Interferon-alpha receptor-1 (IFNAR1) variants are associated with protection against cerebral malaria in the Gambia

The chromosome 21q22.11 cytokine receptor cluster contains four genes that encode subunits of the receptors for the cytokines interleukin-10 and interferon-alpha, -beta and -gamma that may have a role in malaria pathogenesis. A total of 15 polymorphic markers located within these genes were initially genotyped in 190 controls and 190 severe malaria cases from The Gambia. Two interferon-alpha receptor-1 (IFNAR1) gene SNPs (17470 and L168 V) showed evidence for an association with severe malaria phenotypes and were typed in a larger series of samples comprising 538 severe malaria cases, 338 mild malaria cases and 562 controls. Both the 17470-G/G and L168V-G/G genotypes were associated with protection against severe malaria, in general, and cerebral malaria, in particular (P=0.004 and 0.003, respectively). IFNAR1 diplotypes were then constructed for these two markers using the PHASE software package. The (17470-G L168V-G/17470-G L168V-G) diplotype was found to be associated with a reduced risk of cerebral malaria and the (17470-C L168V-C/17470-G L168V-G) diplotype with an increased risk of cerebral malaria (overall 3 x 2 chi(2)=12.8, d.f.=2, P=0.002 and 3 x 2 chi(2)=15.2, d.f.=2, P=0.0005, respectively). These data suggest a role for the type I interferon pathway in resistance to cerebral malaria.

Enhanced T-cell immunogenicity of plasmid DNA vaccines boosted by recombinant modified vaccinia virus Ankara in humans

In animals, effective immune responses against malignancies and against several infectious pathogens, including malaria, are mediated by T cells. Here we show that a heterologous prime-boost vaccination regime of DNA either intramuscularly or epidermally, followed by intradermal recombinant modified vaccinia virus Ankara (MVA), induces high frequencies of interferon (IFN)-gamma-secreting, antigen-specific T-cell responses in humans to a pre-erythrocytic malaria antigen, thrombospondin-related adhesion protein (TRAP). These responses are five- to tenfold higher than the T-cell responses induced by the DNA vaccine or recombinant MVA vaccine alone, and produce partial protection manifest as delayed parasitemia after sporozoite challenge with a different strain of Plasmodium falciparum. Such heterologous prime-boost immunization approaches may provide a basis for preventative and therapeutic vaccination in humans.

Safety of DNA and modified vaccinia virus Ankara vaccines against liver-stage P. falciparum malaria in non-immune volunteers

A series of phase I clinical studies were conducted to evaluate the safety of plasmid DNA and modified vaccinia virus Ankara malaria vaccines. The vaccines each encoded a polyepitope string fused to whole Plasmodium falciparum TRAP antigen. Forty-three healthy adult volunteers received the vaccines alone or in DNA/MVA prime-boost combinations. The DNA vaccine was administered either intramuscularly by needle or intradermally by a needleless delivery device. The MVA vaccine was administered intradermally by needle. The vaccines were well-tolerated by all three routes and in various DNA/MVA immunisation regimes. There were no severe or serious adverse events.

Ex vivo interferon-gamma immune response to thrombospondin-related adhesive protein in coastal Kenyans: longevity and risk of Plasmodium falciparum infection

Thrombospondin-related adhesive protein (TRAP) of Plasmodium falciparum is currently being tested in human vaccine studies. However, its natural reactivity in the field remains poorly characterized. More than 40% of 217 Kenyan donors responded in an ex vivo interferon-gamma (IFN-gamma) enzyme-linked immunospot (ELISPOT) assay to at least one of 14 20mer peptides spanning 42% of the antigen. Reactivity was comparable from early childhood (>1 year of age) to old age, and the maximal precursor frequency of TRAP-specific cells to all 14 peptides was 1 in 4,000. Prospective follow-up for one year indicated that these low-level ex vivo responses to TRAP did not protect against the subsequent development of malaria. Retesting of selected donors after one year showed a complete change in the reactivity pattern, suggesting that malaria-specific ex vivo IFN-gamma ELISPOT assay responses are short lived in naturally exposed donors, even to conserved epitopes. This study provides important information regarding natural reactivity to a key malaria antigen.

Progress with new malaria vaccines

Malaria is a parasitic disease of major global health significance that causes an estimated 2.7 million deaths each year. In this review we describe the burden of malaria and discuss the complicated life cycle of Plasmodium falciparum, the parasite responsible for most of the deaths from the disease, before reviewing the evidence that suggests that a malaria vaccine is an attainable goal. Significant advances have recently been made in vaccine science, and we review new vaccine technologies and the evaluation of candidate malaria vaccines in human and animal studies worldwide. Finally, we discuss the prospects for a malaria vaccine and the need for iterative vaccine development as well as potential hurdles to be overcome.

Genetics of susceptibility to leprosy

The ancient disease of leprosy can cause severe disability and disfigurement and is still a major health concern in many parts of the world. Only a subset of those individuals exposed to the pathogen will go on to develop clinical disease and there is a broad clinical spectrum amongst leprosy sufferers. The outcome of infection is in part due to host genes that influence control of the initial infection and the host's immune response to that infection. Identification of the host genes that influence host susceptibility/resistance will enable a greater understanding of disease pathogenesis. In turn, this should facilitate development of more effective therapeutics and vaccines. So far at least a dozen genes have been implicated in leprosy susceptibility and a genome-wide linkage study has lead to the identification of at least one positional candidate. These findings are reviewed here.

Association of a polymorphism in the P2X7 gene with tuberculosis in a Gambian population

Adenosine triphosphate (ATP) ligation of P2X(7) receptors expressed on human macrophages that are infected with mycobacteria induces cell death and subsequent loss of intracellular bacterial viability. Marked heterogeneity observed in cell donor ATP responsiveness suggests that this antimycobacterial mechanism may be genetically regulated. Five single-nucleotide polymorphisms (SNPs) previously identified in a putative 1.8-kb promoter region upstream of P2RX7 exon 1 were screened for associations with clinical tuberculosis. The frequencies of these promoter SNPs and a polymorphism in P2RX7 exon 13 at position 1513 were compared among >300 Gambian patients with tuberculosis and >160 ethnically matched control subjects by sequence-specific oligonucleotide hybridization and ligation detection reaction analysis. A significant protective association against tuberculosis was found for 1 promoter SNP, at nucleotide position -762 (odds ratio [OR] for variant C allele, 0.70; 95% confidence interval [CI], 0.54-0.89; P=.003; OR for CC genotype, 0.545; 95% CI, 0.318-0.934; P=.027). This association supports a role for ATP/P2X(7)-mediated host regulation of Mycobacterium tuberculosis infection.

Response heterogeneity of human macrophages to ATP is associated with P2X7 receptor expression but not to polymorphisms in the P2RX7 promoter

A region 2 kb upstream of exon 1 of the P2X7 gene was sequenced using DNA from nine healthy individuals who exhibited three different ATP response phenotypes (i.e. high, low and interferon gamma-inducible). Five single nucleotide polymorphisms were identified within the nine donor promoter sequences but none were associated with a specific ATP response phenotype. A P2X7 loss of function polymorphism (1513 in exon 13) was also screened for within donor DNA but no response associations were identified. ATP response phenotype was positively associated with P2X(7) receptor expression, as assessed by flow cytometry, but not with any identified receptor or promoter gene polymorphisms.

A region of chromosome 20 is linked to leprosy susceptibility in a South Indian population

A major susceptibility locus for leprosy has recently been mapped on chromosome 10 (10p13) by genome-wide linkage analysis. Microsatellite markers from this genome screen that showed suggestive evidence of linkage to leprosy were evaluated in an additional 140 families with affected sib pairs. A second region of linkage has thus been identified on chromosome 20 (20p12). The peak of linkage lies at marker D20S115, which has a significant single-point maximum logarithm of odds score of 3.48 (P=.00003). Transmission disequilibrium testing of the microsatellite markers in 20p12 showed that the marker D20S835 is associated with protection against leprosy (P=.021), which suggests that a locus controlling susceptibility lies close to this marker.

Association of low-density lipoprotein receptor polymorphisms and outcome of hepatitis C infection

The low-density lipoprotein receptor (LDLR) has been proposed to promote hepatitis C virus endocytosis and the cell membrane protein CD81 may also promote HCV host cell entry. The CD81 gene was sequenced to screen for novel polymorphisms, but no SNPs were identified. Polymorphisms within the LDLR gene are associated with the pathogenesis of familial hypercholesterolemia, atherosclerosis and obesity. We therefore studied genetic variation within the LDLR gene and clinical features of hepatitis C infection. An amino acid change in exon 8 was associated with severity of fibrosis; a SNP in exon 10 correlated with viral clearance and overall inflammation, and a SNP in the 3'UTR appeared to influence treatment response. There were no other significant associations between any of the SNPs studied and the clinical measures of hepatitis C infection. We furthermore report on linkage disequilibrium within the gene and haplotype frequencies in our population. Our findings support a possible role for the LDLR in the modulation of disease progression by affecting immune responses, rather than functioning as receptor for HCV.

Fine mapping of a putative tuberculosis-susceptibility locus on chromosome 15q11-13 in African families

Host genetics plays an important role in individual susceptibility and resistance to infectious diseases, but no genes have yet been identified using genome-wide screens. Twin studies have indicated that tuberculosis susceptibility has a significant host genetic component, and several genes appear to be involved. Recently, a genome-wide linkage analysis of 136 African families identified chromosome 15q11-13 as a region with suggestive evidence of linkage, with a LOD score of 2.0. We tested 10 microsatellite markers and 5 positional candidate genes in this chromosomal region for deviation from random transmission from parents to affected offspring. The polymorphisms, lying in a region of 14 cM, were initially typed in the same 79 Gambian families used in the genome screen. A borderline significant association with a 7 bp deletion in UBE3A (P = 0.01) was found. This polymorphism was then evaluated further in a larger series of families with tuberculosis, including 44 Guinean families and 57 families from South Africa. Testing for association between the deletion and tuberculosis across all the families using the exact symmetry test further supported the association (overall P = 0.002). These fine-mapping data suggest that UBE3A or a closely flanking gene may be a tuberculosis-susceptibility locus.

HIV in Africa (Communication arising): chemokine-receptor genes and AIDS risk

Schliekelman et al. have provided a model to quantify the speed at which HIV-resistance haplotypes can become enriched in a susceptible population through a delay in the onset of AIDS, permitting greater lifetime reproduction and the selection of AIDS-delaying haplotypes. But we question their conclusion that there could be a rapid evolution of resistance to AIDS onset in some African populations if the current HIV epidemic persists, as this depends on an untested assumption: that variant forms of the chemokine-receptor-5 (CCR5) gene impart selective advantages or disadvantages in Africa that are comparable to those reported for African Americans. Here we test this premise in a large Ugandan population, and find that CCR5 variants are not associated with HIV/AIDS disease risk in Africa--the origin and centre of the current AIDS pandemic. This gene may therefore not be subject to rapid evolutionary change as a result of the HIV epidemic in Africa.

MBL genotype and risk of invasive pneumococcal disease: a case-control study

BACKGROUND

Streptococcus pneumoniae is a major cause of morbidity and mortality in developed and developing countries. No common genetic determinants of susceptibility have been defined. Mannose-binding lectin (MBL) is a key mediator of innate host immunity that activates the complement pathway and directly opsonises some infectious pathogens. Mutations in three codons in the MBL gene have been identified, and individuals homozygous for a mutant genotype have very little or no serum MBL. We did a case-control study in the UK to assess whether these mutant genotypes were associated with invasive pneumococcal disease.

METHODS

The frequencies of genotypes defined by the three mutations in codons 52, 54, and 57, and a functional promoter polymorphism at -221, were compared in a two-stage study of 337 patients with invasive pneumococcal disease and 1032 controls. All individuals were recruited from an ethnically homogeneous white population in Oxfordshire, UK. Patients had S pneumoniae isolated from a normally sterile site.

FINDINGS

In our initial set of participants, 28 (12%) of 229 patients and 18 (5%) of 353 controls were homozygotes for MBL codon variants (odds ratio 2.59 [95% CI 1.39-4.83], p=0.002). Neither heterozygosity for these codon variants nor the promoter polymorphism was associated with susceptibility. In a confirmatory study, 11 (10%) of 108 patients were MBL homozygotes compared with 36 (5%) of 679 controls (p=0.046).

INTERPRETATION

Homozygotes for MBL codon variants, who represent about 5% of north Europeans and north Americans and larger proportions of populations in many developing countries, could be at substantially increased risk of invasive pneumococcal disease.

Haptoglobin genotypes are not associated with resistance to severe malaria in The Gambia

The influence of haptoglobin polymorphisms on severe malaria occurrence was assessed in 1183 individuals from The Gambia. No significant association was found between severe malaria and either haptoglobin genotypes or phenotypes. The advantages of using a deoxyribonucleic acid-based haptoglobin typing method are discussed.

Induction of T helper type 1 and 2 responses to 19-kilodalton merozoite surface protein 1 in vaccinated healthy volunteers and adults naturally exposed to malaria

Plasmodium falciparum malaria is a major cause of death in the tropics. The 19-kDa subunit of P. falciparum merozoite surface protein 1 (MSP-1(19)), a major blood stage vaccine candidate, is the target of cellular and humoral immune responses in animals and humans. In this phase I trial of MSP-1(19), immunization of nonexposed human volunteers with either of the two allelic forms of recombinant MSP-1(19) induced high levels of antigen-specific Th1 (gamma interferon) and Th2 (interleukin 4 [IL-4] and IL-10) type lymphokines. The adjustment of the antigen dose and number of immunizations regulated the level of specificity of immune responses and Th1/Th2 bias of responses induced by vaccination. Novel conserved and allelic T-cell epitopes which induced cross-strain immune responses were identified. Importantly, responses to many of these novel epitopes were also present in adults exposed to malaria, both in east (Kenya) and west Africa (The Gambia). These data suggest that epitope-specific naturally acquired MSP-1(19) immune responses in endemic populations can be boosted by vaccination.

Protective immunity against Mycobacterium tuberculosis induced by dendritic cells pulsed with both CD8(+)- and CD4(+)-T-cell epitopes from antigen 85A

Immunization with DNA followed by modified vaccinia virus Ankara strain, both expressing the antigen 85A, induced both CD4(+)- and CD8(+)-T-cell responses in BALB/c mice. Following challenge with Mycobacterium tuberculosis, this prime-boost regimen produced protection equivalent to that conferred by Mycobacterium bovis BCG. Following immunization with dendritic cells pulsed with an antigen 85A CD4(+)- or CD8(+)-restricted epitope, alone or in combination, copresentation of both epitopes on the same dendritic cell was required for protection, demonstrating that induced CD8(+) T cells can play a protective role against tuberculosis.

Naturally exposed populations differ in their T1 and T2 responses to the circumsporozoite protein of Plasmodium falciparum

T-cell responses directed against the circumsporozoite protein (CS) of Plasmodium falciparum can mediate protection against malaria. We determined the frequency of T cells reactive to different regions of the CS in the blood of donors naturally exposed to P. falciparum by examining T1 (gamma interferon [IFN-gamma] ELISPOT assay), T2 (interleukin 4 [IL-4] ELISPOT assay), and proliferative T-cell responses. The proliferative responses were weak, which confirmed previous observations. The responses to the CS in the IL-4 and IFN-gamma ELISPOT assays were also weak (<40 responding cells per 10(6) cells), much weaker than the response to the purified protein derivative of Mycobacterium tuberculosis in the same donors. Moreover, a response in one assay could not be used to predict a response in either of the other assays, suggesting that although these assays may measure different responding cells, all of the responses are weakly induced by natural exposure. Interestingly, the two different study populations used had significantly different T1 and T2 biases in their responses in the C terminus of the protein, suggesting that the extent of P. falciparum exposure can affect regulation of the immune system.

Enhanced CD8 T cell immunogenicity and protective efficacy in a mouse malaria model using a recombinant adenoviral vaccine in heterologous prime-boost immunisation regimes

Recombinant replication-defective adenovirus expressing the CS gene from Plasmodium berghei (Ad-PbCS) was found to induce a strong CD8(+) T cell response after intra-dermal or -muscular immunisation. Boosting of an adenovirus-primed immune response with the replication-impaired poxvirus, modified vaccinia virus Ankara (MVA) led to enhanced immunogenicity and substantial protective efficacy. The recombinant adenoviral vaccine was capable of boosting to protective levels a CD8(+) T cell response primed by either a plasmid DNA vaccine, a recombinant Ty virus-like particle vaccine or recombinant MVA each expressing the same epitope or antigen. Complete protective efficacy after intradermal immunisation was observed with the adenovirus prime-MVA boost regime. This study identifies recombinant replication-defective adenovirus as an alternative to recombinant replication-defective poxviruses as boosting agents for the induction of strong protective CD8(+) T cell responses.

Malaria vaccines

Malaria kills one child in Africa every 30 s. After summarising the burden of malaria, the life-cycle of this parasite in humans and female Anopheles mosquitoes is outlined. Important differences between natural immunity and that induced by current candidate vaccines are discussed. In the main part of the review, the recent rapid expansion in evaluation of candidate malaria vaccines in clinical trials across the world is discussed. Subunit vaccine technologies are progressing rapidly with new delivery systems, vectors and antigens under evaluation as well as new polyepitope approaches. Combination vaccination regimens, improved adjuvants and genetic engineering of antigens are all improving the immunogenicity of candidate vaccines. We also discuss particular difficulties in vaccination against malaria, the conduct of field trials of malaria vaccines in non-industrialised countries and the need for even greater co-operation between researchers. Finally, the important concept of iterative vaccine development is raised and the prospects for effective malaria vaccination are discussed.

An empirical exploration of the (delta mu)2 genetic distance for 213 human microsatellite markers

Microsatellites are now used ubiquitously as genetic markers. One important application is to the assessment of population subdivision and phylogenetic relatedness. Such applications require a method of estimation of genetic distance. Here we examine the most widely used measure of microsatellite genetic distance, Goldstein et al.'s delta-mu squared ([delta mu]2), with respect to a large data set of 213 markers typed across samples from four diverse human populations. We find that (delta mu)2 yields plausible interpopulation distances. For the first time, we report significant interpopulation differences in mean microsatellite length, although the effect of these differences on (delta mu)2 is negligible. However, we also show that the method is extremely sensitive to one or two loci that contribute extreme values, even when a sample size of >200 loci is used. Some of these extreme loci can be removed on the grounds that some alleles carry large indels, but for others there is no clear justification for exclusion a priori. Our data suggest a rather recent African/non-African split, with an upper limit of some 70,000-80,000 years ago.

Use of Complete Eluted Peptide Sequence Data from HLA-DR and -DQ Molecules to Predict T Cell Epitopes, and the Influence of the Nonbinding Terminal Regions of Ligands in Epitope Selection

In diseases with a strong association with an HLA haplotype, identification of relevant T cell epitopes may allow alteration of the pathologic process. In this report we use a reverse immunogenetic approach to predict possible HLA class II-restricted T cell epitopes by using complete pool sequencing data. Data from HLA-DR2(B1*1501), -DR3(B1*0301), -DQ2(A1*0501, B1*0201), and -DQ8(A1*0301, B1*0302) alleles were used by a computer program that searches a candidate protein to predict ligands with a relatively high probability of being processed and presented. This approach successfully identified both known T cell epitopes and eluted single peptides from the parent protein. Furthermore, the program identified ligands from proteins in which the binding motif of the HLA molecule was unable to do so. When the information from the nonbinding N- and C-terminal regions in the pool sequence was removed, the ability to predict several ligands was markedly reduced, particularly for the HLA-DQ alleles. This suggests a possible role for these regions in determining ligands for HLA class II molecules. Thus, the use of complete eluted peptide sequence data offers a powerful approach to the prediction of HLA-DQ and -DR peptide ligands and T cell epitopes.