD4⁺/CD8⁺ T cell response was observed. Vaccinated Syrian hamsters showed reduced lung histopathology, lower virus proliferation, lower lung weight relative to the body, and faster body weight recovery. Convacell^® thus is shown to be effective and may augment the existing armamentarium of vaccines against COVID-19.

Cell Surface Nucleocapsid Protein Expression: A Betacoronavirus Immunomodulatory Strategy

We recently reported that SARS-CoV-2 Nucleocapsid (N) protein is abundantly expressed on the surface of both infected and neighboring uninfected cells, where it enables activation of Fc receptor-bearing immune cells with anti-N antibodies (Abs) and inhibits leukocyte chemotaxis by binding chemokines (CHKs). Here, we extend these findings to N from the seasonal human coronavirus (HCoV)-OC43, which is also robustly expressed on the surface of infected and non-infected cells by binding heparan-sulfate/heparin (HS/H). HCoV-OC43 N binds with high affinity to the same set of 11 human CHKs as SARS-CoV-2 N, but also to a non-overlapping set of 6 cytokines (CKs). As with SARS-CoV-2 N, HCoV-OC43 N inhibits CXCL12β-mediated leukocyte migration in chemotaxis assays, as do all highly pathogenic and endemic HCoV N proteins. Together, our findings indicate that cell surface HCoV N plays important evolutionary conserved roles in manipulating host innate immunity and as a target for adaptive immunity.

On the Need to Determine the Contribution of Anti-Nucleocapsid Antibodies as Potential Contributors to COVID-19 Convalescent Plasma Efficacy

Historically the therapeutic potential of polyclonal passive immunotherapies in viral diseases has been related to antiviral neutralizing antibodies, but there is also considerable evidence that non-neutralizing antibodies can translate into clinical benefit as well. In the setting of SARS-CoV-2 infection, we review here in vitro and in vivo evidence supporting a contributing role for anti-nucleocapsid antibodies. Retrospective investigation of anti-nucleocapsid antibody levels in randomized clinical trials of COVID-19 convalescent plasma is warranted to better understand whether there is an association with efficacy or lack thereof.

Cell surface SARS-CoV-2 nucleocapsid protein modulates innate and adaptive immunity

SARS-CoV-2 nucleocapsid protein (N) induces strong antibody (Ab) and T cell responses. Although considered to be localized in the cytosol, we readily detect N on the surface of live cells. N released by SARS-CoV-2-infected cells or N-expressing transfected cells binds to neighboring cells by electrostatic high-affinity binding to heparan sulfate and heparin, but not other sulfated glycosaminoglycans. N binds with high affinity to 11 human chemokines, including CXCL12β, whose chemotaxis of leukocytes is inhibited by N from SARS-CoV-2, SARS-CoV-1, and MERS-CoV. Anti-N Abs bound to the surface of N-expressing cells activate Fc receptor-expressing cells. Our findings indicate that cell surface N manipulates innate immunity by sequestering chemokines and can be targeted by Fc-expressing innate immune cells. This, in combination with its conserved antigenicity among human CoVs, advances its candidacy for vaccines that induce cross-reactive B and T cell immunity to SARS-CoV-2 variants and other human CoVs, including novel zoonotic strains.

Cell Surface SARS-CoV-2 Nucleocapsid Protein Modulates Innate and Adaptive Immunity

SARS-CoV-2 nucleocapsid protein (N) induces strong antibody and T cell responses. Although considered to be localized in the cytosol, we readily detect N on the surface of live cells. N released by SARS-CoV-2 infected cells or N-expressing transfected cells binds to neighboring cells by electrostatic high-affinity binding to heparan sulfate and heparin, but not other sulfated glycosaminoglycans. N binds with high affinity to 11 human chemokines, including CXCL12β, whose chemotaxis of leukocytes is inhibited by N from SARS-CoV-2, SARS-CoV-1, and MERS CoV. Anti-N Abs bound to the surface of N expressing cells activate Fc receptor-expressing cells. Our findings indicate that cell surface N manipulates innate immunity by sequestering chemokines and can be targeted by Fc expressing innate immune cells. This, in combination with its conserved antigenicity among human CoVs, advances its candidacy for vaccines that induce cross-reactive B and T cell immunity to SARS-CoV-2 variants and other human CoVs, including novel zoonotic strains.

Development of IFN-γ secretory ELISPOT based assay for screening of ADCC responses

Antibody dependent cell mediated cytotoxicity has been established as one of the important protective immune mechanisms against HIV making it essential to evaluate it while testing immunogenicity of emerging vaccine candidates. IFN-γ secretory ELISPOT assay, widely used for evaluation of CTL response in HIV vaccine trials, was adapted for measuring ADCC responses and the results were compared with the standard ICS based assays. IFN-γ responses elicited by plasma samples of 23 HIV infected individuals against Env and Gag peptides using granulocytes as antigen presenting cells were assessed by both the methods. Supernatants of the activated cells in ELISPOT assay were also assessed for cytokine/chemokine estimation. ELISPOT assays detected significantly more ADCC responders against HIV-Env and Gag peptide pools than ICS assay. The magnitude of IFN-γ response in both the assay correlated significantly (p=0.002). NK cells were found to be the predominant cell type secreting IFN-γ in the assay. Although IFN-γ and IL-6 levels were significantly higher in supernatants of Env peptides stimulated cells, IP-10 and MCP-1α levels were found to be more against Gag peptides. Thus, IFN-γ secretory ELISPOT assay was found to be more sensitive in detecting ADCC responders than ICS assay making it a valuable tool for screening of ADCC responses in future vaccine trials. Differences in cytokine pattern of Env versus Gag stimulated cells warrants a need for investigating their role in protection against HIV infection.

HIV-specific CD4-induced Antibodies Mediate Broad and Potent Antibody-dependent Cellular Cytotoxicity Activity and Are Commonly Detected in Plasma From HIV-infected humans

HIV-specific antibodies (Abs) can reduce viral burden by blocking new rounds of infection or by destroying infected cells via activation of effector cells through Fc–FcR interaction. This latter process, referred to as antibody-dependent cellular cytotoxicity (ADCC), has been associated with viral control and improved clinical outcome following both HIV and SIV infections. Here we describe an HIV viral-like particle (VLP)-based sorting strategy that led to identification of HIV-specificmemory B cells encoding Abs that mediate ADCC froma subtype A-infected Kenyan woman at 914 days post-infection. Using this strategy, 12 HIV-envelope-specific monoclonal antibodies (mAbs) were isolated and three mediated potent ADCC activitywhen compared to well-characterized ADCC mAbs. The ADCC-mediating Abs also mediated antibody-dependent cell-mediated virus inhibition (ADCVI), which provides a net measure of Fc receptor-triggered effects against replicating virus. Two of the three ADCC-mediating Abs targeted a CD4-induced (CD4i) epitope also bound by the mAb C11; the third antibody targeted the N-terminus of V3. Both CD4i Abs identified here demonstrated strong cross-clade breadth with activity against 10 of 11 envelopes tested, including those from clades A, B, C, A/D and C/D, whereas the V3-specific antibody showed more limited breadth. Variants of these CD4i, C11-like mAbs engineered to interrupt binding to FcγRs inhibited a measurable percentage of the donor's ADCC activity starting as early as 189 days post-infection. C11-like antibodies also accounted for between 18–78% of ADCC activity in 9 chronically infected individuals from the same cohort study. Further, the two CD4i Abs originated from unique B cells, suggesting that antibodies targeting this epitope can be commonly produced. Taken together, these data provide strong evidence that CD4i, C11-like antibodies develop within the first 6 months of infection and they can arise fromunique B-cell lineages in the same individual. Further, thesemAbsmediate potent plasma IgG-specificADCC breadth and potency and contribute to ADCC activity in other HIV-infected individuals.

HIV-Specific Antibody Responses in HIV-Infected Patients: From a Monoclonal to a Polyclonal View

HIV infections represent a major global health threat, affecting more than 35 million individuals worldwide. High infection rates and problems associated with lifelong antiretroviral treatment emphasize the need for the development of prophylactic and therapeutic immune intervention strategies. It is conceivable that insights for the design of new immunogens capable of eliciting protective immune responses may come from the analysis of HIV-specific antibody responses in infected patients. Using sophisticated technologies, several monoclonal neutralizing antibodies were isolated from HIV-infected individuals. However, the majority of polyclonal antibody responses found in infected patients are nonneutralizing. Comprehensive analyses of the molecular targets of HIV-specific antibody responses identified that during natural infection antibodies are mainly misdirected towards gp120 epitopes outside of the CD4-binding site and against regions and proteins that are not exposed on the surface of the virus. We therefore argue that vaccines aiming to induce protective responses should include engineered immunogens, which are capable of focusing the immune response towards protective epitopes.

Nuclear and cytoplasmic effects of human CRM1 on HIV-1 production in rat cells

The human immunodeficiency virus type 1 (HIV-1) regulatory protein, Rev, mediates the nuclear export of unspliced gag and singly spliced env mRNAs by bridging viral RNA and the export receptor, CRM1. Recently, rat CRM1 was found to be less efficient than human CRM1 in supporting Rev function in rats. In this study, to understand the role of CRM1 in HIV propagation, the mechanism underlying the function of human and rat CRM1 in HIV-1 replication was investigated in rat cells. The production of viral particles, represented by the p24 Gag protein, was greatly enhanced by hCRM1 expression in rat cells; however, this effect was not simply because of the enhanced export of gag mRNA. The translation initiation rate of gag mRNA was not increased, nor was the Gag protein stabilized in the presence of hCRM1. However, the processing of the p55 Gag precursor and the release of viral particles were facilitated. These results indicated that hCRM1 exports gag mRNA to the cytoplasm, not only more efficiently than rCRM1 but also correctly, leading to efficient processing of Gag proteins and particle formation.

Immunofluorescence assay in India for confirmation of HIV-1 infection using a T-cell line infected with defective HIV-1

BACKGROUND

In India, the enzyme immunoassay (EIA)/rapid test is used for screening and confirmatory antibody testing of HIV infection, and all HIV reactive samples are further confirmed by two other rapid tests working on different principles; however, Western blotting (WB) and immunofluorescence (IF) assays are not routinely performed in this country.

METHODS

A total of 2104 sera from Indian subjects were tested for the presence of HIV-1 antibody using EIA/rapid tests, according to the guidelines of the National AIDS Control Organization of India, and were also subjected to IF test using L-2 cells persistently infected with defective HIV-1. WB and a nested reverse transcriptase polymerase chain reaction (RT-PCR) were performed on discrepant samples.

RESULTS

IF results were 100% concordant with EIA/rapid tests for 212 HIV-1-positive samples and 1889 HIV-1-negative samples. Interestingly, three (0.14%) samples negative by EIA/rapid tests were weakly or moderately positive (1+/2+) by IF test. All three of these samples were confirmed to be negative by WB (reactive with Gag/Pol, but not with Env), but positive by RT-PCR with primers targeting the C2-V5 fragment of the env gene. These three samples were from individuals who voluntarily reported for HIV testing because of high-risk practices, and they may have been at an early stage of HIV infection.

CONCLUSIONS

These results confirm that the IF test using L-2 cells is a sensitive and specific alternative method for confirmation of HIV-1 infection and could be included in the diagnostic algorithm in reference laboratories in developing countries.

Inhibitory effect of human TRIM5alpha on HIV-1 production

Tripartite motif-containing 5 isoform-alpha (TRIM5alpha), a host restriction factor, blocks infection of some retroviruses at a post-entry, pre-integration stage in a species-specific manner. A recent report by Sakuma et al. describes a second antiretroviral activity of rhesus macaque TRIM5alpha, which blocks HIV-1 production through rapid degradation of HIV-1 Gag polyproteins. Here, we find that human TRIM5alpha limits HIV-1 production. Transient expression of TRIM5alpha decreased HIV-1 production, whereas knockdown of TRIM5alpha in human cells increased virion release. A single amino acid substitution (R437C) in the SPRY domain diminished the restriction effect. Moderate levels of human wild-type TRIM5alpha and a little amount of R437C mutant were incorporated into HIV-1 virions. The R437C mutant also lost restriction activity against N-tropic murine leukemia virus infection. However, the corresponding R to C mutation in rhesus macaque TRIM5alpha had no effect on the restriction ability. Our findings suggest human TRIM5alpha is an intrinsic immunity factor against HIV-1 infection. The importance of arginine at 437 aa in SPRY domain for the late restriction is species-specific.

Superinfection of defective human immunodeficiency virus type 1 with different subtypes of wild-type virus efficiently produces infectious variants with the initial viral phenotypes by complementation followed by recombination

Superinfection rates of human immunodeficiency virus type 1 (HIV-1) have increasingly been leading to more variation in HIV-1, as evidenced by the emergence of circulating recombinant forms (CRFs). We recently reported complementation in a persistently replication-defective subtype B-infected cell clone, L-2, by superinfection with CRF15_01B. The L-2 cells continuously produce immature particles due to a one-base insertion at pol protease. Proviruses in the superinfected cells carried both subtypes and produced particles with a mature morphology. In this study, we examined possible recombination following complementation to generate replication-competent variants by using three cell clones prepared from superinfected L-2 cells. The individual clones predominantly expressed the initial subtype B-derived mature Gag proteins. However, the viral particles carried both subtype B with the mutation and wild-type CRF15_01B at pol, suggesting the generation of virions with heterozygous RNAs. Interestingly, with cell-free passages of the progeny, defective particles disappeared, and were replaced with heterogeneous recombinants in the pol region with sequences derived from CRF15_01B that expressed subtype B phenotype. Thus, even a defective form of persistent HIV-1 can become replication-competent through superinfection-mediated complementation followed by recombination. These findings suggest the significance of long-lived infected cells as recipients for superinfection.

Suppression of HIV replication using RNA interference against HIV-1 integrase

RNA interference (RNAi) has become one of the most powerful and popular approach on gene silencing in clinical research study especially in virology due to the gene-specific suppression property of small interfering RNA (siRNA). In this report, we demonstrate that expression of vector-mediated small hairpin RNA (shRNA) against human immunodeficiency virus type 1 (HIV-1) integrase (IN), one of the three important enzymes in HIV infection by controlling the integration of viral RNA to host DNA, could suppress the protein synthesis of EGFP-tagged IN in HeLa cell model efficiently. Furthermore, we show that IN shRNA can successfully reduce the HIV particles production in 293T cells at the level similar to the positive control of HIV-1 tat shRNA. These results provide the therapeutic possibility of HIV replication using RNAi against HIV-1 integrase.

Superinfection of human immunodeficiency virus type 1 (HIV-1) to cell clone persistently infected with defective virus induces production of highly cytopathogenic HIV-1

Superinfection with human immunodeficiency virus type 1 (HIV-1) in human subjects, defined as reinfection with a heterologous strain of HIV-1, has become a topic of great interest. To illustrate the significance of this occurrence, we performed HIV-1 superinfection of L-2 cells, which were isolated from MT-4 cells persistently infected with subtype B HIV-1 as a cell clone continuously producing defective HIV-1 particles. L-2 cells carrying provirus with a one-base insertion in the pol protease were superinfected with HIV-1 derived from primary isolates of subtype B or CRF01_AE. The kinetics of the superinfection in L-2 were very slow compared with those of primary infections in MT-4. Interestingly, L-2 shifted after superinfection to become a producer of highly cytopathogenic HIV-1. Molecular characterization revealed that superinfection occurred in only about 10% of the CRF01_AE-superinfected L-2, which carried provirus of both subtypes and produced viral particles containing genomic RNA of both subtypes. Surprisingly, such cytopathogenic HIV-1 showed predominantly the original subtype B phenotype. Thus, the mechanism of the production of cytopathic HIV-1 seemed to be mediated by trans complementation with pol products of superinfected CRF01_AE. These findings suggest the significance of long-lived infected cells as recipients for superinfection that could result in the generation of new HIV-1 variants with high virulence in patients who are off therapy or do not adhere to treatment, and may indicate the need for precautions against such superinfection.

HIV replication elicits little cytopathic effects in vivo: Analysis of surrogate markers for virus production, cytotoxic T cell response and infected cell death

Several potential mechanisms for viral destruction of HIV-infected cells have been described. The hypothesis was examined that if HIV were cytopathic, a positive relation between the in vivo virus production or CTL activity and infected cell death should be observed. In a regression analysis no significant relation was found between surrogate markers for in vivo virus production or the virus-specific CTL response and death rates of productively infected cells. In a subgroup of patients the hypothesis is rejected that HIV replication elicits a large (R(2) > 0.25) cytopathic effect (P < 0.05, N = 36). It is concluded that HIV replication elicits little cytopathic effect in productively infected cells and that CD4(+) T lymphocytes are eroded by other mechanisms.

Establishment of Vero E6 cell clones persistently infected with severe acute respiratory syndrome coronavirus

Little information is available on persistent infection of severe acute respiratory syndrome (SARS) coronavirus (CoV). In this study, we established persistent infection of SARS-CoV in the Vero E6 cell line. Acute infection of Vero E6 with SARS-CoV produced a lytic infection with characteristic rounding cytopathic effects (CPE) and the production of a large number of infectious particles in the culture fluid within 3 days post-infection. Upon subsequent culturing of the remaining adherent cells, the cells gradually proliferated and recovered normal morphology similar to that of the parental cells, and continued to produce large numbers of infectious viral particles during the observation period of 5 months. Among a total of 87 cell clones obtained from the persistently infected Vero E6, only four cell clones (named #13, #18, #21, and #34) were positive for viral RNA. Clones #13, #18, and #34 shifted to viral RNA-negative during subsequent cultures, while #21 continuously produced infectious particles at a high rate. The SARS-CoV receptor, angiotensin-converting enzyme 2, was almost completely down regulated from the cell surface of persistently infected cells. Western blot analysis as well as electron microscopy indicated that the ratios of spike to nucleocapsid protein in clone #21 as well as its parental persistently infected cells were lower than that in the cells in the acute phase of infection. These Vero E6 cells persistently infected with SARS-CoV may be useful for clarifying the mechanism of the persistent infection and also for elucidating the possible pathophysiologic significance of such long-term maintenance of this virus.

Different susceptibility of human immunodeficiency virus type 1 to Env gp41-derived synthetic peptides corresponding to the C-terminal heptad repeat region

Two functional domains, alpha-helical heptad repeat 1 (HR-1) and HR-2, located in the N-terminal and C-terminal regions of human immunodeficiency virus type 1 (HIV-1) Env gp41, respectively, play an important role in the fusion process. Synthetic 34-amino-acid peptide that contains the HR-2 region, named C34, has been shown to inhibit the HIV-1 fusion process. Here, we prepared six representative peptides (C34-B1, -B2, -A, -C1, -C2, and -E from subtypes B, A, C, and E, respectively) according to the sequences from the HIV sequence database of Los Alamos. All the C34 peptides had lower ability to inhibit the primary isolates (subtypes B and CRF01_AE) than subtype B laboratory strain LAI. On the other hand, the L-2 cell clone, isolated from persistently LAI-infected MT-4 cells (MT-4/LAI), showed unique C34 peptide sensitivities. L-2 virus has the same sequences at HR-1 and HR-2 regions as LAI, but showed higher syncytia formation activity than LAI. Interestingly, the sensitivity of L-2 was higher to C34-B2 and -A but slightly lower to C34-C1 at higher concentrations than MT-4/LAI, while C34-B1, -C2, and -E showed similar activity against both viruses. Thus, in addition to the sequences of the C34 peptide as well as of the HR-1 and HR-2 regions in target viruses used for fusion assays, the fusion inhibitory activities of C34 peptides seem to be affected by viral factor(s) other than the gp41 alpha-helical heptad repeats.

Enhancement of human immunodeficiency virus type 1 infectivity by replacing the region including Env derived from defective particles with an ability to form particle-mediated syncytia in CD4+T cells

The infection and subsequent replication rates of human immunodeficiency virus type 1 (HIV-1) affect the pathogenicity. The initial stage of HIV-1 infection is largely regulated by viral envelope sequence. We previously reported that the defective doughnut-shaped particles produced from a persistently infected cell clone, named L-2, obtained from human CD4+ T-cell line MT-4 that was persistently infected with HIV-1 LAI strain, efficiently form particle-mediated syncytia with uninfected human CD4+ T-cell line, MOLT-4. Here, we prepared a molecular clone (pL2) containing the L-2 provirus to characterize the viral genetic region contributing to this activity to form particle-mediated syncytia. Several recombinants were constructed with pNL4-3 by replacing the pL2-derived region including full-length env. Characterization of the particles obtained by transfection with these recombinant clones confirmed that pL2-derived env carried the particle-mediated syncytia formation activity. It is noteworthy that the pL2-derived env region could also contribute to enhancement of infectivity in CD4+ T-cell lines as well as primary peripheral blood mononuclear cells (PBMCs). Thus, the HIV-1 particle-mediated syncytium formation activity could also contribute to the enhancement of HIV-1 infectivity.

Antibody-Dependent Cellular Cytotoxicity via Humoral Immune Epitope of Nef Protein Expressed on Cell Surface

Antibodies against various proteins of HIV type 1 (HIV-1) can be detected in HIV-1-infected individuals. We previously reported that the level of Ab response against one Nef epitope is correlated with HIV-1 disease progression. To elucidate the mechanism for this correlation, we examined Ab-dependent cellular cytotoxicity (ADCC) against target cells expressing Nef. We observed efficient cytotoxicity against Nef-expressing target cells in the presence of patient plasma and PBMCs. This ADCC activity was correlated with the dilution of plasma from HIV-1-infected patients. Addition of a specific synthetic peptide (peptide 31:FLKEKGGLE) corresponding to the Nef epitope reduced cell lysis to approximately 50%. These results suggest that PBMCs of HIV-1-infected patients may exert ADCC via anti-Nef Abs in the patients' own plasma and serve as a mechanism used by the immune system to regulate HIV-1 replication.

Construction and characterization of an infectious molecular clone derived from the CRF01_AE primary isolate of HIV type 1

An infectious molecular clone (named p95TNIH022) was constructed using long-range polymerase chain reaction products derived from a clinical isolate (95TNIH022) of HIV-1 CRF01_AE obtained from an asymptomatic Thai carrier in 1995. The virus in the supernatant from p95TNIH022-transfected 293T cells showed infectivity in peripheral blood mononuclear cells (PBMCs) as well as in MAGIC5 cells, which express CD4 and CCR5, but not in the original MAGI cells, indicating that p95TNIH022 is an infectious molecular clone with CCR5 tropism. Interestingly, p95TNIH022-derived virus induced profound cell killing in infected PBMCs, as in cells infected with the parental isolate.

Extracellular Nef protein activates signal transduction pathway from Ras to mitogen-activated protein kinase cascades that leads to activation of human immunodeficiency virus from latency

We previously reported that viral antigen expression was markedly up-regulated by stimulation with extracellular Nef, similar to the effects of tumor necrosis factor (TNF)-alpha and phorbol myristate acetate, in model cells for HIV-1 latency. In this study, we examined the molecular mechanism of this novel Nef function. Flow cytometry revealed specific binding of Nef on the surface of latently infected cells. Furthermore, activation of Ras in the cells was detected after treatment with Nef, indicating the involvement of Ras in Nef-mediated activation of HIV-1 from latency. This was also confirmed by the observations that HIV-1 long-terminal repeat-luciferase (LTR-Luc) activity was significantly up-regulated by introduction of the active Ras into uninfected cells, and that LTR-Luc activity observed in Nef-treated cells was specifically inhibited by introduction of a dominant negative Ras. In addition, PD98059 inhibited the activation of HIV-1 by Nef, but not by TNF-alpha. Thus, Nef-mediated reactivation of HIV-1 in latent model cells occurs by signal transduction from Ras to mitogen-activated protein kinase cascades.

Low or no antibody responses to human immunodeficiency virus type 1 Nef in infected carriers with subtype E, in contrast to subtype B that showed antibodies preferentially recognizing subtype-specific Nef epitopes

The viral accessory gene product Nef has been shown to play an important role in human immunodeficiency virus type 1 (HIV-1)-induced pathogenesis. Only little information is available regarding the differences in the host immune responses against Nef protein and its function in vivo among different subtypes of HIV-1. In the present study, we showed marked differences in the immune responses to Nef protein between subtypes B and E. The amino acid sequence in subtype E Nef showed 72% homology with that in subtype B. Most murine monoclonal antibodies obtained by immunization with subtype B or E Nef protein showed cross-reactivity with both Nef proteins (80 and 67%, respectively). Next, we focused on the immune responses among infected Japanese and Thai individuals. Subtyping of the individuals into B and E was carried out by enzyme-linked immunosorbent assay (ELISA) using synthetic peptides corresponding to the V3 loop representing the principal neutralizing domain. Most of the sera from these individuals reacted strongly with Gag p24 proteins derived from subtypes B and E at similar levels. However, the immune responses among these individuals to Nef protein were markedly different. Some subtype B-infected Japanese and Thai individuals (40 and 35%, respectively) showed higher levels of anti-Nef antibodies, although these antibodies preferentially recognized epitopes specific to subtype B. On the other hand, most of the subtype E-infected Japanese and Thai individuals showed low or no antibody responses to Nef proteins. Thus, immune responses to Nef were markedly different between subtypes B- and E-infected carriers, suggesting different function(s) for Nef in AIDS pathogenesis. Further, vaccine design must take into account the different subtypes of HIV-1.

Molecular ratio between borna disease viral-p40 and -p24 proteins in infected cells determined by quantitative antigen capture ELISA

We developed the antigen capture enzyme-linked immunosorbent assay (ELISA) systems for quantification of Borna disease virus (BDV) major antigens, p40 and p24. Using these ELISAs, we quantified the two proteins in various BDV-infected materials, including the cell lysates and culture supernatants as well as the homogenates of experimental animal brains. The ELISAs were also applied to measure the infectious titer of BDV in persistently infected cell lines. Quantitative analysis with these ELISAs allowed us to measure the molecular ratio between the p40 and p24 in infected samples. Interestingly, the ratio of p24 to p40 in persistently infected cells was much higher than that observed in acutely infected cells although the ratios in the supernatants from both cell lines were quite similar. BDV-inoculated gerbil brain cells showed a relatively high ratio of p24 to p40 as compared with acutely infected cells. These observations suggested that the molecular ratio between the proteins strongly depended on the infectious status of BDV in the host cells. The ELISA system developed here could be a convenient method for the quantification of BDV infection and may also be beneficial for understanding viral replication and infectious status in the host cells.

Targets of a protease inhibitor, KNI-272, in HIV-1-infected cells

The targets of a protease inhibitor, KNI-272, in the HIV-1 life cycle were investigated in this study. Neither expression of HIV-1 Gag proteins nor production of virus particles was detected in cells infected acutely with HIV-1 cultured in the presence of KNI-272. Although HIV-1 proviral DNA was detected in the cells by PCR, the inhibitor depressed the amount of the proviral DNA in a concentration dependent manner. These results indicate that one of the targets of KNI-272 occurs in the stage before the expression of viral structural proteins. No direct inhibition of reverse transcription was found with the inhibitor. To confirm the inhibition of viral protease, persistently HIV-1-infected cells were cultured in the presence of the inhibitor and examined by electron microscopy for the morphology of HIV-1 particles. Doughnut-shaped immature particles were observed in the extracellular space of the cells, and disrupted semicircular shaped particles were also seen at the higher concentration of KNI-272. A bioassay for infectivity showed that the virus particles were not infectious, and immunofluorescent assay using anti-p17 antibody, that does not react with the precursor of Gag protein, revealed that Gag precursor p55 protein in the cells was not processed. Thus, KNI-272 blocked the maturation of viral particles. Consequently, KNI-272 has at least two inhibition targets in the stages of the HIV-1 life cycle.

Translation initiation of a bicistronic mRNA of Borna disease virus: a 16-kDa phosphoprotein is initiated at an internal start codon

We examined translational initiation of a bicistronic 0.8-kb mRNA of Borna disease virus (BDV) using a cDNA clone of the mRNA. Upon transfection with the clone, COS-7 cells produced a 16-kDa protein (P'), in addition to the previously identified products of BDV, 24- (P) and 14.5-kDa proteins. The 16-kDa product was detected by anti-P monoclonal antibody and was shown to exist in BDV-infected cell lines as well as in infected animal brain cells. Transient expression analysis of mutated cDNA clones encoding the BDV 0.8-kb mRNA revealed that the 16-kDa protein was initiated at the second AUG codon on the same open reading frame of the P protein. The mutational analysis also demonstrated that the first AUG within the 0.8-kb mRNA is not optimal, although the signal contains a better Kozak's motif. These results demonstrated the presence of three functional AUG codons in the smallest mRNA of BDV and also suggested that a leaky scanning mechanism is involved in translational initiation at AUG codons downstream of the bicistronic mRNA of BDV. Furthermore, the 16-kDa protein was located in the BDV-specific nuclear foci and was found to associate with the other viral proteins in BDV-infected cells, demonstrating an important role of the novel identified BDV protein in viral replication.

Fusion of uninfected T-cells occurs with immature HIV-1 protease-mutant, but not morphologically similar protease inhibitor derived particles

Protease inhibitors are widely used in the treatment of human immunodeficiency virus type 1 (HIV-1)-infected individuals and show a drastic effect on the reduction of virus load. We previously reported that doughnut-shaped, protease-defective gp120-containing HIV-1 particles from an L-2 cell clone, carrying a provirus with mutations at the pol (protease), env (gp41) and nef genes, rapidly and more effectively induces virus particle-mediated syncytia formation of uninfected T-cells, than a parental wild-type laboratory strain of HIV-1 (LAI). In this study, we examined the possibility of whether enhanced syncytia formation is mediated by morphologically similar doughnut-shaped particles obtained after treatment of LAI-infected cells with the protease inhibitors L-689, 502, DMP-323, RO-31-8959, and KNI-272. Utilizing such protease inhibitor-induced particles and a clone of MOLT-4 cells, we could not detect any enhancement of syncytia formation, over that seen with wild-type LAI particles. This result should alleviate concerns of patients on highly active antiretroviral therapy (HAART), that protease inhibitors might accelerate progression of the disease through enhanced production of defective, 'immature'-appearing particles.

The life-cycle of human immunodeficiency virus type 1

The life-cycle of human immunodeficiency virus type 1 (HIV-1) has been studied using several techniques including immunoelectron microscopy and cryomicroscopy. The HIV-1 particle consists of an envelope, a core and the region between the core and the envelope (matrix). Virus particles in the extracellular space are observed as having various profiles: a central or an eccentric round electron-dense core, a bar-shaped electron-dense core, and immature doughnut-shaped particle. HIV-1 particles in the hydrated state were observed by high-resolution electron cryomicroscopy to be spherical and the lipid membrane was clearly resolved as a bilayer. Projections around the circumference were seen to be knob-like. The shapes and sizes of the projections, especially the head parts, were found to vary with each projection. HIV-1 cores were isolated with a mixture of Nonidet P40 and glutaraldehyde, and were confirmed to consist of HIV-1 Gag p24 protein by immunogold labelling. On infection, the HIV-1 virus was found to enter the cell in two ways: membrane fusion and endocytosis. After viral entry, no structures resembling virus particles could be seen in the cytoplasm. In the infected cells, positive reactions by immunolabelling suggest that HIV-1 Gag is produced in membrane-bound structures and transported to the cell surface by the cytoskeletons. A crescent electron-dense layer is then formed underneath the cell membrane. Finally, the virus particle is released from the cell surface and found extracellularly to be a complete virus particle with an electron-dense core. However, several cell clones producing defective mature, doughnut-shaped (immature) or teardrop-shaped particles were found to be produced in the extracellular space. In the doughnut-shaped particles, Gag p17 and p24 proteins exist facing each other against an inner electron-dense ring, suggesting that the inner ring consists of a precursor Gag protein showing a defect at the viral proteinase.

A chain section containing epitopes for cytotoxic T, B and helper T cells within a highly conserved region found in the human immunodeficiency virus type 1 Gag protein

Cell-mediated immune responses constitute a major defense against the spread of human immunodeficiency virus type 1 (HIV-1). However, multiple alterations within a particular epitope may accumulate during disease progression, allowing the virus to escape cytotoxic T lymphocytes (CTLs). Therefore, the best candidate for a peptide vaccine that would prevent the onset of the disease might be a chain section containing epitopes for the generation of CTLs in regions of conserved sequences among different HIV-1 isolates. We previously showed that immunizing mice with synthetic peptides consisting of 23-amino acids (Gag-23mer; 287-309 amino acid residues) in a highly conserved region derived from the major core protein p24 of HIV-1 generates specific CTLs as well as antibodies. Here, we identified one CTL (T-1; 291-300) and two B-cell (B-1; 290-299 and B-2; 300-309) epitopes, all of which consisted of 10 amino acids within the region. In addition, helper T cells primed by the Gag-23mer peptide were proliferated by in vitro stimulation with a 21mer (H-1; 289-309) or a 19mer (H-2; 291-309) peptide, but not with a 17mer peptide (293-309) or 19mer peptide (287-305). Immunization with the H-1 peptide generated an antibody reactive to B-1, but not B-2, whereas that with H-2 generated an antibody reactive to B-2, but not B-1. CTLs were not generated by immunization with these peptides, indicating that the entire sequence of Gag-23mer is the helper epitope for CTLs. Thus, the Gag-23mer is a chain section containing epitopes for cytotoxic T, B and helper T-cells within a highly conserved region of HIV-1 Gag protein.

Decoy approach using RNA-DNA chimera oligonucleotides to inhibit the regulatory function of human immunodeficiency virus type 1 Rev protein

Human immunodeficiency virus type 1 (HIV-1) encodes two regulatory proteins, Tat and Rev, that bind to target RNA sequences. These are the trans-activation responsive (TAR) RNA and the Rev-responsive element (RRE), respectively. The Rev protein shifts RNA synthesis to viral transcripts by binding to the RRE within the env gene. In the present study we prepared a RNA-DNA chimera consisting of 29 or 31 nucleotides to inhibit the Rev regulatory function by means of the decoy approach. The chimera oligonucleotides (anti-Rev oligonucleotides [AROs]) contained an RNA "bubble" structure (13 oligonucleotides; the Rev-binding element in RRE) that bound Rev with a high affinity in an in vitro assay. The controls were RNA-DNA chimera oligonucleotides (negative control oligonucleotides [NCOs]) similar to ARO, but without the bubble structure, that bound with considerably less affinity to Rev. When the inhibitory effects of these decoys on HIV-1 replication were examined, we found that AROs, but no NCOs, reduced more than 90% of the HIV-1 production generated by productively infected human T-cell lines. The production of primary HIV-1 isolates in healthy donor-derived peripheral blood mononuclear cells was also similarly inhibited by AROs. In addition, the induction of viral mRNAs and antigens in latently HIV-1-infected ACH-2 cells by tumor necrosis factor alpha was specifically inhibited by AROs, but not by NCOs. No apparent cytotoxicity was caused by either decoy. Thus, the use of a Rev-binding element-based decoy, the RNA-DNA chimera oligonucleotide, may represent a safer approach to gene therapy for reducing the virus load in HIV-1-infected individuals.

Protease-defective, gp120-containing human immunodeficiency virus type 1 particles induce apoptosis more efficiently than does wild-type virus or recombinant gp120 protein in healthy donor-derived peripheral blood T cells

Apoptosis and syncytium formation are two mechanisms by which human immunodeficiency virus type 1 (HIV-1) impairs uninfected CD4+ T-cell function and are mainly involved in the progression of the disease to AIDS. Previously, we showed that gp120-containing, protease-deficient HIV-1 (L-2) particles generated syncytia by particle-mediated fusion with uninfected cultured CD4+ T cells. Here, we present evidence that such L-2 particles can induce apoptosis in 40 to 50% of T cells which were enriched from HIV-1-negative healthy donor-derived peripheral blood mononuclear cells (PBMC-Ts). Activation of PBMC-Ts with phytohemagglutinin, concanavalin A, or ionomycin after incubation with L-2 particles resulted in the loss of proliferative capacity and gradual induction of apoptosis over 3 days. Wild-type strain LAI particles or recombinant gp120 were markedly less efficient (< or = 15%) at inducing such apoptosis. Western blot (immunoblot) analysis revealed that L-2 particles contained a larger amount of Env gp120 than LAI particles. Either preincubation of PBMC-Ts with a Fas antagonist or preincubation of L-2 particles with soluble CD4 blocked most of the apoptosis. This suggests that L-2-like particles can play a major role in HIV-1-induced apoptosis of uninfected bystander cells.

B cell epitopes of HIV type 1 p24 capsid protein: a reassessment

The objective of the present study was to identify p24 antigenic domains recognized during natural human immunodeficiency virus type 1 (HIV-1) infection, the determination of the major epitopes of p24 having significant applications for both the improvement of diagnostic approaches and the development of vaccines. Reactivity of 20 HIV-1-infected patients and 8 HIV-1-negative patients was analyzed using an enzyme-linked immunosorbent assay (ELISA) developed with 45 overlapping synthetic pentadecapeptides, spanning amino acids 133 to 363 of HIV-1 p55gag precursor. Two peptides covering aa 178-192 and 288-302 of p55 were recognized by 40 and 45% of HIV-1 antibody-positive human samples, respectively. A peptide covering aa 272-322 of p55 was synthesized and recognized by most human sera in indirect ELISA. However, inhibition assays indicated that this sequence does not contain all of the immunodominant domains of p24 since it was not sufficient to block binding of human sera to whole p24. A three-dimensional model of p24 derived from the Mengovirus VP2 suggests that the two distant sequences recognized by human sera containing antibodies to HIV-1 could possibly be a part of a conformational epitope built up by two loops corresponding to aa 183-186 and 289-292.

High susceptibility of U937-derived subclones to human immunodeficiency virus type 1 infection correlates with accumulation of unintegrated circular viral DNA

Our previous report showed that U937-derived subclones were differentiated into at least three types (high, middle, and low types), even in the subclones expressing similar levels of surface CD4, in terms of the kinetics of the appearance of viral antigens and virus production after infection with human immunodeficiency virus type 1 (HIV-1). Here we showed the evidence that high susceptibility to HIV-1 infection, which was confirmed by the profound expression of viral messages and antigens, was exclusively associated with a high number of the unintegrated extrachromosomal form of viral DNA, but not with the amounts of adsorbed virus RNA nor those of integrated DNA form. The difference in the amounts of extrachromosomal form of viral DNA was also observed in the culture with 3'-azido-3'-deoxythymidine (AZT), indicating that the susceptibility is essentially unrelated to multiple infection events. Thus, the susceptibility of U937-derived subclones to HIV-1 infection seems to be affected by the occurrence of specific events involved in the accumulation of unintegrated viral DNA after viral adsorption.

Stimulation of human immunodeficiency virus type 1 infected cells with superoxide enhances the chemotactic motile response of CD4+ human T cells: implication for virus transmission by cell-to-cell interaction

We previously showed that superoxide (O2-) significantly enhanced human immunodeficiency virus type 1 (HIV-1)-induced syncytia formation in co-cultured infected and uninfected human T cells. In this study, we describe a novel chemotactic response of uninfected CD4+ T cells by stimulating infected T cells with O2-. Syncytia formation was amplified only when persistently infected cells were stimulated by O2-. When the infected cells in lower well of microplate were cultured with uninfected cells in the upper well of a Boyden chamber with 8.0 microns pores, uninfected cell migration to the porous membrane was significantly amplified by stimulating infected cells with O2-. In contrast, similar functions were slight under the same assay conditions in the presence of known chemokines such as human RANTES and macrophage inflammatory protein 1 (MIP-1 alpha and beta), which all activate T lymphocytes. In addition, it is unlikely that the O2(-)-induced chemotactic response is due to soluble HIV-1 proteins from infected cells or to amplified expression levels of cell surface functional molecules such as CD4 and LFA-1 (CD11a and CD18) as well as HIV-1 Env gp120 on uninfected and/or infected cells. Thus, an unknown chemotactic factor could be generated from infected T cells by stimulation with O2- and it might contribute to viral transmission by activating cell-to-cell interactions.

Three antigenic regions in p17 of human immunodeficiency virus type 1 (HIV-1) revealed by mouse monoclonal antibodies and human antibodies in HIV-1 carrier sera

We investigated the murine antibody response to recombinant p17 (rp17) of human immunodeficiency virus type 1 (HIV-1) and the human antibody response directed to p17 in HIV-1 infection. Three large peptides covering residues 12-29, 53-87 and 87-115 of p17 were synthesized. The cysteine residues 57 and 87 of peptide 53-87 were reoxidized to form a disulfide bridge. Eighteen out of 19 murine monoclonal anti-rp17 antibodies had relatively high affinities (KA = 1.9 x 10(5)-1.4 x 10(8) M-1) with one of the 3 p17 peptides in the liquid phase. Each monoclonal antibody reacted only with one particular peptide and had no reactivity with the other 2 p17 peptides. All the monoclonal antibodies reacted with rp17 in the liquid phase with a reasonable degree of affinity (KA = 2.0 x 10(5)-1.8 x 10(7) M-1). Four HIV-1 carrier sera, which were positive in ELISA using rp17 as the antigen, reacted positively in an ELISA using 3 p17 peptides which were used to titrate murine monoclonal antibodies. Murine monoclonal antibodies having specificity for the 3 p17 peptides stained live HIV-1-infected cells by means of indirect membrane immunofluorescence, irrespective of their specificity. This suggests that the various portions of p17 (at least 3 regions of p17) were exposed on the surface of live infected cells, probably as short polypeptide chains.

Human immunodeficiency virus type 1 neutralization is determined by epitope exposure on the gp120 oligomer

The major target of the neutralizing antibody response to infection by the human immunodeficiency virus type 1 (HIV-1) is the outer envelope glycoprotein, gp120. The spectrum of HIV-1 neutralization specificity is currently represented by monoclonal antibodies (mAbs) that can be divided broadly into five groups. We have studied the binding of these mAbs to functional oligomeric and soluble monomeric gp120 derived from the molecular clone of a cell line-adapted isolate of HIV-1, and compared these binding properties with virus neutralization. Binding of all mAbs except those reactive with the V3 loop was much weaker to oligomeric than to monomeric gp120. This reduction in binding to oligomeric gp120 was determined mostly by a slower relative rate of association, although the dissociation rate also had some influence on relative variation in mAb affinity. Virus neutralization correlated broadly with mAb binding to the oligomeric rather than to the monomeric form of gp120, and neutralization potency was related to the estimated association rate. Thus, with the exception of the hypervariable V3 loop, regions of HIV-1 gp120 with the potential to induce a neutralization response are likely to be poorly presented for antibody recognition on the surface of cell line-adapted virions.

High susceptibility of U937-derived subclones to infection with human immunodeficiency virus type 1 is correlated with virus-induced cell differentiation and superoxide generation

The promonocytic human leukemic cell line U937, when infected with lymphotropic human immunodeficiency virus type 1 (HIV-1), becomes a continuous virus producer. A total of 46 U937-derived subclones in suspension was isolated and classified into three (2 high, 42 middle, and 2 low) types based on their susceptibility to the infection. By analyzing subclones before infection, we found that the high-type subclones expressed LFA-1 antigens at a relatively low level. In addition, the ability of these subclones to induce adherence after exposure to phorbol 12-myristate 13-acetate (PMA) was reduced. In contrast, a transition by HIV-1 infection to adherent macrophage-like cells was induced only in the high-type, but not in the low-type subclones. The high-type adherent cells obtained by HIV-1 infection were followed by further lineage to become retrodifferentiated suspension cells showing reduced syncytia formation ability. Superoxide was generated in the high-type subclones, without PMA-mediated differentiation, from the early stage of infection before HIV-1 replication, as well as during undifferentiated, differentiated and retrodifferentiated stages. In contrast, it was only transiently generated at acute phase of HIV-1 replication in low-type subclones. Long-term culture of the low-type subclones decreased the expression of major structural viral protein Gag and also virus production. Thus, the mechanism by which PMA differentiates U937 cells is not the same as that induced by HIV-1 infection. The latter mechanism results in high susceptibility to infection. The HIV-1 phenotypes of finally obtained persistently infected cells were also affected by the cell stages at the time of infection.

Viral activation from latency during retrodifferentiation of U937 cells exposed to phorbol ester followed by infection with human immunodeficiency virus type 1

To determine the mechanism underlying the human immunodeficiency virus type 1 (HIV-1) latency and its activation in monocyte/macrophage lineage, the human promonocytic cell line U937 was infected with HIV-1 after differentiation with varied doses of phorbol 12-myristate 13-acetate (PMA). Variously differentiated intermediate stages were generated in U937 cells in a dose-dependent manner. When these cells were infected with lymphotropic HIV-1, the kinetics of the production of HIV-1 DNA, the appearance of HIV-1 antigen-positive cells, and viral production in the conditioned media were slower at higher doses of PMA. This different susceptibility to the infection was not due to the rate of HIV-1 adsorption. Viral replication from latency in the differentiated cells was activated in proportion with the retrodifferentiation observed in long-term cultures of the host cells. Thus, our data demonstrate the close correlation between the regulation of HIV-1 replication and the differentiation stage of monocyte/macrophage lineage cells at the time of HIV-1 infection. The retrodifferentiation phenomenon in infected cells seems to be particularly important for understanding the mechanisms for HIV-1 activation from latency.

The feline lymphoid cell line MBM and its use for feline immunodeficiency virus isolation and quantitation

We report on the development of a feline T lymphoblastoid cell line obtained from the peripheral blood mononuclear cells (PBMC) of a specific pathogen free cat and designated MBM. The cells are pan-T+, CD4- and CD8- and remained interleukin-2-dependent and concanavalin A-dependent throughout the period of observation. MBM cells have proved at least as sensitive as fresh blasts to infection with cell-free stocks of three feline immunodeficiency virus (FIV) isolates. Upon infection, they exhibit a lytic cytopathic effect. Repeated attempts to establish a chronic infection have failed. Using a limiting cell dilution method, it has been shown that MBM cells may be more sensitive than fresh blasts as substrate for isolating FIV from the PBMC of infected cats. These studies have also shown that considerable individual variations exist in the virus loads present in the PBMC of naturally infected cats, and that load size does not appear to correlate with cat age, clinical status, CD4/CD8 ratio and titer of serum neutralizing antibody.

Monoclonal antibodies to p24-core protein of HIV-1 mediate ADCC and inhibit virus spread in vitro

BACKGROUND

Certain antigens of the HIV-1, e.g., gp120-envelop proteins, can be expressed on the membrane of HIV-infected cells. Little is known about the membrane expression of other HIV-antigens and their interaction with specific antibodies.

OBJECTIVE

To develop murine monoclonal antibodies (mAbs) to the p24-core protein of HIV-1 and to characterise their binding sites and biological activities on HIV-infected T cells.

METHODS

Monoclonal antibodies were developed from mice hyperimmunised with a recombinant p24-core protein from HIV-1. Two mAbs were epitope-mapped on overlapping peptides and characterised for their reactivity with non-fixed HIV-infected T cells by immunofluorescence staining and flow cytometric analysis. Their biological activities were studied for antibody-dependent cellular cytotoxicity (ADCC) and suppression of viral spread in vitro.

RESULTS

The epitopes of two selected mAbs were located on the amino terminal region of p24 in the regions 147-152 aa and 178-187 aa, respectively. The antibodies were able to react with living HIV-1 infected cells. The expression of the antigens was time-dependent after the infection of certain cell lines by HIV-1. The mAbs mediated a strong HIV-1-specific ADCC and were able to delay the spread of HIV-1 for about 6 days in cell cultures.

CONCLUSIONS

Certain epitopes of the p24-core protein of HIV-1 can be expressed on living, HIV-infected T cells and are recognised by specific antibodies. Such antibodies can destroy infected cells by ADCC or delay the virus spread, and therefore, should be considered in immunisation strategies against HIV.

Superinfection of a defective human immunodeficiency virus type 1 provirus-carrying T cell clone with vif or vpu mutants gives cytopathic virus particles by homologous recombination

The partially CD4-expressing T cell clone, Vpr-1, which carries a latent vpr-defective HIV-1 genome and expresses HIV-1 Nef protein only, was permissive to superinfection by HIV-1. Superinfection of Vpr-1 with vif- or vpu-defective mutants, which were noncytopathic, reactivated the vpr-defective virus and led to homologous recombination and cytopathogenesis. The data provide an experimental model for homologous recombination being an important mechanism whereby HIV-1 acquires genetic heterogeneity, and when occurring among defective virus in vivo bestows novel biological activities and virulence.

Mechanism of enzyme inhibition mediated by anti-reverse transcriptase antibodies from HIV type 1-infected individuals

We have examined the mechanisms of reverse transcriptase (RT) inhibition mediated by anti-RT antibodies, isolated by affinity chromatography, from four HIV-1-positive individuals. In kinetics assays, anti-RT immunoglobulin (Ig) obtained from three of the sera mediated a noncompetitive type of inhibition against template primer; two of these three also mediated noncompetitive inhibition with respect to deoxyribonucleoside triphosphate. Such inhibition did not require that the Ig be preincubated with RT prior to the addition of reaction components. In contrast, a more complicated pattern of inhibition was exhibited by anti-RT Ig from the fourth serum. Preincubation of this Ig with enzyme markedly enhanced the inhibition. The results demonstrate that the specificities of RT-inhibiting antibodies vary among HIV-1-infected individuals, but that one prevalent mechanism of inhibition involves interactions with epitopes outside of the enzyme active site.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

In vivo induction of human immunodeficiency virus type 1-specific cytotoxic T lymphocytes and delayed-type hypersensitivity by a 23-amino acid peptide from the highly conserved region in major core protein p24

Cell-mediated immune responses are a major immune defence mechanism against the spread of human immunodeficiency virus type 1 (HIV-1) which may lead to acquired immune deficiency syndrome (AIDS). Therefore, the best candidate for a peptide vaccine preventive from the onset of the disease might be a chain section containing both B- and T-cell epitopes in regions of conserved sequences between the different HIV-1 isolates. We previously identified the highly conserved linear B-cell epitope (23 amino acids in the major core protein p24). Since the epitopes of cytotoxic T lymphocytes (CTLs) can be defined by short synthetic peptides, we examined whether this highly conserved region can elicit viral-specific, cell-mediated immune responses. The results showed specific induction of CD8+ CTLs in mice by immunization with the Gag 13-mer peptide. Lysis of targets is specific since unpulsed cells with the same MHC haplotype or cells with a different MHC haplotype pulsed with the peptide were resistant to lysis. This in vivo response induced by the Gag 23-mer peptide was almost the same as that induced by the 15-amino acid peptide from the HIV-1 Env gp120 which is an immunodominant domain in the V3 loop. Lymphocyte proliferation of T-cell fraction from immune spleen cells was observed after in vitro stimulation with the Gag 23-mer peptide, whereas there was no apparent lymphocyte proliferation with the Env 15-mer peptide. In addition, specific antibodies were raised against Gag p24 in mice immunized with the Gag 23-mer peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple effects of CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity on HIV-1 gp120 functions

The interaction of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 with CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity has been studied. Conformational changes in gp120, which could affect its interaction with CD4 and its shedding from virions, were detected by fluorescence spectrum analysis of tryptophan residues after addition of peptide representative of the CD4 CDR3-related region, but not the CD4 CDR2-related region. Interestingly, the addition of scrambled peptide, S1 (with altered amino acid sequence compared with the native CDR3-related peptide but unaltered overall composition), which we recently showed to have stronger anti-HIV-1 activity than the original CDR3-related peptide, had no effects on the conformational change in gp120 or on its interaction with CD4 and its shedding from HIV-1 virions. However, all of the CDR3-related peptides, including S1, showed blocking effects on the binding of antibodies against gp120 V3 loop and C-terminus regions. Thus, we concluded that there were at least two separable activities of the CDR3-related peptides in anti-HIV-1 activity, i.e. induction of conformational changes in gp120, which could affect its binding to CD4 and to gp41 (as observed in native CDR3-related peptides), and inactivation of V3 loop and C-terminus regions in gp120 (as observed in all of the CDR3-related peptides, including S1).

Superoxide enhances the spread of HIV-1 infection by cell-to-cell transmission

Oxidative stress is thought to be involved in the progression of human immunodeficiency virus type 1 (HIV-1)-induced disease. We examined the effect of superoxide (O2-) on HIV-1 spread in cultured human CD4+ cell lines. The O2- significantly enhanced cell-to-cell transmission of HIV-1, although its effect on HIV-1 replication was not evident, presumably due to its cytostatic activity. The effect was notable on the HIV-1 transmission from macrophages to T lymphocytes by endogenous, macrophage-generated O2-. This amplification was specifically reduced to the steady-state level by antioxidants, and further to the basal level by anti-CD4 antibodies, indicating the specificity of O2- for enhancing HIV-1 spread by cell-to-cell transmission.

Protective immunity of Hantaan virus nucleocapsid and envelope protein studied using baculovirus-expressed proteins

Recombinant Hantaan virus nucleocapsid protein (rNP) and recombinant envelope (rEnv) proteins were prepared using a baculovirus expression system to examine the role of Hantaan virus structural proteins in protective immunity. Passive transfer of spleen cells from mice immunized with rNP conferred partial protection or prolongation of time to death from fatal Hantaan virus infection in suckling mice which were challenged with Hantaan virus at 40 LD50 (survival rate: 43%) or 4 LD50 (survival rate: 43%). The T cell-enriched fraction protected one mouse from lethal infection but the B cell-enriched fraction had no such effect on fatal HTN infection. The protective effects of the antibody against HTN challenge were examined by passive immunization. The monoclonal antibody ECO 2 directed to NP also conferred partial survival and significant difference in time to death. Although rEnv antigen failed to induce neutralizing antibody, both immune spleen cells and immune serum to rEnv conferred partial protection upon suckling mice. These results indicate that both nucleocapsid and envelope proteins of Hantaan virus were responsible for induction of cell mediated protective immunity. Vero E 6 cells infected with Hantaan virus expressed envelope protein on the surface, as determined by flow cytometry. However, there was only negligible expression of nucleocapsid protein.

Expression of human immunodeficiency virus type 1 Nef antigen on the surface of acutely and persistently infected human T cells

The authors have previously shown the role of human immunodeficiency virus type 1 (HIV-1) Nef protein as a growth inhibitor to CD4+ T lymphocytes. We now report that the Nef antigen is partly expressed on both acutely and persistently infected human T cells. To investigate the cell surface expression of the Nef antigen, murine monoclonal antibodies (mAbs) were prepared by immunization with a recombinant Nef protein. The recombinant Nef was expressed by a baculovirus expression system as a truncated Nef protein with 22 kDa containing the middle to the C-terminus. Three clones were found to produce mAbs with IgM isotype against Nef protein by ELISA with the same truncated Nef. All these mAbs reacted on immunoblotting with two forms of Nef, 25 kDa and 27 kDa, in an HIV-1-infected human T-cell line and with the 27 kDa Nef in retroviral vector-infected mouse fibroblasts expressing a full-length Nef protein. Membrane immunofluorescence and flow cytometry with these mAbs revealed the Nef antigen to be expressed, at least in part, on the surface of these cells. Thus, the cell-surface form of Nef might play an important role in the selective depletion of CD4+ cells in HIV-1 infection.

Characterization of murine monoclonal antibodies directed against the submembrane protein p17 of HIV-1

Monoclonal antibodies (MAbs) were raised against the gag protein of human immunodeficiency virus type 1 (HIV-1). The reactivity of the selected Mabs with the matrix protein p17 of HIV-1 were investigated in several tests, i.e. ELISA, immunostaining of Western blots, and alkaline phosphatase anti-alkaline phosphatase immunocytochemistry (APAAP). All three Mabs reacted exclusively with HIV-1 and showed specific binding to the virus surface in pre-embedding immuno-electron-microscopy and useful as diagnostic agents. In an "in vitro" cultivation experiment the MAbs showed antiviral activity in concentrations in the range of 25-100 micrograms/ml. No binding region could be defined using overlapping peptides consisting of the p17 protein sequence of HIV-1 in an epitope mapping system and therefore we concluded, that the MAbs recognize a conformational epitope.