Correlates Of Vaccine Protection Research Articles

A new day for human challenge trials?

Two years into the coronavirus disease 2019 (COVID-19) pandemic and following several hot debates, the world’s first COVID-19 human challenge trial has recently been published by Killingley et al. We review its findings and explain why this particular juncture in time makes additional challenge trials for COVID-19 and for other diseases justified and important.

A correlate of protection for SARS-CoV-2 vaccines is urgently needed.

Recent studies suggest that neutralizing antibodies could serve as a correlate of protection for vaccines against SARS-CoV-2 in humans.

Adjuvant Strategies for More Effective Tuberculosis Vaccine Immunity

Tuberculosis (TB) caused by Mycobacterium tuberculosis infection is responsible for the most deaths by a single infectious agent worldwide, with 1.6 million deaths in 2017 alone. The World Health Organization, through its “End TB” strategy, aims to reduce TB deaths by 95% by 2035. In order to reach this goal, a more effective vaccine than the Bacillus Calmette-Guerin (BCG) vaccine currently in use is needed. Subunit TB vaccines are ideal candidates, because they can be used as booster vaccinations for individuals who have already received BCG and would also be safer for use in immunocompromised individuals in whom BCG is contraindicated. However, subunit TB vaccines will almost certainly require formulation with a potent adjuvant. As the correlates of vaccine protection against TB are currently unclear, there are a variety of adjuvants currently being used in TB vaccines in preclinical and clinical development. This review describes the various adjuvants in use in TB vaccines, their effectiveness, and their proposed mechanisms of action. Notably, adjuvants with less inflammatory and reactogenic profiles that can be administered safely via mucosal routes, may have the biggest impact on future directions in TB vaccine design.

Qualified Biolayer Interferometry Avidity Measurements Distinguish the Heterogeneity of Antibody Interactions with Plasmodium falciparum Circumsporozoite Protein Antigens.

Ab avidity is a measure of the overall strength of Ab–Ag interactions and hence is important for understanding the functional efficiency of Abs. In vaccine evaluations, Ab avidity measurements can provide insights into immune correlates of protection and generate hypotheses regarding mechanisms of protection to improve vaccine design to achieve higher levels of efficacy. The commonly used Ab avidity assays require the use of chaotropic reagents to measure avidity index. In this study, using real-time detection of Ab–Ag binding by biolayer interferometry (BLI) technique, we have developed a qualified assay for measuring avidity of vaccine-induced Abs specific for Plasmodium falciparum circumsporozoite protein (CSP) Ags. Human mAb derived from plasmablasts of recipients of RTS,S/AS01 (RTS,S), the most advanced malaria vaccine candidate, were used in the assay development to measure Ag-specific binding responses and rate constants of association and dissociation. The optimized BLI binding assay demonstrated 1) good precision (percentage of coefficient of variation <20), 2) high specificity, 3) a lower limit of detection and quantitation in the 0.3–3.3 nM range, and 4) a range of linearity up to 50–100 nM for the CSP Ags tested. Analysis of polyclonal sera of malaria vaccinees demonstrated the suitability of this method to distinguish among vaccinees and rank Ab responses by avidity. These results demonstrate that precise, specific, and sensitive BLI measurements of Ab avidity in polyclonal sera from malaria vaccinees can map Ab response heterogeneity and potentially help to determine the role of Ab avidity as an immune correlate of protection for vaccines.

Abstract SY23-02: Host immune responses to HPV and HPV vaccines

Abstract Human papillomavirus (HPV) infection is the cause of multiple cancers in both women and men. There are three different licensed HPV prophylactic vaccines. Currently licensed L1 virus-like particles (VLP)–based HPV vaccines target two to nine different HPV types and have demonstrated excellent efficacy against the targeted HPV types and associated disease. Host immune responses to HPV vaccines and infection will be presented and discussed. Briefly, HPV vaccines induce nearly 100% seroconversion to all HPV types included in the vaccine. High titers of high-affinity anti-L1 IgG antibodies, to the HPV types included in the vaccine, are generated after vaccination, and these persist for several years after vaccination at levels considerably higher than those observed in natural infection. Although correlates of vaccine protection have not been identified, neutralizing antibodies are believed to be the main effectors of protection against HPV infection. Vaccine-induced antibodies are detectable not only in serum but also at mucosal sites of infection, such as the cervix and oral cavity. Local HPV-specific antibody levels, although lower, correlate well with circulating levels. Immunogenicity data have shown noninferiority of antibody responses after two doses of HPV vaccine, and the recommended number of vaccine doses was recently reduced from three to two. There is some evidence suggesting that even one dose of the bivalent HPV vaccine may provide similar protection to the recommended dose regimens. Antibody levels following one dose of the bivalent vaccine, although lower, have been shown to be stable over 4 years of follow-up and vaccine recipients remained HPV16/18 seropositive, suggesting that a single dose of the bivalent vaccine may provide durable protection against infection. The robust immunogenicity of the HPV vaccines contrasts with the immune responses observed after natural infection, in which seroconversion has been shown in only a percentage of individuals with incident HPV infection. Furthermore, levels of antibodies induced by vaccination are much higher than the levels observed after infection and appear to be preserved over time. A better understanding of long-term protection of the vaccine as well as a more comprehensive evaluation of correlates of immune protection against HPV infection are still needed. Citation Format: Ligia Pinto. Host immune responses to HPV and HPV vaccines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr SY23-02. doi:10.1158/1538-7445.AM2017-SY23-02

What Are the Most Powerful Immunogen Design Vaccine Strategies?

Functional antibodies, i.e., those with antipathogen activity in in vitro assays, are generally the best correlate of vaccine protection. Mimics of natural infection, including live attenuated and killed pathogens, which induce such antibodies in vivo, have generated highly successful vaccines. However, pathogens that induce functional antibodies at lower levels or more sporadically have been more refractory to vaccine design. Such pathogens are being tackled by more systematic approaches involving identifying functional antibodies, templating immunogens from the antibodies, and then evaluating the immunogens iteratively. I believe this is a powerful new approach to vaccine design as discussed below.

Modeling HIV vaccine trials of the future.

Models of implementation of known-effective interventions for HIV prevention indicate that an efficacious vaccine to prevent HIV infection would be critical for controlling the HIV pandemic. Key issues in the design of future HIV vaccine trials are: first, how to develop reliable immunological correlates of vaccine efficacy, second, how to down-select candidate vaccine regimens into efficacy trials, and third, how to learn about vaccine efficacy in the context of the evolving HIV prevention landscape. Whereas in the past phase-I/-II HIV vaccine trials have addressed the first and second points using a small set of immunological assays and readouts, recently they have used a battery of assays with highly multivariate readouts. In addition, systems vaccinology studies of other pathogens measuring PBMC transcriptomics and other immunological features pre- and postfirst vaccination are demonstrating value, for example, providing discoveries that preimmunization and early postimmunization cell population markers can predict the influenza-specific antibody titer that is a correlate of vaccine protection. The HIV prevention landscape continues to evolve, and the design and analysis of vaccine trials is evolving alongside, to accommodate increasingly dynamic and regional standards of HIV prevention. Development of interpretable and robust functional assays, in addition to the associated bioinformatics and statistical analytic tools, is needed to improve the assessment of correlates of protection in efficacy trials and the down-selection of candidate vaccine regimens into efficacy trials. Moreover, high-priority trials should integrate systems vaccinology, including the analysis of prevaccination and early postvaccination markers.

FCGR2C Polymorphisms Associated with HIV-1 Vaccine Protection Are Linked to Altered Gene Expression of Fc-γ Receptors in Human B Cells.

The phase III Thai RV144 vaccine trial showed an estimated vaccine efficacy (VE) to prevent HIV-1 infection of 31.2%, which has motivated the search for immune correlates of vaccine protection. In a recent report, several single nucleotide polymorphisms (SNPs) in FCGR2C were identified to associate with the level of VE in the RV144 trial. To investigate the functional significance of these SNPs, we utilized a large scale B cell RNA sequencing database of 462 individuals from the 1000 Genomes Project to examine associations between FCGR2C SNPs and gene expression. We found that the FCGR2C SNPs that associated with vaccine efficacy in RV144 also strongly associated with the expression of FCGR2A/C and one of them also associated with the expression of Fc receptor-like A (FCRLA), another Fc-γ receptor (FcγR) gene that was not examined in the previous report. These results suggest that the expression of FcγR genes is influenced by these SNPs either directly or in linkage with other causal variants. More importantly, these results motivate further investigations into the potential for a causal association of expression and alternative splicing of FCGR2C and other FcγR genes with the HIV-1 vaccine protection in the RV144 trial and other similar studies.

Reduction of Streptococcus pneumoniae Colonization and Dissemination by a Nonopsonic Capsular Polysaccharide Antibody.

ABSTRACTStreptococcus pneumoniae colonization of the nasopharynx (NP) is a prerequisite for invasive pneumococcal disease (IPD). The marked reduction in IPD that followed the routine use of pneumococcal polysaccharide conjugate vaccines (PCVs) has been linked to reduced NP colonization with vaccine-included serotypes (STs), with the caveat that PCVs are less effective against pneumonia than against IPD. Although PCV-elicited opsonic antibodies that enhance phagocytic killing of the homologous ST are considered a key correlate of PCV-mediated protection, recent studies question this relationship for some STs, including ST3. Studies with monoclonal antibodies (MAbs) to the pneumococcal capsular polysaccharide (PPS) of ST3 (PPS3) have shown that nonopsonic, as well as opsonic, antibodies can each protect mice against pneumonia and sepsis, but the effect of these types of MAbs on NP colonization is unknown. In this study, we determined the effects of protective opsonic and nonopsonic PPS3 MAbs on ST3 NP colonization in mice. Our results show that a nonopsonic MAb reduced early NP colonization and prevented ST3 dissemination to the lungs and blood, but an opsonic MAb did not. Moreover, the opsonic MAb induced a proinflammatory NP cytokine response, but the nonopsonic MAb had an antiinflammatory effect. The effect of the nonopsonic MAb on colonization did not require its Fc region, but its antiinflammatory effect did. Our findings challenge the paradigm that opsonic MAbs are required to prevent NP colonization and suggest that further studies of the activity of nonopsonic antibodies could advance our understanding of mechanisms of PCV efficacy and provide novel correlates of protection.

HIV Vaccine Development: Current Scenario and Future Prospects

The search for a preventive vaccine that can halt the global pandemic is the ultimate goal of HIV research. Development of a vaccine against HIV-1 has been plagued by many insurmountable challenges. Different vaccine concepts have been tested to overcome these challenges. Experimental vaccines that showed promise in preclinical studies were advanced into clinical trials. Yet, human trials proved to be a huge disappointment until the results of the RV144 trial in Thailand. This trial not only provided the first evidence of a modest efficacy, but also valuable insights on the possible immunologic correlates of vaccine protection. Currently, the HIV vaccine research landscape has been rejuvenated by two major developments. On one hand, there has been a resurgence of interest in T cell-based vaccines with the promising results obtained using a recombinant cytomegalovirus vector vaccine. On the other hand, the discovery of more than a hundred broadly neutralizing antibodies from cohorts of HIV-infected individuals has led to the development of antibody-based preventive vaccines. Innovations in the design of vaccines, vaccination strategies and clinical trial programs will be necessary to accelerate the search for a prophylactic HIV vaccine. This article reviews the current status of HIV vaccine research and provides a roadmap to future efforts in vaccine development.

Similar repertoires of T follicular helper and conventional CD4 T cells are generated in response to protein immunization. (P772) (106.47)

Abstract T follicular helper (Tfh) cells potentiate high-affinity, class-switched B cell responses, the predominant correlate of protection for vaccines. However, little is known about the epitope specificities of Tfh cells generated during polyclonal responses. To date, studies of epitope-specific Tfh cells have relied on either the transfer of TcR transgenic cells or assays involving reagents to stain TcR (tetramer or antibody). In order to comprehensively evaluate polyclonal responses generated from the natural endogenous TcR repertoire, we developed a sorting strategy to separate Tfh cells from CD44highnon-Tfh cells and measured epitope-specific responses with cytokine-specific ELISPOT assays. The immunodominance hierarchies of both Tfh and non-Tfh cells generated in response to immunization with several model antigens (MalE, HEL, and OVA) were remarkably similar. Strikingly, the frequency of cytokine producing Tfh cells at the peak of expansion, independent of the epitope measured, was approximately 6 to 12-fold higher then non-Tfh cells. In agreement with previous studies of unfractionated CD4 cells from our group, increasing the kinetic stability of peptide-MHC class II complexes enhances the priming of Tfh cells. By understanding the parameters that control epitope specific priming, vaccines may be tailored to enhance or focus Tfh responses to facilitate optimal B cell responses.

Multifunctional TH1 cells define a correlate of vaccine-mediated protection against Leishmania major

CD4+ T cells have a crucial role in mediating protection against a variety of pathogens through production of specific cytokines. However, substantial heterogeneity in CD4+ T-cell cytokine responses has limited the ability to define an immune correlate of protection after vaccination. Here, using multiparameter flow cytometry to assess the immune responses after immunization, we show that the degree of protection against Leishmania major infection in mice is predicted by the frequency of CD4+ T cells simultaneously producing interferon-gamma, interleukin-2 and tumor necrosis factor. Notably, multifunctional effector cells generated by all vaccines tested are unique in their capacity to produce high amounts of interferon-gamma. These data show that the quality of a CD4+ T-cell cytokine response can be a crucial determinant in whether a vaccine is protective, and may provide a new and useful prospective immune correlate of protection for vaccines based on T-helper type 1 (TH1) cells.