LTA1 and dmLT enterotoxin-based proteins activate antigen-presenting cells independent of PKA and despite distinct cell entry mechanisms

Vaccine-elicited IL-1R signaling results in Th17 TRM-mediated immunity

Lung tissue resident memory (TRM) cells are thought to play crucial roles in lung host defense. We have recently shown that immunization with the adjuvant LTA1 (derived from the A1 domain of E. coli heat labile toxin) admixed with OmpX from K. pneumoniae can elicit antigen specific lung Th17 TRM cells that provide serotype independent immunity to members of the Enterobacteriaceae family. However, the upstream requirements to generate these cells are unclear. Single-cell RNA-seq showed that vaccine-elicited Th17 TRM cells expressed high levels of IL-1R1, suggesting that IL-1 family members may be critical to generate these cells. Using a combination of genetic and antibody neutralization approaches, we show that Th17 TRM cells can be generated independent of caspase-1 but are compromised when IL-1α is neutralized. Moreover IL-1α could serve as a molecular adjuvant to generate lung Th17 TRM cells independent of LTA1. Taken together, these data suggest that IL-1α plays a major role in vaccine-mediated lung Th17 TRM generation.

Intranasal SARS-CoV-2 RBD decorated nanoparticle vaccine enhances viral clearance in the Syrian hamster model

Multiple vaccines have been developed and licensed for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). While these vaccines reduce disease severity, they do not prevent infection. To prevent infection and limit transmission, vaccines must be developed that induce immunity in the respiratory tract. Therefore, we performed proof-of-principle studies with an intranasal nanoparticle vaccine against SARS-CoV-2. The vaccine candidate consisted of the self-assembling 60-subunit I3-01 protein scaffold covalently decorated with the SARS-CoV-2 receptor-binding domain (RBD) using the SpyCatcher-SpyTag system. We verified the intended antigen display features by reconstructing the I3-01 scaffold to 3.4 A using cryogenicelectron microscopy. Using this RBD-grafted SpyCage scaffold (RBD + SpyCage), we performed two intranasal vaccination studies in the "gold-standard" pre-clinical Syrian hamster model. The initial study focused on assessing the immunogenicity of RBD + SpyCage combined with the LTA1 intranasal adjuvant. These studies showed RBD + SpyCage vaccination induced an antibody response that promoted viral clearance but did not prevent infection. Inclusion of the LTA1 adjuvant enhanced the magnitude of the antibody response but did not enhance protection. Thus, in an expanded study, in the absence of an intranasal adjuvant, we evaluated if covalent bonding of RBD to the scaffold was required to induce an antibody response. Covalent grafting of RBD was required for the vaccine to be immunogenic, and animals vaccinated with RBD + SpyCage more rapidly cleared SARS-CoV-2 from both the upper and lower respiratory tract. These findings demonstrate the intranasal SpyCage vaccine platform can induce protection against SARS-CoV-2 and, with additional modifications to improve immunogenicity, is a versatile platform for the development of intranasal vaccines targeting respiratory pathogens.IMPORTANCEDespite the availability of efficacious COVID vaccines that reduce disease severity, SARS-CoV-2 continues to spread. To limit SARS-CoV-2 transmission, the next generation of vaccines must induce immunity in the mucosa of the upper respiratory tract. Therefore, we performed proof-of-principle, intranasal vaccination studies with a recombinant protein nanoparticle scaffold, SpyCage, decorated with the RBD of the S protein (SpyCage + RBD). We show that SpyCage + RBD was immunogenic and enhanced SARS-CoV-2 clearance from the nose and lungs of Syrian hamsters. Moreover, covalent grafting of the RBD to the scaffold was required to induce an immune response when given via the intranasal route. These proof-of-concept findings indicate that with further enhancements to immunogenicity (e.g., adjuvant incorporation and antigen optimization), the SpyCage scaffold has potential as a versatile, intranasal vaccine platform for respiratory pathogens.

Recent advances in enterotoxin vaccine adjuvants

Enterotoxin adjuvants have been researched for their ability to promote immunity to co-delivered antigens. Outside of cholera vaccines, however, these proteins have yet to be included in any currently licensed vaccines. They include molecules derived from the bacterial toxins of Vibrio cholerae, cholera toxin, or Escherichia coli, heat-labile toxin, such as detoxified mutants or subunits. This class of adjuvants is distinguished by their delivery possibilities, which include parenteral injection, skin applications, or direct mucosal administration by oral, sublingual, or nasal routes. In addition, inclusion of an enterotoxin adjuvant is associated with development of multifaceted cellular and humoral immune responses to vaccination. Here, we review exciting progress in the past few years in clinical trials for safety and efficacy, preclinical vaccines studies, and new mechanistic insights for enterotoxin adjuvants. This includes recent reports of their use in vaccines targeting microbial infections (bacterial, viral, parasitic) or substance abuse drugs.

Route and antigen shape immunity to dmLT-adjuvanted vaccines to a greater extent than biochemical stress or formulation excipients

A key aspect to vaccine efficacy is formulation stability. Biochemical evaluations provide information on optimal compositions or thermal stability but are routinely validated by ex vivo analysis and not efficacy in animal models. Here we assessed formulations identified to improve or reduce stability of the mucosal adjuvant dmLT being investigated in polio and enterotoxigenic E. coli (ETEC) clinical vaccines. We observed biochemical changes to dmLT protein with formulation or thermal stress, including aggregation or subunit dissociation or alternatively resistance against these changes with specific buffer compositions. However, upon injection or mucosal vaccination with ETEC fimbriae adhesin proteins or inactivated polio virus, experimental findings indicated immunization route and co-administered antigen impacted vaccine immunogenicity more so than dmLT formulation stability (or instability). These results indicate the importance of both biochemical and vaccine-derived immunity assessment in formulation optimization. In addition, these studies have implications for use of dmLT in clinical settings and for delivery in resource poor settings.

Enterotoxigenic Escherichia coli enterotoxins regulate epithelial to immune relay of IL-33 and IL-1Ra cytokines

Enterotoxigenic Escherichia coli (ETEC) remain a major cause of diarrheal mortality and morbidity in children in low-resource settings. Few studies have explored the consequences of simultaneous intoxication with heat-stable (ST) and heat-labile (LT) enterotoxins despite the increased prevalence of wild ETEC isolates expressing both toxins. We therefore used a combination of tissue culture and murine models to explore the impact of simultaneous ST+LT intoxication of epithelial and myeloid cell responses. We report that LT induces sustained IL-33 and IL-1Ra responses in T84 intestinal epithelial cells via cAMP-production and protein kinase A activation. We demonstrate that combined ST+LT intoxication hastens epithelial transcriptional responses induced more slowly by LT alone. ST- and LT-mediated luminal fluid accumulation in vivo correlates with significant increases in IL-33 and IL-1Ra in small intestinal mucosal scrapings. Additionally, IL-33 receptor (IL-33R)-deficient mice are less susceptible to ST-mediated secretion. In the immune compartment, IL-33 is sensed by myeloid cells, and LT suppresses IL-33-induced TNFα secretion from macrophages but amplifies IL-33-mediated induction of IL-6 from bone marrow-derived dendritic cells. In conclusion, our studies suggest that enterotoxin-induced IL-33 and IL-1Ra modulate intestinal inflammation and IL-1 receptor signaling in the intestinal mucosa in response to ETEC enterotoxins.

L-DBF Elicits Cross Protection Against Different Serotypes of Shigella spp

Shigellosis is a severe diarrheal disease caused by members of the genus Shigella, with at least 80 million cases and 700,000 deaths annually around the world. The type III secretion system (T3SS) is the primary virulence factor used by the shigellae, and we have previously demonstrated that vaccination with the type T3SS proteins IpaB and IpaD, along with an IpaD/IpaB fusion protein (DBF), protects mice from Shigella infection in a lethal pulmonary model. To simplify the formulation and development of the DBF Shigella vaccine, we have genetically fused LTA1, the active subunit of heat-labile toxin from enterotoxigenic E. coli, with DBF to produce the self-adjuvanting antigen L-DBF. Here we immunized mice with L-DBF via the intranasal, intramuscular, and intradermal routes and challenged them with a lethal dose of S. flexneri 2a. While none of the mice vaccinated intramuscularly or intradermally were protected, mice vaccinated with L-DBF intranasally were protected from lethal challenges with S. flexneri 2a, S. flexneri 1b, S. flexneri 3a, S. flexneri 6, and S. sonnei. Intranasal L-DBF induced both B cell and T cell responses that correlated with protection against Shigella infection. Our results suggest that L-DBF is a candidate for developing an effective serotype-independent vaccine against Shigella spp.

Mucosal vaccines - fortifying the frontiers

Mucosal vaccines offer the potential to trigger robust protective immune responses at the predominant sites of pathogen infection. In principle, the induction of adaptive immunity at mucosal sites, involving secretory antibody responses and tissue-resident T cells, has the capacity to prevent an infection from becoming established in the first place, rather than only curtailing infection and protecting against the development of disease symptoms. Although numerous effective mucosal vaccines are in use, the major advances seen with injectable vaccines (including adjuvanted subunit antigens, RNA and DNA vaccines) have not yet been translated into licensed mucosal vaccines, which currently comprise solely live attenuated and inactivated whole-cell preparations. The identification of safe and effective mucosal adjuvants allied to innovative antigen discovery and delivery strategies is key to advancing mucosal vaccines. Significant progress has been made in resolving the mechanisms that regulate innate and adaptive mucosal immunity and in understanding the crosstalk between mucosal sites, and this provides valuable pointers to inform mucosal adjuvant design. In particular, increased knowledge on mucosal antigen-presenting cells, innate lymphoid cell populations and resident memory cells at mucosal sites highlights attractive targets for vaccine design. Exploiting these insights will allow new vaccine technologies to be leveraged to facilitate rational mucosal vaccine design for pathogens including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and for cancer.

Fentanyl conjugate vaccine by injected or mucosal delivery with dmLT or LTA1 adjuvants implicates IgA in protection from drug challenge

Fentanyl is a major contributor to the devastating increase in overdose deaths from substance use disorders (SUD). A vaccine targeting fentanyl could be a powerful immunotherapeutic. Here, we evaluated adjuvant and delivery strategies for conjugate antigen vaccination with fentanyl-based haptens. We tested adjuvants derived from the heat-labile toxin of E. coli including dmLT and LTA1 by intramuscular, sublingual or intranasal delivery. Our results show anti-fentanyl serum antibodies and antibody secreting cells in the bone-marrow after vaccination with highest levels observed with an adjuvant (alum, dmLT, or LTA1). Vaccine adjuvanted with LTA1 or dmLT elicited the highest levels of anti-fentanyl antibodies, whereas alum achieved highest levels against the carrier protein. Vaccination with sublingual dmLT or intranasal LTA1 provided the most robust blockade of fentanyl-induced analgesia and CNS penetration correlating strongly to anti-FEN IgA. In conclusion, this study demonstrates dmLT or LTA1 adjuvant as well as mucosal delivery may be attractive strategies for improving the efficacy of vaccines against SUD.