AVOIDING DECEPTIVE IMPRINTING OF THE IMMUNE RESPONSE TO HIV-1 INFECTION IN VACCINE DEVELOPMENT

Original antigen sin and COVID-19: implications for seasonal vaccination

INTRODUCTION

Original antigenic sin describes the phenomenon in which immunity against pathogens or antigens is shaped by the host's first exposure to a related pathogen or antigen.

AREAS COVERED

When primary immunity is boosted not by the homologous but by a cross-reacting vaccine, the newly formed antibodies may react better with the primary antigen than with the antigen actually eliciting the response. This form of immune imprinting, which has been observed with influenza, dengue, human immunodeficiency virus, and other pathogens, has profound implications for the approach to seasonal vaccination against a variety of diseases, including COVID-19.

EXPERT OPINION

Public health agencies and regulatory bodies have consistently recommended repeated vaccination every few months as a way to protect against COVID-19. However, the risks and benefits of this approach requires scrutiny given the concern for original antigenic sin in response to SARS-CoV-2. This manuscript examines what is known about immune imprinting and looks ahead to explore how this phenomenon may impact seasonal vaccination against emerging SARS-CoV-2 subvariants such as BA.4, BA.5, and BA.5.1, which have been associated increased transmissibility due to enhanced immune escape.

Progress and Pitfalls in the Quest for Effective SARS-CoV-2 (COVID-19) Vaccines

There are currently around 200 SARS-CoV-2 candidate vaccines in preclinical and clinical trials throughout the world. The various candidates employ a range of vaccine strategies including some novel approaches. Currently, the goal is to prove that they are safe and immunogenic in humans (phase 1/2 studies) with several now advancing into phase 2 and 3 trials to demonstrate efficacy and gather comprehensive data on safety. It is highly likely that many vaccines will be shown to stimulate antibody and T cell responses in healthy individuals and have an acceptable safety profile, but the key will be to confirm that they protect against COVID-19. There is much hope that SARS-CoV-2 vaccines will be rolled out to the entire world to contain the pandemic and avert its most damaging impacts. However, in all likelihood this will initially require a targeted approach toward key vulnerable groups. Collaborative efforts are underway to ensure manufacturing can occur at the unprecedented scale and speed required to immunize billions of people. Ensuring deployment also occurs equitably across the globe will be critical. Careful evaluation and ongoing surveillance for safety will be required to address theoretical concerns regarding immune enhancement seen in previous contexts. Herein, we review the current knowledge about the immune response to this novel virus as it pertains to the design of effective and safe SARS-CoV-2 vaccines and the range of novel and established approaches to vaccine development being taken. We provide details of some of the frontrunner vaccines and discuss potential issues including adverse effects, scale-up and delivery.

Norovirus vaccines: Correlates of protection, challenges and limitations

Norovirus (NoV) is responsible for at least 50% of all gastroenteritis outbreaks worldwide. NoVs are classified into 6 different genogroups (GGI- GGVI) based on the viral capsid protein with NoV genogroup II genotype 4 (GII.4) being the predominant strain causing human diseases. Supportive therapy involving reversal of dehydration and electrolyte deficiency is the main treatment of NoV gastroenteritis. However, the worldwide increased recognition of NoV as an important agent of diarrheal gastroenteritis prompted researchers to focus on establishing preventive strategies conferring long-lasting immunity. This review describes the current status of animal and human vaccine models/studies targeting NoV and addresses the factors hampering the development of a broadly effective vaccine.

Original antigenic sin with human bocaviruses 1-4

Human bocavirus (HBoV) 1 is a widespread parvovirus causing acute respiratory disease in young children. In contrast, HBoV2 occurs in the gastrointestinal tract and is potentially associated with gastroenteritis, whilst HBoV3 and -4 infections are less frequent and have not yet been linked with human disease. Due to HBoV1 DNA persistence in the nasopharynx, serology has been advocated as a better alternative for diagnosing acute infections. In constitutionally healthy children, we previously noted that pre-existing HBoV2 immunity in a subsequent HBoV1 infection typically resulted in low or non-existent HBoV1-specific antibody responses. A phenomenon describing such immunological events among related viruses has been known since the 1950s as 'original antigenic sin' (OAS). The aim of this study was to characterize this putative OAS phenomenon in a more controlled setting. Follow-up sera of 10 rabbit pairs, inoculated twice with HBoV1-4 virus-like particles (VLPs) or control antigens, in various combinations, were analysed with HBoV1-4 IgG enzyme immunoassays with and without depletion of heterotypic HBoV antibodies. There were no significant IgG boosts after the second inoculation in either the heterologously or the homologously HBoV-inoculated rabbits, but a clear increase in cross-reactivity was seen with time. We could, however, distinguish a distinct OAS pattern from plain cross-reactivity: half of the heterologously inoculated rabbits showed IgG patterns representative of the OAS hypothesis, in line with our prior results with naturally infected children. HBoVs are the first parvoviruses to show the possible existence of OAS. Our findings provide new information on HBoV1-4 immunity and emphasize the complexity of human bocavirus diagnosis.

Immunogenicity of infectious pathogens and vaccine antigens

The concept of the immunogenicity of an antigen is frequently encountered in the context of vaccine development, an area of intense interest currently due to the emergence or re-emergence of infectious pathogens with the potential for worldwide spread. However, the theoretical notion of immunogenicity as discussed in older textbooks of immunology needs reconsideration due to advances in our understanding of immunologic responses. Immunogenicity is a property that can either be a desirable attribute, for example in the generation of an effective protective immunity against infectious pathogens or an undesirable trait, for example when it relates to novel therapeutic compounds and drugs, where an immune response needs to be prevented or inhibited. In this Forum Article, we aimed to revisit the issue of immunogenicity to discuss a series of simple questions relevant to the concept that are frequently rephrased but incompletely resolved in the immunologic literature.

Humans and ferrets with prior H1N1 influenza virus infections do not exhibit evidence of original antigenic sin after infection or vaccination with the 2009 pandemic H1N1 influenza virus

The hypothesis of original antigenic sin (OAS) states that the imprint established by an individual's first influenza virus infection governs the antibody response thereafter. Subsequent influenza virus infection results in an antibody response against the original infecting virus and an impaired immune response against the newer influenza virus. The purpose of our study was to seek evidence of OAS after infection or vaccination with the 2009 pandemic H1N1 (2009 pH1N1) virus in ferrets and humans previously infected with H1N1 viruses with various antigenic distances from the 2009 pH1N1 virus, including viruses from 1935 through 1999. In ferrets, seasonal H1N1 priming did not diminish the antibody response to infection or vaccination with the 2009 pH1N1 virus, nor did it diminish the T-cell response, indicating the absence of OAS in seasonal H1N1 virus-primed ferrets. Analysis of paired samples of human serum taken before and after vaccination with a monovalent inactivated 2009 pH1N1 vaccine showed a significantly greater-fold rise in the titer of antibody against the 2009 pH1N1 virus than against H1N1 viruses that circulated during the childhood of each subject. Thus, prior experience with H1N1 viruses did not result in an impairment of the antibody response against the 2009 pH1N1 vaccine. Our data from ferrets and humans suggest that prior exposure to H1N1 viruses did not impair the immune response against the 2009 pH1N1 virus.

Live-virus exposure of vaccine-protected macaques alters the anti-HIV-1 antibody repertoire in the absence of viremia

BACKGROUND

We addressed the question whether live-virus challenges could alter vaccine-induced antibody (Ab) responses in vaccinated rhesus macaques (RMs) that completely resisted repeated exposures to R5-tropic simian-human immunodeficiency viruses encoding heterologous HIV clade C envelopes (SHIV-Cs).

RESULTS

We examined the Ab responses in aviremic RMs that had been immunized with a multi-component protein vaccine (multimeric HIV-1 gp160, HIV-1 Tat and SIV Gag-Pol particles) and compared anti-Env plasma Ab titers before and after repeated live-virus exposures. Although no viremia was ever detected in these animals, they showed significant increases in anti-gp140 Ab titers after they had encountered live SHIVs. When we investigated the dynamics of anti-Env Ab titers during the immunization and challenge phases further, we detected the expected, vaccine-induced increases of Ab responses about two weeks after the last protein immunization. Remarkably, these titers kept rising during the repeated virus challenges, although no viremia resulted. In contrast, in vaccinated RMs that were not exposed to virus, anti-gp140 Ab titers declined after the peak seen two weeks after the last immunization. These data suggest boosting of pre-existing, vaccine-induced Ab responses as a consequence of repeated live-virus exposures. Next, we screened polyclonal plasma samples from two of the completely protected vaccinees by peptide phage display and designed a strategy that selects for recombinant phages recognized only by Abs present after - but not before - any SHIV challenge. With this "subtractive biopanning" approach, we isolated V3 mimotopes that were only recognized after the animals had been exposed to live virus. By detailed epitope mapping of such anti-V3 Ab responses, we showed that the challenges not only boosted pre-existing binding and neutralizing Ab titers, but also induced Abs targeting neo-antigens presented by the heterologous challenge virus.

CONCLUSIONS

Anti-Env Ab responses induced by recombinant protein vaccination were altered by the multiple, live SHIV challenges in vaccinees that had no detectable viral loads. These data may have implications for the interpretation of "vaccine only" responses in clinical vaccine trials.

Multiantibody strategies for HIV

Vaccination strategies depend entirely on the appropriate responsiveness of our immune system against particular antigens. For this active immunization to be truly effective, neutralizing antibodies (nAbs) need to efficiently counter the infectivity or propagation of the pathogen. Some viruses, including HIV, are able to take advantage of this immune response in order to evade nAbs. This review focuses on viral immune evasion strategies that result directly from a robust immune response to infection or vaccination. A rationale for multi-Ab therapy to circumvent this phenomenon is discussed. Progress in the formulation, production, and regulatory approval of monoclonal antibodies (mAbs) is presented.

Viral shape-shifting: norovirus evasion of the human immune system

Noroviruses are a major cause of gastroenteritis, and there are currently no vaccines or antiviral treatments available to treat or prevent the >260 million gastroenteritis cases that are reported globally each year. Noroviruses have proven difficult to work with in the laboratory owing to the lack of cell culture systems and animal models, and therefore little is known about the pathogenesis caused by this virus, which has hampered the development of efficacious therapeutics.

The norovirus family contains two genogroups (GI and GII) that are most commonly associated with enteric disease in humans, and these genogroups contain more than 25 different genotypes that account for most human norovirus cases. However, outbreaks caused by the GII.4 genotype occur much more frequently than those caused by other genotypes in the GII genogroup, and GI outbreaks occur even less frequently. Although the majority of norovirus outbreaks are caused by the GII.4 genotype, the molecular and biological factors that regulate this disease burden are only partially understood.

The GII.4 genotype seems to operate in a similar fashion as influenza virus, whereby evolution of novel immune escape variants allows the virus to escape the predominant memory immune response. By contrast, the prototypic GI.1 noroviruses have remained relatively static over the same time period, evolving variants with identical histo-blood group antigen binding capabilities and similar antigenic properties. The molecular mechanisms governing differential evolution patterns remain a key mystery in the norovirus field.

Immunity against noroviruses has been difficult to assess owing to the complex effects of host pre-exposure histories and differential host susceptibility, which is correlated with blood group and secretor status. However, recent work has suggested that the GI and GII genogroups may use different mechanisms to escape immunological memory and that this is perhaps directly related to the plasticity of and complex evolutionary-related sequence information encoded in the P2 subdomain of the capsid protein.

The GII genogroup contains more amino acid sequence in the P2 subdomain, which may allow increased capsid plasticity and a tolerance for more amino acid variation or insertions and deletions. This would provide a larger repertoire of sequence targets for natural selection and adaptation to complex environmental selection processes, like herd immunity. By contrast, the GI genogroup contains less sequence information with more conserved, surface-exposed residues that are probably recognized by homologous antibodies as well as antibodies generated against heterologous GI strains. Thus, complex patterns of GI pre-exposure history, antibody cross reactivity and original antigenic sin may facilitate secondary infections of GI strains, whereas antigenic drift and receptor switching allow GII noroviruses, especially GII.4 viruses, to persist in human populations.

Noroviruses are the most common cause of food-borne gastroenteritis worldwide; however, the development of effective vaccines and antiviral therapies has proved to be challenging. In this Review, Baric and colleagues discuss the molecular and structural mechanisms underlying the persistence of noroviruses in human populations.

Noroviruses are the most common cause of food-borne gastroenteritis worldwide, and explosive outbreaks frequently occur in community settings, where the virus can immobilize large numbers of infected individuals for 24–48 hours, making the development of effective vaccines and antiviral therapies a priority. However, several challenges have hampered therapeutic design, including: the limitations of cell culture and small-animal model systems; the complex effects of host pre-exposure histories; differential host susceptibility, which is correlated with blood group and secretor status; and the evolution of novel immune escape variants. In this Review, we discuss the molecular and structural mechanisms that facilitate the persistence of noroviruses in human populations.

Heterotypic humoral and cellular immune responses following Norwalk virus infection

Norovirus immunity is poorly understood as the limited data available on protection after infection are often contradictory. In contrast to the more prominent GII noroviruses, GI norovirus infections are less frequent in outbreaks. The GI noroviruses display very complex patterns of heterotypic immune responses following infection, and many individuals are highly susceptible to reinfection. To study the immune responses and mechanisms of GI.1 persistence, we built structural models and recombinant virus-like particles (VLPs) of five GI strains: GI.1-1968, GI.1-2001, GI.2-1999, GI.3-1999, and GI.4-2000. Structural models of four GI genotype capsid P domain dimers suggested that intragenotype structural variation is limited, that the GI binding pocket is mostly preserved between genotypes, and that a conserved, surface-exposed epitope may allow for highly cross-reactive immune responses. GI VLPs bound to histo-blood group antigens (HBGAs) including fucose, Lewis, and A antigens. Volunteers infected with GI.1-1968 (n = 10) had significant increases between prechallenge and convalescent reactive IgG for all five GI VLPs measured by enzyme immunoassay. Potential cross-neutralization of GI VLPs was demonstrated by convalescent-phase serum cross-blockade of GI VLP-HBGA interaction. Although group responses were significant for all GI VLPs, each individual volunteer demonstrated a unique VLP blockade pattern. Further, peripheral blood mononuclear cells (PBMCs) were stimulated with each of the VLPs, and secretion of gamma interferon (IFN-gamma) was measured. As seen with blockade responses, IFN-gamma secretion responses differed by individual. Sixty percent responded to at least one GI VLP, with only two volunteers responding to GI.1 VLP. Importantly, four of five individuals with sufficient PBMCs for cross-reactivity studies responded more robustly to other GI VLPs. These data suggest that preexposure history and deceptive imprinting may complicate PBMC and B-cell immune responses in some GI.1-1968-challenged individuals and highlight a potential complication in the design of efficacious norovirus vaccines.

A diversity-covering approach to immunization with Plasmodium falciparum apical membrane antigen 1 induces broader allelic recognition and growth inhibition responses in rabbits

Plasmodium falciparum apical membrane antigen 1 (PfAMA1), a candidate malaria vaccine, is polymorphic. This polymorphism is believed to be generated predominantly under immune selection pressure and, as a result, may compromise attempts at vaccination. Alignment of 355 PfAMA1 sequences shows that around 10% of the 622 amino acid residues can vary between alleles and that linkages between polymorphic residues occur. Using this analysis, we have designed three diversity-covering (DiCo) PfAMA1 sequences that take account of these linkages and, when taken together, on average incorporate 97% of amino acid variability observed. For each of the three DiCo sequences, a synthetic gene was constructed and used to transform the methylotrophic yeast Pichia pastoris, allowing recombinant expression. All three DiCo proteins were reactive with the reduction-sensitive monoclonal antibody 4G2, suggesting the DiCo sequences had conformations similar to those of naturally occurring PfAMA1. Rabbits were immunized with FVO strain PfAMA1 or with the DiCo proteins either individually or as a mixture. Antibody titers and the ability to inhibit parasite growth in vitro were determined. Animals immunized with the DiCo mix performed similarly to animals immunized with FVO AMA1 when measured against FCR3 strain parasites but outperformed animals immunized with FVO AMA1 when assessed against other strains. The levels of growth inhibition (approximately 70%) induced by the mix of three DiCo proteins were comparable for FVO, 3D7, and HB3, suggesting that a considerable degree of diversity in AMA1 is adequately covered. This suggests that vaccines based upon the DiCo mix approach provide a broader functional immunity than immunization with a single allele.

AIDS vaccine: efficacy, safety and ethics

Human immunodeficiency virus (HIV) has infected 50 million people worldwide and killed 16 million so far, and the epidemic is still spreading with 16,000 new cases of HIV infection daily and a projection of 100 million infected individuals by the end of the next decade. There is no question that a safe and effective acquired immunodeficiency syndrome (AIDS) vaccine is urgently needed to bring the current AIDS pandemic under control. But, is preventive AIDS vaccine an attainable goal? Unfortunately, the results of many laboratory and clinical studies over the past two decades are not encouraging. We comment on the efficacy, safety and ethics of AIDS vaccine, and the urgent need for a new strategy for AIDS vaccine development.