Tuberculosis and chronic hepatitis B virus infection in Africans and variation in the vitamin D receptor gene

The active metabolite of vitamin D, 1,25 dihydroxyvitamin D3, is an important immunoregulatory hormone [1]. Its effects are exerted by interaction with the vitamin D receptor, which is present on human monocytes and activated T and B lymphocytes. Variation in the vitamin D receptor gene was typed in 2015 subjects from large case-control studies of three major infectious diseases: tuberculosis, malaria, and hepatitis B virus. Homozygotes for a polymorphism at codon 352 (genotype tt) were significantly underrepresented among those with tuberculosis (chi2=6.22, 1 df, P=. 01) and persistent hepatitis B infection (chi2=6.25, 1 df, P=.01) but not in subjects with clinical malaria compared with the other genotypes. Therefore, this genetic variant, which predisposes to low bone mineral density in many populations, may confer resistance to certain infectious diseases.

Effective induction of HIV-specific CTL by multi-epitope using gene gun in a combined vaccination regime

Reliable and effective induction of cytotoxic T-lymphocytes (CTL) is one of the prime objectives of vaccine research. Previously, novel HIV vaccine candidates were constructed as a string of CTL epitopes (20 human, 3 macaque and 1 mouse) delivered using a DNA vector [Hanke T, Schneider J, Gilbert SG, Hill AVS, McMichael A. DNA multi-CTL epitope vaccines for HIV and Plasmodium falciparum: immunogenicity in mice. Vaccine 1998;16:426-435.] or modified vaccinia Ankara (MVA [Hanke T, Blanchard TJ, Schneider J, Ogg GS, Tan R, Becker MSC, Gilbert SG, Hill AVS, Smith GL, McMichael A. Immunogenicities of intravenous and intramuscular administrations of MVA-based multi-CTL epitope vaccine for HIV in mice. J Gen Virol 1998;79:83-90.]), i.e. vaccine vehicles acceptable for use in humans. In mice, a single intramuscular (i.m.) needle injection of either vaccine alone elicited good CTL responses. Here, it is demonstrated that the multi-epitope DNA also induced CTL when delivered intradermally using the Accell gene gun. The CTL responses increased after re-immunization and after three deliveries were comparable to those induced by a single i.m. injection. Recent evidence indicates that combining routes and vaccine vehicles enhances the immunogenicity of vaccine-delivered or -encoded antigens. Here, it is shown that administration of DNA by an i.m. priming/gene gun boosting more efficiently induced CTL than gene gun priming/i.m. boosting. A similar increment was obtained by sequential vaccinations using a gene gun-delivered DNA followed by recombinant MVA. Thus particular sequences of routes or vaccine vehicles rather than simple prime-boost delivery of a single vaccine is critical for an effective elicitation of CTL.

Host genetic susceptibility to human tuberculosis

Convincing evidence exists that host genes influence the outcome of infection in human tuberculosis. We are employing two complementary strategies to find the genes involved: a linkage-based, comprehensive genome screen and an association-based candidate gene study. In a genome screen of 282 markers on 92 affected sib pairs we have found evidence of co-segregation of disease with five markers, but further studies are required to replicate these results. The absence of a single strongly linked marker demonstrates that susceptibility to human tuberculosis is not controlled by a single major gene. Using a candidate gene approach investigating over 400 tuberculosis cases and 400 ethnically matched healthy controls we have found evidence that NR AMP1 and vitamin D receptor gene (VDR) polymorphisms are associated with tuberculosis. It is hoped that by identifying the genes that account for why only a minority of those exposed to tuberculosis develop disease, we will develop new insights into potential therapeutic and preventative strategies.

Severe malarial anemia and cerebral malaria are associated with different tumor necrosis factor promoter alleles

Experimental evidence implicates tumor necrosis factor (TNF) in the pathogenesis of malarial anemia, but there are few data relating to this hypothesis. This study found that severely anemic children with Plasmodium falciparum infection have low plasma TNF levels, in contrast to the high levels found in cerebral malaria. A previous case-control study in The Gambia found cerebral malaria, but not severe malarial anemia, was associated with the TNF-308 A allele. This study found that in the same population, severe malarial anemia was associated with the TNF-238 A allele, with an odds ratio of 2.5 (P<.001) after stratification for HLA type. These findings suggest that severe malarial anemia and cerebral malaria are influenced by separate genetic factors situated near the TNF gene.

Genetics and genomics of infectious disease susceptibility

Human genetic variation is a major determinant of susceptibility to many common infectious diseases. Malaria was the first disease to be studied extensively and many susceptibility and resistance loci have been identified. However, genes for other diseases such as HIV/AIDS and mycobacterial infections are now being identified using a variety of approaches. A large number of genes appear to influence susceptibility to infectious pathogens and defining these can provide insights into pathogenic and protective mechanisms and identify new molecular targets for prophylactic and therapeutic interventions. Immunogenetic associations with infectious diseases have considerable potential to guide immunomodulatory interventions and vaccine design.

Association of vitamin D receptor genotype with leprosy type

Host genetic factors including major histocompatibility complex (MHC) polymorphisms influence both susceptibility to leprosy per se and also to leprosy type. Non-MHC genes may play an important role, but such genes remain undefined. The influence of two non-MHC candidate genes was assessed in a case-control study of Bengali leprosy patients from Calcutta. Recent studies have implicated variation in the vitamin D receptor (VDR) gene in susceptibility to several diseases, including osteoporosis and pulmonary tuberculosis. In this population, homozygotes for the alternate alleles of the VDR polymorphism are associated, respectively, with lepromatous and tuberculoid leprosy. The NRAMP1 (natural resistance associated macrophage protein 1) gene may influence human mycobacterial disease susceptibility based on studies with the murine homologue Nramp1. However, no significant association was found between NRAMP1 and leprosy susceptibility. This study suggests that the VDR polymorphism may influence susceptibility to some diseases by affecting the type and the strength of the host immune response.

Protection from Plasmodium berghei infection by priming and boosting T cells to a single class I-restricted epitope with recombinant carriers suitable for human use

The desirability of inducing cytotoxic T cell responses to defined epitopes in humans has led to the development of a variety of recombinant delivery systems. Recombinant protein particles derived from a yeast retrotransposon (Ty) and the modified Ankara vaccinia (MVA) virus can deliver large epitope strings or even whole proteins. Both have previously been administered safely in humans. Immunization with recombinant Ty and MVA containing a single Plasmodium berghei class I-binding epitope provided 95% sterile protection against malaria in mice. The sequence of immunization, Ty followed by MVA, was critical to elicit high levels of IFN-gamma-producing cells and protection. The reciprocal sequence (MVA/TY) or homologous boosting was not protective. Both constructs (Ty and MVA) contain the H-2Kd-restricted pb9 CTL epitope from the circumsporozoite protein of P. berghei among a string of 8-15 human P. falciparum-derived CTL epitopes restricted through 7 common HLA alleles as well as widely recognized CD4 T cell epitopes. Thus, the novel recombinant Ty/MVA prime/boost combination with these constructs provides a safe alternative for evaluation for human vaccination against P. falciparum malaria.

1,25-Dihydroxyvitamin D3 induces nitric oxide synthase and suppresses growth of Mycobacterium tuberculosis in a human macrophage-like cell line

Inducible synthesis of nitric oxide (NO) by macrophages is an important mechanism of the host defense against intracellular infection in mice, but the evidence for significant levels of inducible NO production by human macrophages is controversial. Here we report that the human promyelocytic cell line HL-60, when differentiated to a macrophage-like phenotype, acquires the ability to produce substantial amounts of NO on stimulation with LPS or 1, 25-dihydroxyvitamin D3 (1,25-D3) in the absence of activating factors such as gamma interferon. Expression of the inducible nitric oxide synthase (NOS2) was confirmed by sequencing of the reverse transcription-PCR product from stimulated HL-60 cells. Kinetic studies after lipopolysaccharide stimulation show that NOS2 mRNA levels rise within 3 to 6 h, that conversion of [14C]arginine to [14C]citrulline is maximal at 5 to 6 days, and that levels of reactive nitrogen intermediates stabilize at around 20 microM at 7 to 8 days. We find that 1,25-D3 acts to suppress the growth of Mycobacterium tuberculosis in these cells and that this effect is inhibited by NG-monomethyl-L-arginine, suggesting that vitamin D-induced NO production may play a role in the host defense against human tuberculosis.

Cytotoxic T-lymphocytes against malaria and tuberculosis: from natural immunity to vaccine design

1. Mycobacterium tuberculosis and the liver stage of Plasmodium falciparum are intracellular pathogens which are potentially susceptible to cytotoxic T-lymphocytes, a crucial component of the protective immune response to viral infections. Evidence from animal models points to a protective role for cytotoxic T-lymphocytes against M. tuberculosis and P. falciparum, but cytotoxic T-lymphocytes specific for these pathogens have been difficult to identify in man.2. Using a reverse immunogenetic approach, candidate epitopes from selected antigens of P. falciparum and M. tuberculosis were used to detect peptide-specific cytotoxic T-lymphocyte responses in individuals exposed to these pathogens. Cytotoxic T-lymphocyte activity was detected by the 51Cr release cytotoxicity assay and a sensitive ELISPOT assay for single-cell interferon-gamma release.3. In naturally exposed, partially immune Africans in The Gambia, eight largely conserved cytotoxic T-lymphocyte epitopes in P. falciparum, restricted by several different HLA class I alleles, were identified. Several epitopes were also recognized in Tanzanians and cytotoxic T-lymphocytes recognized endogenously processed antigen.4. In tuberculosis patients with HLA-B52, a CD8+ cytotoxic T-lymphocyte epitope was identified in ESAT-6, a secreted antigen specific for M. tuberculosis complex but absent in BCG. Cytotoxic T-lymphocytes exhibited HLA-B52-restricted peptide-specific interferon-gamma release and lytic activity and recognized endogenously processed antigen.5. These studies demonstrate that CD8+ cytotoxic T-lymphocytes specific for mycobacterial and protozoal antigens are induced during natural infections in humans. The identification of these T-cells endorses current strategies to develop cytotoxic T-lymphocyte-inducing vaccines against P. falciparum and M. tuberculosis and highlights candidate antigens for inclusion in subunit vaccines.

Genetic susceptibility to mycobacteria and other infectious pathogens in humans

Substantial evidence exists that host genes are important in determining the outcome of infection with mycobacteria and other intracellular pathogens. Geographical variation in the prevalence of malaria has facilitated the recognition of many genes important in determining interindividual variability in susceptibility to the severe forms of this infection. This success has, however, been difficult to achieve in other infectious diseases. Recently a variety of study designs including large-scale association-based population case/control studies of candidate genes, family-based linkage studies, investigation of rare individuals with exceptional mycobacterial susceptibility and comparison with mouse models of disease has enabled identification of host genes which contribute to susceptibility to mycobacterial disease. This work demonstrates that a large number of genes are probably important in susceptibility to mycobacterial pathogens and provides a model for the investigation of genetic susceptibility to other infectious diseases.

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.

Phase I/IIa safety, immunogenicity, and efficacy trial of NYVAC-Pf7, a pox-vectored, multiantigen, multistage vaccine candidate for Plasmodium falciparum malaria

Candidate malaria vaccines have failed to elicit consistently protective immune responses against challenge with Plasmodium falciparum. NYVAC-Pf7, a highly attenuated vaccinia virus with 7 P. falciparum genes inserted into its genome, was tested in a phase I/IIa safety, immunogenicity, and efficacy vaccine trial in human volunteers. Malaria genes inserted into the NYVAC genome encoded proteins from all stages of the parasite's life cycle. Volunteers received three immunizations of two different dosages of NYVAC-Pf7. The vaccine was safe and well tolerated but variably immunogenic. While antibody responses were generally poor, cellular immune responses were detected in >90% of the volunteers. Of the 35 volunteers challenged with the bite of 5 P. falciparum-infected Anopheles mosquitoes, 1 was completely protected, and there was a significant delay in time to parasite patency in the groups of volunteers who received either the low or high dose of vaccine compared with control volunteers.

The immunogenetics of human infectious diseases

Twin and adoptee studies have indicated that host genetic factors are major determinants of susceptibility to infectious disease in humans. Twin studies have also found high heritabilities for many humoral and cellular immune responses to pathogen antigens, with most of the genetic component mapping outside of the major histocompatibility complex. Candidate gene studies have implicated several immunogenetic polymorphisms in human infectious diseases. HLA variation has been associated with susceptibility or resistance to malaria, tuberculosis, leprosy, AIDS, and hepatitis virus persistence. Variation in the tumor necrosis factor gene promoter has also been associated with several infectious diseases. Chemokine receptor polymorphism affects both susceptibility ot HIV-1 infection and the rate of progression to AIDS. Inactivating mutations of the gamma-interferon receptor lead to increased susceptibility to typical mycobacteria and disseminated BCG infection in homozygous children. The active form of vitamin D has immunomodulatory effects, and allelic variants of the vitamin D receptor appear to be associated with differential susceptibility to several infectious diseases. NRAMP1, a macrophage gene identified by positional cloning of its murine homologue, has been implicated in susceptibility to tuberculosis in Africans. Whole genome linkage analysis of multi-case families is now being used to map and identify new loci affecting susceptibility to infectious diseases. It is likely that susceptibility to most microorganisms is determined by a large number of polymorphic genes, and identification of these should provide insights into protective and pathogenic mechanisms in infectious diseases.

Immune evasion in malaria: altered peptide ligands of the circumsporozoite protein

T cells are central to immunity in malaria. CD4+ helper T cells favour the generation of high-affinity antibodies that are effective against blood stages and they are necessary to establish immunological memory. The intrahepatic stage of infection can be eliminated by specific CD8+ cytotoxic T cells (CTL). Cytokines secreted by CD4+ T cells may also contribute to liver stage immunity. Evolution has selected varied mechanisms in pathogens to avoid recognition by T cells. T cells recognize foreign epitopes as complexes with host major histocompatibility (MHC) molecules. Thus, a simple form of evasion is to mutate amino acid residues which allow binding to an MHC allele. Recently, more sophisticated forms of polymorphic evasion have been described. In altered peptide ligand (APL) antagonism, the concurrent presentation of particular closely related epitope variants can prevent memory T cell effector functions such as cytotoxicity, lymphokine production and proliferation. In immune interference, the effect of the concurrent presentation of such related epitope variants can go a step further and prevent the induction of memory T cells from naive precursors. The analysis of immune responses to a protein of P. falciparum, the circumsporozoite protein (CSP), indicates that the malaria parasite may utilize these evasion strategies.

Absence of an association between intercellular adhesion molecule 1, complement receptor 1 and interleukin 1 receptor antagonist gene polymorphisms and severe malaria in a West African population

Many genes have been shown to be involved in host susceptibility to the severe forms of Plasmodium falciparum malaria but it is likely that a large number of malaria-susceptibility genes remain to be determined. We conducted a large case-control study of children with the severe forms of this disease-cerebral malaria and severe malarial anaemia--to attempt to identify these genes. Over 1200 children in The Gambia were typed for polymorphisms of the intercellular adhesion molecule 1 (ICAM-1), complement receptor 1 (CR-1) and interleukin 1 receptor antagonist (IL-IRA) genes. None of the polymorphisms typed was significantly associated with severe disease. These data differed significantly from the results of a previous study (Chi 2 = 8.81; P = 0.003) in which the ICAM-1 gene polymorphism was shown to be significantly associated with cerebral malaria in a case-control study of 547 subjects in Kenya. This suggests that there may be heterogeneity in genetic susceptibility to this condition between these 2 African populations.

Enhanced immunogenicity for CD8+ T cell induction and complete protective efficacy of malaria DNA vaccination by boosting with modified vaccinia virus Ankara

Immunization with irradiated sporozoites can protect against malaria infection and intensive efforts are aimed at reproducing this effect with subunit vaccines. A particular sequence of subunit immunization with pre-erythrocytic antigens of Plasmodium berghei, consisting of single dose priming with plasmid DNA followed by a single boost with a recombinant modified vaccinia virus Ankara (MVA) expressing the same antigen, induced unprecedented complete protection against P. berghei sporozoite challenge in two strains of mice. Protection was associated with very high levels of splenic peptide-specific interferon-gamma-secreting CD8+ T cells and was abrogated when the order of immunization was reversed. DNA priming followed by MVA boosting may provide a general immunization regime for induction of high levels of CD8+ T cells.

Variations in the NRAMP1 gene and susceptibility to tuberculosis in West Africans

BACKGROUND

Genetic factors may affect the susceptibility to tuberculosis, but no specific genes governing susceptibility have been identified. In mice, natural resistance to infection with some mycobacteria is influenced by the gene for natural-resistance-associated macrophage protein 1 (Nramp1), but the role of the human homologue of this gene, NRAMP1, in tuberculosis is unknown. We typed polymorphisms in NRAMP1 in a case-control study of tuberculosis in the Gambia, West Africa.

METHODS

Sequence-specific oligonucleotide hybridization and microsatellite analysis were used to type NRAMP1 polymorphisms in 410 adults (mean age, 34.7 years) with smear-positive pulmonary tuberculosis and 417 ethnically matched, healthy controls. Patients with human immunodeficiency virus infection were excluded.

RESULTS

Four NRAMP1 polymorphisms were each significantly associated with tuberculosis. Subjects who were heterozygous for two NRAMP1 polymorphisms in intron 4 and the 3' untranslated region of the gene were particularly overrepresented among those with tuberculosis, as compared with those with the most common NRAMP1 genotype (odds ratio, 4.07; 95 percent confidence interval, 1.86 to 9.12; chi-square= 14.58; P<0.001).

CONCLUSIONS

Genetic variation in NRAMP1 affects susceptibility to tuberculosis in West Africans.

Enhancement of MHC class I-restricted peptide-specific T cell induction by a DNA prime/MVA boost vaccination regime

Human immunodeficiency virus (HIV) vaccine candidates were previously constructed as a string of cytotoxic T lymphocyte (CTL) epitopes delivered and expressed using DNA and modified virus Ankara (MVA; an attenuated vaccinia virus) vectors. These vaccines were shown to induce interferon (IFN)-gamma-producing and cytolytic CD8+ T cells after a single vaccine administration. In the course of this work, immunization protocols were sought which would improve the levels of induced HIV-specific T cells. It was found that previous immunological exposure to MVA reduced the efficiency of subsequent priming and boosting using the same vaccine vehicle. However, a combined regime whereby the animals were first primed with the DNA vaccine and then boosted with MVA was the most potent protocol for the induction of both interferon-gamma-producing and cytolytic T cells against two CTL epitopes simultaneously. The general applicability of this novel vaccination method for induction of major histocompatibility complex class I-restricted T cells is discussed.

Association of malaria parasite population structure, HLA, and immunological antagonism

Host-parasite coevolution has been likened to a molecular arms race, with particular parasite genes evolving to evade specific host defenses. Study of the variants of an antigenic epitope of Plasmodium falciparum that induces a cytotoxic T cell response supports this view. In African children with malaria, the variants present are influenced by the presence of a human leukocyte antigen (HLA) type that restricts the immune response to this epitope. The distribution of parasite variants may be further influenced by the ability of cohabiting parasite strains to facilitate each other's survival by down-regulating cellular immune responses, using altered peptide ligand antagonism.

DNA multi-CTL epitope vaccines for HIV and Plasmodium falciparum: immunogenicity in mice

The potential of building multi-cytotoxic T lymphocyte (CTL) epitope antigens in combination with the nucleic acid immunization technology is explored for development of acquired immunodeficiency syndrome (AIDS) and malaria vaccines. A novel minimal vector pTH for direct gene transfer was constructed for efficient expression of vaccine antigens and used as a vehicle for human immunodeficiency virus (HIV)- and Plasmodium falciparum-derived polyepitope genes. Two murine epitopes were included into these constructs to allow for testing of vaccine immunogenicity in small animals. The results showed that a single DNA injection generated CTL responses in all 15 vaccinated mice. The elicited CTL precursor frequencies were estimated in an interferon-gamma (IFN-gamma)-based ELISPOT assay and found to be an average of 300 (range 4-1346) peptide-responding cells per 10(6) splenocytes.

T-cell receptor variable alpha (TCRAV) polymorphisms in European, Chinese, South American, AfroCaribbean, and Gambian populations

Interactions involving the T-cell receptor (TCR) and major histocompatibility complex (MHC) are fundamental to the generation of a specific immune response. The study of interpopulation differences in TCR genes may identify those genes which are subject to selection, and also provides useful information for future genetic studies in these populations. In this study we present analysis of five TCRAV polymorphisms, for V5S1, V6S1, V8S1, V17S1, and V21S1 loci in five human populations by single-strand conformational polymorphism (SSCP) analysis. Caucasian, Chinese, Gambian, AfroCaribbean, and South American Indians (Mapuches) showed marked interpopulation variation for both the silent (V5S1, V17S1, and V21S1) and coding (V6S1 and V8S1) polymorphisms. In general the alleles were conserved in the different populations, but new, additional variants were found for V5S1 and V17S1 in Gambians and Caucasians. V6S1 overall showed the highest nucleotide diversity, and V6S1 genotype distributions were skewed away from expected values in Chinese and Mapuches. Analysis of allelic associations showed a general lack of linkage disequilibrium between the loci, which was reflected by the absence of strong population-specific haplotypes.

Human cytolytic and interferon gamma-secreting CD8+ T lymphocytes specific for Mycobacterium tuberculosis

Protective immunity to Mycobacterium tuberculosis is poorly understood, but mounting evidence, at least in animal models, implicates major histocompatibility complex class I-restricted CD8+ T cells as an essential component. By using a highly sensitive assay for single cell interferon gamma release, we screened an array of M. tuberculosis antigen-derived peptides congruent with HLA class I allele-specific motifs. We identified CD8+ T cells specific for epitopes in the early secretory antigenic target 6 during active tuberculosis, after clinical recovery and in healthy contacts. Unrestimulated cells exhibited peptide-specific interferon gamma secretion, whereas lines or clones recognized endogenously processed antigen and showed cytolytic activity. These results provide direct evidence for the involvement of CD8+ cytotoxic T lymphocytes in host defense against M. tuberculosis in humans and support current attempts to generate protective cytotoxic T lymphocyte responses against M. tuberculosis by vaccination.

Immunogenicities of intravenous and intramuscular administrations of modified vaccinia virus Ankara-based multi-CTL epitope vaccine for human immunodeficiency virus type 1 in mice

A vaccine against human immunodeficiency virus (HIV) is still awaited. Although the correlates of protection remain elusive, it is likely that CD8+ T cells play an important role in the control of this infection. To firmly establish the importance of these cells in protective immunity, a means of efficient elicitation of CD8+ T cell responses in the absence of antibody is needed and, when available, might represent a crucial step towards a protective vaccine. Here, a novel vaccine candidate was constructed as a multi-cytotoxic T lymphocyte (CTL) epitope gene delivered and expressed using modified vaccinia virus Ankara (MVA). The immunogen consists of 20 human, one murine and three rhesus macaque epitopes. The non-human epitopes were included so that the vaccine can be tested for immunogenicity and optimal vaccination doses, routes and regimes in experimental animals. Mice were immunized intravenously (i.v.) or intramuscularly (i.m.) using a single dose of 10(6) p.f.u. of the recombinant MVA and the induction of CTL was assessed. It was demonstrated that both administration routes induced specific CTL responses and that the i.v. route was moderately more immunogenic than the i.m. route. The frequencies of ex vivo splenocytes producing interferon-y upon MHC class I-restricted peptide stimulation were determined using an ELISPOT assay. Also, the correct processing and presentation of some HLA-restricted epitopes in human cells was confirmed.