Skin scarification with Plasmodium falciparum peptide vaccine using synthetic TLR agonists as adjuvants elicits malaria sporozoite neutralizing immunity

TLR7 enhancing follicular helper T (Tfh) cells response in C57BL/6 mice infected with Plasmodium yoelii NSM TLR7 mediated Tfh cells in P. yoelii infected mice

Toll-like receptors (TLRs) play an important role in inducing innate and acquired immune responses against infection. However, the effect of Toll-like receptor 7 (TLR7) on follicular helper T (Tfh) cells in mice infected with Plasmodium is still not clear. The results showed that the splenic CD4+ CXCR5+ PD-1+ Tfh cells were accumulated after Plasmodium yoelii NSM infection, the content of splenic Tfh cells was correlated to parasitemia and/or the red blood cells (RBCs) counts in the blood. Moreover, the expression of TLR7 was found higher than TLR2, TLR3 and TLR4 in splenic Tfh cells of the WT mice. TLR7 agonist R848 and the lysate of red blood cells of infected mice (iRBCs) could induce the activation and differentiation of splenic Tfh cells. Knockout of TLR7 leads to a decrease in the proportion of Tfh cells, down-regulated expression of functional molecules CD40L, IFN-γ, IL-21 and IL-10 in Tfh cells; decreased the proportion of plasma cells and antibody production and reduces the expression of STAT3 and Ikzf2 in Tfh cells. Administration of R848 could inhibit parasitemia, enhance splenic Tfh cell activation and increase STAT3 and Ikzf2 expression in Tfh cells. In summary, this study shows that TLR7 could regulate the function of Tfh cells, affecting the immune response in the spleen of Plasmodium yoelii NSM-infected mice.

Sustained delivery of CpG oligodeoxynucleotide by acetalated dextran microparticles augments effector response to Computationally Optimized Broadly Reactive Antigen (COBRA) influenza hemagglutinin

A subunit or protein-based influenza vaccine can be a safer alternative to live attenuated vaccine (Flumist) and require fewer boosts than an inactivated vaccine (e.g. Fluzone). However, to form an effective subunit vaccine, an adjuvant is often needed. In this work we used electrospray to encapsulate the hydrophilic adjuvant CpG into microparticles made from the hydrophobic biodegradable polymer acetalated dextran. To understand the rate of particle degradation on CpG release, polymer that was slow (21 h at phagosomal pH 5) and fast (0.25 h at pH 5) degrading was used to encapsulate the adjuvant. The slow-degrading particles exhibited the greatest degree of innate immune stimulation of antigen-presenting cells in vitro. In mice, the broadly acting Computationally Optimized Broadly Reactive Antigen (COBRA) Y2 influenza hemagglutinin (HA) antigen was used with CpG particles, soluble CpG, or MF-59 like adjuvant Addavax. Particles and soluble CpG elicited similar induction of anti-HA antibodies and protection against lethal influenza challenge, but the sustained release particles elicited the highest levels antibody effector functions. These results demonstrate a suitable method for encapsulation of CpG oligonucleotide in a hydrophobic particle matrix, and suggest that sustained release of CpG from Ace-DEX microparticles could potentially be used to induce potent antibody effector functions.

Innate Immune Responses and P. falciparum CS Repeat-Specific Neutralizing Antibodies Following Vaccination by Skin Scarification

The skin is the site of host invasion by the mosquito-borne Plasmodium parasite, which caused an estimated 229 million infections and 409,000 deaths in 2019 according to WHO World Malaria report 2020. In our previous studies, we have shown that skin scarification (SS) with a P. falciparum circumsporozoite (CS) peptide in the oil-in-water adjuvant AddaVax containing a combination of TLR 7/8 and TLR 9 agonists can elicit sporozoite neutralizing antibodies. SS with AddaVax + TLR agonists, but not AddaVax alone, elicited CD4+ Th1 cells and IgG2a/c anti-repeat antibody. To explore the innate immune responses that may contribute to development of adaptive immunity following SS, we examined the skin at 4h and 24h post priming with CS peptide in AddaVax with or without TLR agonists. H&E stained and IHC-labeled dorsal skin sections obtained 24h post SS demonstrated a marked difference in the pattern of infiltration with F4/80+, CD11b+ and Ly6G+ cells at the immunization site, with the lowest intensity noted following SS with AddaVax + TLR agonists. Serum collected at 4h post SS, had reproducible increases in IL-6, MIP-3α, IL-22 and IP-10 (CXCL10) following SS with AddaVax + TLR agonists, but not with AddaVax alone. To begin to decipher the complex roles of these pro-inflammatory cytokines/chemokines, we utilized IP-10 deficient (IP-10 -/-) mice to examine the role of this chemokine in the development of anti-repeat antibody response following SS. In the absence of IP-10, the levels of Th1-type IgG2a/c antibody and kinetics of the primary anti-repeat antibody response were reduced following prime and boost. The IP-10 chemokine, present as early as 4h post prime, may provide an early serological marker for rapid screening of adjuvant formulations and delivery platforms to optimize SS-induced humoral immunity to CS repeats as well as other pathogens.

A review of combination adjuvants for malaria vaccines: a promising approach for vaccine development

It is obvious that there is a critical need for an efficient malaria vaccine to accelerate malaria eradication. Currently, recombinant subunit vaccination against malaria using proteins and peptides is gaining attention. However, one of the major drawbacks of this approach is the lack of an efficient and durable immune response. Therefore, subunit vaccines require adjuvants to make the vaccine sufficiently immunogenic. Considering the history of the RTS,S vaccine, it seems likely that no single adjuvant is capable of eliciting all the protective immune responses required in many malarial subunit vaccines and the use of combination adjuvants will be increasingly important as the science of malaria vaccines advances. In light of this, it appears that identifying the most effective mixture of adjuvants with minimal adverse effects offers tremendous opportunities in improving the efficacy of vaccines against malaria. Owing to the importance of a multi-adjuvanted approach in subunit malaria vaccine development, this review paper outlines some of the best known combination adjuvants used in malaria subunit vaccines, focusing on their proposed mechanisms of action, their immunological properties, and their notable results. The aim of the present review is to consolidate these findings to aid the application of these combination adjuvants in experimental malaria vaccines.

Repurposing of existing therapeutics to combat drug-resistant malaria

In the era of drug repurposing, speedy discovery of new therapeutic options for the drug-resistant malaria is the best available tactic to reduce the financial load and time in the drug discovery process. Six anticancer drugs, three immunomodulators and four antibiotics were selected for the repositioning against experimental malaria owing to their mode of action and published literature. The efficacy of existing therapeutics was evaluated against chloroquine-resistant in vitro and in vivo strains of Plasmodium falciparum and P. yoelii, respectively. All the pre-existing FDA-approved drugs along with leptin were primarily screened against chloroquine-resistant (PfK1) and drug-sensitive (Pf3D7) strains of P. falciparum using SYBR green-based antiplasmodial assay. Cytotoxic profiling of these therapeutics was achieved on Vero and HepG2 cell lines, and human erythrocytes. Percent blood parasitemia and host survival was determined in chloroquine-resistant P. yoelii N67-infected Swiss mice using appropriate doses of these drugs/immunomodulators. Antimalarial screening together with cytotoxicity data revealed that anticancer drugs, idelalisib and 5-fluorouracil acquired superiority over their counterparts, regorafenib, and tamoxifen, respectively. ROS-inducer anticancer drugs, epirubicin and bleomycin were found toxic for the host. Immunomodulators (imiquimod, lenalidomide and leptin) were safest but less active in in vitro system, however, in P. yoelii-infected mice, they exhibited modest parasite suppression at their respective doses. Among antibiotics, moxifloxacin exhibited better antimalarial prospective than levofloxacin, roxithromycin and erythromycin. 5-Fluorouracil, imiquimod and moxifloxacin displayed 97.64, 81.18 and 91.77 % parasite inhibition in treated animals and attained superiority in their respective groups thus could be exploited further in combination with suitable antimalarials.

Identification of a neutralizing epitope within minor repeat region of Plasmodium falciparum CS protein

Malaria remains a major cause of morbidity and mortality worldwide with 219 million infections and 435,000 deaths predominantly in Africa. The infective Plasmodium sporozoite is the target of a potent humoral immune response that can protect murine, simian and human hosts against challenge by malaria-infected mosquitoes. Early murine studies demonstrated that sporozoites or subunit vaccines based on the sporozoite major surface antigen, the circumsporozoite (CS) protein, elicit antibodies that primarily target the central repeat region of the CS protein. In the current murine studies, using monoclonal antibodies and polyclonal sera obtained following immunization with P. falciparum sporozoites or synthetic repeat peptides, we demonstrate differences in the ability of these antibodies to recognize the major and minor repeats contained in the central repeat region. The biological relevance of these differences in fine specificity was explored using a transgenic P. berghei rodent parasite expressing the P. falciparum CS repeat region. In these in vitro and in vivo studies, we demonstrate that the minor repeat region, comprised of three copies of alternating NANP and NVDP tetramer repeats, contains an epitope recognized by sporozoite-neutralizing antibodies. In contrast, murine monoclonal antibodies specific for the major CS repeats (NANP)_n could be isolated from peptide-immunized mice that had limited or no sporozoite-neutralizing activity. These studies highlight the importance of assessing the fine specificity and functions of antirepeat antibodies elicited by P. falciparum CS-based vaccines and suggest that the design of immunogens to increase antibody responses to minor CS repeats may enhance vaccine efficacy.

Toll-like receptor 4, Toll-like receptor 7 and Toll-like receptor 9 agonists enhance immune responses against blood-stage Plasmodium chabaudi infection in BALB/c mice

BACKGROUND

Toll-like receptor (TLR) signals play vital roles during the blood-stage of malaria infections. However, the roles of TLR agonists in the regulation of immune responses and the development of protective immunity to malaria remain poorly understood.

METHOD

BALB/c mice were pre-treated with TLR4, TLR7 and TLR9 agonists, followed by infection with Plasmodium chabaudi. After infection, splenic dendritic cells (DCs), Th1 cells and programmed death-1 (PD-1) expressed on Th1 cells, as well as regulatory T cells (Tregs) were analyzed by flow cytometry. The levels of IFN-γ, TNF-α, TGF-β and IL-10 in splenocytes and IgG1 and IgG2a in serum were measured by ELISA.

RESULT

Administration of TLR4, TLR7 and TLR9 agonists prior to infection improved disease outcomes. All TLR agonists promoted DC activation, and the proportions of Th1 cells increased. In TLR4, TLR7 and TLR9 agonist treated groups the levels of pro-inflammatory cytokines IFN-γ and TNF-α were elevated, and IgG1 and IgG2a serum levels were also significantly increased. TLR4, TLR7 and TLR9 agonists diminished the activation of Tregs and down-regulated the anti-inflammatory cytokines TGF-β and IL-10. Finally, PD-1 expressed on Th1 cells were decreased in TLR4, TLR7 and TLR9 agonist treated groups compared with control groups.

CONCLUSION

TLR4, TLR7 and TLR9 agonists activated DC-mediated innate immune responses and adaptive immune response, which against the blood-stage of Plasmodium and might be applied to malaria protection and treatment.

Progress in the Development of Subunit Vaccines against Malaria

Malaria is a life-threatening disease and one of the main causes of morbidity and mortality in the human population. The disease also results in a major socio-economic burden. The rapid spread of malaria epidemics in developing countries is exacerbated by the rise in drug-resistant parasites and insecticide-resistant mosquitoes. At present, malaria research is focused mainly on the development of drugs with increased therapeutic effects against Plasmodium parasites. However, a vaccine against the disease is preferable over treatment to achieve long-term control. Trials to develop a safe and effective immunization protocol for the control of malaria have been occurring for decades, and continue on today; still, no effective vaccines are available on the market. Recently, peptide-based vaccines have become an attractive alternative approach. These vaccines utilize short protein fragments to induce immune responses against malaria parasites. Peptide-based vaccines are safer than traditional vaccines, relatively inexpensive to produce, and can be composed of multiple T- and B-cell epitopes integrated into one antigenic formulation. Various combinations, based on antigen choice, peptide epitope modification and delivery mechanism, have resulted in numerous potential malaria vaccines candidates; these are presently being studied in both preclinical and clinical trials. This review describes the current landscape of peptide-based vaccines, and addresses obstacles and opportunities in the production of malaria vaccines.

Time to Micromanage the Pathogen-Host-Vector Interface: Considerations for Vaccine Development

The current increase in vector-borne disease worldwide necessitates novel approaches to vaccine development targeted to pathogens delivered by blood-feeding arthropod vectors into the host skin. A concept that is gaining traction in recent years is the contribution of the vector or vector-derived components, like salivary proteins, to host-pathogen interactions. Indeed, the triad of vector-host-pathogen interactions in the skin microenvironment can influence host innate and adaptive responses alike, providing an advantage to the pathogen to establish infection. A better understanding of this "bite site" microenvironment, along with how host and vector local microbiomes immunomodulate responses to pathogens, is required for future vaccines for vector-borne diseases. Microneedle administration of such vaccines may more closely mimic vector deposition of pathogen and saliva into the skin with the added benefit of near painless vaccine delivery. Focusing on the 'micro'⁻from microenvironments to microbiomes to microneedles⁻may yield an improved generation of vector-borne disease vaccines in today's increasingly complex world.

Applications of Immunomodulatory Immune Synergies to Adjuvant Discovery and Vaccine Development

Pathogens comprise a diverse set of immunostimulatory molecules that activate the innate immune system during infection. The immune system recognizes distinct combinations of pathogenic molecules leading to multiple immune activation events that cooperate to produce enhanced immune responses, known as 'immune synergies'. Effective immune synergies are essential for the clearance of pathogens, thus inspiring novel adjuvant design to improve vaccines. We highlight current vaccine adjuvants and the importance of immune synergies to adjuvant and vaccine design. The focus is on new technologies used to study and apply immune synergies to adjuvant and vaccine development. Finally, we discuss how recent findings can be applied to the future design and characterization of synergistic adjuvants and vaccines.

A novel multi-peptide subunit vaccine admixed with AddaVax adjuvant produces significant immunogenicity and protection against Proteus mirabilis urinary tract infection in mice model

Proteus mirabilis is a common pathogen in urinary tract infections (UTIs). There is no vaccine against P. mirabilis, thus a novel multi-peptide vaccine of MrpA, UcaA and Pta factors of P. mirabilis we designed and a mice model was used to evaluate its efficacy in combination with AddaVax adjuvant. According to the bioinformatics studies, 7 fragments of MrpA (31-75, 112-146), UcaA (68-117, 132-156) and Pta (210-265, 340-400, 496-570) with B and T cell epitope regions were selected for fusion construction. Mice subcutaneously vaccinated with the fusion MrpA.Pta.UcaA induced a significant increase in serum and mucosal IgG and IgA responses. The fusion also showed a significant induction in cellular responses (Th1 and Th2). The addition of AddaVax to fusion and the mixture of MrpA, UcaA, and Pta (MUP) improved the humoral and cellular responses, especially the IgG2a and IFN-γ (Th1 responses) levels. Fusion with and without AddaVax and MUP + AddaVax could maintain significant humoral responses until 6 months after the first vaccine dose. All vaccine combinations with and without adjuvant showed high effectiveness in the protection of the bladder and kidney against experimental UTI; this could be attributed to the significant humoral and cellular responses. The present study suggests that the AddaVax-based vaccine formulations especially the fusion Pta.MrpA.UcaA admixed with AddaVax as potential vaccine candidates for protection against P. mirabilis. Furthermore, AddaVax could be considered as an effective adjuvant in designing other vaccines against UTI pathogens.