Blood-stage challenge for malaria vaccine efficacy trials: a pilot study with discussion of safety and potential value

There is increasing interest in malaria vaccines targeting the asexual blood stage of Plasmodium falciparum. Without accepted immunologic correlates of clinical protection, challenge studies are useful for assessing the efficacy of candidate vaccines in vivo in healthy volunteers. We report a pilot study of a safe and robust challenge protocol using a blood-stage inoculum. We have applied well-validated trial endpoints and twice daily real-time quantitative polymerase chain reaction monitoring of parasitemia to blood-stage challenge, which enabled direct comparison with sporozoite challenge. We found that greater accuracy in quantification of blood-stage growth rates can be achieved with blood-stage challenge. This finding may provide greater power to detect partial efficacy of many blood-stage candidate vaccines. We discuss the potential utility of blood-stage challenge studies in accelerating malaria vaccine development.

Single-dose immunogenicity and protective efficacy of simian adenoviral vectors against Plasmodium berghei

Simian adenoviral vectors (SAd) offer an attractive alternative to standard human adenovirus serotype 5 (AdH5) subunit vaccination, due to pre-existing immunity affecting vaccine performance. We have used a mouse model of liver-stage malaria to test the efficiency of three chimpanzee-origin adenoviral vectors, AdC6, AdC7 and AdC9 containing ME.TRAP as an insert. AdC7 and AdC9 elicited strong immunogenicity ( approximately 20% of CD8(+) T cells in spleen), equivalent to or outperforming AdH5 and inducing sterile protection in 92% (C9), 83% (H5 and C7) and 67% (C6) of the mice, providing the first evidence of single-dose protection to Plasmodium berghei. Protection was afforded by the SAd despite high levels of pre-existing immunity to AdH5. Phenotypic analysis showed that all adenoviral vectors (Ad) elicited CD8(+) T cell responses with an effector memory T cell (T(EM)) phenotype. By contrast, vaccination with poxviral vectors did not confer protection to P. berghei and induced a predominantly CD8(+) central memory T cell (T(CM)) response. Multifunctional CD8(+) T cell responses (co-expressing IFN-gamma, TNF-alpha and IL-2) were also induced by the Ad in higher percentages than the poxviral vectors. Our data suggest that T(EM) cells are important as a first line of defense against fast-replicating pathogens such as murine Plasmodium and demonstrate the potential of replication-defective SAd as future malaria vaccines for humans.

Correlation of memory T cell responses against TRAP with protection from clinical malaria, and CD4 CD25 high T cells with susceptibility in Kenyans

Background

Immunity to malaria develops naturally in endemic regions, but the protective immune mechanisms are poorly understood. Many vaccination strategies aim to induce T cells against diverse pre-erythrocytic antigens, but correlates of protection in the field have been limited. The objective of this study was to investigate cell-mediated immune correlates of protection in natural malaria. Memory T cells reactive against thrombospondin-related adhesive protein (TRAP) and circumsporozoite (CS) protein, major vaccine candidate antigens, were measured, as were frequencies of CD4⁺ CD25^high T cells, which may suppress immunity, and CD56⁺ NK cells and γδ T cells, which may be effectors or may modulate immunity.

Methodology and Principal Findings

112 healthy volunteers living in rural Kenya were entered in the study. Memory T cells reactive against TRAP and CS were measured using a cultured IFNγ ELISPOT approach, whilst CD4⁺ CD25^high T cells, CD56⁺ NK cells, and γδ T cells were measured by flow cytometry. We found that T cell responses against TRAP were established early in life (<5 years) in contrast to CS, and cultured ELISPOT memory T cell responses did not correlate with ex-vivo IFNγ ELISPOT effector responses. Data was examined for associations with risk of clinical malaria for a period of 300 days. Multivariate logistic analysis incorporating age and CS response showed that cultured memory T cell responses against TRAP were associated with a significantly reduced incidence of malaria (p = 0.028). This was not seen for CS responses. Higher numbers of CD4⁺ CD25^high T cells, potentially regulatory T cells, were associated with a significantly increased risk of clinical malaria (p = 0.039).

Conclusions

These data demonstrate a role for central memory T cells in natural malarial immunity and support current vaccination strategies aimed at inducing durable protective T cell responses against the TRAP antigen. They also suggest that CD4⁺ CD25^high T cells may negatively affect naturally acquired malarial immunity.

Mapping of a novel susceptibility locus suggests a role for MC3R and CTSZ in human tuberculosis

RATIONALE

Tuberculosis remains a major cause of morbidity and mortality in the developing world. A better understanding of the mechanisms of disease protection could allow novel strategies to disease management and control.

OBJECTIVES

To identify human genomic loci with evidence of linkage to tuberculosis susceptibility and, within these loci, to identify individual genes influencing tuberculosis susceptibility.

METHODS

Affected sibling pair analysis in South African and Malawian populations. Independent case-control study in West Africa.

MEASUREMENTS AND MAIN RESULTS

Two novel putative loci for tuberculosis susceptibility are identified: chromosome 6p21-q23 and chromosome 20q13.31-33--the latter with the strongest evidence for any locus reported to date in human tuberculosis (single point LOD score of 3.1, P = 10(-4), with a maximum likelihood score [MLS] of 2.8). An independent, multistage genetic association study in West African populations mapped this latter region in detail, finding evidence that variation in the melanocortin 3 receptor (MC3R) and cathepsin Z (CTSZ) genes play a role in the pathogenesis of tuberculosis.

CONCLUSIONS

These results demonstrate how a genomewide approach to the complex phenotype of human tuberculosis can identify novel targets for further research.

Recombination-mediated genetic engineering of a bacterial artificial chromosome clone of modified vaccinia virus Ankara (MVA)

The production, manipulation and rescue of a bacterial artificial chromosome clone of Vaccinia virus (VAC-BAC) in order to expedite construction of expression vectors and mutagenesis of the genome has been described (Domi & Moss, 2002, PNAS99 12415–20). The genomic BAC clone was ‘rescued’ back to infectious virus using a Fowlpox virus helper to supply transcriptional machinery. We apply here a similar approach to the attenuated strain Modified Vaccinia virus Ankara (MVA), now widely used as a safe non-replicating recombinant vaccine vector in mammals, including humans. Four apparently full-length, rescuable clones were obtained, which had indistinguishable immunogenicity in mice. One clone was shotgun sequenced and found to be identical to the parent. We employed GalK recombination-mediated genetic engineering (recombineering) of MVA-BAC to delete five selected viral genes. Deletion of C12L, A44L, A46R or B7R did not significantly affect CD8⁺ T cell immunogenicity in BALB/c mice, but deletion of B15R enhanced specific CD8⁺ T cell responses to one of two endogenous viral epitopes (from the E2 and F2 proteins), in accordance with published work (Staib et al., 2005, J. Gen. Virol.86, 1997–2006). In addition, we found a higher frequency of triple-positive IFN-γ, TNF-α and IL-2 secreting E3-specific CD8+ T-cells 8 weeks after vaccination with MVA lacking B15R. Furthermore, a recombinant vaccine capable of inducing CD8⁺ T cells against an epitope from Plasmodium berghei was created using GalK counterselection to insert an antigen expression cassette lacking a tandem marker gene into the traditional thymidine kinase locus of MVA-BAC. MVA continues to feature prominently in clinical trials of recombinant vaccines against diseases such as HIV-AIDS, malaria and tuberculosis. Here we demonstrate in proof-of-concept experiments that MVA-BAC recombineering is a viable route to more rapid and efficient generation of new candidate mutant and recombinant vaccines based on a clinically deployable viral vector.

CD209 genetic polymorphism and tuberculosis disease

Background

Tuberculosis causes significant morbidity and mortality worldwide, especially in sub-Saharan Africa. DC-SIGN, encoded by CD209, is a receptor capable of binding and internalizing Mycobacterium tuberculosis. Previous studies have reported that the CD209 promoter single nucleotide polymorphism (SNP)-336A/G exerts an effect on CD209 expression and is associated with human susceptibility to dengue, HIV-1 and tuberculosis in humans. The present study investigates the role of the CD209 -336A/G variant in susceptibility to tuberculosis in a large sample of individuals from sub-Saharan Africa.

Methods and Findings

A total of 2,176 individuals enrolled in tuberculosis case-control studies from four sub-Saharan Africa countries were genotyped for the CD209 -336A/G SNP (rs4804803). Significant overall protection against pulmonary tuberculosis was observed with the -336G allele when the study groups were combined (n = 914 controls vs. 1262 cases, Mantel-Haenszel 2x2 χ² = 7.47, P = 0.006, odds ratio = 0.86, 95%CI 0.77–0.96). In addition, the patients with -336GG were associated with a decreased risk of cavitory tuberculosis, a severe form of tuberculosis disease (n = 557, Pearson's 2×2 χ² = 17.34, P = 0.00003, odds ratio = 0.42, 95%CI 0.27–0.65). This direction of association is opposite to a previously observed result in a smaller study of susceptibility to tuberculosis in a South African Coloured population, but entirely in keeping with the previously observed protective effect of the -336G allele.

Conclusion

This study finds that the CD209 -336G variant allele is associated with significant protection against tuberculosis in individuals from sub-Saharan Africa and, furthermore, cases with -336GG were significantly less likely to develop tuberculosis-induced lung cavitation. Previous in vitro work demonstrated that the promoter variant -336G allele causes down-regulation of CD209 mRNA expression. Our present work suggests that decreased levels of the DC-SIGN receptor may therefore be protective against both clinical tuberculosis in general and cavitory tuberculosis disease in particular. This is consistent with evidence that Mycobacteria can utilize DC-SIGN binding to suppress the protective pro-inflammatory immune response.

The induction and persistence of T cell IFN-gamma responses after vaccination or natural exposure is suppressed by Plasmodium falciparum

Epidemiological observations suggest that T cell immunity may be suppressed in malaria-endemic areas. In vitro studies, animal models, and limited data in humans link immunosuppression with malaria, malnutrition, and other parasitic infections. However, there are no data to determine whether malaria-induced immunosuppression is significant in the long-term, or relative data comparing it with other factors in malaria-endemic areas, so as to measure the impact of malaria, other parasitic disease, nutritional status, age. and location on the acquisition and longevity of IFN-gamma responses in children in Kenya. We studied these factors in two cohorts of 1- to 6-year-old children in a malaria-endemic area. T cell responses were induced by vaccination in one cohort, and acquired as a result of natural exposure in a second cohort. Serial ELISPOT assays conducted over a 1-year period measured the induction and kinetics of IFN-gamma production in response to the malaria Ag thrombospondin-related adhesion protein. Induced responses in both cohorts and the longevity of response in the vaccinated cohort were fitted to potential explanatory variables. Parasitemia was prospectively associated with reduced IFN-gamma-producing T cells in both cohorts (by 15-25%), and both parasitemia and episodes of febrile malaria were associated with 19 and 31% greater attrition of T cell responses, respectively. Malaria may reduce the efficacy vaccinations such as bacillus Calmette-Guérin and investigational T cell-inducing vaccines, and may delay the acquisition of immunity following natural exposure to malaria and other pathogens.

Immunisation with BCG and recombinant MVA85A induces long-lasting, polyfunctional Mycobacterium tuberculosis-specific CD4+ memory T lymphocyte populations

In the search for effective vaccines against intracellular pathogens such as HIV, tuberculosis and malaria, recombinant viral vectors are increasingly being used to boost previously primed T cell responses. Published data have shown prime-boost vaccination with BCG-MVA85A (modified vaccinia virus Ankara expressing antigen 85A) to be highly immunogenic in humans as measured by ex vivo IFN-gamma ELISPOT. Here, we used polychromatic flow cytometry to investigate the phenotypic and functional profile of these vaccine-induced Mycobacterium tuberculosis (M.tb) antigen 85A-specific responses in greater detail. Promisingly, antigen 85A-specific CD4(+) T cells were found to be highly polyfunctional, producing IFN-gamma, TNF-alpha, IL-2 and MIP-1beta. Surface staining showed the responding CD4(+) T cells to be relatively immature (CD45RO(+) CD27(int)CD57(-)); this observation was supported by the robust proliferative responses observed following antigenic stimulation. Furthermore, these phenotypic and functional properties were independent of clonotypic composition and epitope specificity, which was maintained through the different phases of the vaccine-induced immune response. Overall, these data strongly support the use of MVA85A in humans as a boosting agent to expand polyfunctional M.tb-specific CD4(+) T cells capable of significant secondary responses.

Boosting BCG with recombinant modified vaccinia ankara expressing antigen 85A: different boosting intervals and implications for efficacy trials

Objectives

To investigate the safety and immunogenicity of boosting BCG with modified vaccinia Ankara expressing antigen 85A (MVA85A), shortly after BCG vaccination, and to compare this first with the immunogenicity of BCG vaccination alone and second with a previous clinical trial where MVA85A was administered more than 10 years after BCG vaccination.

Design

There are two clinical trials reported here: a Phase I observational trial with MVA85A; and a Phase IV observational trial with BCG. These clinical trials were all conducted in the UK in healthy, HIV negative, BCG naïve adults. Subjects were vaccinated with BCG alone; or BCG and then subsequently boosted with MVA85A four weeks later (short interval). The outcome measures, safety and immunogenicity, were monitored for six months. The immunogenicity results from this short interval BCG prime–MVA85A boost trial were compared first with the BCG alone trial and second with a previous clinical trial where MVA85A vaccination was administered many years after vaccination with BCG.

Results

MVA85A was safe and highly immunogenic when administered to subjects who had recently received BCG vaccination. When the short interval trial data presented here were compared with the previous long interval trial data, there were no significant differences in the magnitude of immune responses generated when MVA85A was administered shortly after, or many years after BCG vaccination.

Conclusions

The clinical trial data presented here provides further evidence of the ability of MVA85A to boost BCG primed immune responses. This boosting potential is not influenced by the time interval between prior BCG vaccination and boosting with MVA85A. These findings have important implications for the design of efficacy trials with MVA85A. Boosting BCG induced anti-mycobacterial immunity in either infancy or adolescence are both potential applications for this vaccine, given the immunological data presented here.

Trial Registration

ClinicalTrials.gov NCT00427453 (short boosting interval), NCT00427830 (long boosting interval), NCT00480714 (BCG alone)

Evaluation of liquid culture for quantitation of Mycobacterium tuberculosis in murine models

Quantitation of bacterial load in tissues is essential for experimental investigation of Mycobacterium tuberculosis infection and immunity. We have used an automated liquid culture system to determine the number of colony forming units (CFU) in murine tissues and compared the results to those obtained by conventional plating on Middlebrook agar. There is an overall good correlation between results obtained by the two methods. Although less consistency and more contamination was observed in the automated liquid culture, the method is more sensitive, less labour intensive and allows the processing of large numbers of samples.

Positive replication and linkage disequilibrium mapping of the chromosome 21q22.1 malaria susceptibility locus

Four cytokine receptor genes are located on Chr21q22.11, encoding the alpha and beta subunits of the interferon-alpha receptor (IFNAR1 and IFNAR2), the beta subunit of the interleukin 10 receptor (IL10RB) and the second subunit of the interferon-gamma receptor (IFNGR2). We previously reported that two variants in IFNAR1 were associated with susceptibility to malaria in Gambians. We now present an extensive fine-scale mapping of the associated region utilizing 45 additional genetic markers obtained from public databases and by sequencing a 44 kb region in and around the IFNAR1 gene in 24 Gambian children (12 cases/12 controls). Within the IFNAR1 gene, a newly studied C --> G single-nucleotide polymorphism (IFNAR1 272354c-g) at position -576 relative to the transcription start was found to be more strongly associated with susceptibility to severe malaria. Association was observed in three populations: in Gambian (P=0.002), Kenyan (P=0.022) and Vietnamese (P=0.005) case-control studies. When all three studies were combined, using the Mantel-Haenszel test, the presence of IFNAR1 -576G was associated with a substantially elevated risk of severe malaria (N=2444, OR=1.38, 95% CI: 1.17-1.64; P=1.7 x 10(-4)). This study builds on previous work to further highlight the importance of the type-I interferon pathway in malaria susceptibility and illustrates the utility of typing SNPs within regions of high linkage disequilibrium in multiple populations to confirm initial positive associations.

Combination of protein and viral vaccines induces potent cellular and humoral immune responses and enhanced protection from murine malaria challenge

The search for an efficacious vaccine against malaria is ongoing, and it is now widely believed that to confer protection a vaccine must induce very strong cellular and humoral immunity concurrently. We studied the immune response in mice immunized with the recombinant viral vaccines fowlpox strain FP9 and modified virus Ankara (MVA), a protein vaccine (CV-1866), or a combination of the two; all vaccines express parts of the same preerythrocytic malaria antigen, the Plasmodium berghei circumsporozoite protein (CSP). Mice were then challenged with P. berghei sporozoites to determine the protective efficacies of different vaccine regimens. Two immunizations with the protein vaccine CV-1866, based on the hepatitis B core antigen particle, induced strong humoral immunity to the repeat region of CSP that was weakly protective against sporozoite challenge. Prime-boost with the viral vector vaccines, FP9 followed by MVA, induced strong T-cell immunity to the CD8+ epitope Pb9 and partially protected animals from challenge. Physically mixing CV-1866 with FP9 or MVA and then immunizing with the resultant combinations in a prime-boost regimen induced both cellular and humoral immunity and afforded substantially higher levels of protection (combination, 90%) than either vaccine alone (CV-1866, 12%; FP9/MVA, 37%). For diseases such as malaria in which different potent immune responses are required to protect against different stages, using combinations of partially effective vaccines may offer a more rapid route to achieving deployable levels of efficacy than individual vaccine strategies.

Extended follow-up following a phase 2b randomized trial of the candidate malaria vaccines FP9 ME-TRAP and MVA ME-TRAP among children in Kenya

Background

“FFM ME-TRAP” is sequential immunisation with two attenuated poxvirus vectors (FP9 and modified vaccinia virus Ankara) delivering the pre-erythrocytic malaria antigen ME-TRAP. Over nine months follow-up in our original study, there was no evidence that FFM ME-TRAP provided protection against malaria. The incidence of malaria was slightly higher in children who received FFM ME-TRAP, but this was not statistically significant (hazard ratio 1.5, 95% CI 1.0-2.3). Although the study was unblinded, another nine months follow-up was planned to monitor the incidence of malaria and other serious adverse events.

Methods and Findings

405 children aged 1–6 yrs were initially randomized to vaccination with either FFM ME-TRAP or control (rabies vaccine). 380 children were still available for follow-up after the first nine months. Children were seen weekly and whenever they were unwell for nine months monitoring. The axillary temperature was measured, and blood films taken when febrile. The primary analysis was time to parasitaemia >2,500/µl. During the second nine months monitoring, 49 events met the primary endpoint (febrile malaria with parasites >2,500/µl) in the Intention To Treat (ITT) group. 23 events occurred among the 189 children in the FFM ME-TRAP group, and 26 among the 194 children in the control group. In the full 18 months of monitoring, there were 63 events in the FFM ME-TRAP group and 60 in the control group (HR = 1.2, CI 0.84-1.73, p = 0.35). There was no evidence that the HR changed over the 18 months (test for interaction between time and vaccination p = 0.11).

Conclusions

Vaccination with FFM ME-TRAP was not protective against malaria in this study. Malaria incidence during 18 months of surveillance was similar in both vaccine groups.

Trial Registration

Controlled-Trials.com ISRCTN88335123

Clearing asymptomatic parasitaemia increases the specificity of the definition of mild febrile malaria

In clinical trials, the specificity of the disease endpoint is critical to an accurate estimate of vaccine efficacy. We used a logistic regression model to analyse parasite densities among children before and after treatment with antimalarials, in order to estimate the impact that clearing asymptomatic parasitaemia had on the specificity of the endpoint of febrile malaria. The malaria attributable fever fraction was higher after antimalarial treatment (i.e. fever and parasitaemia were more likely to be causally related), implying that drug treatment prior to monitoring decreased the misclassification of febrile malaria. In intervention studies with febrile malaria as an endpoint, clearing asymptomatic parasitaemia increases the study's power more effectively than raising the threshold parasitaemia.

IκB Genetic Polymorphisms and Invasive Pneumococcal Disease

RATIONALE

Increasing evidence supports a key role for the transcription factor nuclear factor (NF)-kappaB in the host response to pneumococcal infection. Control of NF-kappaB activity is achieved through interactions with the IkappaB family of inhibitors, encoded by the genes NFKBIA, NFKBIB, and NFKBIE. Rare NFKBIA mutations cause immunodeficiency with severe bacterial infection, raising the possibility that common IkappaB gene polymorphisms confer susceptibility to common bacterial disease.

OBJECTIVES

To determine whether polymorphisms in NFKBIA, NFKBIB, and NFKBIE associate with susceptibility to invasive pneumococcal disease (IPD) and thoracic empyema.

METHODS

We studied the frequencies of 62 single-nucleotide polymorphisms (SNPs) across NFKBIA, NFKBIB, and NFKBIE in individuals with IPD and control subjects (n=1,060). Significantly associated SNPs were then studied in a group of individuals with thoracic empyema and a second control group (n=632).

MEASUREMENTS AND MAIN RESULTS

Two SNPs in the NFKBIA promoter region were associated with protection from IPD in both the initial study group and the pneumococcal empyema subgroup. Significant protection from IPD was observed for carriage of mutant alleles at these two loci on combining the groups (SNP rs3138053: Mantel-Haenszel 2x2 chi2=13.030, p=0.0003; odds ratio [OR], 0.60; 95% confidence interval [CI], 0.45-0.79; rs2233406: Mantel-Haenszel 2x2 chi2=18.927, p=0.00001; OR, 0.55; 95% CI, 0.42-0.72). An NFKBIE SNP associated with susceptibility to IPD but not pneumococcal empyema. None of the NFKBIB SNPs associated with IPD susceptibility.

CONCLUSIONS

NFKBIA polymorphisms associate with susceptibility to IPD. Genetic variation in an inhibitor of NF-kappaB therefore not only causes a very rare immunodeficiency state but may also influence the development of common infectious disease.

Polymorphisms in IRF-1 associated with resistance to HIV-1 infection in highly exposed uninfected Kenyan sex workers

OBJECTIVE

To determine the correlation between polymorphisms in the IL-4 gene cluster and resistance to HIV-1 infection.

DESIGN

: A cross-sectional genetic analysis of polymorphisms within the IL-4 gene cluster was conducted in a well-described female sex worker cohort from Nairobi, Kenya, known to exhibit differential susceptibility to HIV-1 infection.

METHODS

Microsatellite genotyping was used to screen six microsatellite markers in the IL-4 gene cluster for associations with HIV-1 resistance. Further analysis of the interferon regulatory factor 1 (IRF-1) gene was conducted by genomic sequencing. Associations between IRF-1 gene polymorphisms and the HIV-1 resistance phenotype were determined using the chi-square test and Kaplan-Meier survival analysis. The functional consequence of IRF-1 polymorphism was conducted by quantitative Western blot.

RESULTS

Three polymorphisms in IRF-1, located at 619, the microsatellite region and 6516 of the gene, showed associations with resistance to HIV-1 infection. The 619A, 179 at IRF-1 microsatellite and 6516G alleles were associated with the HIV-1-resistant phenotype and a reduced likelihood of seroconversion. Peripheral blood mononuclear cells from patients with protective IRF-1 genotypes exhibited significantly lower basal IRF-1 expression and reduced responsiveness to exogenous IFN-gamma stimulation.

CONCLUSION

Polymorphisms in the IRF-1 gene are associated with resistance to infection by HIV-1 and a lowered level of IRF-1 protein expression. This study adds IRF-1, a transcriptional immunoregulatory gene, to the list of genetic correlates of altered susceptibility to HIV-1. This is the first report suggesting that a viral transcriptional regulator might contribute to resistance to HIV-1. Further functional analysis on the role of IRF-1 polymorphisms and HIV-1 resistance is underway.

Functional polymorphisms in the FCN2 gene are not associated with invasive pneumococcal disease

L-ficolin is a pattern-recognition molecule which binds lipoteichoic acid and Gram-positive bacteria and activates the lectin pathway of complement. Five common functional polymorphisms have recently been identified in the FCN2 gene which encodes L-ficolin: three promoter polymorphisms (at positions -986, -602 and -4) which affect serum L-ficolin concentration, and two non-synonymous polymorphisms (Thr236Met and Ala258Ser) which influence carbohydrate binding. We studied the frequencies of these polymorphisms in individuals with invasive pneumococcal disease (IPD) and a control group. Although the five FCN2 polymorphisms were each present in the UK Caucasian population studied, no significant associations were observed between the FCN2 polymorphisms and susceptibility to IPD. This is in contrast to mannose-binding lectin deficiency, which we have previously shown to be associated with increased susceptibility to IPD. Although we are unable to exclude small effects of FCN2 genetic variation on susceptibility to IPD, the result suggests that L-ficolin may not be critical for host defence against pneumococcal infection.

Aspects of genetic susceptibility to human infectious diseases

Host genetic factors play a major role in determining differential susceptibility to major infectious diseases of humans, such as malaria, HIV/AIDS, tuberculosis, and invasive pneumococcal disease. Progress in identifying the relevant genetic loci has come from a variety of approaches. Most convincing associations have been identified by case-control studies assessing biologically plausible candidate genes. All six of the genes that have a major effect on infectious disease susceptibility in humans have been identified in this way. However, recently genome-wide linkage analysis of affected sibling pairs has identified susceptibility loci for chronic infections such as leprosy and chronic hepatitis B virus persistence. Other approaches used successfully have included assessment in humans of the homologues of susceptibility genes mapped and identified in murine models. However, the great majority of susceptibility loci remain to be identified and the advent of large-scale genome-wide association scans offers a new approach to defining many of these.

Low-level malaria infections detected by a sensitive polymerase chain reaction assay and use of this technique in the evaluation of malaria vaccines in an endemic area

The feasibility of using a sensitive polymerase chain reaction (PCR) to evaluate malaria vaccines in small group sizes was tested in 102 adult Gambian volunteers who received either the malaria vaccine regimen FP9 ME-TRAP/MVA ME-TRAP or rabies vaccine. All volunteers received the antimalarial drugs primaquine and Lapdap plus artesunate to eliminate malaria parasites. Volunteers in a further group received an additional single treatment with sulfadoxine-pyrimethamine (SP) to prevent new infections. There was substantially lower T-cell immunogenicity than in previous trials with this vaccine regimen and no protection against infection in the malaria vaccine group. Using the primary endpoint of 20 parasites per mL, no difference was found in the prevalence of low-level infections in volunteers who received SP compared with those who did not, indicating that SP did not reduce the incidence of very low-density infection. However, SP markedly reduced the incidence of higher density infections. These findings support the feasibility and potential of this approach to screen pre-erythrocytic vaccines for efficacy against infection in small numbers of vaccinees in endemic areas.

A Mal functional variant is associated with protection against invasive pneumococcal disease, bacteremia, malaria and tuberculosis

Toll-like receptors (TLRs) and members of their signaling pathway are important in the initiation of the innate immune response to a wide variety of pathogens. The adaptor protein Mal (also known as TIRAP), encoded by TIRAP (MIM 606252), mediates downstream signaling of TLR2 and TLR4 (refs. 4-6). We report a case-control study of 6,106 individuals from the UK, Vietnam and several African countries with invasive pneumococcal disease, bacteremia, malaria and tuberculosis. We genotyped 33 SNPs, including rs8177374, which encodes a leucine substitution at Ser180 of Mal. We found that heterozygous carriage of this variant associated independently with all four infectious diseases in the different study populations. Combining the study groups, we found substantial support for a protective effect of S180L heterozygosity against these infectious diseases (N = 6,106; overall P = 9.6 x 10(-8)). We found that the Mal S180L variant attenuated TLR2 signal transduction.

A DNA prime-modified vaccinia virus ankara boost vaccine encoding thrombospondin-related adhesion protein but not circumsporozoite protein partially protects healthy malaria-naive adults against Plasmodium falciparum sporozoite challenge

The safety, immunogenicity, and efficacy of DNA and modified vaccinia virus Ankara (MVA) prime-boost regimes were assessed by using either thrombospondin-related adhesion protein (TRAP) with a multiple-epitope string ME (ME-TRAP) or the circumsporozoite protein (CS) of Plasmodium falciparum. Sixteen healthy subjects who never had malaria (malaria-naive subjects) received two priming vaccinations with DNA, followed by one boosting immunization with MVA, with either ME-TRAP or CS as the antigen. Immunogenicity was assessed by ex vivo gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) and antibody assay. Two weeks after the final vaccination, the subjects underwent P. falciparum sporozoite challenge, with six unvaccinated controls. The vaccines were well tolerated and immunogenic, with the DDM-ME TRAP regimen producing stronger ex vivo IFN-gamma ELISPOT responses than DDM-CS. One of eight subjects receiving the DDM-ME TRAP regimen was completely protected against malaria challenge, with this group as a whole showing significant delay to parasitemia compared to controls (P = 0.045). The peak ex vivo IFN-gamma ELISPOT response in this group correlated strongly with the number of days to parasitemia (P = 0.033). No protection was observed in the DDM-CS group. Prime-boost vaccination with DNA and MVA encoding ME-TRAP but not CS resulted in partial protection against P. falciparum sporozoite challenge in the present study.

A phase 2b randomised trial of the candidate malaria vaccines FP9 ME-TRAP and MVA ME-TRAP among children in Kenya

Objective:

The objective was to measure the efficacy of the vaccination regimen FFM ME-TRAP in preventing episodes of clinical malaria among children in a malaria endemic area. FFM ME-TRAP is sequential immunisation with two attenuated poxvirus vectors (FP9 and modified vaccinia virus Ankara), which both deliver the pre-erythrocytic malaria antigen construct multiple epitope–thrombospondin-related adhesion protein (ME-TRAP).

Design:

The trial was randomised and double-blinded.

Setting:

The setting was a rural, malaria-endemic area of coastal Kenya.

Participants:

We vaccinated 405 healthy 1- to 6-year-old children.

Interventions:

Participants were randomised to vaccination with either FFM ME-TRAP or control (rabies vaccine).

Outcome Measures:

Following antimalarial drug treatment children were seen weekly and whenever they were unwell during nine months of monitoring. The axillary temperature was measured, and blood films taken when febrile. The primary analysis was time to a parasitaemia of over 2,500 parasites/μl.

Results:

The regime was moderately immunogenic, but the magnitude of T cell responses was lower than in previous studies. In intention to treat (ITT) analysis, time to first episode was shorter in the FFM ME-TRAP group. The cumulative incidence of febrile malaria was 52/190 (27%) for FFM ME-TRAP and 40/197 (20%) among controls (hazard ratio = 1.52). This was not statistically significant (95% confidence interval [CI] 1.0–2.3; p = 0.14 by log-rank). A group of 346 children were vaccinated according to protocol (ATP). Among these children, the hazard ratio was 1.3 (95% CI 0.8–2.1; p = 0.55 by log-rank). When multiple malaria episodes were included in the analyses, the incidence rate ratios were 1.6 (95% CI 1.1–2.3); p = 0.017 for ITT, and 1.4 (95% CI 0.9–2.1); p = 0.16 for ATP. Haemoglobin and parasitaemia in cross-sectional surveys at 3 and 9 mo did not differ by treatment group. Among children vaccinated with FFM ME-TRAP, there was no correlation between immunogenicity and malaria incidence.

Conclusions:

No protection was induced against febrile malaria by this vaccine regimen. Future field studies will require vaccinations with stronger immunogenicity in children living in malarious areas.

Background: Malaria kills over a million people a year worldwide, and young children in sub-Saharan are particularly at risk. Cheap, safe, and effective vaccines are needed. One strategy involves a double-vaccination process. This approach (termed “prime-boost”) uses two different delivery methods to transmit the same antigen (part of a protein from the malaria parasite that can trigger an immune response). The two-step vaccination is designed to achieve a greater immune response than with just one vaccination. One research group, based in Oxford in the UK, is using an antigen called “ME-TRAP,” which is delivered using first a strain of modified fowlpox virus (called FP9), then a weakened vaccinia virus (called MVA). Previous studies done in adult UK volunteers have been promising, achieving an immune response and some protection against malaria when volunteers were deliberately infected. However, this approach has not been tested in the group most in need of a vaccine—young African children. Therefore a field trial was conducted among 405 healthy children aged 1–6 years, in a region of Kenya with year-round malaria transmission. Children were randomized to receive either the sequence of vaccines delivering ME-TRAP or to receive a rabies vaccine (as control, but which still gives the children some benefit for taking part in the trial). The children were followed up for nine months, and the primary aim of the trial was to compare the occurrence of clinical malaria (fever combined with malaria parasites in the blood) in the two groups.

What this trial shows: In the 387 children receiving vaccine and having at least one follow-up visit the vaccine did produce an immune response; however, this immune response did not seem to be protective, as the occurrence of malaria was slightly higher in the group receiving the candidate vaccine—although this difference was not statistically significant. Safety data were also collected; the number and severity of adverse events were similar between volunteers receiving the rabies vaccine and those receiving the candidate malaria vaccine, and any serious events were not judged to be linked to the vaccines by the trial's data safety monitoring board.

Strengths and limitations: The methods used in the trial were robust, using appropriate randomization procedures and blinding of participants and researchers. Outcome measures (clinical malaria, defined as fever together with parasites in the blood over 2,500/microliter) were clinically relevant. In order to detect cases of malaria in vaccinated children, health workers visited children weekly, and children with a temperature over 37.5 °C were tested for parasites in the blood. (In between the weekly visits, self-report and referral for assessment also allowed detection of cases.) This process of active detection of malaria cases (as opposed to obtaining data on clinical malaria only from self-report or referral) enables a smaller sample size to be used in the trial, but it is not clear whether this approach is more or less specific at picking up malaria cases than are passive methods. The researchers aimed to ensure that their case detection methods were specific; for children with normal temperature, but reported by their parents as feverish, parasite tests were done only if subsequent temperature readings were high.

Contribution to the evidence: Previous studies of ME-TRAP using the FP9 and MVA vectors have shown the candidate vaccine is safe and induces a strong immune response. The safety result was also supported by the findings of the current trial, conducted in young Kenyan children, but a 5-fold lower immune response was found compared to previous studies. The trial showed that this weak immune response was not effective at preventing clinical cases of malaria in this group of children, although it is not clear why the immune response was lower than expected.