Antibody-mediated neutralization of primary human immunodeficiency virus type 1 isolates: investigation of the mechanism of inhibition

The multiple functions of miR-574-5p in the neuroblastoma tumor microenvironment

Neuroblastoma is the most common extracranial solid tumor in childhood and arises from neural crest cells of the developing sympathetic nervous system. Prostaglandin E2 (PGE2) has been identified as a key pro-inflammatory mediator of the tumor microenvironment (TME) that promotes neuroblastoma progression. We report that the interaction between the microRNA miR-574-5p and CUG-binding protein 1 (CUGBP1) induces the expression of microsomal prostaglandin E2 synthase 1 (mPGES-1) in neuroblastoma cells, which contributes to PGE2 biosynthesis. PGE2 in turn specifically induces the sorting of miR-574-5p into small extracellular vesicles (sEV) in neuroblastoma cell lines. sEV are one of the major players in intercellular communication in the TME. We found that sEV-derived miR-574-5p has a paracrine function in neuroblastoma. It acts as a direct Toll-like receptor 7/8 (TLR7/8) ligand and induces α-smooth muscle actin (α-SMA) expression in fibroblasts, contributing to fibroblast differentiation. This is particularly noteworthy as it has an opposite function to that in the TME of lung carcinoma, another PGE2 dependent tumor type. Here, sEV-derived miR-574-5p has an autokrine function that inhibits PGE2 biosynthesis in lung cancer cells. We report that the tetraspanin composition on the surface of sEV is associated with the function of sEV-derived miR-574-5p. This suggests that the vesicles do not only transport miRs, but also appear to influence their mode of action.

Nanobody-Mediated Neutralization Reveals an Achilles Heel for Norovirus

Human norovirus frequently causes outbreaks of acute gastroenteritis. Although discovered more than five decades ago, antiviral development has, until recently, been hampered by the lack of a reliable human norovirus cell culture system. Nevertheless, a lot of pathogenesis studies were accomplished using murine norovirus (MNV), which can be grown routinely in cell culture. In this study, we analyzed a sizeable library of nanobodies that were raised against the murine norovirus virion with the main purpose of developing nanobody-based inhibitors. We discovered two types of neutralizing nanobodies and analyzed the inhibition mechanisms using X-ray crystallography, cryo-electron microscopy (cryo-EM), and cell culture techniques. The first type bound on the top region of the protruding (P) domain. Interestingly, this nanobody binding region closely overlapped the MNV receptor-binding site and collectively shared numerous P domain-binding residues. In addition, we showed that these nanobodies competed with the soluble receptor, and this action blocked virion attachment to cultured cells. The second type bound at a dimeric interface on the lower side of the P dimer. We discovered that these nanobodies disrupted a structural change in the capsid associated with binding cofactors (i.e., metal cations/bile acid). Indeed, we found that capsids underwent major conformational changes following addition of Mg²⁺ or Ca²⁺ Ultimately, these nanobodies directly obstructed a structural modification reserved for a postreceptor attachment stage. Altogether, our new data show that nanobody-based inhibition could occur by blocking functional and structural capsid properties.IMPORTANCE This research discovered and analyzed two different types of MNV-neutralizing nanobodies. The top-binding nanobodies sterically inhibited the receptor-binding site, whereas the dimeric-binding nanobodies interfered with a structural modification associated with cofactor binding. Moreover, we found that the capsid contained a number of vulnerable regions that were essential for viral replication. In fact, the capsid appeared to be organized in a state of flux, which could be important for cofactor/receptor-binding functions. Blocking these capsid-binding events with nanobodies directly inhibited essential capsid functions. Moreover, a number of MNV-specific nanobody binding epitopes were comparable to human norovirus-specific nanobody inhibitors. Therefore, this additional structural and inhibition information could be further exploited in the development of human norovirus antivirals.

New virologic reagents for neutralizing antibody assays

PURPOSE OF REVIEW

This review summarizes current and novel virologic reagents employed for the development and application of in-vitro assays that assess neutralizing activity of antibodies against HIV-1. Characteristics of several virologic approaches are placed in context with various cellular targets and assay read-outs intended to determine potency and breadth of neutralization in patient cohorts and clinical vaccine trials.

RECENT FINDINGS

New molecular virologic reagents developed for in-vitro primary cell-based assays promise to facilitate rigorous and standardized assessment of anti-HIV-1-neutralizing antibody responses elicited by vaccine immunogens.

SUMMARY

Comprehensive assessment of anti-HIV-1 antibody potency and breadth is essential for evaluating vaccine immunogens, the advancement of vaccine candidates into clinical trials, and ultimately the development of effective vaccine strategies. Env-pseudovirion and recombinant reporter cell line neutralization assays are important tools for rapid and standardized measurement of neutralizing antibody activity. However, recent studies indicate that reporter cell lines fail to detect neutralization activity of certain antibodies observed when analyzed in peripheral blood mononuclear cells and may yield results on neutralizing antibody breadth that are discordant with peripheral blood mononuclear cell assays. Importantly, it remains unknown whether current in-vitro assays may be predictive of a protective neutralizing antibody response elicited by vaccine immunogens. This situation underscores the significance of standardizing existing, complementary methods as well as developing new assay concepts that assess neutralization in primary cells. Thus, this chapter focuses on new virologic reagents that promise to facilitate reaching this goal.

Herpes simplex virus as a tool to define the role of complement in the immune response to peripheral infection

A complex network of interactions exist between the innate and adaptive immune pathways, which act together to elicit a broad and durable host response following pathogen infection. The importance of the complement system in the host's defense against viruses has become increasingly clear as a result of detailed studies using transgenic mouse models that disrupt specific components of this host immune mechanism. We have utilized herpes simplex virus and replication-defective mutant strains to examine the impact of the complement system on development and maintenance of humoral immune responses. Here we review work from our group and others that highlights the central role that complement proteins C3 and C4 and complement receptors Cr1/Cr2 play during viral infection. We discuss the implications of these results in the context of pathogen infection and current vaccine strategies.

Relationship of HIV-1 and SIV envelope glycoprotein trimer occupation and neutralization

Insights into the process of HIV-1 neutralization may assist rational vaccine design. Here, we compared antibody neutralization against the JR-FL primary isolate and trimer binding affinities judged by native PAGE. Monovalent Fab-trimer binding and neutralization showed a direct quantitative relationship, implying that neutralization begins as each trimer is occupied by one antibody. At saturation, three Fab or soluble CD4 molecules engaged each trimer. In contrast, a maximum of one soluble CD4 molecule bound to functional SIV trimers with a truncated a gp41 tail. Remarkably, soluble CD4 was found to trigger dramatic enhancement of this virus. Unlike Fabs, a quantitative correlation between JR-FL trimer binding and neutralization was unclear for some, but not all IgGs, as neutralization was markedly increased, but trimer affinity was largely unchanged. In addition, only one molecule of certain gp41-specific IgGs appeared to be able to bind each trimer. We discuss the implications of these findings in weighing the relative contributions of size, multivalent binding and other possible effects of IgGs to explain their increased potency.

Time frames for neutralization during the human immunodeficiency virus type 1 entry phase, as monitored in synchronously infected cell cultures

Characterization of the neutralizing interaction between antibody and virus is hindered by the nonsynchronized progression of infection in cell cultures. Discrete steps of the viral entry sequence cannot be discerned, and thus, the mode of antibody-mediated interference with virus infectivity remains undefined. Here, we magnetically synchronize the motion and cell attachment of human immunodeficiency virus type 1 (HIV-1) to monitor the progression of neutralization, both in solution and following virus attachment to the cell. By simultaneous transfer of all viral particles from reaction solution with antibody to the cell-bound state, the precise rate of neutralization of cell-free virus could be determined for each antibody. HIV-1 neutralization by both monoclonal and polyclonal antibody preparations followed distinct pseudo-first-order kinetics. For all antibodies, cell types, and HIV-1 strains examined, postattachment interference served a major role in the neutralizing effect. To monitor the progression of postattachment interference, we synchronized the entry process at initiation and measured the escape of cell-bound virus from antibody. We found that different antibodies neutralized the virus over different time frames during the entry phase. Virus was observed to progress through a sequence of shifting sensitivities to different antibodies during entry, suggested here to correlate with the exposure time of the target epitope on receptor-activated viral envelope proteins. Thus, by monitoring the progression of HIV-1 entry under synchronized conditions, we identify a new and significant determinant of antibody neutralization capacity, namely, the time frames for neutralization during the course of the viral entry phase.

Antigenicity and immunogenicity of HIV-1 consensus subtype B envelope glycoproteins

"Centralized" (ancestral and consensus) HIV-1 envelope immunogens induce broadly cross-reactive T cell responses in laboratory animals; however, their potential to elicit cross-reactive neutralizing antibodies has not been fully explored. Here, we report the construction of a panel of consensus subtype B (ConB) envelopes and compare their biologic, antigenic, and immunogenic properties to those of two wild-type Env controls from individuals with early and acute HIV-1 infection. Glycoprotein expressed from full-length (gp160), uncleaved (gp160-UNC), truncated (gp145), and N-linked glycosylation site deleted (gp160-201N/S) versions of the ConB env gene were packaged into virions and, except for the fusion defective gp160-UNC, mediated infection via the CCR5 co-receptor. Pseudovirions containing ConB Envs were sensitive to neutralization by patient plasma and monoclonal antibodies, indicating the preservation of neutralizing epitopes found in contemporary subtype B viruses. When used as DNA vaccines in guinea pigs, ConB and wild-type env immunogens induced appreciable binding, but overall only low level neutralizing antibodies. However, all four ConB immunogens were significantly more potent than one wild-type vaccine at eliciting neutralizing antibodies against a panel of tier 1 and tier 2 viruses, and ConB gp145 and gp160 were significantly more potent than both wild-type vaccines at inducing neutralizing antibodies against tier 1 viruses. Thus, consensus subtype B env immunogens appear to be at least as good as, and in some instances better than, wild-type B env immunogens at inducing a neutralizing antibody response, and are amenable to further improvement by specific gene modifications.

Neutralization kinetics of sensitive and resistant subtype B primary human immunodeficiency virus type 1 isolates

The aim of the study was to determine if sensitive and resistant human immunodeficiency virus type 1 (HIV-1) subtype B primary isolates have different neutralization kinetics. Neutralization assays were undertaken where either the time allowed for virus to react with antibodies or the subsequent period of this mixture's exposure to target cells were varied. The relative neutralization sensitivity/resistance is a reproducible property of the isolates. In a minority of combinations, the titre falls exponentially for as long as the free virions are exposed to antibody. In the remainder, neutralization kinetics shows deviations which may be attributed to events occurring after the virus-antibody mixture is added to the target cells: significant neutralization with minimal exposure of the free virions to antibody; a plot where reduction in virus titre is parallel to the duration of the incubation phase of the assay. Neutralization rate constants are similar for primary HIV-1 SF33, HIV-1 SF162, and HIV-1 89.6, reaching 5 x 10(5)-1 x 10(6)/M sec for the monoclonal antibody IgG1 b12. However, although increased antibody levels produced greater reductions in virus titre the rate of neutralization was not proportional to the antibody concentration. Neutralization of either the free virion or cell-associated virus does not correlate with the resistance/sensitivity of primary subtype B isolates. The target cells play an active role, so that in designing neutralization assays with primary isolates of HIV-1, events following the virus-antibody complex binding to the cell surface have to be taken into consideration.

Inhibition of HIV Env binding to cellular receptors by monoclonal antibody 2G12 as probed by Fc-tagged gp120

During natural HIV infection, an array of host receptors are thought to influence virus attachment and the kinetics of infection. In this study, to probe the interactions of HIV envelope (Env) with various receptors, we assessed the inhibitory properties of various anti-Env monoclonal antibodies (mAbs) in binding assays. To assist in detecting Env in attachment assays, we generated Fc fusions of full-length wild-type gp120 and several variable loop-deleted gp120s. Through investigation of the inhibition of Env binding to cell lines expressing CD4, CCR5, DC-SIGN, syndecans or combinations thereof, we found that the broadly neutralizing mAb, 2G12, directed to a unique carbohydrate epitope of gp120, inhibited Env-CCR5 binding, partially inhibited Env-DC-SIGN binding, but had no effect on Env-syndecan association. Furthermore, 2G12 inhibited Env attachment to primary monocyte-derived dendritic cells, that expressed CD4 and CCR5 primary HIV receptors, as well as DC-SIGN, and suggested that the dual activities of 2G12 could be valuable in vivo for inhibiting initial virus dissemination and propagation.

Failure of HIV-exposed CD4+ T cells to activate dendritic cells is reversed by restoration of CD40/CD154 interactions

Because interactions between activated CD4+ T cells and antigen-presenting cells (APCs) are crucial for optimal APC function, defective CD4+ T-cell activation may contribute to APC dysregulation in HIV infection. Here, we show that CD4+ T cells exposed during stimulation to noninfectious HIV having functional envelope glycoproteins failed to provide activation signals to autologous dendritic cells (DCs). Consequently, important DC functions, including production of immunoregulatory cytokines (interleukin-12 p40 and interleukin-10) and up-regulation of costimulatory molecules (CD86, CD40, CD83), as well as the capacity to stimulate naive allogeneic T cells, were all adversely affected. The blunted up-regulation of CD154 in CD4+ T cells that were activated in the presence of noninfectious viruses is likely to be the major underlying mechanism for these defects. Addition of recombinant trimeric CD154 could restore production of cytokines by DCs cocultured with HIV-exposed T cells. Moreover, the functional defects mediated by coculture with HIV-exposed T cells were similar to those following antibody blockade of CD40-CD154 interactions. HIV-mediated blunted CD154 expression may thus play an important role in the suppression of cell-mediated immunity seen in HIV infection.

Persistence of HIV-1 structural proteins and glycoproteins in lymph nodes of patients under highly active antiretroviral therapy

Here we report a long-term persistence of HIV-1 structural proteins and glycoproteins in germinal centers (GCs) of lymph nodes (LNs) in the absence of detectable virus replication in patients under highly active antiretroviral therapy (HAART). The persistence of viral structural proteins and glycoproteins in GCs was accompanied by specific antibody responses to HIV-1. Seven patients during the chronic phase of HIV-1 infection were analyzed for the presence of the capsid protein (HIV-1p24), matrix protein (HIV-1p17), and envelope glycoproteins (HIV-1gp120/gp41), as well as for viral RNA (vRNA) in biopsy specimens from LNs obtained before initiation of therapy and during HAART that lasted from 5 to 13 months. In parallel, these patients were also monitored for viremia and specific anti-HIV-1 antibody responses to structural proteins and glycoproteins both before and during treatment. Before-therapy viral levels, as determined by RT-PCR, ranged from 3 x 10(3) to 6.3 x 10(5) copies of vRNA per ml, whereas during treatment, vRNA was under detectable levels (<25 copies per ml). The pattern of vRNA detection in peripheral blood was concordant with in situ hybridization results of LN specimens. Before treatment, vRNA associated with follicular dendritic cells (FDCs) was readily detected in GCs of LNs of the patients, whereas during therapy, vRNA was consistently absent in the GCs of LN biopsies of treated patients. In contrast to vRNA hybridization results, viral structural proteins and glycoproteins, evaluated by immunohistochemical staining, were present and persisted in the GC light zone of LNs in abundant amounts not only before initiation of therapy but also during HAART, when no vRNA was detected in GCs. Consistent with immunohistochemical findings, specific antibody responses to HIV-1p17, -p24, and -gp120/gp41, as evaluated by ELISA and virus neutralization, persisted in patients under therapy for up to 13 months of follow-up. The implications of these findings are discussed in relation to HIV-1 persistence in infected individuals and the potential role of chronic antigenic stimulation by the deposited structural proteins in GCs for AIDS-associated B cell malignancies.

Interactions between natural killer cells and antibody Fc result in enhanced antibody neutralization of human immunodeficiency virus type 1

Antibodies can prevent lentivirus infections in animals and may play a role in controlling viral burden in established infection. In preventing and particularly in controlling infection, antibodies likely function in the presence of large quantities of virus. In this study, we explored the mechanisms by which antibodies neutralize large inocula of human immunodeficiency virus type 1 (HIV-1) on different target cells. Immunoglobulin G (IgG) from HIV-infected patients was tested for neutralizing activity against primary R5 strains of HIV-1 at inocula ranging from 100 to 20,000 50% tissue culture infective doses. At all virus inocula, inhibition by antibody was enhanced when target cells for virus growth were monocyte-depleted, peripheral blood mononuclear cells (PBMCs) rather than CD4(+) lymphocytes. However, enhanced inhibition on PBMCs was greatest with larger amounts of virus. Depleting PBMCs of natural killer (NK) cells, which express Fc receptors for IgG (FcgammaRs), abrogated the enhanced antibody inhibition, whereas adding NK cells to CD4(+) lymphocytes restored inhibition. There was no enhanced inhibition on PBMCs when F(ab')(2) was used. Further experiments demonstrated that the release of beta-chemokines, most likely through FcgammaR triggering of NK cells, contributed modestly to the antiviral activity of antibody on PBMCs and that antibody-coated virus adsorbed to uninfected cells provided a target for NK cell-mediated inhibition of HIV-1. These results indicate that Fc-FcgammaR interactions enhance the ability of antibody to neutralize HIV-1. Since FcgammaR-bearing cells are always present in vivo, FcgammaR-mediated antibody function may play a role in the ability of antibody to control lentivirus infection.

Neutralizing as well as non-neutralizing polyclonal immunoglobulin (Ig)G from infected patients capture HIV-1 via antibodies directed against the principal immunodominant domain of gp41

We analyzed the factors influencing the binding of polyclonal immunoglobulin (Ig)G from HIV-infected patients to primary isolates (PI) in capture assays and a potential correlation between this binding and neutralization. The fixation of antibodies (Abs) to viral particles was measured by quantifying the capture of 4 PI by purified IgG immobilized onto a plate or by analyzing the capture of IgG-virus complexes formed in solution. We found that the capture of virus and the formation of immune complexes is mainly achieved by Abs directed against the principal immunodominant domain (PID) of gp41. We have further compared the binding measured by these two methods and the neutralizing activity of our polyclonal IgG and found no correlation. Thus, capture assays, including the immune complex capture assay that is more representative of "physiological" conditions, cannot be used as surrogate method for the investigation of the neutralizing activity of Abs.

Molecular strategies to inhibit HIV-1 replication

The human immunodeficiency virus type 1 (HIV-1) is the primary cause of the acquired immunodeficiency syndrome (AIDS), which is a slow, progressive and degenerative disease of the human immune system. The pathogenesis of HIV-1 is complex and characterized by the interplay of both viral and host factors. An intense global research effort into understanding the individual steps of the viral replication cycle and the dynamics during an infection has inspired researchers in the development of a wide spectrum of antiviral strategies. Practically every stage in the viral life cycle and every viral gene product is a potential target. In addition, several strategies are targeting host proteins that play an essential role in the viral life cycle. This review summarizes the main genetic approaches taken in such antiviral strategies.

The prolonged culture of human immunodeficiency virus type 1 in primary lymphocytes increases its sensitivity to neutralization by soluble CD4

Primary strains of human immunodeficiency virus type 1 (HIV-1) are known to adapt to replication in cell lines in vitro by becoming sensitive to soluble CD4 (sCD4) and neutralizing antibodies (NAb). T-cell lines favor isolation of variants that use CXCR4 as a co-receptor, while primary isolates predominantly use CCR5. We have now studied how a primary R5 isolate, CC1/85, adapts to prolonged replication in primary human peripheral blood mononuclear cells (PBMC). After 19 passages, a variant virus, CCcon.19, had increased sensitivity to both sCD4 and NAb b12 that binds to a CD4-binding site (CD4BS)-associated epitope, but decreased sensitivity to anti-CD4 antibodies. CCcon.19 retains the R5 phenotype, its resistance to other NAbs was unaltered, its sensitivity to various entry inhibitors was unchanged, and its ability to replicate in macrophages was modestly increased. We define CCcon.19 as a primary T-cell adapted (PTCA) variant. Genetic sequence analysis combined with mutagenesis studies on clonal, chimeric viruses derived from CC1/85 and the PTCA variant showed that the most important changes were in the V1/V2 loop structure, one of them involving the loss of an N-linked glycosylation site. Monomeric gp120 proteins expressed from CC1/85 and the PTCA variant did not differ in their affinities for sCD4, suggesting that the structural consequences of the sequence changes were manifested at the level of the native, trimeric Env complex. Overall, the adaptation process probably involves selection for variants with higher CD4 affinity and hence greater fusion efficiency, but this also involves the loss of some resistance to neutralization by agents directed at or near to the CD4BS. The loss of neutralization resistance is of no relevance under in vitro conditions, but NAbs would presumably be a counter-selection pressure against such adaptive changes in vivo, at least when the humoral immune response is intact.

Neutralization of primary HIV-1 SF13 can be detected in extended incubation phase assays with sera from monkeys immunized with recombinant HIV-1 SF2 gp120

Phase III efficacy trials of recombinant human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins were postponed. In Phase I and II trials these candidate vaccines had failed to induce neutralizing antibodies against virus which had been isolated by co-culture with human peripheral blood mononuclear cells (PBMC). The aim of the present study was to determine assay conditions for detecting neutralization of primary HIV-1 isolates with sera from immunized individuals. We show that in two immunogenicity trials in rhesus macaques, recombinant HIV-1 SF2 gp120 induced antibodies which neutralized the primary HIV-1 SF13 isolate. Statistically significant in vitro neutralization required assays in which the incubation phase was extended. Sterile immunity was only seen with the highest level of neutralization, induced by a recombinant prime, peptide boost strategy. We recommend that neutralization assays with extended incubation phases should be used to monitor Phase III efficacy trials.

Polyclonal immunoglobulin G from patients neutralizes human immunodeficiency virus type 1 primary isolates by binding free virions, but without interfering with an initial CD4-independent attachment of the virus to primary blood mononuclear cells

We investigated the relationship between human immunodeficiency virus type 1 (HIV-1) primary isolate (PI) antibody-mediated neutralization and attachment to primary blood mononuclear cells (PBMC). Incubation of PIs with immunoglobulin G (IgG) purified from infected patients did not inhibit attachment of the viruses with PBMC, but partial to complete neutralization was achieved. Neutralization of PIs already fixed on the cells was achieved by some IgG samples only and was of limited intensity compared to the former neutralization protocol. On the contrary, the binding of IgG to free virions was shown to be sufficient to reach potent neutralization, as the infectivity of IgG-PI complexes purified from the bulk of antibodies before addition to PBMC was strongly diminished compared to mock-treated controls. Monoclonal antibodies to the CDR2 domain of CD4 completely inhibited the infection of PBMC without interfering with the attachment of PIs to the cells, suggesting that, under these experimental conditions, the initial attachment of viruses to PBMC involves alternative cellular receptors. This initial interaction may also involve other components of the viral envelope than gp120, as partial depletion of the surface glycoproteins of primary viral particles that resulted in an almost complete loss of infectivity did not impair attachment to PBMC. A limited inhibition of attachment was observed when interfering with putative interactions with cellular heparan sulfate, whereas no effect was observed for cellular CD147 or nucleolin or for virion-incorporated cyclophilin A. Altogether, our results favor a mechanism of neutralization of HIV-1 PIs by polyclonal IgG where antibodies predominantly bind free virions and neutralize without interfering with the attachment to PBMC, which, in this model, is mainly CD4 independent.

Epitopes corresponding to the envelope genetic subtype are present on the surface of free virions of HIV-1 group M primary isolates and can be detected in neutralization assays with extended incubation phases

The hypothesis is that there are neutralizing epitopes on the surface of free virions of human immunodeficiency virus type 1 (HIV-1) that correspond to the genetic subtype of the envelope glycoprotein. Assays with extended incubation and reduced absorption phases are required to demonstrate neutralization with antibodies to these epitopes. These assays quantify virus infectivity, rather than reductions in release of antigen into culture supernatants. Neutralizing antibodies reduce virus infectivity by at least 80%, as scored by the presence/absence of antigen released after 14 days in culture of mitogen-transformed peripheral blood mononuclear cells (PBMCs). The epitopes are shared within different subtypes of group M, but not group O, isolates. Individual plasma, selected from three, independent panels of seropositive individuals, cross-neutralize within each subtype as well as the combinations of A with C, B with D or G, and C with CRF01_AE. Isolates within subtype B show the greatest variation in their resistance to neutralization, ranging from highly sensitive to highly resistant. No highly sensitive subtype D isolates were identified. Isolates from subtypes A, C, and CRF01_AE were all resistant. The strategic implication for vaccine design is that antibodies to a limited number of epitopes can neutralize more than 90% of the HIV-1 isolates that are circulating currently in the world. Also, since only antibodies that produce an all-or-nothing loss in virus infectivity can reasonably be expected to prevent the viremic phase after in vivo infection, assays with extended incubation, and culture phases should be used to monitor current efficacy trials.

Tropism-restricted neutralization by secretory IgA from parotid saliva of HIV type 1-infected individuals

In this study we examined secretory IgA, isolated from the parotid saliva of 10 HIV-1-infected subjects, for its ability to influence HIV-1 infection of peripheral blood mononuclear cells with two R5 and two X4 primary isolates. Salivary IgA from four subjects was found to inhibit both R5 viruses but not the X4 viruses. In another subject, salivary IgA inhibited both X4 viruses but not the R5 viruses. The specificity of these antibodies seemed to be directed against, but not restricted to, gp160 and gp120. Compared with subjects whose salivary IgA did not inhibit HIV-1 infection, subjects who displayed neutralizing activity were in relatively early stages of disease and had CD4(+) T cell counts greater than 200 cells/microl. Our data indicate the presence of tropism-specific (more frequently R5-specific) neutralizing antibodies in HIV-1-infected subjects. Because mucosal transmission of HIV-1 occurs exclusively in R5 viruses, and X4 viruses often emerge in established infection and account for viral persistence later in disease, our data suggest a potential role for secretory IgA in preventing viral transmission, but a less likely effect on chronic infection.

Inter-subtype cross-neutralizing antibodies recognize epitopes on cell-associated HIV-1 virions

HIV-1 infected individuals with cross-neutralizing antibodies against primary HIV-1 isolates belonging to Group M (envA-H) and O, are identified. To investigate the neutralization-kinetics of primary isolates with these antibodies, different neutralization assay conditions are compared. Each set is summarized as a/b/c where a is the time in hours for which antibody is incubated with virus, b is the time in hours allowed for virus to absorb to cells, c is the total culture period in days, from the cells' first exposure to virus, before antigen production (peripheral blood mononuclear cells) or number of fluorescent cells (GHOST) are measured. In HIV-infected individuals, neutralizing antibodies can be detected against a wide range of primary isolates (Group M; A-H and Group O) in PBMC-assays with short incubation phases (1/2/7 or 1/24/7). If cultures are extended (1/2/14 or 1/24/14), however, neutralization can be lost. In kinetic experiments, neutralization can even be seen without pre-incubation (a=0 hr). This study shows that neutralization of primary HIV isolates by cross-reactive antibodies can continue after the virus has bound to its target cell. This neutralization, however, is not an all or nothing loss in virus infectivity. Most often it leads only to a reduction in viral replication rates.

Human immunodeficiency virus type 1 attachment, coreceptor, and fusion inhibitors are active against both direct and trans infection of primary cells

Inhibitors of human immunodeficiency virus type 1 attachment (CD4-immunoglobulin G subclass 2), CCR5 usage (PRO 140), and fusion (T-20) were tested on diverse primary cell types that represent the major targets both for infection in vivo and for the inhibition of trans infection of target cells by virus bound to dendritic cells. Although minor cell-type-dependent differences in potency were observed, each inhibitor was active on each cell type and trans infection was similarly vulnerable to inhibition at each stage of the fusion cascade.

Nonneutralizing antibodies to the CD4-binding site on the gp120 subunit of human immunodeficiency virus type 1 do not interfere with the activity of a neutralizing antibody against the same site

We have investigated whether nonneutralizing monoclonal antibodies (MAbs) to the gp120 subunit of the envelope glycoprotein (Env) complex of human immunodeficiency virus type 1 (HIV-1) can interfere with HIV-1 neutralization by another anti-gp120 MAb. We used neutralizing (b12) and nonneutralizing (205-42-15, 204-43-1, 205-46-9) MAbs to the epitope cluster overlapping the CD4-binding site (CD4BS) on gp120. All the MAbs, neutralizing or otherwise, cross-competed for binding to monomeric gp120, indicating the close topological proximity of their epitopes. However, the nonneutralizing CD4BS MAbs did not interfere with the neutralization activity of MAb b12. In contrast, in a binding assay using oligomeric Env expressed on the surface of Env-transfected cells, the nonneutralizing MAbs did partially compete with b12 for Env binding. The surface of Env-transfected cells contains two categories of binding site for CD4BS MAbs. One type of site is recognized by both b12 and nonneutralizing CD4BS MAbs; the other is recognized by only b12. Binding assays for Env-gp120 interactions based on the use of monomeric gp120 or Env-transfected cells do not predict the outcome of HIV-1 neutralization assays, and they should therefore be used only with caution when gauging the properties of anti-Env MAbs.

Occupancy and mechanism in antibody-mediated neutralization of animal viruses

Neutralization of virus infectivity by antibodies is an important component of immunity to several virus infections. Here, the immunochemical basis for the action of neutralizing antibodies, and what role their induction of conformational changes in the antigen might play, is reviewed. Theories of the mechanisms by which antibodies neutralize virus infectivity in vitro are also presented. The theoretical and empirical foundation of the hypothesis that viruses are neutralized by a single antibody per virion is critically reviewed. The relationship between antibody occupancy on virions and the mechanism of neutralization is explored. Examples of neutralization mediated through antibody interference with virus attachment and entry are discussed and test implications of refined theories of neutralization by antibody coating of virions are formulated.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: failure to protect and possible enhancement of challenge infection by four cell-based vaccines prepared with autologous lymphoblasts

Immunogenicity and protective activity of four cell-based feline immunodeficiency virus (FIV) vaccines prepared with autologous lymphoblasts were investigated. One vaccine was composed of FIV-infected cells that were paraformaldehyde fixed at the peak of viral expression. The other vaccines were attempts to maximize the expression of protective epitopes that might become exposed as a result of virion binding to cells and essentially consisted of cells mildly fixed after saturation of their surface with adsorbed, internally inactivated FIV particles. The levels of FIV-specific lymphoproliferation exhibited by the vaccinees were comparable to the ones previously observed in vaccine-protected cats, but antibodies were largely directed to cell-derived constituents rather than to truly viral epitopes and had very poor FIV-neutralizing activity. Moreover, under one condition of testing, some vaccine sera enhanced FIV replication in vitro. As a further limit, the vaccines proved inefficient at priming animals for anamnestic immune responses. Two months after completion of primary immunization, the animals were challenged with a low dose of homologous ex vivo FIV. Collectively, 8 of 20 vaccinees developed infection versus one of nine animals mock immunized with fixed uninfected autologous lymphoblasts. After a boosting and rechallenge with a higher virus dose, all remaining animals became infected, thus confirming their lack of protection.

Cellular factors influence the binding of HIV type 1 to cells

The goal of this study was to determine the importance of cellular factors for binding of HIV to cells. HIV primary isolates (PIs) produced in peripheral blood mononuclear cells (PBMCs) bound at relatively high levels to PBMCs but at low levels to cell lines, whereas T cell line-adapted (TCLA) virus produced in the H9 T cell line bound at high levels to both cell lines and PBMCs. Expression of CD4 in CD4-negative cells or blocking CD4 with antibody on CD4-positive cells did not affect virus binding. Blocking of gp120/gp41 with antibodies or a lack of expression of gp120/gp41 in virus particles also did not affect virus binding. However, the cell type from which virus was produced did affect virus binding. Thus, the binding pattern of TCLA virus shifted to that of a PI virus when produced in PBMCs. A PI binding pattern also occurred when a cloned TCLA virus (NL4-3) was produced in PBMCs, indicating that the virus-producing cell type has more of an effect on virus binding than the virus strain. These experiments show that both the virus-producing cell and the target cell have a major influence on HIV binding and suggest that host cell factors incorporated into virions are important for virus binding.

Immunoglobulin G (IgG) and IgA, but also nonantibody factors, account for in vitro neutralization of human immunodeficiency virus (HIV) type 1 primary isolates by serum and plasma of HIV-infected patients

The factors present in serum and plasma samples of human immunodeficiency virus (HIV)-infected patients that are responsible for the neutralization of four HIV type 1 (HIV-1) primary isolates in vitro have been analyzed. Purification of immunoglobulins (Ig) by affinity chromatography showed that the activities were mostly attributable to IgG and less frequently to IgA. For two samples, we have shown that the high-level and broad-spectrum inhibitory activity was essentially caused by non-Ig factors interfering with the measurement of antibody-specific neutralizing activity.