Antibodies against viruses: passive and active immunization

Is malaria immunity a possible protection against severe symptoms and outcomes of COVID-19?

Malaria-endemic areas of the world are noted for high morbidity and mortality from malaria. Also noted in these areas is the majority of persons in the population having acquired malaria immunity. Though this acquired malaria immunity does not prevent infection, it resists the multiplication of Plasmodium parasites, restricting disease to merely uncomplicated cases or asymptomatic infections. Does this acquired malaria immunity in endemic areas protect against other diseases, especially outbreak diseases like COVID-19? Does malaria activation of innate immunity resulting in trained or tolerance immunity contribute to protection against COVID-19? In an attempt to answer these questions, this review highlights the components of malaria and viral immunity and explores possible links with immunity against COVID-19. With malaria-endemic areas of the world having a fair share of cases of COVID-19, it is important to direct research in this area to evaluate and harness any benefits of acquired malaria immunity to help mitigate the effects of COVID-19 and any possible future outbreaks.

Funding

None declared.

Role of Host Immune and Inflammatory Responses in COVID-19 Cases with Underlying Primary Immunodeficiency: A Review

Coronavirus disease 2019 (COVID-19) has spread rapidly and become a pandemic. Caused by a novel human coronavirus called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), severe COVID-19 is characterized by cytokine storm syndromes due to innate immune activation. Primary immunodeficiency (PID) cases represent a special patient population whose impaired immune system might make them susceptible to severe infections, posing a higher risk to COVID-19, but this could also lead to suppressed inflammatory responses and cytokine storm. It remains an open question as to whether the impaired immune system constitutes a predisposing or protective factor for PID patients when facing SARS-CoV-2 infection. After literature review, it was found that, similar to other patient populations with different comorbidities, PID patients may be susceptible to SARS-CoV-2 infection. Their varied immune status, however, may lead to different disease severity and outcomes after SARS-CoV-2 infection. PID patients with deficiency in antiviral innate immune signaling [eg, Toll-like receptor (TLR)3, TLR7, or interferon regulatory factor 7 (IRF7)] or interferon signaling (IFNAR2) may be linked to severe COVID-19. Because of its anti-infection, anti-inflammatory, and immunomodulatory effects, routine intravenous immunoglobulin therapy may provide some protective effects to the PID patients.

From therapeutic antibodies to immune complex vaccines

In recent years, therapeutic monoclonal antibodies have made impressive progress, providing great benefit by successfully treating malignant and chronic inflammatory diseases. Monoclonal antibodies with broadly neutralizing effects against specific antigens, or that target specific immune regulators, manifest therapeutic effects via their Fab fragment specificities. Subsequently therapeutic efficacy is mediated mostly by interactions of the Fc fragments of the antibodies with their receptors (FcR) displayed on cells of the immune system. These interactions can trigger a series of immunoregulatory responses, involving both innate and adaptive immune systems and including cross-presentation of antigens, activation of CD₈⁺ T cells and CD₄⁺ T cells, phagocytosis, complement-mediated antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). The nature of the triggered effector functions of the antibodies is markedly affected by the glycosylation patterns of the Fc fragments. These can cause differences in the conformation of the heavy chains of antibodies, with resultant changes in antibody binding affinity and activation of the complement system. Studies of the Fc glycosylation profiles together with the associated Fc effector functions and FcR/CR interactions promoted interest and progress in engineering therapeutic antibodies. Furthermore, because antigen–antibody immune complexes (ICs) have shown similar actions, in addition to certain novel immunoregulatory mechanisms that also reshape immune responses, the properties of ICs are being explored in new approaches for prevention and therapy of diseases. In this review, both basic studies and experimental/clinical applications of ICs leading to the development of preventive and therapeutic vaccines are presented.

Dendrimeric peptides can confer protection against foot-and-mouth disease virus in cattle

Foot-and-mouth disease virus (FMDV) causes a highly contagious disease in cloven-hoofed animals. A synthetic vaccine candidate consisting of dendrimeric peptides harbouring two copies of a B-epitope [VP1(136–154)] linked to a T-cell epitope [3A(21–35)] of FMDV confers protection to type O FMDV challenge in pigs. Herein we show in cattle that novel dendrimeric peptides bearing a T-cell epitope [VP1(21–40] and two or four copies of a B-cell epitope [VP1(135–160)] from type O1 Campos FMDV (termed B₂T and B₄T, respectively) elicited FMDV specific immune responses to similar levels to a commercial vaccine. Animals were challenged with FMDV and 100% of vaccinated cattle with B₂T or B₄T were protected to podal generalization. Moreover, bovines immunized with B₄T were completely protected (with no clinical signs) against FMDV challenge after three vaccine doses, which was associated with titers of viral neutralizing antibodies in serum higher than those of B₂T group (p< 0.05) and levels of opsonic antibodies similar to those of animals immunized with one dose of FMDV commercial vaccine. Bovines vaccinated with both dendrimeric peptides presented high levels of IgG1 anti FMDV in sera and in mucosa. When IgA in nasal secretions was measured, 20% or 40% of the animals in B₂T or B₄T groups respectively, showed anti-FMDV IgA titers. In addition, B₂T and B₄T peptides evoked similar consistent T cell responses, being recognized in vitro by lymphocytes from most of the immunized cattle in the proliferation assay, and from all animals in the IFN-γ production assay. Taken together, these results support the potential of dendrimers B₂T or B₄T in cattle as a highly valuable, cost-effective FMDV candidate vaccine with DIVA potential.

Induction of high titred, non-neutralising antibodies by self-adjuvanting peptide epitopes derived from the respiratory syncytial virus fusion protein

Respiratory syncytial virus (RSV) causes severe lower respiratory tract illness in infants and young children. The significant morbidity and mortality rates associated with RSV infection make an effective RSV vaccine development a priority. Two neutralising antibody binding sites, Ø and II, located on the pre-fusion RSV F glycoprotein are prime candidates for epitope-focused vaccine design. We report on a vaccine strategy that utilises a lipid core peptide (LCP) delivery system with self-adjuvanting properties in conjunction with either the antigenic site Ø or II (B cell epitopes) along with PADRE as a T helper cell epitope. These LCP constructs adopted the desired helical conformation in solution and were recognised by their cognate antibodies D25 and Motavizumab, specific for site Ø and II on RSV F protein, respectively. The LCP constructs were capable of eliciting higher levels of antigen specific antibodies than those induced by antigens administered with complete Freund's adjuvant, demonstrating the potent adjuvanting properties of LCP delivery. However, the antibodies induced failed to recognise native F protein or neutralise virus infectivity. These results provide a note of caution in assuming that peptide vaccines, successfully designed to structurally mimic minimal linear B cell epitopes, will necessarily elicit the desired immune response.

Post-exposure Treatment with Anti-rabies VHH and Vaccine Significantly Improves Protection of Mice from Lethal Rabies Infection

Post-exposure prophylaxis (PEP) against rabies infection consists of a combination of passive immunisation with plasma-derived human or equine immune globulins and active immunisation with vaccine delivered shortly after exposure. Since anti-rabies immune globulins are expensive and scarce, there is a need for cheaper alternatives that can be produced more consistently. Previously, we generated potent virus-neutralising VHH, also called Nanobodies, against the rabies glycoprotein that are effectively preventing lethal disease in an in vivo mouse model. The VHH domain is the smallest antigen-binding functional fragment of camelid heavy chain-only antibodies that can be manufactured in microbial expression systems. In the current study we evaluated the efficacy of half-life extended anti-rabies VHH in combination with vaccine for PEP in an intranasal rabies infection model in mice. The PEP combination therapy of systemic anti-rabies VHH and intramuscular vaccine significantly delayed the onset of disease compared to treatment with anti-rabies VHH alone, prolonged median survival time (35 versus 14 days) and decreased mortality (60% versus 19% survival rate), when treated 24 hours after rabies virus challenge. Vaccine alone was unable to rescue mice from lethal disease. As reported also for immune globulins, some interference of anti-rabies VHH with the antigenicity of the vaccine was observed, but this did not impede the synergistic effect. Post exposure treatment with vaccine and human anti-rabies immune globulins was unable to protect mice from lethal challenge. Anti-rabies VHH and vaccine act synergistically to protect mice after rabies virus exposure, which further validates the possible use of anti-rabies VHH for rabies PEP.

Rabies is an infectious disease causing 59,000 deaths and millions are exposed each year worldwide. Post-exposure prophylaxis (PEP) against rabies consists of a combination of passive (immune globulins) and active immunisation (vaccine) directly after viral exposure. Currently used plasma-derived anti-rabies immune globulins are expensive and scarce, urging the development of alternatives. Nanobodies or VHH are the smallest antigen-binding fragments of camelid heavy chain antibodies and are easy to produce with intrinsic good thermal stability and solubility. Combined treatment with anti-rabies VHH and vaccine gave significantly better protection than either compound alone in an intranasal rabies challenge model in mice, which validates the potential use of anti-rabies VHH as replacement of immune globulins in PEP.

Virus-specific antibodies allow viral replication in the marginal zone, thereby promoting CD8(+) T-cell priming and viral control

Clinically used human vaccination aims to induce specific antibodies that can guarantee long-term protection against a pathogen. The reasons that other immune components often fail to induce protective immunity are still debated. Recently we found that enforced viral replication in secondary lymphoid organs is essential for immune activation. In this study we used the lymphocytic choriomeningitis virus (LCMV) to determine whether enforced virus replication occurs in the presence of virus-specific antibodies or virus-specific CD8(+) T cells. We found that after systemic recall infection with LCMV-WE the presence of virus-specific antibodies allowed intracellular replication of virus in the marginal zone of spleen. In contrast, specific antibodies limited viral replication in liver, lung, and kidney. Upon recall infection with the persistent virus strain LCMV-Docile, viral replication in spleen was essential for the priming of CD8(+) T cells and for viral control. In contrast to specific antibodies, memory CD8(+) T cells inhibited viral replication in marginal zone but failed to protect mice from persistent viral infection. We conclude that virus-specific antibodies limit viral infection in peripheral organs but still allow replication of LCMV in the marginal zone, a mechanism that allows immune boosting during recall infection and thereby guarantees control of persistent virus.

Post-exposure passive immunisation for preventing rubella and congenital rubella syndrome

BACKGROUND

Control of rubella is desired because infection in early pregnancy can result in miscarriage, foetal death or congenital abnormality. Primary studies examining the effectiveness of immunoglobulins for post-exposure prophylaxis of rubella have small sample sizes and varying results. National public health recommendations suggest a degree of effectiveness.

OBJECTIVES

To assess the effectiveness of intramuscular injection or intravenous infusion of polyclonal immunoglobulins of human sera or plasma origin for preventing rubella and congenital rubella syndrome when administered to exposed susceptible people before the onset of disease.

SEARCH METHODS

We searched CENTRAL (2014, Issue 7), MEDLINE (1946 to August week 2, 2014), EMBASE (1974 to August 2014), CINAHL (1981 to August 2014), LILACS (1982 to August 2014) and Web of Science (1955 to August 2014). We searched ClinicalTrials.gov and the World Health Organization International Clinical Trials Registry on 16 October 2014. We searched the reference lists of relevant retrieved reviews and studies and identified national public health guidelines.

SELECTION CRITERIA

For the outcome 'preventing cases of rubella', we included randomised controlled trials (RCTs) and quasi-RCTs. We found several studies addressing this outcome where the design was a controlled clinical trial (CCT) (with exposure to rubella virus controlled by the investigators) but the method of allocation of participants to groups was not reported. We found an alternative report of one of these studies that indicated participants were assigned to groups randomly. We therefore included such studies as meeting criteria for RCTs or quasi-RCTs and undertook sensitivity analyses. For the outcomes, 'congenital rubella infection' and 'congenital rubella syndrome', we included RCTs, quasi-RCTs and prospective controlled (cohort) studies. Participants were necessarily susceptible and exposed to rubella. Polyclonal immunoglobulins derived from human sera or plasma must have been administered intramuscularly or intravenously as the only intervention in at least one group.

DATA COLLECTION AND ANALYSIS

We used the standard methodological procedures expected by The Cochrane Collaboration.

MAIN RESULTS

We included 12 studies (430 participants) in the review: seven RCTs and five CCTs where it was not clear whether participants were randomly allocated to groups. We did not include any unpublished studies. Participants included children and adults of both sexes. Only one study included pregnant women. All studies were conducted in high-income countries.The quality of the 11 studies in the initial meta-analysis was moderate, although we classified no study as having a low risk of bias on all criteria.We included 11 studies in the initial meta-analysis of gamma-globulin (concentrated polyclonal immunoglobulins) versus control (saline or no treatment) for rubella cases. The result favoured the intervention group (risk ratio (RR) 0.61, 95% confidence interval (CI) 0.45 to 0.83) but was heterogenous (Chi² test = 36.59, df = 10 (P value < 0.0001); I² statistic = 73%). Heterogeneity was explained by subgrouping studies according to the estimated volume of gamma-globulin administered per pound of bodyweight and then removing those studies where the intervention was administered more than five days after participant exposure to rubella (post hoc analysis). The test of subgroup differences demonstrated heterogeneity between subgroups according to our protocol definition (P value < 0.1; I² statistic > 60%) and there appeared to be greater effectiveness of the intervention when a greater volume of gamma-globulin was administered ('0.027 to 0.037 ml/lb' RR 1.60 (95% CI 0.57 to 4.52); '0.1 to 0.15 ml/lb' RR 0.53 (95% CI 0.29 to 0.99); '0.2 to 0.5 ml/lb' RR 0.20 (95% CI 0.04 to 1.00)).None of the studies reported the outcome 'congenital rubella infection'. One included study reported on congenital rubella syndrome, with no cases among participants who were fewer than nine weeks pregnant at enrolment and who were randomised to one of two gamma-globulin groups ('high' or 'low' rubella titre). However, the study did not report how congenital rubella syndrome was measured and did not report the length of follow-up according to intervention group. This study did not include a non-treatment group.No included study measured adverse events.

AUTHORS' CONCLUSIONS

Compared to no treatment, polyclonal immunoglobulins seem to be of benefit for preventing rubella. The available evidence suggests that this intervention may be of benefit up to five days after exposure, and that effectiveness is dependent on dose. Considering the attack rate for rubella cases in the control group of the highest volume gamma-globulin subgroup (333 per 1000), the absolute risk reduction (calculated from the RR) for this volume of gamma-globulin was 266 (95% CI 0 to 320) and the number needed to treat to benefit is four (95% CI 3 to incalculable).The included studies did not measure rubella-specific antibodies in the immunoglobulin products used in a standard way and thus estimation of the dose of rubella-specific antibodies in international units administered was not possible. As the concentration of rubella-specific antibodies in today's polyclonal immunoglobulin products may vary from those products used in the studies in the review, the volume required per pound of bodyweight to produce similar results may also vary.There is insufficient evidence to make direct conclusions about the effectiveness of polyclonal immunoglobulins for preventing congenital rubella syndrome. This is an area requiring further research.

Interaction of Virus-Like Particles with Vesicles Containing Glycolipids: Kinetics of Detachment

Many viruses interact with their host cells via glycosphingolipids (GSLs) and/or glycoproteins present on the outer cell membrane. This highly specific interaction includes virion attachment and detachment. The residence time determined by the detachment is particularly interesting, since it is directly related to internalization and infection as well as to virion egress and spreading. In an attempt to deepen the understanding of virion detachment kinetics, we have used total internal reflection fluorescence (TIRF) microscopy to probe the interaction between individual fluorescently labeled GSL-containing lipid vesicles and surface-bound virus-like particles (VLPs) of a norovirus genotype II.4 strain. The distribution of the VLP-vesicle residence time was investigated for seven naturally occurring GSLs, all of which are candidates for the not yet identified receptor(s) mediating norovirus entry into host cells. As expected for interactions involving multiple GSL binding sites at a viral capsid, the detachment kinetics displayed features typical for a broad activation-energy distribution for all GSLs. Detailed inspection of these distributions revealed significant differences among the different GSLs. The results are discussed in terms of strength of the interaction, vesicle size, as well as spatial distribution and clustering of GSLs in the vesicle membrane.

Antibody avidity following secondary dengue virus type 2 infection across a range of disease severity

BACKGROUND

The four dengue virus serotypes (DENV1-4) are responsible for the most prevalent mosquito-borne viral illness in humans. DENV causes a spectrum of disease from self-limiting dengue fever (DF) to severe, life-threatening dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Antibodies from one infection can contribute to either protection or increased disease severity in a subsequent infection with a distinct DENV serotype. The effectiveness of the antibody response is modulated by both the affinity and avidity of the antibody/antigen interaction.

OBJECTIVES

We investigated how antibody avidity developed over time following secondary DENV2 infection across different disease severities.

STUDY DESIGN

We analyzed sera from 42 secondary DENV2-infected subjects (DF, n=15; DHF, n=16; DSS, n=11) from a pediatric hospital-based dengue study in Nicaragua. IgG avidity against DENV2 virions was measured in samples collected during acute and convalescent phases as well as 3, 6, and 18 months post-illness using a urea enzyme-linked immunosorbent assay.

RESULTS

The data show a significant increase in avidity from acute to convalescent phase followed by a decrease from convalescent phase to 3 months post-symptom onset, then a plateau. Linear regression analysis comparing antibody avidity between disease severity groups over time indicate that individuals with more severe disease (DHF/DSS) experienced greater decay in antibody avidity over time compared to less severe disease (DF), and ROC curve analysis showed that at 18 months post-illness, lower avidity was associated with previously having experienced more severe disease.

CONCLUSIONS

These data suggest that increased dengue disease severity is associated with lower antibody avidity at later time-points post-illness.

Protective effect of different anti-rabies virus VHH constructs against rabies disease in mice

Rabies virus causes lethal brain infection in about 61000 people per year. Each year, tens of thousands of people receive anti-rabies prophylaxis with plasma-derived immunoglobulins and vaccine soon after exposure. Anti-rabies immunoglobulins are however expensive and have limited availability. VHH are the smallest antigen-binding functional fragments of camelid heavy chain antibodies, also called Nanobodies. The therapeutic potential of anti-rabies VHH was examined in a mouse model using intranasal challenge with a lethal dose of rabies virus. Anti-rabies VHH were administered directly into the brain or systemically, by intraperitoneal injection, 24 hours after virus challenge. Anti-rabies VHH were able to significantly prolong survival or even completely rescue mice from disease. The therapeutic effect depended on the dose, affinity and brain and plasma half-life of the VHH construct. Increasing the affinity by combining two VHH with a glycine-serine linker into bivalent or biparatopic constructs, increased the neutralizing potency to the picomolar range. Upon direct intracerebral administration, a dose as low as 33 µg of the biparatopic Rab-E8/H7 was still able to establish an anti-rabies effect. The effect of systemic treatment was significantly improved by increasing the half-life of Rab-E8/H7 through linkage with a third VHH targeted against albumin. Intraperitoneal treatment with 1.5 mg (2505 IU, 1 ml) of anti-albumin Rab-E8/H7 prolonged the median survival time from 9 to 15 days and completely rescued 43% of mice. For comparison, intraperitoneal treatment with the highest available dose of human anti-rabies immunoglobulins (65 mg, 111 IU, 1 ml) only prolonged survival by 2 days, without rescue. Overall, the therapeutic benefit seemed well correlated with the time of brain exposure and the plasma half-life of the used VHH construct. These results, together with the ease-of-production and superior thermal stability, render anti-rabies VHH into valuable candidates for development of alternative post exposure treatment drugs against rabies.

Glycan variants of a respiratory syncytial virus antibody with enhanced effector function and in vivo efficacy

Respiratory syncytial virus (RSV) can cause devastating lower respiratory tract infections in preterm infants or when other serious health problems are present. Immunoprophylaxis with palivizumab (Synagis), a humanized IgG1 mAb, is the current standard of care for preventing RSV infection in at-risk neonates. We have explored the contribution of effector function to palivizumab efficacy using a plant-based expression system to produce palivizumab N-glycan structure variants with high homogeneity on different antibody isotypes. We compared these isotype and N-glycoform variants with commercially available palivizumab with respect to both in vitro receptor and C1q binding and in vivo efficacy. Whereas the affinity for antigen and neutralization activity of each variant were indistinguishable from those of palivizumab, their Fcγ receptor binding profiles were very different, which was reflected in either a reduced or enhanced ability to influence the RSV lung titer in challenged cotton rats. Enhanced Fcγ receptor binding was associated with reduced viral lung titers compared with palivizumab, whereas abrogation of receptor binding led to a drastic reduction in efficacy. The results support the hypotheses that classic antibody neutralization is a minor component of efficacy by palivizumab in the cotton rat and that antibody-dependent cell-mediated cytotoxicity activity can significantly enhance the efficacy of this antiviral mAb.

Mapping the AAV Capsid Host Antibody Response toward the Development of Second Generation Gene Delivery Vectors

The recombinant adeno-associated virus (rAAV) gene delivery system is entering a crucial and exciting phase with the promise of more than 20 years of intense research now realized in a number of successful human clinical trials. However, as a natural host to AAV infection, anti-AAV antibodies are prevalent in the human population. For example, ~70% of human sera samples are positive for AAV serotype 2 (AAV2). Furthermore, low levels of pre-existing neutralizing antibodies in the circulation are detrimental to the efficacy of corrective therapeutic AAV gene delivery. A key component to overcoming this obstacle is the identification of regions of the AAV capsid that participate in interactions with host immunity, especially neutralizing antibodies, to be modified for neutralization escape. Three main approaches have been utilized to map antigenic epitopes on AAV capsids. The first is directed evolution in which AAV variants are selected in the presence of monoclonal antibodies (MAbs) or pooled human sera. This results in AAV variants with mutations on important neutralizing epitopes. The second is epitope searching, achieved by peptide scanning, peptide insertion, or site-directed mutagenesis. The third, a structure biology-based approach, utilizes cryo-electron microscopy and image reconstruction of AAV capsids complexed to fragment antibodies, which are generated from MAbs, to directly visualize the epitopes. In this review, the contribution of these three approaches to the current knowledge of AAV epitopes and success in their use to create second generation vectors will be discussed.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

Passive immunization for the public health control of communicable diseases: current status in four high-income countries and where to next

The practice of passive immunization with human immune globulin (IG) for the control of communicable diseases (measles, rubella and hepatitis A) differs somewhat between Australia, the United States of America, the United Kingdom, and New Zealand despite the many similarities of these countries, including disease incidence rates and population immunity. No minimum effective dose of IG has been identified for protecting susceptible contacts of measles or hepatitis A. Recommended passive immunization practice for susceptible pregnant contacts of rubella is based on limited evidence in all countries. We suggest that gaps in the evidence base need to be addressed to appropriately inform the role of passive immunization in public health practice into the future.

Capsid antibodies to different adeno-associated virus serotypes bind common regions

Interactions between viruses and the host antibody immune response are critical in the development and control of disease, and antibodies are also known to interfere with the efficacy of viral vector-based gene delivery. The adeno-associated viruses (AAVs) being developed as vectors for corrective human gene delivery have shown promise in clinical trials, but preexisting antibodies are detrimental to successful outcomes. However, the antigenic epitopes on AAV capsids remain poorly characterized. Cryo-electron microscopy and three-dimensional image reconstruction were used to define the locations of epitopes to which monoclonal fragment antibodies (Fabs) against AAV1, AAV2, AAV5, and AAV6 bind. Pseudoatomic modeling showed that, in each serotype, Fabs bound to a limited number of sites near the protrusions surrounding the 3-fold axes of the T=1 icosahedral capsids. For the closely related AAV1 and AAV6, a common Fab exhibited substoichiometric binding, with one Fab bound, on average, between two of the three protrusions as a consequence of steric crowding. The other AAV Fabs saturated the capsid and bound to the walls of all 60 protrusions, with the footprint for the AAV5 antibody extending toward the 5-fold axis. The angle of incidence for each bound Fab on the AAVs varied and resulted in significant differences in how much of each viral capsid surface was occluded beyond the Fab footprints. The AAV-antibody interactions showed a common set of footprints that overlapped some known receptor-binding sites and transduction determinants, thus suggesting potential mechanisms for virus neutralization by the antibodies.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

Human broadly neutralizing antibodies to the envelope glycoprotein complex of hepatitis C virus

Hepatitis C virus (HCV) infects ∼2% of the world's population. It is estimated that there are more than 500,000 new infections annually in Egypt, the country with the highest HCV prevalence. An effective vaccine would help control this expanding global health burden. HCV is highly variable, and an effective vaccine should target conserved T- and B-cell epitopes of the virus. Conserved B-cell epitopes overlapping the CD81 receptor-binding site (CD81bs) on the E2 viral envelope glycoprotein have been reported previously and provide promising vaccine targets. In this study, we isolated 73 human mAbs recognizing five distinct antigenic regions on the virus envelope glycoprotein complex E1E2 from an HCV-immune phage-display antibody library by using an exhaustive-panning strategy. Many of these mAbs were broadly neutralizing. In particular, the mAb AR4A, recognizing a discontinuous epitope outside the CD81bs on the E1E2 complex, has an exceptionally broad neutralizing activity toward diverse HCV genotypes and protects against heterologous HCV challenge in a small animal model. The mAb panel will be useful for the design and development of vaccine candidates to elicit broadly neutralizing antibodies to HCV.

Class-specific effector functions of therapeutic antibodies

Physiology usually combines polyclonal antibodies of multiple classes in a single humoral response. Beyond their common ability to bind antigens, these various classes of human immunoglobulins carry specific functions which can each serve specific goals. In many cases, the function of a monoclonal therapeutic antibody may thus be modulated according to the class of its constant domains. Depending on the immunoglobulin class, different functional assays will be used in order to evaluate the functional activity of a monoclonal antibody.

Examining the cross-reactivity and neutralization mechanisms of a panel of mAbs against adeno-associated virus serotypes 1 and 5

Neutralizing antibodies play a central role in the prevention and clearance of viral infections, but can be detrimental to the use of viral capsids for gene delivery. Antibodies present a major hurdle for ongoing clinical trials using adeno-associated viruses (AAVs); however, relatively little is known about the antigenic epitopes of most AAV serotypes or the mechanism(s) of antibody-mediated neutralization. We developed panels of AAV mAbs by repeatedly immunizing mice with AAV serotype 1 (AAV1) capsids, or by sequentially immunizing with AAV1 followed by AAV5 capsids, in order to examine the efficiency and mechanisms of antibody-mediated neutralization. The antibodies were not cross-reactive between heterologous AAV serotypes except for a low level of recognition of AAV1 capsids by the AAV5 antibodies, probably due to the initial immunization with AAV1. The neutralization efficiency of different IgGs varied and Fab fragments derived from these antibodies were generally poorly neutralizing. The antibodies appeared to display various alternative mechanisms of neutralization, which included inhibition of receptor-binding and interference with a post-attachment step.

Role of macrophages in early protective immune responses induced by two vaccines against foot and mouth disease

Foot and Mouth Disease (FMD) is an acute disease of cloven-hoofed species. We studied the protection and early immune response induced in the murine model by vaccines formulated with inactivated virus and two different adjuvants. The presence of IMS12802PR or ISA206VG adjuvants yielded protection against viral challenge at early times post vaccination and induced FMDV-specific, but non neutralizing, antibody titers. In vivo macrophage depletion in vaccinated mice severely decreased the protection levels after virus challenge, indicating a central role of this cell population in the response elicited by the vaccines. Accordingly, opsonophagocytosis of FITC-labelled virus was augmented in 802-FMDVi and 206-FMDVi vaccinated mice. These results demonstrate the ability of the studied adjuvants to enhance the protective responses of these inactivated vaccines without the increase in seroneutralizing antibodies and the main role of opsonization and phagocytosis in the early protective immune responses against FMD infection in the murine model.

Antibodies to the envelope glycoprotein of human T cell leukemia virus type 1 robustly activate cell-mediated cytotoxic responses and directly neutralize viral infectivity at multiple steps of the entry process

Infection of human cells by human T cell leukemia virus type 1 (HTLV-1) is mediated by the viral envelope glycoproteins. The gp46 surface glycoprotein binds to cell surface receptors, including heparan sulfate proteoglycans, neuropilin 1, and glucose transporter 1, allowing the transmembrane glycoprotein to initiate fusion of the viral and cellular membranes. The envelope glycoproteins are recognized by neutralizing Abs and CTL following a protective immune response, and therefore, represent attractive components for a HTLV-1 vaccine. To begin to explore the immunological properties of potential envelope-based subunit vaccine candidates, we have used a soluble recombinant surface glycoprotein (gp46, SU) fused to the Fc region of human IgG (sRgp46-Fc) as an immunogen to vaccinate mice. The recombinant SU protein is highly immunogenic and induces high titer Ab responses, facilitating selection of hybridomas that secrete mAbs targeting SU. Many of these mAbs recognize envelope displayed on the surface of HTLV-1-infected cells and virions and several of the mAbs robustly antagonize envelope-mediated membrane fusion and neutralize pseudovirus infectivity. The most potently neutralizing mAbs recognize the N-terminal receptor-binding domain of SU, though there is considerable variation in neutralizing proficiency of the receptor-binding domain-targeted mAbs. By contrast, Abs targeting the C-terminal domain of SU tend to lack robust neutralizing activity. Importantly, we find that both neutralizing and poorly neutralizing Abs strongly stimulate neutrophil-mediated cytotoxic responses to HTLV-1-infected cells. Our data demonstrate that recombinant forms of SU possess immunological features that are of significant utility to subunit vaccine design.

Developments in high-yield system expressed vaccines and immunotherapy

Conventional vaccine production techniques are outdated, leaving the world defenseless to viruses and pathogens. Successful protection necessitates the innovation of strategies that can generate an induced defensive humoral and cellular response with: ease of mass production, nominal side-effects, and controlled design specificity, all while being cost effective. Fortunately, technology exists to facilitate such advances in this billion dollar industry and this review is focused on recent publications and patents which hold promise to revolutionize the fight against pathogenic illnesses.

Coated cross-species antibodies by mannosamine-biotin adduct confer protection against snake venom without eliciting humoral immune response

Passive immunization with cross-species antibodies triggers the patient's immune response, thereby preventing repeated treatment. Mannosamine-biotin adduct (MBA) has been described as a masking agent for immunogenic reduction and here, the immunogenicity and biological activity of MBA-coated horse anti-viper venom (hsIgG) were compared to those of uncoated or PEGylated hsIgG. In in vitro tests, hsIgG binding was not affected by MBA conjugation. The immune response to hsIgG-MBA was about 8-fold and 32-fold lower than to PEG-coated and uncoated hsIgG, respectively. In vivo, hsIgG-MBA showed efficient venom-neutralization activity. We thus demonstrate the feasibility of using MBA as a masking agent for passive immunization with cross-species antibodies.

Respiratory syncytial virus-neutralizing monoclonal antibodies motavizumab and palivizumab inhibit fusion

Respiratory syncytial virus (RSV) is a major cause of virus-induced respiratory disease and hospitalization in infants. Palivizumab, an RSV-neutralizing monoclonal antibody, is used clinically to prevent serious RSV-related respiratory disease in high-risk infants. Motavizumab, an affinity-optimized version of palivizumab, was developed to improve protection against RSV. These antibodies bind RSV F protein, which plays a role in virus attachment and mediates fusion. Determining how these antibodies neutralize RSV is important to help guide development of new antibody drugs against RSV and, potentially, other viruses. This study aims to uncover the mechanism(s) by which palivizumab and motavizumab neutralize RSV. Assays were developed to test the effects of these antibodies at distinct steps during RSV replication. Pretreatment of virus with palivizumab or motavizumab did not inhibit virus attachment or the ability of F protein to interact with the target cell membrane. However, pretreatment of virus with either of these antibodies resulted in the absence of detectable viral transcription. These results show that palivizumab and motavizumab act at a point after F protein initiates interaction with the cell membrane and before virus transcription. Palivizumab and motavizumab also inhibited F protein-mediated cell-to-cell fusion. Therefore, these results strongly suggest that these antibodies block both cell-to-cell and virus-to-cell fusion, since these processes are likely similar. Finally, palivizumab and motavizumab did not reduce viral budding. Based on models developed from numerous studies of viral fusion proteins, our results indicate that these antibodies may prevent conformational changes in F protein required for the fusion process.

Carrier induced epitopic suppression of antibody responses induced by virus-like particles is a dynamic phenomenon caused by carrier-specific antibodies

Pre-existing immunity against vaccine carrier proteins has been reported to inhibit the immune response against antigens conjugated to the same carrier by a process termed carrier induced epitopic suppression (CIES). Hence understanding the phenomenon of CIES is of major importance for the development of conjugate vaccines. Virus-like particles (VLPs) are a novel class of potent immunological carriers which have been successfully used to enhance the antibody response to virtually any conjugated antigen. In the present study we investigated the impact of a pre-existing VLP-specific immune response on the development of antibody responses against a conjugated model peptide after primary, secondary and tertiary immunization. Although VLP-specific immune responses led to reduced peptide-specific antibody titers, we showed that CIES against peptide-VLP conjugates could be overcome by high coupling densities, repeated injections and/or higher doses of conjugate vaccine. Furthermore we dissected VLP-specific immunity by adoptively transferring VLP-specific antibodies, B-cells or T(helper) cells separately into naïve mice and found that the observed CIES against peptide-VLP conjugates was mainly mediated by carrier-specific antibodies.

Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Differentiation of naïve B cells, including immunoglobulin class-switch DNA recombination, is critical for the immune response and depends on the extensive integration of signals from the B-cell receptor (BCR), tumor necrosis factor (TNF) family members, Toll-like receptors (TLRs), and cytokine receptors. TLRs and BCR synergize to induce class-switch DNA recombination in T cell-dependent and T cell-independent antibody responses to microbial pathogens. BCR triggering together with simultaneous endosomal TLR engagement leads to enhanced B-cell differentiation and antibody responses. Te requirement of both BCR and TLR engagement would ensure appropriate antigen-specific activation in an infection. Co-stimulation of TLRs and BCR likely plays a significant role in anti-microbial antibody responses to contain pathogen loads until the T cell-dependent antibody responses peak. Furthermore, the temporal sequence of different signals is also critical for optimal B cell responses, as exemplified by the activation of B cells by initial TLR engagement, leading to the up-regulation of co-stimulatory CD80 and MCH-II receptors, which result in more efficient interactions with T cells, thereby enhancing the germinal center reaction and antibody affinity maturation. Overall, BCR and TLR stimulation and the integration with signals from the pathogen or immune cells and their products determine the ensuing B-cell antibody response.

Identification of neutralizing conformational epitopes on the human papillomavirus type 31 major capsid protein and functional implications

The aim of this study was to characterize the conformational neutralizing epitopes of the major capsid protein of human papillomavirus type 31. Analysis of the epitopes was performed by competitive epitope mapping using 15 anti-HPV31 and by reactivity analysis using a HPV31 mutant with an insertion of a seven-amino acid motif within the FG loop of the capsid protein. Fine mapping of neutralizing conformational epitopes on HPV L1 was analyzed by a new approach using a system displaying a combinatorial library of constrained peptides exposed on E. coli flagella. The findings demonstrate that the HPV31 FG loop is dense in neutralizing epitopes and suggest that HPV31 MAbs bind to overlapping but distinct epitopes on the central part of the FG loop, in agreement with the exposure of the FG loop on the surface of HPV VLPs, and thus confirming that neutralizing antibodies are mainly located on the tip of capsomeres. In addition, we identified a crossreacting and partially crossneutralizing conformational epitope on the relatively well conserved N-terminal part of the FG loop. Moreover, our findings support the hypothesis that there is no correlation between neutralization and the ability of MAbs to inhibit VLP binding to heparan sulfate, and confirm that the blocking of virus attachment to the extracellular matrix is an important mechanism of neutralization.

Fc receptor-targeted mucosal vaccination as a novel strategy for the generation of enhanced immunity against mucosal and non-mucosal pathogens

Numerous studies have demonstrated that targeting immunogens to Fcgamma receptors (FcgammaR) on antigen (Ag)-presenting cells (APC) can enhance humoral and cellular immunity in vitro and in vivo. FcgammaR are classified based on their molecular weight, IgG-Fc binding affinities, IgG subclass binding specificity, and cellular distribution and they consist of activating and inhibitory receptors. However, despite the potential advantages of targeting Ag to FcR at mucosal sites, very little is known regarding the role of FcR in mucosal immunity or the efficacy of FcR-targeted mucosal vaccines. In addition, recent work has suggested that FcRn is present in the lungs of adult mice and humans and can transport FcRn-targeted Ag to FcgammaR-bearing APC within mucosal lymphoid tissue. In this review we will discuss the need for new vaccine strategies, the potential for FcR-targeted vaccines to fill this need, the impact of activating versus inhibitory FcgammaR on FcR-targeted vaccination, the significance of focusing on mucosal immunity, as well as caveats that could impact the use of FcR targeting as a mucosal vaccine strategy.