Overcoming original (antigenic) sin

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.

Thyroid Autoantibodies Display both "Original Antigenic Sin" and Epitope Spreading

Evidence for original antigenic sin in spontaneous thyroid autoimmunity is revealed by autoantibody interactions with immunodominant regions on thyroid autoantigens, thyroglobulin (Tg), thyroid peroxidase (TPO), and the thyrotropin receptor (TSHR) A-subunit. In contrast, antibodies induced by immunization of rabbits or mice recognize diverse epitopes. Recognition of immunodominant regions persists despite fluctuations in autoantibody levels following treatment or over time. The enhancement of spontaneously arising pathogenic TSHR antibodies in transgenic human thyrotropin receptor/NOD.H2^h4 mice by injecting a non-pathogenic form of TSHR A-subunit protein also provides evidence for original antigenic sin. From other studies, antigen presentation by B cells, not dendritic cells, is likely responsible for original antigenic sin. Recognition of restricted epitopes on the large glycosylated thyroid autoantigens (60-kDa A-subunit, 100-kDa TPO, and 600-kDa Tg) facilitates exploring the amino acid locations in the immunodominant regions. Epitope spreading has also been revealed by autoantibodies in thyroid autoimmunity. In humans, and in mice that spontaneously develop autoimmunity to all three thyroid autoantigens, autoantibodies develop first to Tg and later to TPO and the TSHR A-subunit. The pattern of intermolecular epitope spreading is related in part to the thyroidal content of Tg, TPO and TSHR A-subunit and to the molecular sizes of these proteins. Importantly, the epitope spreading pattern provides a rationale for future antigen-specific manipulation to block the development of all thyroid autoantibodies by inducing tolerance to Tg, first in the autoantigen cascade. Because of its abundance, Tg may be the autoantigen of choice to explore antigen-specific treatment, preventing the development of pathogenic TSHR antibodies.

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.

Sequential immunization with a subtype B HIV-1 envelope quasispecies partially mimics the in vivo development of neutralizing antibodies

A major goal of human immunodeficiency virus type 1 (HIV-1) vaccine efforts is the design of Envelope (Env)-based immunogens effective at eliciting heterologous or broad neutralizing antibodies (NAbs). We hypothesized that programming the B-cell response could be achieved by sequentially exposing the host to a collection of env variants representing the viral quasispecies members isolated from an individual that developed broad NAbs over time. This ordered vaccine approach (sequential) was compared to exposure to a cocktail of env clones (mixture) and to a single env variant (clonal). The three strategies induced comparable levels of the autologous and heterologous neutralization of tier 1 pseudoviruses. Sequential and mixture exposure to quasispecies led to epitope targeting similar to that observed in the simian-human immunodeficiency virus (SHIV)-infected animal from which the env variants were cloned, while clonal and sequential exposure led to greater antibody maturation than the mixture. Therefore, the sequential vaccine approach best replicated the features of the NAb response observed in that animal. This study is the first to explore the use of a collection of HIV-1 env quasispecies variants as immunogens and to present evidence that it is possible to educate the B-cell response by sequential exposure to native HIV-1 quasispecies env variants derived from an individual with a broadened NAb response.

Immunization with different PfAMA1 alleles in sequence induces clonal imprint humoral responses that are similar to responses induced by the same alleles as a vaccine cocktail in rabbits

Background

Antibodies to key Plasmodium falciparum surface antigens have been shown to be important effectors that mediate clinical immunity to malaria. The cross-strain fraction of anti-malarial antibodies may however be required to achieve

strain-transcending immunity. Such antibody responses against Plasmodium falciparum apical membrane antigen 1 (PfAMA1), a vaccine target molecule that is expressed in both liver and blood stages of the parasite, can be elicited through immunization with a mixture of allelic variants of the parasite molecule. Cross-strain antibodies are most likely elicited against epitopes that are shared by the allelic antigens in the vaccine cocktail.

Methods

A standard competition ELISA was used to address whether the antibody response can be further focused on shared epitopes by exclusively boosting these common determinants through immunization of rabbits with different PfAMA1 alleles in sequence. The in vitro parasite growth inhibition assay was used to further evaluate the functional effects of the broadened antibody response that is characteristic of multi-allele vaccine strategies.

Results

A mixed antigen immunization protocol elicited humoral responses that were functionally similar to those elicited by a sequential immunization protocol (p > 0.05). Sequential exposure to the different PfAMA1 allelic variants induced immunological recall of responses to previous alleles and yielded functional cross-strain antibodies that would be capable of optimal growth inhibition of variant parasites at high enough concentrations.

Conclusions

These findings may have implications for the current understanding of the natural acquisition of clinical immunity to malaria as well as for rational vaccine design.

Overcoming viral escape with vaccines that generate and display antigen diversity in vivo

Background

Viral diversity is a key problem for the design of effective and universal vaccines. Virtually, a vaccine candidate including most of the diversity for a given epitope would force the virus to create escape mutants above the viability threshold or with a high fitness cost.

Presentation of the hypothesis

Therefore, I hypothesize that priming the immune system with polyvalent vaccines where each single vehicle generates and displays multiple antigen variants in vivo, will elicit a broad and long-lasting immune response able to avoid viral escape.

Testing the hypothesis

To this purpose, I propose the use of yeasts that carry virus-like particles designed to pack the antigen-coding RNA inside and replicate it via RNA-dependent RNA polymerase. This would produce diversity in vivo limited to the target of interest and without killing the vaccine vehicle.

Implications of the hypothesis

This approach is in contrast with peptide cocktails synthesized in vitro and polyvalent strategies where every cell or vector displays a single or definite number of mutants; but similarly to all them, it should be able to overcome original antigenic sin, avoid major histocompatibility complex restriction, and elicit broad cross-reactive immune responses. Here I discuss additional advantages such as minimal global antagonism or those derived from using a yeast vehicle, and potential drawbacks like autoimmunity. Diversity generated by this method could be monitored both genotypically and phenotypically, and therefore selected or discarded before use if needed.

Viral peptide immunogens: current challenges and opportunities

Synthetic peptide vaccines have potential to control viral infections. Successful experimental models using this approach include the protection of mice against the lethal Sendai virus infection by MHC class I binding CTL peptide epitope. The main benefit of vaccination with peptide epitopes is the ability to minimize the amount and complexity of a well-defined antigen. An appropriate peptide immunogen would also decrease the chance of stimulating a response against self-antigens, thereby providing a safer vaccine by avoiding autoimmunity. In general, the peptide vaccine strategy needs to dissect the specificity of antigen processing, the presence of B-and T-cell epitopes and the MHC restriction of the T-cell responses. This article briefly reviews the implications in the design of peptide vaccines and discusses the various approaches that are applied to improve their immunogenicity.

Combined single-clade candidate HIV-1 vaccines induce T cell responses limited by multiple forms of in vivo immune interference

We assessed in mice whether broad CD8+ T cell responses capable of efficient recognition of multiple HIV-1 clades could be induced using current single-clade vaccine constructs that were or will be used in clinical trials in Europe and Africa. We found that single-clade A, B and C vaccines applied alone induced only limited cross-clade reactivity and that the epitope hierarchy varied according to the immunizing clade. However, combining single-clade HIV-1 vaccines into multi-clade formulations resulted in multiple forms of in vivo immune interference such as original antigenic sin and antagonism, which dampened or even abrogated induction of responses to epitope variants and reduced the breadth of induced T cell responses. Simultaneous administration of individual clade-specific vaccines into anatomically separated sites on the body alleviated antagonism and increased the number of detectable epitope responses. Although cross-reactivity of murine CD8+ T cells does not directly translate to humans, the molecular interactions involved in triggering T cell responses are the same in mouse and man. Thus, these results have important ramifications for the design of both prophylactic and therapeutic vaccines against HIV-1 and other highly variable pathogens.

Timing of retroviral infection influences anamnestic immune response in vaccinated primates

Using simian immunodeficiency virus (SIV) infection of rhesus macaques to model human immunodeficiency virus (HIV) infection of humans, we assessed whether broadly reactive vaccine-induced humoral immunity would remain broadly reactive after viral challenge, and whether there would be significant differences in anamnestic antibody responses if animals were challenged when predominately effector or memory lymphocyte populations were present. Animals immunized over a prolonged period and challenged 11 months after vaccination mounted more broadly reactive and stronger humoral immunity than those rapidly vaccinated and challenged 2 weeks after their final vaccinations. These data suggest that vaccination schedule and the timing of virus challenge should be considered when evaluating future candidate HIV vaccines.

Neutralizing and hemagglutination-inhibiting activities of antibodies elicited by the 2004-2005 influenza vaccine against drifted viruses

Evaluation of the antibody responses induced by the 2004-2005 influenza vaccine strain against the homologous variant, the 2004-2005 field isolates, and a previous circulating strain showed that a correlation between neutralizing and hemagglutination-inhibiting activities exists only when the antigen is very close to the vaccine strain.

Strain-specific T-cell suppression and protective immunity in patients with chronic hepatitis C virus infection

Hepatitis C virus (HCV) frequently persists with an apparently ineffective antiviral T-cell response. We hypothesized that some patients may be exposed to multiple HCV subtypes and that strain-specific T cells could contribute to the viral dynamics in this setting. To test this hypothesis, CD4 T-cell responses to three genotype 1a-derived HCV antigens and HCV antibody serotype were examined in chronically HCV infected (genotypes 1a, 1b, 2, 3, and 4) and spontaneously HCV recovered subjects. Consistent with multiple HCV exposure, 63% of patients infected with genotypes 2 to 4 (genotypes 2-4) and 36% of those infected with genotype 1b displayed CD4 T-cell responses to 1a-derived HCV antigens, while 29% of genotype 2-4-infected patients showed serotype responses to genotype 1. Detection of 1a-specific T cells in patients without active 1a infection suggested prior self-limited 1a infection with T-cell-mediated protection from 1a but not from non-1a viruses. Remarkably, CD4 T-cell responses to 1a-derived HCV antigens were weakest in patients with homologous 1a infection and greater in non-1a-infected patients: proportions of patients responding were 19% (1a), 36% (1b), and 63% (2-4) (P = 0.0006). Increased 1a-specific CD4 T-cell responsiveness in non-1a-infected patients was not due to increased immunogenicity or cross-reactivity of non-1a viruses but directly related to sequence divergence. We conclude that the T-cell response to the circulating virus is either suppressed or not induced in a strain-specific manner in chronically HCV infected patients and that, despite their ability to clear one HCV strain, patients may be reinfected with a heterologous strain that can then persist. These findings provide new insights into host-virus interactions in HCV infection that have implications for vaccine development.

Immunisation with a combination of two complementary feline calicivirus strains induces a broad cross-protection against heterologous challenges

Feline calicivirus (FCV) is characterised by a high degree of antigenic variation potentially compromising vaccine efficacy. Inclusion of several FCV strains or antigens in current vaccines could be a means to improve protection against antigenically distinct isolates. This study evaluated the synergy between two FCV strains (FCVG1 and FCV431) by comparing immunity induced by either strain with that provided by a combination of the two strains against an heterologous challenge with antigenically distant FCV strains (FCV393 and FCV220). Thirty-two SPF kittens were randomly allocated to four groups of eight cats in each group. Groups B, C and D cats were vaccinated once subcutaneously with strains FCVG1, FCV431, and FCVG1 + FCV431, respectively. Each kitten received a total dose of 10(3.4) CCID50 of FCV. Control group A was not immunised. On day 31, four cats from each group were challenged oronasally with FCV220 and four cats with FCV393. Following challenge, the cats were monitored for clinical signs, viral shedding and antibody responses. FCV220 and FCV393 induced severe clinical signs in control cats typical of FCV infection. Immunisation with both strains mixed together induced higher neutralizing antibody titres against FCV220 and FCV393 strains on average. Protection was observed in all groups, however combination of the two strains resulted in a better clinical protection and reduction of virus shedding after heterologous challenge. A moderate correlation was observed between neutralizing antibody titres at the time of challenge and protection against clinical signs. These results indicated that vaccines combining antigens from different FCV strains may induce a broader heterologous protection.

The role of T cell antagonism and original antigenic sin in genetic immunization

To counter highly mutable pathogens like HIV-1, a number of vaccines are being developed to deliver multiple mutant forms of viral Ags to provoke multivalent antiviral CTLs. However, it is uncertain whether such multiple mutant epitope vaccines will generate the diverse CTL responses desired or will instead create immune interference. To characterize the role of immune interference by mutant epitopes in this process, we have tested a "worst case" scenario in which the immunodominant epitope of OVA (SIINFEKL) and its in vitro TCR antagonist (SIINFEDL) have been used to genetically immunize C57BL/6 mice. We demonstrate here that sequential delivery of these mutant epitopes provokes original antigenic sin in CD8 T cells as demonstrated by attenuation of CTLs, intracellular IFN-gamma production, and MHC I peptide-tetramer staining. By contrast, simultaneous exposure of the immune system to this agonist/antagonist pair not only fails to generate T cell antagonism in vivo, but also avoids original antigenic sin. These observations suggest that simultaneous immunization with vaccines containing mutant epitopes, even T cell antagonists, can indeed generate a diverse array of T cell responses and that at least some immune interference can be avoided by delivering mutant Ags to the immune system simultaneously.