The 2022 Vaccines Against Shigella and Enterotoxigenic Escherichia coli (VASE) Conference: Summary of breakout workshops

The global public health nonprofit organization PATH hosted the third Vaccines Against Shigella and Enterotoxigenic Escherichia coli (VASE) Conference in Washington, DC, from November 29 to December 1, 2022. This international gathering focused on cutting-edge research related to the development of vaccines against neglected diarrheal pathogens including Shigella, enterotoxigenic Escherichia coli (ETEC), Campylobacter, and non-typhoidal Salmonella. In addition to the conference's plenary content, the agenda featured ten breakout workshops on topics of importance to the enteric vaccine field. This unique aspect of VASE Conferences allows focused groups of attendees to engage in in-depth discussions on subjects of interest to the enteric vaccine development community. In 2022, the workshops covered a range of topics. Two focused on the public health value of enteric vaccines, with one examining how to translate evidence into policy and the other on the value proposition of potential combination vaccines against bacterial enteric pathogens. Two more workshops explored new tools for the development and evaluation of vaccines, with the first on integrating antigen/antibody technologies for mucosal vaccine and immunoprophylactic development, and the second on adjuvants specifically for Shigella vaccines for children in low- and middle-income countries. Another pair of workshops covered the status of vaccines against two emerging enteric pathogens, Campylobacter and invasive non-typhoidal Salmonella. The remaining four workshops examined the assessment of vaccine impact on acute and long-term morbidity. These included discussions on the nature and severity of intestinal inflammation; cellular immunity and immunological memory in ETEC and Shigella infections; clinical and microbiologic endpoints for Shigella vaccine efficacy studies in children; and intricacies of protective immunity to enteric pathogens. This article provides a brief summary of the presentations and discussions at each workshop in order to share these sessions with the broader enteric vaccine field.

The SeroNet Clinical and Translational Serology Task Force (CTTF) SARS-CoV-2 mucosal immunity methodological considerations and best practices workshop

SARS-CoV-2 persists in certain populations, even with vaccination and boosters. Emerging evidence suggests that reductions in virus transmission and infection will likely require involvement of the mucosal immune system, especially secretory antibodies in the upper respiratory tract. The Clinical and Translational Serology Task Force (CTTF) within The National Cancer Institute (NCI)'s Serological Sciences Network for COVID-19 (SeroNet) hosted a workshop to review the status of development and standardization of mucosal sample collection methods and assays, identify challenges, and develop action plans to bridge gaps. Speakers presented data underscoring a role for secretory IgA in protection, mucosal markers as correlates of protection, methods for tracking and assessing mucosal antibodies, and lessons learned from other infectious agents. Perspectives from regulators and industry were put forward to guide mucosal vaccine development. Methodological considerations for optimizing collection protocols and assays and harmonizing data were highlighted. Rigorous studies, standardized protocols, controls, standards, and assay validation were identified as necessary to gain momentum in expanding SARS-CoV-2 vaccines to the mucosa.

Abstract P540: Cardiovascular Disease Risk Factors and Antibody Response to COVID-19 Vaccination: The C4R Study

Background: Adults with cardiovascular co-morbidities and risk factors are at greater risk of severe COVID-19. These same risk factors may also be associated with an attenuated antibody response to COVID-19 vaccines, although studies in diverse, U.S. population-based cohorts have been limited.

Methods: The Collaborative Cohort of Cohorts for COVID-19 Research (C4R) conducted a serosurvey for SARS-CoV-2 antibodies via dried blood spot (DBS) in 14 U.S. cohorts. IgG antibodies to SARS-CoV-2 spike subunit 1 (S1) and nucleocapsid (N) were measured from DBS using a semi-quantitative microsphere immunoassay and reported as median fluorescence intensity (MFI). Multivariable adjusted linear models regressed log-transformed anti-S1 MFI on age, sex, race/ethnicity, education attainment, self-reported diabetes, hypertension, cardiovascular disease (CVD), chronic kidney disease, smoking history, body mass index (BMI), asthma, obstructive lung diseases, DBS batch, anti-N MFI, vaccine type, time between vaccine and DBS, and vaccine dose at time of DBS collection. Results are presented as the percent difference in anti-S1 MFI compared with a reference group.

Results: There were 6614 vaccinated participants prior to booster regimens and DBS collection (April 2021-July 2022) with 50%, 48%, and 2% of the cohort who received BNT162b2, mRNA-1273, or other vaccines, respectively. The mean (SD) time between vaccination and DBS was 3.8 (1.8) months. Over 10% of the cohort had self-reported a history of diabetes, 55% had hypertension, and 74% had a BMI>25 kg/m 2 . Anti-S1 MFI decreased as the time between vaccine dose and DBS collection increased. Diabetes was associated with a 16.1% lower anti-S1 MFI (95%CI:-22.4,-9.5) whereas neither hypertension (-3.8%;95%CI:-9.3,2.1), nor cardiovascular disease history (-5.3%;95%CI:-16.3,7.4) were associated with anti-S1 MFI. Former and current smoking history were each associated with a lower anti-S1 MFI: (-6.6%;95%CI:-12.1,-0.8) and (-16.1%;95%CI:-24.7,-6.6), respectively. Participants with a BMI 25-29.9 kg/m 2 had a 7.6% higher anti-S1 MFI (95%CI:0.3,15.4) whereas those with a BMI of 30-35 kg/m 2 and >35 kg/m 2 had 6.2% (95%CI:-2.4,15.5) higher and 8.9% lower (95%CI:-17.6,0.7) MFI levels, respectively. Older age and male sex were each associated with a lower anti-S1 MFI and mRNA-1273 vaccine, Asian subgroup, higher anti-N titer, and prior COVID-19 infection were each associated with higher anti-S1 MFI. Chronic kidney disease, education attainment, and lung disease were not associated with anti-S1 MFI.

Conclusions: Several traditional cardiovascular disease risk factors were associated with diminished humoral responses to the initial COVID-19 vaccine regimens in a diverse U.S. population-based cohort and may have implications on strategies to improve vaccine responses.

Mission, Organization and Future Direction of the Serological Sciences Network for COVID-19 (SeroNet) Epidemiologic Cohort Studies

Global efforts are needed to elucidate the epidemiology of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the underlying cause of coronavirus disease 2019 (COVID-19) including seroprevalence, risk factors and long-term sequelae, as well as immune responses following vaccination across populations and the social dimensions of prevention and treatment strategies. In the U.S., the National Cancer Institute in partnership with the National Institute of Allergy and Infectious Diseases, established the SARS-CoV-2 Serological Sciences Network (SeroNet) as the nation’s largest coordinated effort to study COVID-19. The network is comprised of multidisciplinary researchers bridging gaps and fostering collaborations between immunologists, epidemiologists, virologists, clinicians and clinical laboratories, social and behavioral scientists, policy makers, data scientists, and community members. In total, 49 institutions form the SeroNet consortium to study individuals with cancer, autoimmune disease, inflammatory bowel diseases, cardiovascular diseases, HIV, transplant recipients, as well as otherwise healthy pregnant women, children, college students, and high-risk occupational workers (including health care workers and first responders). Several studies focus on underrepresented populations, including ethnic minorities and rural communities. To support integrative data analyses across SeroNet studies, efforts are underway to define common data elements for standardized serology measurements, cellular and molecular assays, self-reported data, treatment, and clinical outcomes. In this paper, we discuss the overarching framework for SeroNet epidemiology studies, critical research questions under investigation, and data accessibility for the worldwide scientific community. Lessons learned will help inform preparedness and responsiveness to future emerging diseases.

Antibody-mediated toxin aggregation correlates with the ability of heterodimeric camelid antibodies to passively protect mice against ricin (VAC11P.1108)

Ricin belongs to the A-B toxin family that also includes Shiga and cholera toxins. While immunity to ricin is antibody (Ab)-mediated, it is unclear what functional qualities of Abs are associated with immunity to ricin, considering that Abs with potent toxin-neutralizing activity (TNA) in vitro are not always protective in vivo. We recently produced a collection of alpaca heavy-chain only Abs (VHHs) specific for ricin’s enzymatic (RTA) and binding (RTB) subunits. To better define the attributes associated with immunity to ricin, we engineered VHH homodimers and heterodimers and characterized them in vitro and in vivo. Dimeric VHHs had 10-fold improved binding affinities for ricin and had 2-3 times increased TNA as compared to their monomeric constituents. However, neither improved affinity nor increased TNA correlated with a VHH’s ability to protect in vivo. Instead, immunity was associated with heterodimers composed of an RTA and an RTB monomer in which at least one had TNA. Analytical ultracentrifugation showed that all five protective heterodimers identified in this study promoted ricin aggregation in solution. The same five VHHs also prevented ricin from binding to cell surfaces in vitro. We propose that toxin aggregation and/or inhibition of attachment are likely key determinants of antibody-mediated protective immunity to ricin and possibly other A-B family members.

Identification and surface display on liposomes of neutralizing linear B-cell epitopes as a strategy for biodefense vaccine development (113.25)

The design of subunit vaccines for Category A-B toxins, including anthrax and ricin, has proven largely unsuccessful. Recombinant subunit antigens are minimally protective, difficult to manufacture and/or highly unstable. Thus, there is an urgent need to develop alternative, more effective approaches to vaccine design. With ricin toxin as a model system, we are using a two-tiered approach to address this issue. First, we have identified and validated continuous epitopes on ricin’s A (RTA) and B (RTB) subunits that are recognized by toxin neutralizing monoclonal antibodies (mAbs). In the case of mAb CMA2, pepscan analysis identified residues 190-CLTSDSNIRETVVKI-204 on RTB as being the target of antibody binding. Screening of a phage-displayed peptide library revealed that residues DxNxR are the critical contact points between CMA2 and ricin. An RTA-specific mAb, PB10, bound to peptide 97-NQEDAEAITHLF-108 in pepscan analysis, and phage display identified residues QExA as the likely contact points. The second tier of this project involves attaching these specific peptide epitopes polyvalently to the surface of liposomes (100 nm), with or without the TLR4 agonist MPLA present in the liposome bilayer, and then testing whether these liposomes, when injected into mice, are capable of eliciting protective anti-ricin antibodies. In preliminary studies, we have successfully produced liposomes displaying peptide epitopes from RTA and RTB, and have initiated mouse immunization studies.

RTA-specific monoclonal antibodies neutralize ricin toxin intracellularly (53.13)

According to the current paradigm, antibodies neutralize protein toxins by either preventing toxin binding to cell surfaces or promoting toxin clearance via Fc-receptor mediated uptake. In the case of the biothreat agent ricin, although this toxin uses its B-subunit (RTB) to gain cell entry the majority of antibodies are produced against its enzymatic A-subunit (RTA). We have previously demonstrated using a collection of monoclonal antibodies (mAbs) against RTA that neutralizing activity and protection in a mouse model is associated with epitope specificity, and not mAb affinity or Ig subclass. We now put forth evidence to indicate that at least two RTA-specific mAbs, R70 and IB2, neutralize ricin intracellulary. Neither R70 nor IB2 prevented ricin binding to cell surfaces, as measured by flow cytometry. Moreover, R70 (and likely IB2) retained toxin neutralizing activity in a Vero cell assay when added after ricin had attached to cell surfaces. Using in vitro assays, we also found that R70 and IB2 affected the ability of protein disulfide isomerase to reduce the single disulphide bond linking RTA to RTB and interfered with RTA’s capacity to arrest protein synthesis. We are currently using a combination of techniques to define more precisely the mechanism by which these and other RTA-specific mAbs neutralize ricin.

Monoclonal antibodies against the ricin toxin B subunit: Evidence for extracellular and intracellular mechanisms of toxin neutralization. (53.2)

Antibodies are remarkable in their capacity to inactivate even the most potent protein toxins, including anthrax, botulinum, diphtheria, and ricin. However, the underlying mechanisms by which this occurs are poorly understood. In this study, we have produced and characterized a collection of monoclonal antibodies (mAbs) directed against the B subunit (RTB) of ricin toxin. RTB plays two roles in ricin cytotoxicity; it promotes toxin attachment to host cell surfaces, and it mediates the intracellular trafficking of the toxin from the plasma membrane to the endoplasmic reticulum (ER). We screened ~2,000 RTB-specific murine B cell hybridomas, and identified 4 mAbs with toxin neutralizing activity. Two of these mAbs, SylH3 and 24B11, passively protected mice against a 5xLD50 dose of ricin, but only partially prevented ricin binding to cell surfaces, indicating that they must neutralize ricin by a mechanism other than simply interfering with receptor attachment. Using a modified Vero cell cytotoxicity assay we found that both SylH3 and 24B11 were able to completely neutralize ricin in vitro even after the toxin had bound to cell surfaces. These data reveal that SylH3 and 24B11 neutralize ricin downstream of attachment, perhaps by interfering with the ability of the toxin to traffic to the ER. We are currently using biochemical and microscopic techniques to localize where in the cell these mAbs interfere with ricin toxicity.

Role of the mannose receptor (CD206) in ricin toxin pathogenesis (99.16)

The entry of ricin toxin into macrophages and other cell types is proposed to occur through two distinct pathways. The first involves the toxin’s B subunit (RTB), an R-type lectin that binds to cell-surface glycolipids and glycoproteins bearing terminal β1,4-linked galactose residues. The second involves recognition of ricin’s mannose side chains by the mannose receptor (MR; CD206), a C-type lectin that functions in both scavenging of serum glycoproteins and uptake of microbial pathogens. While much is known about the role of RTB in ricin uptake, much less is understood about the contribution of the MR in ricin pathogenesis. In this study, we demonstrate using the human monocyte cell line, THP-1 that the MR does indeed mediate toxin uptake and contribute to ricin-induced cell killing in vitro. To determine the role of the MR in ricin toxicity in vivo, MR knock-out mice (MR-/-) or age-matched wild type controls were challenged with 2.5 or 5 lethal dose 50s (LD50s) of ricin, administered by intraperitoneal injection. We found that mice lacking the MR were more susceptible to toxin-induced death than control animals, suggesting that the MR promotes toxin clearance rather than productive internalization, as has been proposed. These findings have implications for the development of biodefense therapeutics aimed at rapidly ridding the toxin from circulation.

Specific regions of the ricin toxin A subunit are involved in eliciting protective antibodies: implications for vaccine design (53.5)

Efforts to develop an effective vaccine against ricin are focused on engineering attenuated and stable recombinant forms of the toxin’s enzymatic A subunit (RTA). While several candidate antigens are in development, vaccine design and efficacy studies are being undertaken in the absence of a fundamental understanding of the regions of RTA that are critical in eliciting protective immunity. To address this issue, we recently identified six distinct immunodominant linear regions on RTA. Using monoclonal antibodies (MAbs) specific to each region, we determined that protective antibodies were directed against α-helices located in RTA folding domains 1 and 2, whereas non-neutralizing antibodies recognized random coils and loops that were primarily confined to folding domain 3. This suggests that neutralizing and non-neutralizing antibodies target distinct domains on RTA. In support of this hypothesis, we now report that the vast majority of conformation-dependent ricin neutralizing MAbs bind to RTA 1-33/44-198, a recombinant RTA deletion mutant lacking folding domain 3. Furthermore, preliminary studies in mice suggest that RTA 1-33/44-198 is more effective than an attenuated RTA point mutant at eliciting neutralizing antibodies. These data offer insights into the immunodominant and structural determinants on RTA that give rise to protective immunity, and for the first time provide an immunological rationale for ricin vaccine design.

Secretory IgA: coordinating the capture and sampling of enteric pathogens (161.20)

Secretory IgA (SIgA) antibodies directed against the O-antigen (O-Ag) component of lipopolysaccharide are the primary determinants of mucosal immunity to enteric bacterial pathogens, such as Shigella flexneri and Salmonella enteric serovar Typhimurium. In this report, we demonstrate that monoclonal IgA Abs against the O-Ag are effective inhibitors of the bacterial type 3 secretion systems (T3SS) required for bacterial entry into intestinal epithelial cells. While these Abs did not affect bacterial viability, they did transiently de-energize both S. flexneri and S. Typhimurium, possibly accounting for the “paralysis” that has been observed in the case of S. Typhimurium, following Ab treatment. O-Ag specific IgA Abs also elicited long lasting ultrastructural changes in the bacterial outer membranes, suggesting that S. flexneri and S. Typhimurium made undergo specific adaptations in response to SIgA. These findings may have implications for understanding immune surveillance in the gut. We have shown that Peyer’s patch M cells transport SIgA and SIgA-antigen complexes from the intestinal lumen to underlying dendritic cells (DC), and that the uptake of SIgA-immune complexes by DCs may be mediated by the mannose receptor and/or DC-¬SIGN. The fact that SIgA may be capable of transiently rendering S. flexneri and S. Typhimurium avirulent suggests a mechanism by which the normally pathogenic bacteria can be sampled safely by the mucosal immune system.

The B-subunit of ricin toxin is involved in eliciting protective antibodies (106.18)

The ricin toxin B subunit (RTB) is a galactose-specific lectin that promotes attachment and entry of the ricin enzymatic subunit (RTA) into host cells. RTB is an appealing alternative to RTA-based vaccines, because it is non-toxic and inherently antigenic. However, the degree to which RTB elicits protective immunity against ricin remains unresolved. Here we report that parenteral immunization of mice with RTB elicited high ricin-specific serum antibody titers, yet conferred only partial protection against systemic ricin challenge. Analysis of ~2000 RTB-specific B cell hybridomas identified only a handful of neutralizing monoclonal antibodies (mAbs). Two neutralizing mAbs, 24B11 and SylH3, were shown by passive immunization to be sufficient to protect mice against a 5xLD50 dose of ricin. Both 24B11 and SylH3 blocked ricin attachment to terminal galactose residues and prevented toxin binding to primary macrophages, suggesting that they function by steric hindrance and recognize epitopes located on RTB’s galactose binding sub-domains, 1α or 2γ. By contrast, analysis of four non-neutralizing mAbs revealed that they recognized epitopes situated within sub-domains of RTB not involved in galactose recognition (i.e., 1β, 2α). Based on these data, we propose using specific RTB sub-domains as antigens, rather than RTB itself, as a means to more efficiently elicit neutralizing antibodies and protective immunity against ricin.

Salmonella typhimurium induces apoptosis in human monocyte-derived macrophages

Salmonella species represent a leading cause of gastroenteritis worldwide. More recently, they have been proposed as putative vaccine delivery vehicles in humans. Oral infection with Salmonella leads to invasion of the intestinal epithelial barrier and subsequent interaction with mucosal macrophages. In this study, we investigated the fate of Salmonella typhimurium-infected human macrophages differentiated from blood monocytes by GM-CSF. Wild type S. typhimurium strain SL1344 induced macrophage surface blebbing and caused the release of host cytoplasmic lactate dehydrogenase beginning 30 min post-infection. Three hours later more than 80% of the macrophages in the culture were killed. In contrast, during the same period, macrophages infected with the non-invasive S. typhimurium strain BJ66 remained viable. Chromatin fragmentation is a hallmark of cells undergoing apoptosis. Using TUNEL analysis, we observed chromatin fragmentation in macrophages infected with SL1344 but not in BJ66 infected cells. Consistent with this observation, we found that pretreatment of human macrophages with an inhibitor of caspase-3, a member of the pro-apoptotic enzyme family shown to be involved in S. typhimurium-induced killing of mouse macrophages, reduced SL1344-mediated cytotoxicity by 40%. Our study provides the first evidence that invasive S. typhimurium induces apoptosis in human macrophages that were differentiated from blood monocytes by GM-CSF, and that cell death is a caspase-dependent phenomenon.

Immunization of mice with peptomers covalently coupled to aluminum oxide nanoparticles

Subunit vaccines generally require adjuvants to elicit immune responses, but adjuvants may alter the conformation of critical epitopes and reduce vaccine efficacy. We therefore tested an immunization strategy in which antigen is covalently coupled to aluminum oxide nanoparticles using a method that favors preservation of the native conformation. The test antigen consisted of "peptomers" (head-to-tail-linked peptide homopolymers) derived from the 4th conserved region (C4) of HIV-1 gp120 which is believed to be in an alpha-helical conformation prior to binding to CD4. Immune responses in mice to peptomer-nanoparticle conjugates were compared to responses elicited by free C4 peptide and C4 peptomers, with and without the hydrophilic adjuvant muramyl dipeptide (MDP). Highest peptomer-specific serum antibody responses were induced by peptomer-particles without MDP. Serum antibodies induced by peptomer-particles also showed highest reactivity towards recombinant, glycosylated gp120 and HIV-1 infected T cells. The results suggest that this novel vaccine approach could be useful for induction of immune responses against conformation-sensitive viral antigens without the need for additional adjuvants.

A functional role for ezrin during Shigella flexneri entry into epithelial cells

Shigella flexneri is an enteroinvasive bacterium responsible for bacillary dysentery in humans. Bacterial entry into epithelial cells is a crucial step for the establishment of the infection. It is characterized by a transient reorganization of the host cell cytoskeleton at the site of bacterial interaction with the cell membrane, which leads to bacterial engulfment by a macropinocytic process. We show in this study that the membrane-cytoskeleton linker, ezrin, a member of the ERM (ezrin, radixin, moesin) family, plays an active role in the process of Shigella uptake. Ezrin is highly enriched in cellular protrusions induced by the bacterium and is found in close association with the plasma membrane. In addition, Shigella entry is significantly reduced in cells transfected with a dominant negative allele of ezrin with entry foci showing much shorter cellular protrusions. These results indicate that ezrin not only acts as a membrane-cytoskeleton linker, but may also mediate extension of cellular projections in the presence of signals such as those elicited by invading microorganisms.

Analysis of epithelial cell stress response during infection by Shigella flexneri

Shigella flexneri-infected macrophage cells undergo an apoptotic-like death as early as one hour after infection (A. Zychlinsky, M. C. Prévost, and P. J. Sansonetti, Nature [London] 358:167-168, 1992). To determine the fate of infected epithelial cells, we characterized the viability, morphology, and several metabolic activities of HeLa cells after treatment with M90T, an invoffve isolate of S. flexneri serotype 5, or BS176, a noninvasive isolate cured of the 220-kb virulence plasmid. Using standard assays, we found that for at least 4 h after infection with M90T, HeLa cells remained viable and did not detach or lyse. The ultrastructural morphology of HeLa cells heavily infected with M90T was free of hallmarks associated with cells undergoing apoptosis. Consistent with the idea that intracellular bacterial growth is metabolically stressful to the host cell, we observed that, compared with BS176 treated-HeLa cells, M90T-treated HeLa cells showed (i) a significant decrease in the total pool size of nucleoside triphosphates, (ii) a reduced ability to incorporate extracellular radiolabeled methionine into the soluble and insoluble cell fractions, and (iii) a stimulation of glucose uptake. However, there was no detectable increase in expression of the stress-inducible hsp70 gene in M90T-infected HeLa cells or activation of the anaerobic metabolic pathway as determined by measuring total lactate levels. These results demonstrate clearly that the fate of S.flexneri-infected cells can vary dramatically between cell types and agree with the hypothesis that the destruction of epithelial cells observed in experimental models of shigellosis is due to the host inflammatory response and probably not bacterial intracellular multiplication per se.