Role of neutralizing antibodies in HIV infection

Serum antibodies to phosphatidylcholine in MS

OBJECTIVE

To evaluate the value of serum immunoglobulin G (IgG) and immunoglobulin M (IgM) antibodies reactive with phosphatidylcholine (PC) and lactosylceramide (LC) as biomarkers in MS.

METHODS

We developed an ultrasensitive ELISA technique to analyze serum IgG and IgM antibodies to LC and PC, which we used to analyze samples from 362 patients with MS, 10 patients with non-MS myelin diseases (Non-MSMYDs), 11 patients with nonmyelin neurologic diseases (Non-MYNDs), and 80 controls. MS serum samples included clinically isolated syndrome (CIS, n = 17), relapsing-remitting MS (RRMS, n = 62), secondary progressive MS (SPMS, n = 50), primary progressive MS (PPMS, n = 37), and benign MS (BENMS, n = 36).

RESULTS

We detected higher levels of serum IgM antibodies to PC (IgM-PC) in MS than control samples; patients with CIS and RRMS showed higher IgM-PC levels than patients with SPMS, PPMS, and BENMS and controls. MS and control samples did not differ in serum levels of IgM antibodies reactive with LC, nor in IgG antibodies reactive with LC or PC.

CONCLUSIONS

Serum IgM-PC antibodies are elevated in patients with MS, particularly during the CIS and RRMS phases of the disease. Thus, serum IgM-PC is a candidate biomarker for early inflammatory stages of MS.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that serum antibodies to PC are elevated in patients with MS. The study is rated Class III because of the case control design and the risk of spectrum bias: antibody levels in patients with MS were compared with healthy controls.

Natural antibodies bridge innate and adaptive immunity

Natural Abs, belonging to isotypes IgM, IgG3, and IgA, were discovered nearly half a century ago. Despite knowledge about the role of the polyreactive natural IgM in pathogen elimination, B cell survival and homeostasis, inflammatory diseases, and autoimmunity, there is a lack of clarity about the physiological role of natural IgG and natural IgA because they appear incapable of recognizing Ags on their own and are perceived as nonreactive. However, recent research revealed exciting functions of natural IgG in innate immunity. Natural IgG:lectin collaboration swiftly and effectively kills invading pathogens. These advances prompt further examination of natural Abs in immune defense and homeostasis, with the potential for developing novel therapeutics. This review provides new insights into the interaction between natural Abs and lectins, with implications on how interactions between molecules of the innate and adaptive immune systems bridge these two arms of immunity.

Lessons in nonhuman primate models for AIDS vaccine research: from minefields to milestones

Nonhuman primate (NHP) disease models for AIDS have made important contributions to the search for effective vaccines for AIDS. Viral diversity, persistence, capacity for immune evasion, and safety considerations have limited development of conventional approaches using killed or attenuated vaccines, necessitating the development of novel approaches. Here we highlight the knowledge gained and lessons learned in testing vaccine concepts in different virus/NHP host combinations.

Antibody-Dependent Cellular Cytotoxicity and NK Cell-Driven Immune Escape in HIV Infection: Implications for HIV Vaccine Development

The HIV-1 genome is malleable and a difficult target tot vaccinate against. It has long been recognised that cytotoxic T lymphocytes and neutralising antibodies readily select for immune escape HIV variants. It is now also clear that NK cells can also select for immune escape. NK cells force immune escape through both direct Killer-immunoglobulin-like receptor (KIR)-mediated killing as well as through facilitating antibody-dependent cellular cytotoxicity (ADCC). These newer finding suggest NK cells and ADCC responses apply significant pressure to the virus. There is an opportunity to harness these immune responses in the design of more effective HIV vaccines.

Broad antibody mediated cross-neutralization and preclinical immunogenicity of new codon-optimized HIV-1 clade CRF02_AG and G primary isolates

Creation of an effective vaccine for HIV has been an elusive goal of the scientific community for almost 30 years. Neutralizing antibodies are assumed to be pivotal to the success of a prophylactic vaccine but previous attempts to make an immunogen capable of generating neutralizing antibodies to primary “street strain” isolates have resulted in responses of very limited breadth and potency. The objective of the study was to determine the breadth and strength of neutralizing antibodies against autologous and heterologous primary isolates in a cohort of HIV-1 infected Nigerians and to characterize envelopes from subjects with particularly broad or strong immune responses for possible use as vaccine candidates in regions predominated by HIV-1 CRF02_AG and G subtypes. Envelope vectors from a panel of primary Nigerian isolates were constructed and tested with plasma/sera from the same cohort using the PhenoSense HIV neutralizing antibody assay (Monogram Biosciences Inc, USA) to assess the breadth and potency of neutralizing antibodies. The immediate goal of this study was realized by the recognition of three broadly cross-neutralizing sera: (NG2-clade CRF02_AG, NG3-clade CRF02_AG and NG9- clade G). Based on these findings, envelope gp140 sequences from NG2 and NG9, complemented with a gag sequence (Clade G) and consensus tat (CRF02_AG and G) antigens have been codon-optimized, synthesized, cloned and evaluated in BALB/c mice. The intramuscular administration of these plasmid DNA constructs, followed by two booster DNA immunizations, induced substantial specific humoral response against all constructs and strong cellular responses against the gag and tat constructs. These preclinical findings provide a framework for the design of candidate vaccine for use in regions where the HIV-1 epidemic is driven by clades CRF02_AG and G.

Elimination of helminth infection restores HIV-1C vaccine-specific T cell responses independent of helminth-induced IL-10

HIV-1 prevalence is highest in developing countries; similarly helminth parasites are often highly endemic in these same areas. Helminths are strong immune modulators, and negatively impact the ability of the infected hosts to mount protective vaccine-specific T cell immune responses for HIV-1 and other pathogens. Indeed, previously we found that Schistosoma mansoni infected mice had significantly impaired HIV-1C vaccine-specific T cell responses. Anthelminthics are available and inexpensive; therefore, in this study, we evaluated whether elimination of schistosome infection prior to vaccination with an HIV-1C DNA vaccine would increase recipients vaccine-specific responses. As expected, splenocytes from S. mansoni infected mice produced significantly elevated amounts of interleukin (IL)-4 and IL-10, and significantly lower amounts of interferon (IFN)-gamma than splenocytes from naïve mice. Following elimination of parasites by praziquantel (PZQ) treatment, splenomegaly was significantly reduced, though splenocytes produced similar or higher levels of IL-10 than splenocytes from infected mice. However, we found that PZQ treatment significantly increased levels of IFN-gamma in response to concanavalin A or SEA compared to splenocytes from untreated mice. Importantly, PZQ treatment resulted in complete restoration of HIV-1C vaccine-specific T cell responses at 8 weeks post-PZQ treatment. Restoration of HIV-1C vaccine-specific T cell responses following elimination of helminth infection was time dependent, but surprisingly independent of the levels of IL-4 and IL-10 induced by parasite antigens. Our study shows that elimination of worms offers an affordable and a simple means to restore immune responsiveness to T cell based vaccines for HIV-1 and other infectious diseases in helminth endemic settings.

An oligosaccharide-based HIV-1 2G12 mimotope vaccine induces carbohydrate-specific antibodies that fail to neutralize HIV-1 virions

The conserved oligomannose epitope, Man(9)GlcNAc(2), recognized by the broadly neutralizing human mAb 2G12 is an attractive prophylactic vaccine candidate for the prevention of HIV-1 infection. We recently reported total chemical synthesis of a series of glycopeptides incorporating one to three copies of Man(9)GlcNAc(2) coupled to a cyclic peptide scaffold. Surface plasmon resonance studies showed that divalent and trivalent, but not monovalent, compounds were capable of binding 2G12. To test the efficacy of the divalent glycopeptide as an immunogen capable of inducing a 2G12-like neutralizing antibody response, we covalently coupled the molecule to a powerful immune-stimulating protein carrier and evaluated immunogenicity of the conjugate in two animal species. We used a differential immunoassay to demonstrate induction of high levels of carbohydrate-specific antibodies; however, these antibodies showed poor recognition of recombinant gp160 and failed to neutralize a panel of viral isolates in entry-based neutralization assays. To ascertain whether antibodies produced during natural infection could recognize the mimetics, we screened a panel of HIV-1-positive and -negative sera for binding to gp120 and the synthetic antigens. We present evidence from both direct and competitive binding assays that no significant recognition of the glycopeptides was observed, although certain sera did contain antibodies that could compete with 2G12 for binding to recombinant gp120.

Neutralization of HIV-1 by redirection of natural antibodies

The great variability and high glycosylation of gp120 poses a great challenge for the design of a functional immune therapy. The binding region of the CD4 receptor to gp120, however, is well conserved and may constitute a target to limit viral entry and infectivity. Our strategy consists in using a preexisting pool of natural antibodies directed toward the gal(alpha1,3)gal disaccharide and to redirect it to HIV. We here show that using CD4-derived, gp120-binding, synthetic peptides chemically linked to gal(alpha1,3)gal can redirect these natural antibodies and improve the HIV-1 neutralizing activity of the CD4-derived peptides in vitro. Importantly, the binding of the CD4-gal(alpha1,3)gal peptides to HIV-1-infected cells conferred antibody-dependent cellular cytotoxicity after the addition of human sera. Thus, the temporary redirection of naturally occurring antibodies and their biological activities to a new antigen represents a completely new way of targeting a human disease.

Impact of glycosylation on antigenicity of simian immunodeficiency virus SIV239: induction of rapid V1/V2-specific non-neutralizing antibody and delayed neutralizing antibody following infection with an attenuated deglycosylated mutant

Infection of rhesus macaques with a deglycosylation mutant, Delta5G, derived from SIV239, a pathogenic clone of simian immunodeficiency virus (SIV), led to robust acute-phase viral replication followed by a chronic phase with undetectable viral load. This study examined whether humoral responses in Delta5G-infected animals played any role in the control of infection. Neutralizing antibodies (nAbs) were elicited more efficiently in Delta5G-infected animals than in SIV239-infected animals. However, functional nAb measured by 90% neutralization was prominent in only two of the five Delta5G-infected animals, and only at 8 weeks post-infection (p.i.), when viral loads were already below 10(4) copies ml(-1). These results suggest a minimal role for nAbs in the control of the primary infection. In contrast, whilst Ab responses to epitopes localized to the variable loops V1/V2 were detected in all Delta5G-infected animals at 3 weeks p.i., this response was associated with a concomitant reduction in Ab responses to epitopes in gp41 compared with those in SIV239-infected animals. These results suggest that the altered surface glycosylation and/or conformation of viral spikes induce a humoral response against SIV that is distinct from the response induced by SIV239. More interestingly, whereas V1/V2-specific Abs were induced in all animals, these Abs were associated with vigorous Delta5G-specific virion capture ability in only two Delta5G-infected animals that exhibited a functional nAb response. Thus, whereas the deglycosylation mutant infection elicited early virion capture and subsequent nAbs, the responses differed among animals, suggesting the existence of host factors that may influence the functional humoral responses against human immunodeficiency virus/SIV.

Enhancing exposure of HIV-1 neutralization epitopes through mutations in gp41

BACKGROUND

The generation of broadly neutralizing antibodies is a priority in the design of vaccines against HIV-1. Unfortunately, most antibodies to HIV-1 are narrow in their specificity, and a basic understanding of how to develop antibodies with broad neutralizing activity is needed. Designing methods to target antibodies to conserved HIV-1 epitopes may allow for the generation of broadly neutralizing antibodies and aid the global fight against AIDS by providing new approaches to block HIV-1 infection. Using a naturally occurring HIV-1 Envelope (Env) variant as a template, we sought to identify features of Env that would enhance exposure of conserved HIV-1 epitopes.

METHODS AND FINDINGS

Within a cohort study of high-risk women in Mombasa, Kenya, we previously identified a subtype A HIV-1 Env variant in one participant that was unusually sensitive to neutralization. Using site-directed mutagenesis, the unusual neutralization sensitivity of this variant was mapped to two amino acid mutations within conserved sites in the transmembrane subunit (gp41) of the HIV-1 Env protein. These two mutations, when introduced into a neutralization-resistant variant from the same participant, resulted in 3- to >360-fold enhanced neutralization by monoclonal antibodies specific for conserved regions of both gp41 and the Env surface subunit, gp120, >780-fold enhanced neutralization by soluble CD4, and >35-fold enhanced neutralization by the antibodies found within a pool of plasmas from unrelated individuals. Enhanced neutralization sensitivity was not explained by differences in Env infectivity, Env concentration, Env shedding, or apparent differences in fusion kinetics. Furthermore, introduction of these mutations into unrelated viral Env sequences, including those from both another subtype A variant and a subtype B variant, resulted in enhanced neutralization susceptibility to gp41- and gp120-specific antibodies, and to plasma antibodies. This enhanced neutralization sensitivity exceeded 1,000-fold in several cases.

CONCLUSIONS

Two amino acid mutations within gp41 were identified that expose multiple discontinuous neutralization epitopes on diverse HIV-1 Env proteins. These exposed epitopes were shielded on the unmodified viral Env proteins, and several of the exposed epitopes encompass desired target regions for protective antibodies. Env proteins containing these modifications could act as a scaffold for presentation of such conserved domains, and may aid in developing methods to target antibodies to such regions.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Analysis of the neutralization breadth of the anti-V3 antibody F425-B4e8 and re-assessment of its epitope fine specificity by scanning mutagenesis

The identification of cross-neutralizing antibodies to HIV-1 is important for designing antigens aimed at eliciting similar antibodies upon immunization. The monoclonal antibody (mAb) F425-B4e8 had been suggested previously to bind an epitope at the base of V3 and shown to neutralize two primary HIV isolates. Here, we have assessed the neutralization breadth of mAb F425-B4e8 using a 40-member panel of primary HIV-1 and determined the epitope specificity of the mAb. The antibody was able to neutralize 8 clade B viruses (n=16), 1 clade C virus (n=11), and 2 clade D viruses (n=6), thus placing it among the more broadly neutralizing anti-V3 antibodies described so far. Contrary to an initial report, results from our scanning mutagenesis of the V3 region suggest that mAb F425-B4e8 interacts primarily with the crown/tip of V3, notably Ile(309), Arg(315), and Phe(317). Despite the somewhat limited neutralization breadth of mAb F425-B4e8, the results presented here, along with analyses from other cross-neutralizing anti-V3 mAbs, may facilitate the template-based design of antigens that target V3 and permit neutralization of HIV-1 strains in which the V3 region is accessible to antibodies.

GTU®-MultiHIV DNA vaccine results in protection in a novel P815 tumor challenge model

A novel animal model for testing the immunogenicity and protective immune response induced by HIV-1 DNA vaccines was developed. DBA/2 mice were immunized with GTU-MultiHIV DNA encoding multigene for Rev, Nef, Tat, optp17/24 and a stretch of Pol/Env epitopes. A single GTU-MultiHIV B-clade specific plasmid or Auxo-GTU-MultiHIV(mix) (mixture of four plasmids with A, B, C and FGH clade specific MultiHIV antigens) were administered via gene gun and cell-mediated and humoral immune responses were analysed. The protective efficacy of the immune response was evaluated by challenging the mice with syngeneic tumor cells (P815) stably transfected with the MultiHIV fusion gene. Our results show that the strong MultiHIV-specific immune response generated by the GTU-MultiHIV vaccines in DBA/2 mice was able to delay the tumor growth substantially, indicating that the CTL response detected in vitro confers protection in vivo. The model described here is a safe and feasible in vivo assay for assessment of the vaccine potency to induce protective cell-mediated immune responses.

Comparison of heterologous neutralizing antibody responses of human immunodeficiency virus type 1 (HIV-1)- and HIV-2-infected Senegalese patients: distinct patterns of breadth and magnitude distinguish HIV-1 and HIV-2 infections

Neutralizing antibody responses against heterologous isolates in human immunodeficiency virus type 1 (HIV-1) and HIV-2 infections were compared, and their relationships with established clinical markers of progression were examined. Neutralizing responses against 7 heterologous primary isolates and 1 laboratory strain were compared between 32 untreated HIV-1-infected subjects and 35 untreated HIV-2-infected subjects using a pseudotyped reporter virus assay. The breadth of the neutralizing response, defined as the proportion of panel viruses positively neutralized by patient plasma, was significantly greater among HIV-2-infected subjects than among HIV-1-infected subjects. Notably, for fully one-third of HIV-2 subjects, all viruses were effectively neutralized in our panel. Magnitudes of responses, defined as reciprocal 50% inhibitory concentration (IC(50)) titers for positive reactions, were significantly greater among HIV-1-infected subjects than among HIV-2-infected subjects. When plasma samples from HIV-1 patients were tested for cross-neutralization of HIV-2 and vice versa, we found that these intertype responses are very rare and their prevalences comparable in both HIV-1 and HIV-2 infection. The significantly higher magnitude of heterologous responses for HIV-1 compared to HIV-2 prompted us to examine associations with viremia, which is known to be significantly higher in HIV-1 infection. Importantly, there was a significant positive correlation between the IC(50) titer and viral load within both the HIV-1 and HIV-2 groups, suggesting heterologous antibodies may be driven by viral replication. We conclude that HIV-2 infection is characterized by a broad, low-magnitude intratype neutralization response, while HIV-1 is characterized by a narrower but higher-magnitude intratype response and that a significant positive association between the IC(50) titer and viremia is common to both HIV-1 and HIV-2 infections.

The antigenic determinants on HIV p24 for CD4+ T cell inhibiting antibodies as determined by limited proteolysis, chemical modification, and mass spectrometry

In the last decade, mass spectrometry has been employed by more and more researchers for identifying the proteins in a macromolecular complex as well as for defining the surfaces of their binding interfaces. This characterization of protein-protein interfaces usually involves at least one of several different methodologies in addition to the actual mass spectrometry. For example, limited proteolysis is often used as a first step in defining regions of a protein that are protected from proteolysis when the protein of interest is part of a macromolecular complex. Other techniques used in conjunction with mass spectrometry for determining regions of a protein involved in protein-protein interactions include chemical modification, such as covalent cross-linking, acetylation of lysines, hydrogen-deuterium exchange, or other forms of modification. In this report, both limited proteolysis and chemical modification were combined with several mass spectrometric techniques in efforts to define the protein surface on the HIV core protein, p24, recognized by two different monoclonal human antibodies that were isolated from HIV+ patients. One of these antibodies, 1571, strongly inhibits the CD4+ T cell proliferative response to a known epitope (PEVIPMFSALSEGATP), while the other antibody, 241-D, does not inhibit as strongly. The epitopes for both of these antibodies were determined to be discontinuous and localized to the N-terminus of p24. Interestingly, the epitope recognized by the strongly inhibiting antibody, 1571, completely overlaps the T cell epitope PEVIPMFSALSEGATP, while the antibody 241-D binds to a region adjacent to the region of p24 recognized by the antibody 1571. These results suggest that, possibly due to epitope competition, antibodies produced during HIV infection can negatively affect CD4+ T cell-mediated immunity against the virus.

Helminth infection suppresses T-cell immune response to HIV-DNA-based vaccine in mice

A number of HIV-1 vaccines are in various phases of clinical trials and many more are in the developmental pipeline. Vaccines are especially needed for developing countries where morbidity and mortality due to HIV/AIDS is most severe, the prevalence of HIV infection is highest, and its incidence is often still rising dramatically. Individuals living in these regions are often infected with one or more helminth parasites which systemically bias the immune system towards Th2-type as well as drive immune anergy. The goal of this study was to develop a multi-T-cell epitope DNA-based vaccine for HIV-1 subtype C and to determine the impact of helminth infection on the immune response to this vaccine. We found that vaccination of naïve mice with the multi-epitope vaccine, designated TD158, induced a strong HIV-1C-specific T-cell immune response, and that the addition of the Igkappa leader sequence to the TD158 vaccine construct significantly increased the frequencies of IFN-gamma secreting CD8+ T cells. However, the TD158 vaccine specific response of mice infected with the human helminth Schistosoma mansoni was significantly suppressed. The impact of schistosome infection on suppressing the virus-specific immune response was the same whether mice were vaccinated with the TD158 vaccine or with the Igkappa enhanced TD158. The results of this study suggest that helminth infection may pose a serious problem for vaccination with the DNA-based HIV-1 vaccine in developing country populations, and that the prevalence of helminth infections in the vaccine cohorts should be taken into account for HIV-1 vaccine trial design.

A randomized, placebo-controlled phase I trial of DNA prime, recombinant fowlpox virus boost prophylactic vaccine for HIV-1

An HIV-vaccine consisting of a DNA prime, recombinant fowlpox virus (rFPV) boost was evaluated in a double-blind placebo controlled trial. One milligram of pHIS-HIV-B expressing mutated gag, pol, env, vpu, tat and rev was administered at weeks 0 and 4 boosted by 5 x 10(7) pfu rFPV-HIV-B expressing gag/pol at week 8. The vaccine regimen was safe, but there was no difference between vaccine (n = 18) and placebo recipients (n = 6) for Gag or Pol-specific T-cell immune responses at week 9.

Human immunodeficiency virus type 1 variants isolated from single plasma samples display a wide spectrum of neutralization sensitivity

Individuals infected with human immunodeficiency virus type 1 (HIV-1) harbor a mixture of viral variants with different sequences and in some instances with different phenotypic properties. Major and rapid fluctuations in the proportion of viral variants coexisting in an infected individual can be observed under strong pharmacological and immune selective pressure. Because of the short half-life of HIV-infected cells and of HIV virions in the blood, plasma virus populations are highly relevant to HIV evolution in the face of these selective pressures. Here we analyzed the sensitivity to antibody-mediated neutralization of viral variants coexisting in the plasma virus populations of two infected patients. For each patient, several replication-competent viral clones were constructed that carry primary envelope gene sequences obtained from a single plasma sample. Viral clones differed in their tropism and replicative capacity and in the number and positions of glycosylation sites in the envelope glycoproteins. Viruses were tested against heterologous and autologous sera obtained at different time points. Interestingly, we found that viral variants coexisting in each plasma sample were highly heterogeneous in terms of sensitivity to neutralization. The order of sensitivity depended on the serum used and was not associated with virus tropism. The neutralization potency of sera increased with the duration of the infection for both autologous and heterologous neutralization.

Influence of glycosylation on the efficacy of an Env-based vaccine against simian immunodeficiency virus SIVmac239 in a macaque AIDS model

The envelope glycoprotein (Env) of human immunodeficiency viruses (HIVs) and simian immunodeficiency viruses (SIVs) is heavily glycosylated, and this feature has been speculated to be a reason for the insufficient immune control of these viruses by their hosts. In a macaque AIDS model, we demonstrated that quintuple deglycosylation in Env altered a pathogenic virus, SIVmac239, into a novel attenuated mutant virus (delta5G). In delta5G-infected animals, strong protective immunity against SIVmac239 was elicited. These HIV and SIV studies suggested that an understanding of the role of glycosylation is critical in defining not only the virological properties but also the immunogenicity of Env, suggesting that glycosylation in Env could be modified for the development of effective vaccines. To examine the effect of deglycosylation, we constructed prime-boost vaccines consisting of Env from SIVmac239 and delta5G and compared their immunogenicities and vaccine efficacies by challenge infection with SIVmac239. Vaccination-induced immune responses differed between the two vaccine groups. Both Env-specific cellular and humoral responses were higher in wild-type (wt)-Env-immunized animals than in delta5G Env-immunized animals. Following the challenge, viral loads in SIVmac239 Env (wt-Env)-immunized animals were significantly lower than in vector controls, with controlled viral replication in the chronic phase. Unexpectedly, viral loads in delta5G Env-immunized animals were indistinguishable from those in vector controls. This study demonstrated that the prime-boost Env vaccine was effective against homologous SIVmac239 challenge. Changes in glycosylation affected both cell-mediated and humoral immune responses and vaccine efficacy.

Delay of HIV-1 rebound after cessation of antiretroviral therapy through passive transfer of human neutralizing antibodies

To determine the protective potential of the humoral immune response against HIV-1 in vivo we evaluated the potency of three neutralizing antibodies (2G12, 2F5 and 4E10) in suppressing viral rebound in six acutely and eight chronically HIV-1-infected individuals undergoing interruption of antiretroviral treatment (ART). Only two of eight chronically infected individuals showed evidence of a delay in viral rebound during the passive immunization. Rebound in antibody-treated acutely infected individuals upon cessation of ART was substantially later than in a control group of 12 individuals with acute infection. Escape mutant analysis showed that the activity of 2G12 was crucial for the in vivo effect of the neutralizing antibody cocktail. By providing further direct evidence of the potency, breadth and titers of neutralizing antibodies that are required for in vivo activity, these data underline both the potential and the limits of humoral immunity in controlling HIV-1 infection.

New approaches to eliciting protective immunity through T cell repertoire manipulation: the concept of thymic vaccination

Conventional vaccines afford protection against infectious diseases by expanding existing pathogen-specific peripheral lymphocytes, both CD8 cytotoxic effector (CTL) and CD4 helper T cells. The latter induce B cell maturation and antibody production. As a consequence, lymphocytes within the memory pool are poised to rapidly proliferate at the time of a subsequent infection. The "thymic vaccination" concept offers a novel way to alter the primary T cell repertoire through exposure of thymocytes to altered peptide ligands (APL) with reduced T cell receptor (TCR) affinity relative to cognate antigens recognized by those same TCRs. Thymocyte maturation (i.e. positive selection) is enhanced by low affinity interaction between a TCR and an MHC-bound peptide in the thymus and subsequent emigration of mature cells into the peripheral T lymphocyte pool follows. In principal, such variants of antigens derived from infectious agents could be utilized for peptide-driven maturation of thymocytes bearing pathogen-specific TCRs. To test this idea, APLs of gp33-41, a Db-restricted peptide derived from the lymphocytic choriomeningitis virus (LCMV) glycoprotein, and of VSV8, a Kb-restricted peptide from the vesicular stomatitis virus (VSV) nucleoprotein, have been designed and their influence on thymic maturation of specific TCR-bearing transgenic thymocytes examined in vivo using irradiation chimeras. Injection of APL resulted in positive selection of CD8 T cells expressing the relevant viral specificity and in the export of those virus-specific CTL to lymph nodes without inducing T cell proliferation. Thus, exogenous APL administration offers the potential of expanding repertoires in vivo in a manner useful to the organism. To efficiently peripheralize antigen-specific T cells, concomitant enhancement of mechanisms promoting thymocyte migration appears to be required. This commentary describes the rationale for thymic vaccination and addresses the potential prophylactic and therapeutic applications of this approach for treatment of infectious diseases and cancer. Thymic vaccination-induced peptide-specific T cells might generate effective immune protection against disease-causing agents, including those for which no effective natural protection exists.