Pre-clinical evaluation of new adjuvant formulations to improve the immunogenicity of the malaria vaccine RTS,S/AS02A

Potentials of saponins-based adjuvants for nasal vaccines

Respiratory infections are a major public health concern caused by pathogens that colonize and invade the respiratory mucosal surface. Nasal vaccines have the advantage of providing protection at the primary site of pathogen infection, as they induce higher levels of mucosal secretory IgA antibodies and antigen-specific T and B cell responses. Adjuvants are crucial components of vaccine formulation that enhance the immunogenicity of the antigen to confer long-term and effective protection. Saponins, natural glycosides derived from plants, shown potential as vaccine adjuvants, as they can activate the mammalian immune system. Several licensed human vaccines containing saponins-based adjuvants administrated through intramuscular injection have demonstrated good efficacy and safety. Increasing evidence suggests that saponins can also be used as adjuvants for nasal vaccines, owing to their safety profile and potential to augment immune response. In this review, we will discuss the structure-activity-relationship of saponins, their important role in nasal vaccines, and future prospects for improving their efficacy and application in nasal vaccine for respiratory infection.

Developments in Vaccine Adjuvants

Vaccines, including subunit, recombinant, and conjugate vaccines, require the use of an immunostimulator/adjuvant for maximum efficacy. Adjuvants not only enhance the strength and longevity of immune responses but may also influence the type of response. In this chapter, we review the adjuvants that are available for use in human vaccines, such as alum, MF59, AS03, and AS01. We extensively discuss their composition, characteristics, mechanism of action, and effects on the immune system. Additionally, we summarize recent trends in adjuvant discovery, providing a brief overview of saponins, TLRs agonists, polysaccharides, nanoparticles, cytokines, and mucosal adjuvants.

AddaVax Formulated with PolyI:C as a Potential Adjuvant of MDCK-based Influenza Vaccine Enhances Local, Cellular, and Antibody Protective Immune Response in Mice

Poor immune responses to inactivated influenza vaccine can be improved by effective and safe adjuvants to increase antibody titers and cellular protective response. In our study, AddaVax and PolyI:C combined adjuvant (AP adjuvant) were used for influenza vaccine development. After immunizing BALB/c mice and Wistar rats intramuscularly, Split inactivated H3N2 vaccine adjuvanted with AP elicited higher serum hemagglutination-inhibition antibodies and IgG titers. We demonstrated that AP induced a transient innate immune cytokines production at the injection site, induced H3N2 uptake by DCs, increased recruitment of monocytes and DCs in LNs, and promoted H3N2 vaccine migration; AP facilitated vaccines to induce a vigorous adaptive immune response. Besides, AP showed good safety as shown by lymph nodes (LNs) size, spleens index of BALB/c mice, and weight changes and C-reaction protein level of BALB/c mice and Wistar rats after repeated administration of high-dose vaccine with or without adjuvant. These findings indicate that AP is a potential novel adjuvant and can be used as a safe and effective adjuvant for MDCK-based influenza inactivated vaccine to induce cellular and antibody protective response.

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.

Natural and Synthetic Saponins as Vaccine Adjuvants

Saponin adjuvants have been extensively studied for their use in veterinary and human vaccines. Among them, QS-21 stands out owing to its unique profile of immunostimulating activity, inducing a balanced Th1/Th2 immunity, which is valuable to a broad scope of applications in combating various microbial pathogens, cancers, and other diseases. It has recently been approved for use in human vaccines as a key component of combination adjuvants, e.g., AS01b in Shingrix^® for herpes zoster. Despite its usefulness in research and clinic, the cellular and molecular mechanisms of QS-21 and other saponin adjuvants are poorly understood. Extensive efforts have been devoted to studies for understanding the mechanisms of QS-21 in different formulations and in different combinations with other adjuvants, and to medicinal chemistry studies for gaining mechanistic insights and development of practical alternatives to QS-21 that can circumvent its inherent drawbacks. In this review, we briefly summarize the current understandings of the mechanism underlying QS-21’s adjuvanticity and the encouraging results from recent structure-activity-relationship (SAR) studies.

Breadth of humoral immune responses to the C-terminus of the circumsporozoite protein is associated with protective efficacy induced by the RTS,S malaria vaccine

The circumsporozoite protein (CSP) is the main surface antigen of malaria sporozoites, a prime vaccine target, and is known to have polymorphisms in the C-terminal region. Vaccines using a single allele may have lower efficacy against genotypic variants. Recent studies have found evidence suggesting the efficacy of the CSP-based RTS,S malaria vaccine may be limited against P. falciparum CSP alleles that diverge from the 3D7 vaccine allele, particularly in this polymorphic C-terminal region. In order to assess the breadth of the RTS,S-induced antibody responses against CSP C-terminal antigenic variants, we used a novel multiplex assay to measure reactivity of serum samples from a recent RTS,S study against C-terminal peptides from 3D7 and seven additional CSP alleles that broadly represent the genetic diversity found in circulating P. falciparum field isolates. We found that responses to the variants showed, on average, a ~ 30-fold reduction in reactivity relative to the vaccine-matched 3D7 allele. The extent of this reduction, ranging from 21 to 69-fold, correlated with the number of polymorphisms between the variants and 3D7. We calculated antibody breadth of each sample as the median relative reactivity to the seven CSP variants compared to 3D7. Surprisingly, protection from 3D7 challenge in the RTS,S study was associated with higher C-terminal antibody breadth. These findings suggest CSP C-terminal-specific avidity or fine-specificity may play a role in RTS,S-mediated protection and that breadth of C-terminal CSP-specific antibody responses may be a marker of protection.

Breadth of humoral immune responses to the C-terminus of the circumsporozoite protein is associated with protective efficacy induced by the RTS,S malaria vaccine.. [DOI: 10.1101/2020.11.15.20232033]

The circumsporozoite protein (CSP) is the main surface antigen of malaria sporozoites and a prime vaccine target. Responses induced by the CSP-based RTS,S vaccine towards the polymorphic C-terminal region of P.falciparum-CSP raise concerns that vaccines using single alleles may have lower efficacy against genotypic variants. We characterized the extent of C-terminal cross-reactivity of antibodies induced by RTS,S (based on the 3D7 allele) with variants representing seven circulating field isolates through a novel HTS-multiplex assay for screening closely related peptides. Reactivity to variants showed approximately 30-fold reduction in recognition relative to 3D7. The degree of reduced cross-reactivity,ranging from 21 to 69-fold, directly correlated with the number of polymorphisms between variants and 3D7. Surprisingly, protection assessed by challenge with 3D7 parasites was strongly associated with higher C-terminal antibody breadth suggesting that C-terminal specific avidity or fine-specificity may play a role in RTS,S/AS01B-mediated protection and that breadth of C-terminal CSP-specific antibody responses may be a marker of protection.

[Development of malaria vaccines-state of the art]

Weltweit leben 3,1 Mrd. Menschen in Gebieten, in denen Malaria endemisch ist (Tropen, Subtropen). Jährlich erkranken etwa 200 Mio. Menschen, ca. 500.000 sterben daran. Betroffen sind vor allem Kinder. Um die Malaria zu kontrollieren und schlussendlich jegliche Neuinfektion zu verhindern, ist die Entwicklung wirksamer Impfstoffe von großer Bedeutung. In diesem Beitrag werden zunächst Hintergrundinformationen zur Geschichte der Impfstoffentwicklung, zur Malariaerkrankung und zu den Möglichkeiten der Therapie und Ausbreitungskontrolle gegeben. Der Hauptteil widmet sich dem aktuellen Forschungsstand zu Impfstoffen gegen den Erreger Plasmodium falciparum, gefolgt von einer ausführlichen Diskussion.

Malaria ist eine parasitäre Infektionskrankheit, die von Einzellern, sog. Plasmodien, verursacht wird. Es werden 5 humanpathogene Spezies unterschieden, von denen P. falciparum über 99 % der Erkrankungen in Afrika verursacht. Überträger (Vektor) ist die Anophelesmücke. Plasmodium bietet innerhalb seines Lebenszyklus verschiedene Ansatzpunkte für die Wirkung von Impfstoffen. Von den insgesamt ca. 70 Impfstoffkandidaten sind die präerythrozytären Impfstoffe, die in den Leberzyklus des Parasiten eingreifen, aktuell am weitesten entwickelt. Die von der Weltgesundheitsorganisation (WHO) angestrebte Wirksamkeit von mindestens 75 % wurde aber längst nicht erreicht.

Mit RTS,S/AS01 wird derzeit erstmals ein mäßig wirksamer Impfstoff großflächig eingesetzt. Schon jetzt ist offensichtlich, dass die Malaria nur im Zusammenspiel mit anderen Maßnahmen eingedämmt werden kann. Expositionsprophylaxe mit imprägnierten Moskitonetzen, der Einsatz von Insektiziden mit Residualeffekt in Innenräumen (Indoor Residual Spraying), die Vernichtung von Moskitobrutplätzen und schnelle Diagnose und Therapie der Erkrankung sind hier wichtige Elemente ebenso wie eine funktionierende Gesundheitsversorgung, die in den von Armut geprägten Gebieten oft nicht gewährleistet ist.

Comparison of immunogenicity and safety outcomes of a malaria vaccine FMP013/ALFQ in rhesus macaques (Macaca mulatta) of Indian and Chinese origin

Background

Indian-origin rhesus (InR) are preferred for research, but strict export restrictions continue to limit their use. Chinese-origin rhesus (ChR), although easier to procure, are genetically distinct from InR and differ in their immune response to infectious agents, such as the Simian Immunodeficiency Virus. The most advanced malaria vaccine, RTS,S (GlaxoSmithKline), is based on the circumsporozoite protein (CSP) of Plasmodium falciparum. The efficacy of RTS,S vaccine in the field remains low and short-lived; efforts are underway to improve CSP-based vaccines. Rhesus models can accelerate preclinical down-selection of the next generation of malaria vaccines. This study was used to determine if the safety and immunogenicity outcomes following vaccination with a CSP vaccine would differ in the InR and ChR models, given the genetic differences between the two sub-populations of rhesus.

Methods

The FMP013 vaccine, was composed of nearly full-length soluble P. falciparum CSP produced in Escherichia coli and was adjuvanted with the Army liposomal formulation (ALFQ). Three doses of the vaccine were administered in InR and ChR (n = 6) at 1-month intervals and the antibody and T cell responses were assessed.

Results

Local and systemic toxicity profile of FMP013 vaccine in InR and ChR were similar and they revealed that the FMP013 vaccine was safe and caused only mild and transient inflammatory adverse reactions. Following the first 2 vaccines, there was a slower acquisition of antibodies to the CSP repeat region in ChR. However after the 3rd vaccination the titers in the two models were comparable. The ChR group repeat-specific antibodies had higher avidity and ChR group showed higher inhibition of liver stage development activity compared to InR. There was no difference in T-cell responses to the FMP013 vaccine between the two models.

Conclusions

A difference in the quality of serological responses was detected between the two sub-populations of rhesus. However, both models confirmed that FMP013/ALFQ vaccine was safe, highly immunogenic, elicited functional antibodies and T-cell responses. Overall, the data suggests that rhesus of Indian and Chinese origins can be interchangeably used to compare the safety and immunogenicity of next-generation of malaria vaccines and adjuvants.

Safety, toxicity and immunogenicity of a malaria vaccine based on the circumsporozoite protein (FMP013) with the adjuvant army liposome formulation containing QS21 (ALFQ)

Antibodies to Circumsporozoite protein (CSP) confer protection against controlled human malaria infection (CHMI) caused by the parasite Plasmodium falciparum. Although CSP is highly immunogenic, it does not induce long lasting protection and efforts to improve CSP-specific immunological memory and duration of protection are underway. We have previously reported that the clinical grade CSP vaccine FMP013 was immunogenic and protective against malaria challenge in mice when combined with the Army Liposomal Formulation adjuvant containing immune modulators 3D-PHAD™ and QS21 (ALFQ). To move forward with clinical evaluation, we now report the safety, toxicity and immunogenicity of clinical grade FMP013 and ALFQ in Rhesus macaques. Three groups of Rhesus (n = 6) received half or full human dose of FMP013 + ALFQ on a 0-1-2 month schedule, which showed mild local site reactions with no hematologic derangements in red blood cell homeostasis, liver function or kidney function. Immunization induced a transient systemic inflammatory response, including elevated white blood cell counts, mild fever, and a few incidences of elevated creatine kinase, receding to normal range by day 7 post vaccination. Optimal immunogenicity in Rhesus was observed using a 1 mL ALFQ + 20 µg FMP013 dose. Doubling the FMP013 antigen dose to 40 µg had no effect while halving the ALFQ adjuvant dose to 0.5 mL lowered immunogenicity. Similar to data generated in mice, FMP013 + ALFQ induced serum antibodies that reacted to all regions of the CSP molecule and a Th1-biased cytokine response in Rhesus. Rhesus antibody response to FMP013 + ALFQ was found to be non-inferior to historical benchmarks including that of RTS,S + AS01 in humans. A four-dose GLP toxicity study in rabbits confirmed no local site reactions and transient systemic inflammation associated with ALFQ adjuvant administration. These safety and immunogenicity data support the clinical progression and testing of FMP013 + ALFQ in a CHMI trial in the near future.

Vaccine adjuvants CpG (oligodeoxynucleotides ODNs), MPL (3-O-deacylated monophosphoryl lipid A) and naloxone-enhanced Th1 immune response to the Plasmodium vivax recombinant thrombospondin-related adhesive protein (TRAP) in mice

Despite considerable efforts toward vaccine development over decades, there is no available effective vaccine against Plasmodium vivax. Thrombospondin-related adhesive protein of P. vivax (PvTRAP) is essential for sporozoite motility and invasions into mosquito's salivary gland and vertebrate's hepatocyte; hence, it is a promising target for pre-erythrocytic vaccine. In the current investigation, the role of antibodies and cellular immune responses induced by purified recombinant PvTRAP (rPvTRAP) delivered in three adjuvants, naloxone (NLX), CpG oligodeoxynucleotides ODN1826 (CpG-ODN), and 3-O-deacylated monophosphoryl lipid A (MPL), alone and in combination was evaluated in immunized C57BL/6 mice. The highest level and the avidity of anti-PvTRAP IgG (mean OD_490nm 2.55), IgG2b (mean OD_490nm 1.68), and IgG2c (mean OD_490nm 1.466) were identified in the group received rPvTRA/NLX-MPL-CpG. This group also presented the highest IgG2c/IgG1 (2.58) and IgG2b/IgG1 (2.95) ratio when compared to all other groups, and among the adjuvant groups, the lowest IgG2c/IgG1 (1.86) and IgG2b/IgG1 (2.25) ratio was observed in mice receiving rPvTRAP/NLX. Mice receiving rPvTRAP/adjuvants induced significantly the higher levels of interferon gamma (IFN-γ), low level of detectable IL-10, and no detectable IL-4 production. The present result revealed that PvTRAP is immunogenic and its administration with CPG, MPL, and NLX in C57BL/6 mice induced Th1 immune response. Besides, the rPvTRAP delivery in the mixed formulation of those adjuvants had more potential to increase the level, avidity, and persistence of anti-TRAP antibodies. However, it warrants further assessment to test the blocking activity of the produced antibodies in immunized mice with different adjuvant formulations.

Distinct Helper T Cell Type 1 and 2 Responses Associated With Malaria Protection and Risk in RTS,S/AS01E Vaccinees

Background

The RTS,S/AS01E malaria vaccine has moderate efficacy, lower in infants than children. Current efforts to enhance RTS,S/AS01E efficacy would benefit from learning about the vaccine-induced immunity and identifying correlates of malaria protection, which could, for instance, inform the choice of adjuvants. Here, we sought cellular immunity-based correlates of malaria protection and risk associated with RTS,S/AS01E vaccination.

Methods

We performed a matched case-control study nested within the multicenter African RTS,S/AS01E phase 3 trial. Children and infant samples from 57 clinical malaria cases (32 RTS,S/25 comparator vaccinees) and 152 controls without malaria (106 RTS,S/46 comparator vaccinees) were analyzed. We measured 30 markers by Luminex following RTS,S/AS01E antigen stimulation of cells 1 month postimmunization. Crude concentrations and ratios of antigen to background control were analyzed.

Results

Interleukin (IL) 2 and IL-5 ratios were associated with RTS,S/AS01E vaccination (adjusted P ≤ .01). IL-5 circumsporozoite protein (CSP) ratios, a helper T cell type 2 cytokine, correlated with higher odds of malaria in RTS,S/AS01E vaccinees (odds ratio, 1.17 per 10% increases of CSP ratios; P value adjusted for multiple testing = .03). In multimarker analysis, the helper T cell type 1 (TH1)-related markers interferon-γ, IL-15, and granulocyte-macrophage colony-stimulating factor protected from subsequent malaria, in contrast to IL-5 and RANTES, which increased the odds of malaria.

Conclusions

RTS,S/AS01E-induced IL-5 may be a surrogate of lack of protection, whereas TH1-related responses may be involved in protective mechanisms. Efforts to develop second-generation vaccine candidates may concentrate on adjuvants that modulate the immune system to support enhanced TH1 responses and decreased IL-5 responses.

Liposomal vaccine formulations as prophylactic agents: design considerations for modern vaccines

Vaccinology is one of the most important cornerstones in modern medicine, providing better quality of life. The human immune system is composed of innate and adaptive immune processes that interplay when infection occurs. Innate immunity relies on pathogen-associated molecular patterns which are recognized by pathogen recognition receptors localized in antigen presenting cells. After antigen processing and presentation, CD4+ T cell polarization occurs, further leading to B cell and CD8+ activation and humoral and cell-mediated adaptive immune responses. Liposomes are being employed as vaccine technologies and their design is of importance to ensure proper immune responses. Physicochemical parameters like liposome size, charge, lamellarity and bilayer fluidity must be completely understood to ensure optimal vaccine stability and efficacy. Liposomal vaccines can be developed to target specific immune cell types for the induction of certain immune responses. In this review, we will present promising liposomal vaccine approaches for the treatment of important viral, bacterial, fungal and parasitic infections (including tuberculosis, TB). Cationic liposomes are the most studied liposome types due to their enhanced interaction with the negatively charged immune cells. Thus, a special section on the cationic lipid dimethyldioctadecylammonium and TB is also presented.

Development of a Novel Virus-Like Particle Vaccine Platform That Mimics the Immature Form of Alphavirus

Virus-like particles (VLPs) are noninfectious multiprotein structures that are engineered to self-assemble from viral structural proteins. Here, we developed a novel VLP-based vaccine platform utilizing VLPs from the chikungunya virus. We identified two regions within the envelope protein, a structural component of chikungunya, where foreign antigens can be inserted without compromising VLP structure. Our VLP displays 480 copious copies of an inserted antigen on the VLP surface in a highly symmetric manner and is thus capable of inducing strong immune responses against any inserted antigen. Furthermore, by mimicking the structure of the immature form of the virus, we altered our VLP's in vivo dynamics and enhanced its immunogenicity. We used the circumsporozoite protein (CSP) of the Plasmodium falciparum malaria parasite as an antigen and demonstrated that our VLP-based vaccine elicits strong immune responses against CSP in animals. The sera from immunized monkeys protected mice from malaria infection. Likewise, mice vaccinated with P. yoelii CSP-containing VLPs were protected from an infectious sporozoite challenge. Hence, our uniquely engineered VLP platform can serve as a blueprint for the development of vaccines against other pathogens and diseases.

Rhesus macaque and mouse models for down-selecting circumsporozoite protein based malaria vaccines differ significantly in immunogenicity and functional outcomes

BACKGROUND

Non-human primates, such as the rhesus macaques, are the preferred model for down-selecting human malaria vaccine formulations, but the rhesus model is expensive and does not allow for direct efficacy testing of human malaria vaccines. Transgenic rodent parasites expressing genes of human Plasmodium are now routinely used for efficacy studies of human malaria vaccines. Mice have however rarely predicted success in human malaria trials and there is scepticism whether mouse studies alone are sufficient to move a vaccine candidate into the clinic.

METHODS

A comparison of immunogenicity, fine-specificity and functional activity of two Alum-adjuvanted Plasmodium falciparum circumsporozoite protein (CSP)-based vaccines was conducted in mouse and rhesus models. One vaccine was a soluble recombinant protein (CSP) and the other was the same CSP covalently conjugated to the Qβ phage particle (Qβ-CSP).

RESULTS

Mice showed different kinetics of antibody responses and different sensitivity to the NANP-repeat and N-terminal epitopes as compared to rhesus. While mice failed to discern differences between the protective efficacy of CSP versus Qβ-CSP vaccine following direct challenge with transgenic Plasmodium berghei parasites, rhesus serum from the Qβ-CSP-vaccinated animals induced higher in vivo sporozoite neutralization activity.

CONCLUSIONS

Despite some immunologic parallels between models, these data demonstrate that differences between the immune responses induced in the two models risk conflicting decisions regarding potential vaccine utility in humans. In combination with historical observations, the data presented here suggest that although murine models may be useful for some purposes, non-human primate models may be more likely to predict the human response to investigational vaccines.

Tissue Distribution of Memory T and B Cells in Rhesus Monkeys following Influenza A Infection

Studies of influenza-specific immune responses in humans have largely assessed systemic responses involving serum Ab and peripheral blood T cell responses. However, recent evidence indicates that tissue-resident memory T (TRM) cells play an important role in local murine intrapulmonary immunity. Rhesus monkeys were pulmonary exposed to 2009 pandemic H1N1 virus at days 0 and 28 and immune responses in different tissue compartments were measured. All animals were asymptomatic postinfection. Although only minimal memory immune responses were detected in peripheral blood, a high frequency of influenza nucleoprotein-specific memory T cells was detected in the lung at the "contraction phase," 49-58 d after second virus inoculation. A substantial proportion of lung nucleoprotein-specific memory CD8(+) T cells expressed CD103 and CD69, phenotypic markers of TRM cells. Lung CD103(+) and CD103(-) memory CD8(+) T cells expressed similar levels of IFN-γ and IL-2. Unlike memory T cells, spontaneous Ab secreting cells and memory B cells specific to influenza hemagglutinin were primarily observed in the mediastinal lymph nodes. Little difference in systemic and local immune responses against influenza was observed between young adult (6-8 y) and old animals (18-28 y). Using a nonhuman primate model, we revealed substantial induction of local T and B cell responses following 2009 pandemic H1N1 infection. Our study identified a subset of influenza-specific lung memory T cells characterized as TRM cells in rhesus monkeys. The rhesus monkey model may be useful to explore the role of TRM cells in local tissue protective immunity after rechallenge and vaccination.

Delivery strategies to enhance oral vaccination against enteric infections

While the majority of human pathogens infect the body through mucosal sites, most licensed vaccines are injectable. In fact the only mucosal vaccine that has been widely used globally for infant and childhood vaccination programs is the oral polio vaccine (OPV) developed by Albert Sabin in the 1950s. While oral vaccines against Cholera, rotavirus and Salmonella typhi have also been licensed, the development of additional non-living oral vaccines against these and other enteric pathogens has been slow and challenging. Mucosal vaccines can elicit protective immunity at the gut mucosa, in part via antigen-specific secretory immunoglobulin A (SIgA). However, despite their advantages over the injectable route, oral vaccines face many hurdles. A key challenge lies in design of delivery strategies that can protect antigens from degradation in the stomach and intestine, incorporate appropriate immune-stimulatory adjuvants and control release at the appropriate gastrointestinal site. A number of systems including micro and nanoparticles, lipid-based strategies and enteric capsules have significant potential either alone or in advanced combined formulations to enhance intestinal immune responses. In this review we will outline the opportunities, challenges and potential delivery solutions to facilitate the development of improved oral vaccines for infectious enteric diseases.

Ad35.CS.01-RTS,S/AS01 Heterologous Prime Boost Vaccine Efficacy against Sporozoite Challenge in Healthy Malaria-Naïve Adults

Methods

In an observer blind, phase 2 trial, 55 adults were randomized to receive one dose of Ad35.CS.01 vaccine followed by two doses of RTS,S/AS01 (ARR-group) or three doses of RTS,S/AS01 (RRR-group) at months 0, 1, 2 followed by controlled human malaria infection.

Results

ARR and RRR vaccine regimens were well tolerated. Efficacy of ARR and RRR groups after controlled human malaria infection was 44% (95% confidence interval 21%-60%) and 52% (25%-70%), respectively. The RRR-group had greater anti-CS specific IgG titers than did the ARR-group. There were higher numbers of CS-specific CD4 T-cells expressing > 2 cytokine/activation markers and more ex vivo IFN-γ enzyme-linked immunospots in the ARR-group than the RRR-group. Protected subjects had higher CS-specific IgG titers than non-protected subjects (geometric mean titer, 120.8 vs 51.8 EU/ml, respectively; P = .001).

Conclusions

An increase in vaccine efficacy of ARR-group over RRR-group was not achieved. Future strategies to improve upon RTS,S-induced protection may need to utilize alternative highly immunogenic prime-boost regimens and/or additional target antigens.

Trial Registration

ClinicalTrials.gov NCT01366534

Recent developments in malaria vaccinology

The development of a highly effective malaria vaccine remains a key goal to aid in the control and eventual eradication of this devastating parasitic disease. The field has made huge strides in recent years, with the first-generation vaccine RTS,S showing modest efficacy in a Phase III clinical trial. The updated 2030 Malaria Vaccine Technology Roadmap calls for a second generation vaccine to achieve 75% efficacy over two years for both Plasmodium falciparum and Plasmodium vivax, and for a vaccine that can prevent malaria transmission. Whole-parasite immunisation approaches and combinations of pre-erythrocytic subunit vaccines are now reporting high-level efficacy, whilst exciting new approaches to the development of blood-stage and transmission-blocking vaccine subunit components are entering clinical development. The development of a highly effective multi-component multi-stage subunit vaccine now appears to be a realistic ambition. This review will cover these recent developments in malaria vaccinology.

Genome-based vaccine design: the promise for malaria and other infectious diseases

Vaccines are one of the most effective interventions to improve public health, however, the generation of highly effective vaccines for many diseases has remained difficult. Three chronic diseases that characterise these difficulties include malaria, tuberculosis and HIV, and they alone account for half of the global infectious disease burden. The whole organism vaccine approach pioneered by Jenner in 1796 and refined by Pasteur in 1857 with the "isolate, inactivate and inject" paradigm has proved highly successful for many viral and bacterial pathogens causing acute disease but has failed with respect to malaria, tuberculosis and HIV as well as many other diseases. A significant advance of the past decade has been the elucidation of the genomes, proteomes and transcriptomes of many pathogens. This information provides the foundation for new 21st Century approaches to identify target antigens for the development of vaccines, drugs and diagnostic tests. Innovative genome-based vaccine strategies have shown potential for a number of challenging pathogens, including malaria. We advocate that genome-based rational vaccine design will overcome the problem of poorly immunogenic, poorly protective vaccines that has plagued vaccine developers for many years.

Sequential Phase 1 and Phase 2 randomized, controlled trials of the safety, immunogenicity and efficacy of combined pre-erythrocytic vaccine antigens RTS,S and TRAP formulated with AS02 Adjuvant System in healthy, malaria naïve adults

In an attempt to improve the efficacy of the candidate malaria vaccine RTS,S/AS02, two studies were conducted in 1999 in healthy volunteers of RTS,S/AS02 in combination with recombinant Plasmodium falciparum thrombospondin-related anonymous protein (TRAP). In a Phase 1 safety and immunogenicity study, volunteers were randomized to receive TRAP/AS02 (N=10), RTS,S/AS02 (N=10), or RTS,S+TRAP/AS02 (N=20) at 0, 1 and 6-months. In a Phase 2 challenge study, subjects were randomized to receive either RTS,S+TRAP/AS02 (N=25) or TRAP/AS02 (N=10) at 0 and 1-month, or to a challenge control group (N=8). In both studies, the combination vaccine had an acceptable safety profile and was acceptably tolerated. Antigen-specific antibodies, lymphoproliferative responses, and IFN-γ production by ELISPOT assay elicited with the combination vaccine were qualitatively similar to those generated by the single component vaccines. However, post-dose 2 anti-CS antibodies in the RTS,S+TRAP/AS02 vaccine recipients were lower than in the RTS,S/AS02 vaccine recipients. After challenge, 10 of 11 RTS,S+TRAP/AS02 vaccinees, 5 of 5 TRAP/AS02 vaccinees, and 8 of 8 infectivity controls developed parasitemia, with median pre-patent periods of 13.0, 11.0, and 12.0 days, respectively. The absence of any prevention or delay of parasitemia by TRAP/AS02 suggests no apparent added value of TRAP/AS02 as a candidate vaccine. The absence of significant protection or delay of parasitemia in the 11 RTS,S+TRAP/AS02 vaccine recipients contrasts with previous 2 dose studies of RTS,S/AS02. The small sample size did not permit identifying statistically significant differences between the study arms. However, we speculate, within the constraints of the challenge study, that the presence of the TRAP antigen may have interfered with the vaccine efficacy previously observed with this regimen of RTS,S/AS02, and that any future TRAP-based vaccines should consider employing alternative vaccine platforms.

Vaccine Potentiation by Combination Adjuvants

Adjuvants are crucial components of vaccines. They significantly improve vaccine efficacy by modulating, enhancing, or extending the immune response and at the same time reducing the amount of antigen needed. In contrast to previously licensed adjuvants, current successful adjuvant formulations often consist of several molecules, that when combined, act synergistically by activating a variety of immune mechanisms. These "combination adjuvants" are already registered with several vaccines, both in humans and animals, and novel combination adjuvants are in the pipeline. With improved knowledge of the type of immune responses needed to successfully induce disease protection by vaccination, combination adjuvants are particularly suited to not only enhance, but also direct the immune responses desired to be either Th1-, Th2- or Th17-biased. Indeed, in view of the variety of disease and population targets for vaccine development, a panel of adjuvants will be needed to address different disease targets and populations. Here, we will review well-known and new combination adjuvants already licensed or currently in development-including ISCOMs, liposomes, Adjuvant Systems Montanides, and triple adjuvant combinations-and summarize their performance in preclinical and clinical trials. Several of these combination adjuvants are promising having promoted improved and balanced immune responses.

Improving vaccines by incorporating immunological coadjuvants

While vaccination continues to be the most successful interventionist health policy to date, infectious disease remains a significant cause of death worldwide. A primary reason that vaccination is not able to generate effective immunity is a lack of appropriate adjuvants capable of initiating the desired immune response. Adjuvant combinations can potentially overcome this problem; however, the possible permutations to consider, which include the route and kinetics of vaccination, as well as combinations of adjuvants, are practically limitless. This review aims to summarize the current understanding of adjuvants and related immunological processes and how this knowledge can and has been applied to the strategic selection of adjuvant combinations as components of vaccines against human infectious disease.

RTS,S/AS02A for malaria

Malaria prevention and treatment is becoming increasingly difficult as drug-resistant strains of parasites spread globally and affordable antimalarial drugs become ineffective. Therefore, there is a need for a safe and effective vaccine. In recent years, significant technological advances and an increase in funding for malaria vaccine research, including better public-private collaboration, have increased optimism that highly effective vaccines can be developed. RTS,S/AS02A is a novel pre-erythrocytic vaccine based on the Plasmodium falciparum circumsporozoite surface protein. Among all candidate vaccines developed thus far, only the RTS,S/AS02A vaccine has consistently been demonstrated to be well tolerated and provide significant protective efficacy in challenge studies and clinical trials in malaria-endemic countries.

Key roles of adjuvants in modern vaccines

Vaccines containing novel adjuvant formulations are increasingly reaching advanced development and licensing stages, providing new tools to fill previously unmet clinical needs. However, many adjuvants fail during product development owing to factors such as manufacturability, stability, lack of effectiveness, unacceptable levels of tolerability or safety concerns. This Review outlines the potential benefits of adjuvants in current and future vaccines and describes the importance of formulation and mechanisms of action of adjuvants. Moreover, we emphasize safety considerations and other crucial aspects in the clinical development of effective adjuvants that will help facilitate effective next-generation vaccines against devastating infectious diseases.

TLR4 ligand formulation causes distinct effects on antigen-specific cell-mediated and humoral immune responses

The formulation of TLR ligands and other immunomodulators has a critical effect on their vaccine adjuvant activity. In this work, the synthetic TLR4 ligand GLA was formulated with three distinct vaccine delivery system platforms (aqueous suspension, liposome, or oil-in-water emulsion). The effect of the different formulations on the adaptive immune response to protein subunit vaccines was evaluated in the context of a recombinant malaria antigen, Plasmodium berghei circumsporozoite protein (PbCSP). Antibody responses in vaccinated mice were similar for the different formulations of GLA. However, cell-mediated responses differed significantly depending on the adjuvant system; in particular, the emulsion formulation of the TLR4 ligand induced significantly enhanced cellular IFN-γ and TNF-α responses compared to the other formulations. The effects of differences in adjuvant formulation composition and physical characteristics on biological activity are discussed. These results illustrate the importance of formulation of immunostimulatory adjuvants (e.g. TLR ligands) on the resulting immune responses to adjuvanted vaccines and may play a critical role for combating diseases where T cell immunity is advantageous.

Adjuvant formulation structure and composition are critical for the development of an effective vaccine against tuberculosis

One third of the world is infected with Mycobacterium tuberculosis (Mtb) with eight million new cases of active tuberculosis (TB) each year. Development of a new vaccine to augment or replace the only approved TB vaccine, BCG, is needed to control this disease. Mtb infection is primarily controlled by TH1 cells through the production of IFN-γ and TNF which activate infected macrophages to kill the bacterium. Here we examine an array of adjuvant formulations containing the TLR4 agonist GLA to identify candidate adjuvants to pair with ID93, a lead TB vaccine antigen, to elicit protective TH1 responses. We evaluate a variety of adjuvant formulations including alum, liposomes, and oil-in-water emulsions to determine how changes in formulation composition alter adjuvant activity. We find that alum and an aqueous nanosuspension of GLA synergize to enhance generation of ID93-specific TH1 responses, whereas neither on their own are effective adjuvants for generation of ID93-specific TH1 responses. For GLA containing oil-in-water emulsions, the selection of the oil component is critical for adjuvant activity, whereas a variety of lipid components may be used in liposomal formulations of GLA. The composition of the liposome formulation of ID93/GLA does alter the magnitude of the TH1 response. These results demonstrate that there are multiple solutions for an effective formulation of a novel TB vaccine candidate that enhances both TH1 generation and protective efficacy.

Full-length Plasmodium falciparum circumsporozoite protein administered with long-chain poly(I·C) or the Toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion elicits potent antibody and CD4+ T cell immunity and protection in mice

The Plasmodium falciparum circumsporozoite (CS) protein (CSP) is a major vaccine target for preventing malaria infection. Thus, developing strong and durable antibody and T cell responses against CSP with novel immunogens and potent adjuvants may improve upon the success of current approaches. Here, we compare four distinct full-length P. falciparum CS proteins expressed in Escherichia coli or Pichia pastoris for their ability to induce immunity and protection in mice when administered with long-chain poly(I · C) [poly(I · C)LC] as an adjuvant. CS proteins expressed in E. coli induced high-titer antibody responses against the NANP repeat region and potent CSP-specific CD4(+) T cell responses. Moreover, E. coli-derived CS proteins in combination with poly(I · C)LC induced potent multifunctional (interleukin 2-positive [IL-2(+)], tumor necrosis factor alpha-positive [TNF-α(+)], gamma interferon-positive [IFN-γ(+)]) CD4(+) effector T cell responses in blood, in spleen, and particularly in liver. Using transgenic Plasmodium berghei expressing the repeat region of P. falciparum CSP [Pb-CS(Pf)], we showed that there was a 1- to 4-log decrease in malaria rRNA in the liver following a high-dose challenge and ~50% sterilizing protection with a low-dose challenge compared to control levels. Protection was directly correlated with high-level antibody titers but not CD4(+) T cell responses. Finally, protective immunity was also induced using the Toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE) as the adjuvant, which also correlated with high antibody titers yet CD4(+) T cell immunity that was significantly less potent than that with poly(I · C)LC. Overall, these data suggest that full-length CS proteins and poly(I · C)LC or GLA-SE offer a simple vaccine formulation to be used alone or in combination with other vaccines for preventing malaria infection.

Improved CD4⁺ T cell responses to Mycobacterium tuberculosis in PPD-negative adults by M72/AS01 as compared to the M72/AS02 and Mtb72F/AS02 tuberculosis candidate vaccine formulations: a randomized trial

BACKGROUND

The Bacille Calmette-Guérin (BCG) tuberculosis (TB) vaccine provides incomplete protection, necessitating development of an effective vaccine against TB disease. The Mtb72F/AS02 candidate vaccine was previously shown to be clinically well tolerated and immunogenic in Purified Protein Derivative (PPD)-negative adults. To improve the stability of Mtb72F, a point mutation was introduced into a putative serine protease site to give the final M72 construct. AS01 is an Adjuvant System that can potentially improve both humoral and cellular immune responses compared to the AS02 Adjuvant System or unadjuvanted vaccine. This study evaluated the safety and immunogenicity in Mtb-naïve adults of vaccines containing 40 μg of the M72 antigen with AS02 or AS01 and compared the results with Mtb72F/AS02 vaccine (40 μg dose), M72 in saline (40 μg dose) and AS01 alone.

METHODS

In this Phase I/II observer-blind controlled trial, 110 participants were randomized (4:4:1:1:1) to receive M72/AS01, M72/AS02, Mtb72F/AS02, M72/saline or AS01, following a 0, 1-month schedule. Subjects receiving the adjuvanted M72 vaccines were followed up until 3 years post vaccination. Evaluation of the immune response and safety/reactogenicity was performed.

RESULTS

For all vaccines, solicited adverse events (AEs) were predominantly mild to moderate and transient. No vaccine-related serious AEs occurred and no subject withdrew due to an AE. Immune responses induced by Mtb72F and M72 antigens combined with AS02 were similar. M72/AS01 and M72/AS02 induced robust polyfunctional M72-specific CD4(+) T cell and antibody responses persisting at 3 years, with the highest CD4(+) T cell responses found with M72/AS01.

CONCLUSION

This first clinical study with M72/AS01 and M72/AS02 showed that both vaccines were clinically well tolerated and induced high magnitude and persistent cell-mediated and humoral immune responses. The Mtb72F/AS02 and M72/AS02 vaccines were comparably immunogenic with significantly higher immune responses compared to the M72/saline control. Of the formulations tested, M72/AS01 demonstrated significantly higher vaccine specific Th1 CD4(+) T cell responses supporting its further clinical evaluation.

Recent clinical experience with vaccines using MPL- and QS-21-containing adjuvant systems

The immunostimulants 3-O-desacyl-4'-monophosphoryl lipid A (MPL) and the saponin QS-21 are part of licensed or candidate vaccines. MPL and QS-21 directly affect the innate immune response to orchestrate the quality and intensity of the adaptive immune response to the vaccine antigens. The combination of immunostimulants in different adjuvant formulations forms the basis of Adjuvant Systems (AS) as a way to promote appropriate protective immune responses following vaccination. MPL and aluminum salts are present in AS04, and both MPL and QS-21 are present in AS01 and AS02, which are liposome- and emulsion-based formulations, respectively. The recent clinical performance of AS01-, AS02- and AS04-adjuvanted vaccines will be discussed in the context of the diseases being targeted. The licensing of two AS04-adjuvanted vaccines and the initiation of Phase III trials with an AS01-adjuvanted vaccine demonstrate the potential to develop new or improved human vaccines that contain MPL or MPL and QS-21.

Targeting of Toll-like receptors: a decade of progress in combating infectious diseases

Toll-like receptors (TLRs) recognise highly conserved molecular structures, collectively known as pathogen-associated molecular patterns. In the past two decades, development and clinical implementation of TLR ligands-ie, chemically modified synthetic derivatives of naturally occurring ligands and fully synthetic small molecules-have been topics of intense research. Targeted manipulation of TLR signalling has been applied clinically to boost vaccine effectiveness, promote a robust T helper 1-predominant immune response against viral infection, or dampen the exaggerated inflammatory response to bacterial infection. Use of these new therapeutic molecules as adjuncts to conventional pharmacotherapy or stand-alone treatments might offer solutions to unmet clinical needs or could replace existing partly effective therapeutic strategies.

Evaluation of the safety and immunogenicity in rhesus monkeys of a recombinant malaria vaccine for Plasmodium vivax with a synthetic Toll-like receptor 4 agonist formulated in an emulsion

Plasmodium vivax is the major cause of malaria outside sub-Saharan Africa and inflicts debilitating morbidity and consequent economic impacts in developing countries. In order to produce a P. vivax vaccine for global use, we have previously reported the development of a novel chimeric recombinant protein, VMP001, based on the circumsporozoite protein (CSP) of P. vivax. Very few adjuvant formulations are currently available for human use. Our interest is to evaluate second-generation vaccine formulations to identify novel combinations of adjuvants capable of inducing strong, long-lasting immune responses. In this study rhesus monkeys were immunized intramuscularly three times with VMP001 in combination with a stable emulsion (SE) or a synthetic Toll-like receptor 4 (TLR4) agonist (glucopyranosyl lipid A [GLA]) in SE (GLA-SE). Sera and peripheral blood mononuclear cells (PBMCs) were tested for the presence of antigen-specific humoral and cellular responses, respectively. All groups of monkeys generated high titers of anti-P. vivax IgG antibodies, as detected by enzyme-linked immunosorbent assays (ELISAs) and immunofluorescence assays. In addition, all groups generated a cellular immune response characterized by antigen-specific CD4(+) T cells secreting predominantly interleukin-2 (IL-2) and lesser amounts of tumor necrosis factor (TNF). We conclude that the combination of VMP001 and GLA-SE is safe and immunogenic in monkeys and may serve as a potential second-generation vaccine candidate against P. vivax malaria.

T cell responses to the RTS,S/AS01(E) and RTS,S/AS02(D) malaria candidate vaccines administered according to different schedules to Ghanaian children

BACKGROUND

The Plasmodium falciparum pre-erythrocytic stage candidate vaccine RTS,S is being developed for protection of young children against malaria in sub-Saharan Africa. RTS,S formulated with the liposome based adjuvant AS01(E) or the oil-in-water based adjuvant AS02(D) induces P. falciparum circumsporozoite (CSP) antigen-specific antibody and T cell responses which have been associated with protection in the experimental malaria challenge model in adults.

METHODS

This study was designed to evaluate the safety and immunogenicity induced over a 19 month period by three vaccination schedules (0,1-, 0,1,2- and 0,1,7-month) of RTS,S/AS01(E) and RTS,S/AS02(D) in children aged 5-17 months in two research centers in Ghana. Control Rabies vaccine using the 0,1,2-month schedule was used in one of two study sites.

RESULTS

Whole blood antigen stimulation followed by intra-cellular cytokine staining showed RTS,S/AS01(E) induced CSP specific CD4 T cells producing IL-2, TNF-α, and IFN-γ. Higher T cell responses were induced by a 0,1,7-month immunization schedule as compared with a 0,1- or 0,1,2-month schedule. RTS,S/AS01(E) induced higher CD4 T cell responses as compared to RTS,S/AS02(D) when given on a 0,1,7-month schedule.

CONCLUSIONS

These findings support further Phase III evaluation of RTS,S/AS01(E). The role of immune effectors and immunization schedules on vaccine protection are currently under evaluation.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00360230.

Induction of Plasmodium falciparum-specific CD4+ T cells and memory B cells in Gabonese children vaccinated with RTS,S/AS01(E) and RTS,S/AS02(D)

The recombinant circumsporozoite protein (CS) based vaccine, RTS,S, confers protection against Plasmodium falciparum infection in controlled challenge trials and in field studies. The RTS,S recombinant antigen has been formulated with two adjuvant systems, AS01 and AS02, which have both been shown to induce strong specific antibody responses and CD4 T cell responses in adults. As infants and young children are particularly susceptible to malaria infection and constitute the main target population for a malaria vaccine, we have evaluated the induction of adaptive immune responses in young children living in malaria endemic regions following vaccination with RTS,S/AS01(E) and RTS,S/AS02(D). Our data show that a CS-specific memory B cell response is induced one month after the second and third vaccine dose and that CS-specific antibodies and memory B cells persist up to 12 months after the last vaccine injection. Both formulations also induced low but significant amounts of CS-specific IL-2(+) CD4(+) T cells one month after the second and third vaccine dose, upon short-term in vitro stimulation of whole blood cells with peptides covering the entire CS derived sequence in RTS,S. These results provide evidence that both RTS,S/AS01(E) and RTS,S/AS02(D) induced adaptive immune responses including antibodies, circulating memory B cells and CD4(+) T cells directed against P. falciparum CS protein.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00307021.

Role of nonhuman primates in the evaluation of candidate AIDS vaccines: an industry perspective

PURPOSE OF REVIEW

To consider how nonhuman primate (NHP) model systems can best contribute to HIV vaccine development.

RECENT FINDINGS

We review the traditional roles of NHP model systems in vaccine development and compare this with how NHP models have been used in HIV vaccine research and development. Comparisons of the immune responses elicited by cellular immune response-inducing vaccines in macaques and humans illustrate the value of primate studies for the relative ranking of HIV vaccine concepts for their likely immunogenicity in humans. The unusual structures (e.g. long complementarity-determining regions) of known broadly neutralizing HIV antibodies (bNAbs) suggest that it is critical to test candidate env immunogens in NHPs, whose germline antibody repertoires resemble those of humans. Recent clinical efficacy trial results question the utility of existing NHP challenge models in predicting HIV vaccine efficacy in humans, and highlight the need to further develop models in which acquisition of infection can be reliably evaluated. When evaluated in models using low virus dose challenges that better approximate human sexual exposure to HIV - some vaccine and passive NAb interventions appear to protect against acquisition of infection.

SUMMARY

NHP models have important roles in the preclinical evaluation, optimization, and ranking of novel HIV immunogens. The apparent vaccine efficacy observed using low virus dose challenge models provides an opportunity to investigate the correlates of protection.

An adjuvanted polyprotein HIV-1 vaccine induces polyfunctional cross-reactive CD4+ T cell responses in seronegative volunteers

A novel AS01-adjuvanted HIV-1 vaccine candidate consisting of a recombinant fusion protein (F4) containing 4 HIV-1 clade B antigens (Gag p24, Pol reverse transcriptase [RT], Nef and Gag p17) induced long-lasting, polyfunctional cross-reactive CD4⁺ T-cell responses in HIV-seronegative volunteers.

Background. This phase I/II partially blinded, randomized, dose-ranging study assessed the safety and immunogenicity of a novel human immunodeficiency virus type 1 (HIV-1) vaccine candidate consisting of a recombinant fusion protein (F4) containing 4 HIV-1 clade B antigens (Gag p24, Pol reverse transcriptase, Nef, and Gag p17) adjuvanted with AS01 in HIV-seronegative volunteers.

Methods. Two doses of the recombinant F4 protein (10, 30, or 90 μg/dose), adjuvanted with AS01 or reconstituted with water for injection, were administered 1 month apart to 180 healthy volunteers aged 18–40 years. F4-specific CD4⁺ T cell responses were measured using intracellular cytokine staining after in vitro stimulation by overlapping peptide pools covering the 4 individual antigens.

Results. Reactogenicity was higher during the 7-day period after each vaccine dose in the adjuvanted than in the nonadjuvanted groups. In the adjuvanted groups, the overall immune response rate was high after the second vaccine dose, with highest responder rates seen in the 10-μg F4/AS01 group (100% to 3 HIV-1 antigens and 80% to all 4 HIV-1 antigens). High and long-lasting CD4⁺ T cell frequencies were observed (up to a median value of 1.2% F4-specific CD4⁺ T cells at day 44), with strongest responses directed against reverse transcriptase. Antigen-specific CD4⁺ T cells exhibited a polyfunctional phenotype, expressing at least CD40 ligand and interleukin 2, often in combination with tumor necrosis factor α and/or interferon γ. Vaccine-induced CD4⁺ T cell responses were broadly cross-reactive to all 4 antigens derived from HIV-1 clades A and C.

Conclusions. These results support further clinical investigation of this HIV-1 vaccine candidate both in a prophylactic setting (alone, in conjunction with an envelope-based antigen or in combination with other vaccine approaches in a heterologous prime-boost regimen) and as a potentially disease-modifying therapeutic vaccine in HIV-1–infected subjects.

Clinical trials registration. NCT00434512.

Unmet needs in modern vaccinology: adjuvants to improve the immune response

The key objective of vaccination is the induction of an effective pathogen-specific immune response that leads to protection against infection and/or disease caused by that pathogen, and that may ultimately result in its eradication from humanity. The concept that the immune response to pathogen antigens can be improved by the addition of certain compounds into the vaccine formulation was demonstrated about one hundred years ago when aluminium salts were introduced. New vaccine technology has led to vaccines containing highly purified antigens with improved tolerability and safety profiles, but the immune response they induce is suboptimal without the help of adjuvants. In parallel, the development of effective vaccines has been facing more and more important challenges linked to complicated pathogens (e.g. malaria, TB, HIV, etc.) and/or to subjects with conditions that jeopardize the induction or persistence of a protective immune response. A greater understanding of innate and adaptive immunity and their close interaction at the molecular level is yielding insights into the possibility of selectively stimulating immunological pathways to obtain the desired immune response. The better understanding of the mechanism of 'immunogenicity' and 'adjuvanticity' has prompted the research of new vaccine design based on new technologies, such as naked DNA or live vector vaccines and the new adjuvant approaches. Adjuvants can be used to enhance the magnitude and affect the type of the antigen-specific immune response, and the combination of antigens with more than one adjuvant, the so called adjuvant system approach, has been shown to allow the development of vaccines with the ability to generate effective immune responses adapted to both the pathogen and the target population. This review focuses on the adjuvants and adjuvant systems currently in use in vaccines, future applications, and the remaining challenges the field is facing.

Poly(I:C) is an effective adjuvant for antibody and multi-functional CD4+ T cell responses to Plasmodium falciparum circumsporozoite protein (CSP) and αDEC-CSP in non human primates

Development of a fully effective vaccine against the pre-erythrocytic stage of malaria infection will likely require induction of both humoral and cellular immune responses. Protein based vaccines can elicit such broad-based immunity depending on the adjuvant and how the protein is formulated. Here to assess these variables, non human primates (NHP) were immunized three times with Plasmodium falciparum (Pf) circumsporozoite protein (CSP) or CSP cloned into MG38, a monoclonal antibody that targets DEC-205 (αDEC-CSP), an endocytic receptor on dendritic cells (DCs). Both vaccines were administered with or without poly(I:C) as adjuvant. Following three immunizations, the magnitude and quality of cytokine secreting CD4+ T cells were comparable between CSP+poly(I:C) and αDEC-CSP+poly(I:C) groups with both regimens eliciting multi-functional cytokine responses. However, NHP immunized with CSP+poly(I:C) had significantly higher serum titers of CSP-specific IgG antibodies and indirect immunofluorescent antibody (IFA) titers against Pf sporozoites. Furthermore, sera from both CSP or αDEC-CSP+poly(I:C) immunized animals limited sporozoite invasion of a hepatocyte cell line (HC04) in vitro. To determine whether CSP-specific responses could be enhanced, all NHP primed with CSP or αDEC-CSP+poly(I:C) were boosted with a single dose of 150,000 irradiated Pf sporozoites (PfSPZ) intravenously. Remarkably, boosting had no effect on the CSP-specific immunity. Finally, immunization with CSP+poly-ICLC reduced malaria parasite burden in the liver in an experimental mouse model. Taken together, these data showing that poly(I:C) is an effective adjuvant for inducing potent antibody and Th1 immunity with CSP based vaccines offers a potential alternative to the existing protein based pre-erythrocytic vaccines.

Mucosal vaccines: recent progress in understanding the natural barriers

It has long been known that protection against pathogens invading the organism via mucosal surfaces correlates better with the presence of specific antibodies in local secretions than with serum antibodies. The most effective way to induce mucosal immunity is to administer antigens directly to the mucosal surface. The development of vaccines for mucosal application requires antigen delivery systems and immunopotentiators that efficiently facilitate the presentation of the antigen to the mucosal immune system. This review provides an overview of the events within mucosal tissues that lead to protective mucosal immune responses. The understanding of those biological mechanisms, together with knowledge of the technology of vaccines and adjuvants, provides guidance on important technical aspects of mucosal vaccine design. Not being exhaustive, this review also provides information related to modern adjuvants, including polymeric delivery systems and immunopotentiators.

Impact of RTS,S/AS02(A) and RTS,S/AS01(B) on genotypes of P. falciparum in adults participating in a malaria vaccine clinical trial

Objective

RTS,S, a candidate vaccine for malaria, is a recombinant protein expressed in yeast containing part of the circumsporozoite protein (CSP) sequence of 3D7 strain of Plasmodium falciparum linked to the hepatitis B surface antigen in a hybrid protein. The RTS,S antigen is formulated with GSK Biologicals' proprietary Adjuvant Systems AS02_A or AS01_B. A recent trial of the RTS,S/AS02_A and RTS,S/AS01_B vaccines evaluated safety, immunogenicity and impact on the development of parasitemia of the two formulations. Parasite isolates from this study were used to determine the molecular impact of RTS,S/AS02_A and RTS,S/AS01_B on the multiplicity of infection (MOI) and the csp allelic characteristics of subsequent parasitemias.

Design

The distribution of csp sequences and the MOI of the infecting strains were examined at baseline and in break-through infections from vaccinated individuals and from those receiving a non-malarial vaccine.

Setting

The study was conducted in Kombewa District, western Kenya.

Participants

Semi-immune adults from the three study arms provided isolates at baseline and during break-through infections.

Outcome

Parasite isolates used for determining MOI and divergence of csp T cell–epitopes were 191 at baseline and 87 from break-through infections.

Results

Grouping recipients of RTS,S/AS01_A and RTS,S/AS02_B together, vaccine recipients identified as parasite-positive by microscopy contained significantly fewer parasite genotypes than recipients of the rabies vaccine comparator (median in pooled RTS,S groups: 3 versus 4 in controls, P = 0.0313). When analyzed separately, parasitaemic individuals in the RTS,S/AS01_B group, but not the RTS,S/AS02_A group, were found to have significantly fewer genotypes than the comparator group. Two individual amino acids found in the vaccine construct (Q339 in Th2R and D371 in Th3R) were observed to differ in incidence between vaccine and comparator groups but in different directions; parasites harboring Q339 were less common among pooled RTS,S/AS vaccine recipients than among recipients of rabies vaccine, whereas parasites with D371 were more common among the RTS,S/AS groups.

Conclusions

It is concluded that both RTS,S/AS vaccines reduce multiplicity of infection. Our results do not support the hypothesis that RTS,S/AS vaccines elicit preferential effects against pfcsp alleles with sequence similarity to the 3D7 pfcsp sequence employed in the vaccine construct.

A randomized trial assessing the safety and immunogenicity of AS01 and AS02 adjuvanted RTS,S malaria vaccine candidates in children in Gabon

Background

The malaria vaccine candidate antigen RTS,S includes parts of the pre-erythrocytic stage circumsporozoite protein fused to the Hepatitis B surface antigen. Two Adjuvant Systems are in development for this vaccine, an oil-in water emulsion – based formulation (AS02) and a formulation based on liposomes (AS01).

Methods & Principal Findings

In this Phase II, double-blind study (NCT00307021), 180 healthy Gabonese children aged 18 months to 4 years were randomized to receive either RTS,S/AS01_E or RTS,S/AS02_D, on a 0–1–2 month vaccination schedule. The children were followed-up daily for six days after each vaccination and monthly for 14 months. Blood samples were collected at 4 time-points. Both vaccines were well tolerated. Safety parameters were distributed similarly between the two groups. Both vaccines elicited a strong specific immune response after Doses 2 and 3 with a ratio of anti-CS GMT titers (AS02_D/AS01_E) of 0.88 (95% CI: 0.68–1.15) post-Dose 3. After Doses 2 and 3 of experimental vaccines, anti-CS and anti-HBs antibody GMTs were higher in children who had been previously vaccinated with at least one dose of hepatitis B vaccine compared to those not previously vaccinated.

Conclusions

RTS,S/AS01_E proved similarly as well tolerated and immunogenic as RTS,S/AS02_D, completing an essential step in the age de-escalation process within the RTS,S clinical development plan.

Trial Registration

ClinicalTrials.gov. NCT00307021

Evaluation of the safety and immunogenicity of Plasmodium falciparum apical membrane antigen 1, merozoite surface protein 1 or RTS,S vaccines with adjuvant system AS02A administered alone or concurrently in rhesus monkeys

In an effort to broaden the immune response induced by the RTS,S/AS02(A),vaccine, we have evaluated the immunogenicity of the RTS,S antigen when combined with MSP1(42) and with AMA1, antigens derived from the asexual blood stage. The objectives of this study were (i) to determine whether MSP1(42) and AMA1 vaccines formulated with the AS02(A) Adjuvant System were safe and immunogenic in the rhesus monkey model; (ii) to investigate whether MSP1(42) or AMA1 induced immune interference to each other, or to RTS,S, when added singly or in combinations at a single injection site; (iii) in the event of immune interference, to determine if this could be reduced when antigens were administered at separate sites. We found that MSP1(42) and AMA1 were safe and immunogenic, eliciting antibodies, and Th1 and Th2 responses using IFN-gamma and IL-5 as markers. When malaria antigens were delivered together in one formulation, MSP1(42) and RTS,S reduced AMA1-specific antibody responses as measured by ELISA however, only MSP1(42) lowered parasite growth inhibitory activity of anti-AMA1 antibodies as measured by in vitro growth inhibition assay. Unlike RTS,S, MSP1(42) significantly reduced AMA1 IFN-gamma and IL-5 responses. MSP1(42) suppression of AMA1 IFN-gamma responses was not seen in animals receiving RTS,S+AMA1+MSP1(42) suggesting that RTS,S restored IFN-gamma responses. Conversely, AMA1 had no effect on MSP1(42) antibody and IFN-gamma and IL-5 responses. Neither AMA1 alone or combined with MSP1(42) affected RTS,S antibody or IFN-gamma and IL-5 responses. Immune interference by MSP1(42) on AMA1 antibody responses was also evident when AMA1, MSP1(42) and RTS,S were administered concurrently at separate sites. These results suggest that immune interference may be complex and should be considered for the design of multi-antigen, multi-stage vaccines against malaria.

How might infant and paediatric immune responses influence malaria vaccine efficacy?

Naturally acquired immunity to malaria requires repeat infections yet does not engender sterile immunity or long-lasting protective immunologic memory. This renders infants and young children the most susceptible to malaria-induced morbidity and mortality, and the ultimate target for a malaria vaccine. The prevailing paradigm is that infants initially garner protection due to transplacentally transferred anti-malarial antibodies and other intrinsic factors such as foetal haemoglobin. As these wane infants have an insufficient immune repertoire to prevent genetically diverse Plasmodium infections and an inability to control malaria-induced immunopathology. This Review discusses humoral, cell-mediated and innate immune responses to malaria and how each contributes to protection – focusing on how deficiencies in infant and paediatric immune responses might influence malaria vaccine efficacy in this population. In addition, burgeoning evidence suggests a role for inhibitory receptors that limit immunopathology and guide the development of long-lived immunity. Precisely how age or malaria infections influence the function of these regulators is unknown. Therefore the possibility that infants may not have the immune-dexterity to balance effective parasite clearance with timely immune-regulation leading to protective immunologic memory is considered. And thus, malaria vaccines tested in adults and older children may not be predictive for trials conducted in infants.