Impact of Adjuvants on the Antibody Responses to Pre-pandemic H5N1 Influenza Vaccines

Human influenza pandemics occur when influenza viruses to which the population has little or no immunity emerge and acquire the ability to transmit among humans. Since their emergence in 1996, human infections with highly pathogenic avian influenza A (H5N1) viruses presented a serious public health challenge. Additionally, H5N1 viruses caused significant agricultural and economic losses in the communities it has affected. Human infections with these viruses are rare but when they occur, these infections are highly fatal. A greater public health concern stems from the rapid evolution displayed by these viruses so far, which in turn might result in viruses able to cause sustained and widespread human‑to‑human transmission. The development of H5N1 vaccines that can induce protective antibody responses is the cornerstone of the global efforts to address this pandemic threat. However, it was repeatedly shown in clinical trials that (at comparable antigen doses) candidate human H5N1 influenza vaccines generally elicit lower immune responses than seasonal human influenza vaccines. In addition, the evolution of H5N1 viruses into at least 10 antigenically distinct clades and multiple subclades suggests that an optimal H5N1 vaccine should confer cross-reactivity against H5N1 viruses from other clades. Therefore, the WHO has recommended the use of adjuvants especially the oil‑in‑water emulsions (such as MF59) in combination with H5N1 split or subunit vaccines. While the ability of these adjuvants to generally boost the vaccine-specific antibody titers has been well documented, the question of how adjuvants modulate the quality of such responses remains largely unanswered. First, we studied the impact of two adjuvants that are licensed with human influenza vaccines, MF59 and alum on the kinetics of developing protective antibody responses to subunit H5N1 vaccines in the ferret model. With a single immunization regimen, we found that including adjuvants in the vaccine formulation was essential for protection against a lethal H5N1 virus challenge. Adjuvanted vaccines provided protection against lethality when administered as early as 7 days prior to challenge and protection against challenge-associated morbidity when administered 14 days or longer prior to challenge. We also examined the breadth of the antibody responses to adjuvanted vs. unadjuvanted H5N1 vaccines. Previous studies have suggested that the oil‑in‑water emulsion adjuvanted vaccines did not only elicit higher antibody titers against homologous H5N1 strains, but also against representative isolates of different clades. Our data clearly showed that indeed MF59‑adjuvanted H5N1 vaccines elicited a quantitatively greater H5‑specific antibody response than the alum‑adjuvanted or the unadjuvanted vaccine. However, for the most part, the specificity of these antibodies as determined by binding to H5 antigen microarray and competitive ELISA assays was not different than induced by alum or vaccine alone. Finally, we tested the contribution of the cytosolic innate immune sensing complex known as the NLRP3 (also known as cryopyrin, CIAS1, or Nalp3) inflammasome is in the adjuvant effect of MF59. It was recently shown that activation of the NLRP3 inflammasome is essential for the adjuvant effect of alum. Our data clearly demonstrated that while the NLRP3 is dispensable for the antibody responses to MF59‑adjuvanted H5N1 vaccines, ablation of the adapter molecule ASC abrogated these responses

Re-Engaging Cross-Reactive Memory B Cells: The Influenza Puzzle

The emergence of a novel influenza A virus strain into humans poses a continuous public health threat. Vaccination is the most effective means of protection against influenza. The generation of memory B cells and long-lived plasma cells that can maintain protective levels of influenza-specific antibodies for protracted periods of time is the foundation for the success of such vaccines. Influenza vaccines elicit an antibody response that is primarily targeting viral surface glycoproteins. However, frequent amino acid mutations within the immunodominant epitopes allow the virus to efficiently escape neutralization by pre-existing antibodies and consequently cause annual epidemics and occasional pandemics. Recently, monoclonal antibodies (mAbs) that target subdominant influenza epitopes have been extensively characterized. These epitopes are immunogenic, can mediate virus neutralization, and most importantly are conserved among different influenza strains. It remains puzzling, however, that despite being repeatedly exposed to such conserved domains of influenza hemagglutinin (HA) either in the form of vaccination or natural infection, most humans do not develop immunological memory that can provide broad protection against emerging virus strains. Here we will discuss the conditions that may be required for engaging such cross-reactive memory B cells in the immune response to influenza infection and vaccination in humans

Antibodies targeting the neuraminidase active site inhibit influenza H3N2 viruses with an S245N glycosylation site

Contemporary influenza A H3N2 viruses circulating since 2016 have acquired a glycosylation site in the neuraminidase in close proximity to the enzymatic active site. Here, we investigate if this S245N glycosylation site, as a result of antigenic evolution, can impact binding and function of human monoclonal antibodies that target the conserved active site. While we find that a reduction in the inhibitory ability of neuraminidase active site binders is measurable, this class of broadly reactive monoclonal antibodies maintains protective efficacy in vivo

Human B cell lineages associated with germinal centers following influenza vaccination are measurably evolving

The poor efficacy of seasonal influenza virus vaccines is often attributed to pre-existing immunity interfering with the persistence and maturation of vaccine-induced B cell responses. We previously showed that a subset of vaccine-induced B cell lineages are recruited into germinal centers (GCs) following vaccination, suggesting that affinity maturation of these lineages against vaccine antigens can occur. However, it remains to be determined whether seasonal influenza vaccination stimulates additional evolution of vaccine-specific lineages, and previous work has found no significant increase in somatic hypermutation among influenza-binding lineages sampled from the blood following seasonal vaccination in humans. Here, we investigate this issue using a phylogenetic test of measurable immunoglobulin sequence evolution. We first validate this test through simulations and survey measurable evolution across multiple conditions. We find significant heterogeneity in measurable B cell evolution across conditions, with enrichment in primary response conditions such as HIV infection and early childhood development. We then show that measurable evolution following influenza vaccination is highly compartmentalized: while lineages in the blood are rarely measurably evolving following influenza vaccination, lineages containing GC B cells are frequently measurably evolving. Many of these lineages appear to derive from memory B cells. We conclude from these findings that seasonal influenza virus vaccination can stimulate additional evolution of responding B cell lineages, and imply that the poor efficacy of seasonal influenza vaccination is not due to a complete inhibition of vaccine-specific B cell evolution

Polyclonal epitope mapping reveals temporal dynamics and diversity of human antibody responses to H5N1 vaccination

Novel influenza A virus (IAV) strains elicit recall immune responses to conserved epitopes, making them favorable antigenic choices for universal influenza virus vaccines. Evaluating these immunogens requires a thorough understanding of the antigenic sites targeted by the polyclonal antibody (pAb) response, which single-particle electron microscopy (EM) can sensitively detect. In this study, we employ EM polyclonal epitope mapping (EMPEM) to extensively characterize the pAb response to hemagglutinin (HA) after H5N1 immunization in humans. Cross-reactive pAbs originating from memory B cells immediately bound the stem of HA and persisted for more than a year after vaccination. In contrast, de novo pAb responses to multiple sites on the head of HA, targeting previously determined key neutralizing sites on H5 HA, expanded after the second immunization and waned quickly. Thus, EMPEM provides a robust tool for comprehensively tracking the specificity and durability of immune responses elicited by novel universal influenza vaccine candidates

Stalking influenza by vaccination with pre-fusion headless HA mini-stem.

Inaccuracies in prediction of circulating viral strain genotypes and the possibility of novel reassortants causing a pandemic outbreak necessitate the development of an anti-influenza vaccine with increased breadth of protection and potential for rapid production and deployment. The hemagglutinin (HA) stem is a promising target for universal influenza vaccine as stem-specific antibodies have the potential to be broadly cross-reactive towards different HA subtypes. Here, we report the design of a bacterially expressed polypeptide that mimics a H5 HA stem by protein minimization to focus the antibody response towards the HA stem. The HA mini-stem folds as a trimer mimicking the HA prefusion conformation. It is resistant to thermal/chemical stress, and it binds to conformation-specific, HA stem-directed broadly neutralizing antibodies with high affinity. Mice vaccinated with the group 1 HA mini-stems are protected from morbidity and mortality against lethal challenge by both group 1 (H5 and H1) and group 2 (H3) influenza viruses, the first report of cross-group protection. Passive transfer of immune serum demonstrates the protection is mediated by stem-specific antibodies. Furthermore, antibodies indudced by these HA stems have broad HA reactivity, yet they do not have antibody-dependent enhancement activity

SARS-CoV-2 viral RNA shedding for more than 87 days in an individual with an impaired CD8+ T cell response

Prolonged shedding of viral RNA occurs in some individuals following SARS-CoV-2 infection. We perform comprehensive immunologic evaluation of one individual with prolonged shedding. The case subject recovered from severe COVID-19 and tested positive for SARS-CoV-2 viral RNA repeatedly as many as 87 days after the first positive test, 97 days after symptom onset. The subject did not have any associated rise in anti-Spike protein antibody titers or plasma neutralization activity, arguing against re-infection. This index subject exhibited a profoundly diminished circulating CD8+ T cell population and correspondingly low SARS-CoV-2-specific CD8+ T cell responses when compared with a cohort of other recovering COVID-19 subjects. CD4+ T cell responses and neutralizing antibody responses developed as expected in this individual. Our results demonstrate that detectable viral RNA shedding in the upper airway can occur more than 3 months following infection in some individuals with COVID-19 and suggest that impaired CD8+ T cells may play a role in prolonged viral RNA shedding

Functionality of the putative surface glycoproteins of the Wuhan spiny eel influenza virus

A panel of influenza virus-like sequences were recently documented in fish and amphibians. Of these, the Wuhan spiny eel influenza virus (WSEIV) was found to phylogenetically cluster with influenza B viruses as a sister clade. Influenza B viruses have been documented to circulate only in humans, with certain virus isolates found in harbor seals. It is therefore interesting that a similar virus was potentially found in fish. Here we characterize the putative hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins of the WSEIV. Functionally, we show that the WSEIV NA-like protein has sialidase activity comparable to B/Malaysia/2506/2004 influenza B virus NA, making it a bona fide neuraminidase that is sensitive to NA inhibitors. We tested the functionality of the HA by addressing the receptor specificity, stability, preferential airway protease cleavage, and fusogenicity. We show highly specific binding to monosialic ganglioside 2 (GM2) and fusogenicity at a range of different pH conditions. In addition, we found limited antigenic conservation of the WSEIV HA and NA relative to the B/Malaysia/2506/2004 virus HA and NA. In summary, we perform a functional and antigenic characterization of the glycoproteins of WSEIV to assess if it is indeed a bona fide influenza virus potentially circulating in ray-finned fish

An agonistic anti-CD137 antibody disrupts lymphoid follicle structure and T-cell-dependent antibody responses

CD137 is a costimulatory receptor expressed on natural killer cells, T cells, and subsets of dendritic cells. An agonistic monoclonal antibody (mAb) against CD137 has been used to reduce tumor burden or reverse autoimmunity in animal models and clinical trials. Here, we show that mice treated with an agonistic anti-CD137 mAb have reduced numbers of germinal center (GC) B cells and follicular dendritic cells (FDCs) in lymphoid tissues, which impair antibody responses to multiple T-cell-dependent antigens, including infectious virus, viral proteins, and conjugated haptens. These effects are not due to enhanced apoptosis or impaired proliferation of B cells but instead correlate with changes in lymphoid follicle structure and GC B cell dispersal and are mediated by CD137 signaling in CD

An agonistic anti-CD137 antibody disrupts lymphoid follicle structure and T-cell-dependent antibody responses

CD137 is a costimulatory receptor expressed on natural killer cells, T cells, and subsets of dendritic cells. An agonistic monoclonal antibody (mAb) against CD137 has been used to reduce tumor burden or reverse autoimmunity in animal models and clinical trials. Here, we show that mice treated with an agonistic anti-CD137 mAb have reduced numbers of germinal center (GC) B cells and follicular dendritic cells (FDCs) in lymphoid tissues, which impair antibody responses to multiple T-cell-dependent antigens, including infectious virus, viral proteins, and conjugated haptens. These effects are not due to enhanced apoptosis or impaired proliferation of B cells but instead correlate with changes in lymphoid follicle structure and GC B cell dispersal and are mediated by CD137 signaling in CD