Pathways for potentiation of immunogenicity during adjuvant-assisted immunizations with Plasmodium falciparum major merozoite surface protein 1

Trichinella spiralis: immune response and protective immunity elicited by recombinant paramyosin formulated with different adjuvants

Our previous studies showed that immunization with recombinant paramyosin from Trichinella spiralis (rTs-Pmy) formulated with Freund's adjuvant significantly reduced larval burden in mice after T. spiralis larval challenge. Since Freund's adjuvant is toxic and not a suitable adjuvant for clinical vaccine trials, we evaluated the ability of the adjuvants Montanide ISA206 and ISA720 to stimulate immune responses during rTs-Pmy immunization and to enhance protective immunity. The results revealed that immunization of BALB/c mice with rTs-Pmy formulated with either ISA206 or ISA720 triggered Th1 and Th2 immune responses similar to those produced by the conventional Freund's adjuvant formulation and also provided a similar level of protection against T. spiralis larval challenge. This indicates that the recombinant Ts-Pmy formulated with Montanide ISA206 or ISA720 may be an effective and safety vaccine strategy for trichinellosis.

Immune responses of mice with different genetic backgrounds to improved multiepitope, multitarget malaria vaccine candidate antigen FALVAC-1A

FALVAC-1A is a second-generation multitarget, multiepitope synthetic candidate vaccine against Plasmodium falciparum, incorporating elements designed to yield a stable and immunogenic molecule. Characteristics of the immunogenicity of FALVAC-1A were evaluated in congenic (H-2(b), H-2(k), and H-2(d)) and outbred strains of mice. The influences of four adjuvants (aluminum phosphate, QS-21, Montanide ISA-720, and copolymer CRL-1005) on different aspects of the immune response were also assessed. FALVAC-1A generated strong antibody responses in all mouse strains. The highest mean enzyme-linked immunosorbent assay (ELISA) antibody concentrations against FALVAC-1A were observed in the outbred ICR mice, followed by B10.BR, B10.D2, and C57BL/6 mice, though this order varied for the different adjuvants, with no statistical differences between mouse strains. In all mouse strains, the highest anti-FALVAC-1A antibody titers in ELISAs were induced by FALVAC-1A in copolymer and ISA-720 formulations, followed by QS-21 and AlPO4. These antibodies were of all four subclasses, though immunoglobulin G1 (IgG1) predominated, with the exception of FALVAC-1A with the QS-21 adjuvant, which induced predominantly IgG2c responses. Both sporozoites and blood stages of P. falciparum were recognized by anti-FALVAC-1A sera in the immunofluorescence assay. In addition to antibody, cellular immune responses were detected; these responses were studied by examining spleen cells producing gamma interferon and interleukin-4 in enzyme-linked immunospot assays. In summary, FALVAC-1A was found to be highly immunogenic and elicited functionally relevant antibodies that can recognize sporozoites and blood-stage parasites in diverse genetic backgrounds.

Adjuvant formulations possess differing efficacy in the potentiation of antibody and cell mediated responses to a human malaria vaccine under selective immune genes knockout environment

Infections and chronic diseases can alter the host's immunological balance or result in immunodeficiencies. We hypothesize that this may also affect the performance of vaccine adjuvants. Accordingly, the potency and adjuvanticity of eight adjuvant formulations based on Montanide ISA720, MF59, monophosphoryl lipid A (MPL), QS21 (saponin derivative), MPL-SE (stable emulsion of a MPL derivative), and MPL-AF (MPL in aqueous formulation) were studied in immune gene knockout mice, IFN-gamma -/-, IL-4 -/-, and STAT6 -/-, using the P. falciparum MSP1 vaccine, P30P2MSP1-19 as a model immunogen. The adjuvants showed preferential requirements for the immune mediators to induce immune responses to MSP1-19, and the effects were formulation-specific. While emulsion-type adjuvants were highly effective in mice, their potency was more readily suppressed by immune knockouts; and additions of immunomodulators were required to restore efficacy. Formulated adjuvants had characteristics distinct from their individual components, and multi-components formulations were not necessarily superior. We conclude that perturbation of immune environments will have measurable impact on adjuvants' potency. Evaluation of adjuvants in immune knockout models may be a supplementary approach to measure and compare adjuvants' efficacy, and to further unveil their distinct biological activities.

The requirement of CD80, CD86, and ICAM-1 on the ability of adjuvant formulations to potentiate antibody responses to a Plasmodium falciparum blood-stage vaccine

Many adjuvants are known to enhance expression of co-stimulatory and adhesion molecules secondarily to the activation of immune cells. Whether interactions via these molecules are obligatory in adjuvants' ability to potentiation vaccine immunogenicity is less clear. We investigated the ability of eight adjuvant formulations to potentiate the immunogenicity of a malaria vaccine in mice deficient in the prominent co-stimulatory molecules, CD80 and CD86; and the adhesion ligand, ICAM-1. While no adjuvants could bypass co-stimulatory requirements, more formulations exhibited dependency for CD86 than for CD80. In CD80 or CD86 KO mice, formulations with the saponin derivative, QS21 could efficiently default to the other B7 molecule. This effect was dominant over other adjuvant constituents. The requirement for ICAM-1 could be readily bypassed using adjuvant formulations containing immunomodulators; whereas this was not the case with emulsion-type adjuvants in which reduction in adjuvanticity was associated with decreases in antigen-specific IFN-gamma responses. These studies may help to guide the formulation of vaccine adjuvants to maintain effectiveness in hosts with altered immunological environment that often result from infections.

Role of interferon-γ during CpG oligodeoxynucleotide-adjuvanted immunization with recombinant proteins

Synthetic oligonucleotides (ODNs) containing immunostimulatory CpG motifs (CpG) are a new class of adjuvants suitable for the development of recombinant vaccines. Here we describe that endogenous interferon (IFN) was critical for the adjuvant activity of CpG ODN as genetically deficient mice developed significantly lower IgG antibody titers following immunization with recombinant proteins. In contrast, the absence of endogenous IL-12/IL-23 or IL-4 had little impact on the magnitude of the antibody response but instead caused a dramatic change in the pattern of IgG isotypes. The dependence on IFN-gamma was specific for CpG ODN and it was not observed with other adjuvants tested. IFN-gamma was produced by NK, dendritic cells, CD4+ and CD8+ T cells stimulated in vitro with CpG ODN. Adoptive transfer experiments confirmed that CD4+ or CD8+ T cells were in fact relevant sources of IFN-gamma in vivo. Following CpG ODN injection, splenic dendritic cells from IFN-gamma deficient mice did not up-regulate CD86 or CD40 expression, suggesting a role for these molecules. The importance of CD28 (CD86 ligand) was confirmed using CD28 deficient mice which presented severely impaired immune responses following CpG ODN-assisted immunization.

Interleukin-6 has differential influence on the ability of adjuvant formulations to potentiate antibody responses to a Plasmodium falciparum blood-stage vaccine

The efficacy of vaccine adjuvants can be influenced by the immunological environment of the host, depending on the mechanism(s) by which they exert their immunopotentiating activities. Interleukin-6 is a pleiotropic cytokine that has a broad range of biological activities on immune and non-immune cells. We investigated the role of IL-6 on the ability of nine adjuvant formulations to induce antibody responses to the Plasmodium falciparum MSP1-19 malaria vaccine, using IL-6-/- (KO) mice. Results showed that some adjuvants, i.e. MPL-SE, CFA/IFA, ISA720/QS21/MPL, depended on IL-6 for their efficacy, while others exhibited increased potency in its absence. The efficacy of adjuvants in the IL-6 KO environment cannot be solely attributed to their ability to stimulate antigen-specific cellular responses, suggesting that other biological activities of IL-6 are also important. The results further suggest that two adjuvants utilized dissimilar pathways to potentiate the same type of immune response.

Plasmodium falciparum: immunization with MSP1-42 induced non-inhibitory antibodies that have no blocking activities but enhanced the potency of inhibitory anti-MSP1-42 antibodies

Hyperimmunization with Plasmodium falciparum MSP1-42 could induce antibodies that have little or no parasite growth inhibitory activities. These antisera had no blocking activities as determined by their ability to interfere with the in vitro activities of growth inhibitory anti-MSP1-42 sera. Equally important, they enhanced the potency of growth inhibitory anti-MSP1-42 sera.

Immunogenicity and protective efficacy of Escherichia coli expressed Plasmodium falciparum merozoite surface protein-1(42) using human compatible adjuvants

The C-terminal 42-kDa fragment of the merozoite surface protein-1 of Plasmodium falciparum (PfMSP-1(42)) was expressed as a recombinant protein in Escherichia coli and purified to near homogeneity. We tested the immunogenicity of recombinant PfMSP-1(42) in three clinically acceptable adjuvants (Montanide ISA 720, alum and MF59) in mice and in rabbits. High antibody responses were obtained with two adjuvant formulations with IgGl being the predominant immunoglobulin isotype. Significant T-cell proliferation responses were also observed. Competitive enzyme linked immunosorbant assay (ELISA) showed the presence of both invasion and processing inhibitory antibodies in sera obtained from the immunized rabbits. Passive immunizations of mice with anti-PfMSP-1(42) IgG purified from the rabbit-sera were found to be protective against a parasite challenge with P. berghei/P. falciparum chimeric line (Pb-PfM19) that expresses Plasmodium falciparum MSP-1(19). These findings may be useful for the development of a malaria vaccine based on Plasmodium falciparum MSP-1(42).

Progress in the development of recombinant and synthetic blood-stage malaria vaccines

The use of asexual blood-stage proteins as malaria vaccines is strongly supported by experimental data directly implicating antibodies induced by these antigens in parasite clearance and protection from re-challenge. The selection of blood-stage antigens is based on their ability to interfere with the pathogenesis of clinical malaria by reducing parasitemias. These vaccines could complement other vaccines aimed at preventing infection, such as those targeted at pre-erythrocytic or mosquito stages of the parasite. Asexual blood-stage vaccines may reduce disease by blockade of red blood cell invasion, inhibition of parasite growth in red cells or interference in cytoadherence of infected red cells. Clearance of blood-stage parasites is dependent primarily on antibody-mediated mechanisms, but CD4 T cells may also play an important role in help for B cells and probably have a direct effector function in the clearance of blood-stage parasites. Since asexual blood-stage parasites reside within erythrocytes, they are accessible to immune clearance mechanisms only for a short time, which imposes special requirements on vaccines. For example, immunity that induces high titers of antibody will be required. Antigenic variation and extensive polymorphism of malarial proteins also needs to be addressed. Several recombinant antigens derived from blood-stage proteins have moved beyond basic research and are now poised for phase I trials in endemic countries. In this review we discuss the state of asexual blood-stage vaccines, focusing on recombinant antigens from Plasmodium falciparum. The significance of polymorphism and antigenic variation, the relevance of parasite immune evasion mechanisms, the need for reliable measures of successful intervention and new adjuvants are reviewed. Results from trials of asexual blood stage vaccine that support the continued effort to develop these antigens as key ingredients of multicomponent,multistage malaria vaccines are documented.

Endogenous interleukin-4 downregulates the type 1 CD4 T cell-mediated immune response induced by intramuscular DNA immunization

Intramuscular (i.m.) administration of eukaryotic plasmid vectors containing foreign genes is a general immunization strategy capable of inducing protective type 1 immune responses against viral, bacterial, fungal, and parasitic infections. We have described that immunization with a plasmid containing a gene encoding a parasite antigen elicits specific type 1 protective immune responses against experimental infection with the human protozoan parasite Trypanosoma cruzi. However, we had evidence suggesting that DNA immunization concomitantly activated specific type 2 immune responses. To determine precisely the influence of the type 2 cytokine interleukin-4 (IL-4) during DNA immunization, we compared the immune responses of genetically modified IL-4-deficient or wild-type (wt) BALB/c mice. IL-4-deficient mice had a significantly lower ratio of specific serum IgG1/IgG2a, and on in vitro restimulation with antigen, their spleen cells secreted significantly higher amounts of interferon-gamma (IFN-gamma). In contrast, absence of IL-4 did not affect total serum antibody response, T cell proliferative responses, or activation of IFN-gamma-producing CD8(+) T cells. Our results suggested that in contrast to conventional adjuvants, such as alum and complete Freund's adjuvant, specific IgG1 in DNA-immunized BALB/c mice was highly dependent on IL-4. To our knowledge, our study provides the first evidence that endogenous IL-4 selectively downregulates the type 1 CD4(+) T cell-mediated immune response induced by i.m. genetic immunization, a fact that may have implications for the design of certain DNA vaccines.

Immunity to asexual blood stage malaria and vaccine approaches

The development of a malaria vaccine seems to be a definite possibility despite the fact that even individuals with a life time of endemic exposure do not develop sterile immunity. An effective malaria vaccine would be invaluable in preventing malaria-associated deaths in endemic areas, especially amongst children less than 5 years of age and pregnant women. This review discusses our current understanding of immunity against the asexual blood stage of malaria - the stage that is responsible for the symptoms of the disease - and approaches to the design of an asexual blood stage vaccine.

Construction of a tetR-integrated Salmonella enterica serovar Typhi CVD908 strain that tightly controls expression of the major merozoite surface protein of Plasmodium falciparum for applications in human Vaccine production

Attenuated Salmonella strains are an attractive live vector for delivery of a foreign antigen to the human immune system. However, the problem with this vector lies with plasmid segregation and the low level of expression of the foreign gene in vivo when constitutive expression is employed, leading to a diminished immune response. We have established inducible expressions of foreign genes in the Salmonella enterica serovar Typhi CVD908 vaccine strain using the tetracycline response regulatory promoter. To set up this system, a tetracycline repressor (tetR) was integrated into a defined Delta aroC locus of the chromosome via suicide plasmid pJG12/tetR-neo. To remove the neo gene conferring kanamycin resistance from the locus, a cre expression vector under the control of the tetracycline response promoter was transformed into the clone; expression of the Cre recombinase excised the neo gene and generated the end strain CVD908-tetR. Expression of the luciferase reporter gene in this strain is dependent on the presence of tetracycline in the medium and can be regulated up to 4,773-fold. Moreover, the tightly controlled expression of major merozoite surface protein 1 (MSP1) and parts of Plasmodium falciparum was achieved, and the product yield was increased when the inducible expression system was employed. Inoculation of bacteria harboring plasmid pZE11/MSP1(42) in mice produced the protein in liver and spleen controlled by the inducer. The persistence of the plasmid-carrying bacteria in mice was determined. Peak colonization of both liver and spleen was detected on the third day postinoculation and was followed by a decline in growth curves. After 14 days postinfection, the majority of the bacteria (>90%) recovered from the liver and spleen of the mice retained the plasmid when expression was induced; this clearly indicated that stability of the expression vector in vivo was improved by inducible expression. Establishment of the regulatory system in the vaccine strain may broaden the range of its use by enhancing plasmid stability and expression levels in vivo. Moreover, the availability of the vaccine strain inducibly expressing the entire MSP1 provides possibilities for examining its immunogenicity, particularly the cellular response in animal models.

Linkage of exogenous T-cell epitopes to the 19-kilodalton region of Plasmodium yoelii merozoite surface protein 1 (MSP1(19)) can enhance protective immunity against malaria and modulate the immunoglobulin subclass response to MSP1(19)

The degree of protection against Plasmodium yoelii asexual blood stages induced by immunization of mice with the 19-kDa region of merozoite surface protein 1 (MSP1(19)) is H-2 dependent. As a strategy to improve the protection, mouse strains with disparate H-2 haplotypes were immunized with glutathione S-transferase (GST)-MSP1(19) proteins including either a universal T-cell epitope from tetanus toxin (P2) or an I-A(k)-restricted T-cell epitope (P8) from Plasmodium falciparum Pf332. In H-2(k) mice which are poorly protected following immunization with GST-MSP1(19), GST-P2-MSP1(19) significantly improved the protection. In mice partially (H-2(k/b)) or well protected by GST-MSP1(19) (H-2(d) and H-2(b)), P2 did not further increase the protection. However, the protection of H-2(k/b) mice and to some extent H-2(k) mice was improved by immunization with GST-P8-MSP1(19). The magnitudes of immunoglobulin G1 (IgG1) and IgG2a responses in mice immunized with the GST-MSP1(19) variants correlated with low peak parasitemia, indicating a protective capacity of these IgG subclasses. In H-2(k) mice immunized with GST-P2-MSP1(19), both IgG1 and IgG2a responses were significantly enhanced. The epitope P2 appeared to have a general ability to modulate the IgG subclass response since all four mouse strains displayed elevated IgG2a and/or IgG2b levels after immunization with GST-P2-MSP1(19). In contrast, GST-P8-MSP1(19) induced a slight enhancement of IgG responses in H-2(k/b) and H-2(k) mice without any major shift in IgG subclass patterns. The ability to improve the protective immunity elicited by P. yoelii MSP1(19) may have implications for improvement of human vaccines based on P. falciparum MSP1(19).