Identification of frequently recognized dimorphic T-cell epitopes in plasmodium falciparum merozoite surface protein-1 in West and East Africans: lack of correlation of immune recognition and allelic prevalence

The merozoite surface protein-1 (MSP1) is the most studied malaria blood-stage vaccine candidate. Lymphokines such as interferon gamma (IFN-gamma) and interleukin 4 (IL-4) may mediate blood-stage specific protection. Here we identify Plasmodiumfalciparum MSP1 T-cell epitopes capable of rapid induction of IFN-gamma and/or IL-4 from peripheral blood mononuclear cells of East and West African donors. Both allelic forms of these novel MSP1 T-cell epitopes were stimulatory. An unusually high numbers of Gambian responders (> 80%) to these epitopes were observed, suggesting that MSPI reactivity may have been underestimated previously in this population. Surprisingly, IFN-gamma responses to allelic T-cell epitopes failed to correlate with differential antigenic exposure in The Gambia compared to Kenya. These results suggest an unexpected level of immunoregulation of IFN-gamma response with variable allelic T-cell reactivity independent of the level of antigenic exposure. Further analysis of the mechanisms determining this response pattern may be required if vaccines are to overcome this allelic reactivity bias in malaria-exposed populations.

Enhanced immunogenicity of CD4(+) t-cell responses and protective efficacy of a DNA-modified vaccinia virus Ankara prime-boost vaccination regimen for murine tuberculosis

DNA vaccines whose DNA encodes a variety of antigens from Mycobacterium tuberculosis have been evaluated for immunogenicity and protective efficacy. CD8(+) T-cell responses and protection achieved in other infectious disease models have been optimized by using a DNA immunization to prime the immune system and a recombinant virus encoding the same antigen(s) to boost the response. A DNA vaccine (D) and recombinant modified vaccinia virus Ankara (M) in which the DNA encodes early secreted antigenic target 6 and mycobacterial protein tuberculosis 63 synthesized, and each was found to generate specific gamma interferon (IFN-gamma)-secreting CD4(+) T cells. Enhanced CD4(+) IFN-gamma T-cell responses were produced by both D-M and M-D immunization regimens. Significantly higher levels of IFN-gamma were seen with a D-D-D-M immunization regimen. The most immunogenic regimens were assessed in a challenge study and found to produce protection equivalent to that produced by Mycobacterium bovis BCG. Thus, heterologous prime-boost regimens boost CD4(+) as well as CD8(+) T-cell responses, and the use of heterologous constructs encoding the same antigen(s) may improve the immunogenicity and protective efficacy of DNA vaccines against tuberculosis and other diseases.

Enumeration of T cells specific for RD1-encoded antigens suggests a high prevalence of latent Mycobacterium tuberculosis infection in healthy urban Indians

Knowledge of the prevalence of latent Mycobacterium tuberculosis infection is crucial for effective tuberculosis control, but tuberculin skin test surveys have major limitations, including poor specificity because of the broad antigenic cross-reactivity of tuberculin. The M. tuberculosis RD1 genomic segment encodes proteins, such as early secretory antigenic target (ESAT)-6, that are absent from M. bovis bacille Calmette-Guérin (BCG) and most environmental mycobacteria. We recently identified circulating ESAT-6-specific T cells as an accurate marker of M. tuberculosis infection. Here, interferon-gamma-secreting T cells specific for peptides derived from ESAT-6 and a second RD1 gene product, CFP10, were enumerated in 100 prospectively recruited healthy adults in Bombay (Mumbai), India. Eighty percent responded to >/=1 antigen, and many donors had high frequencies of T cells that were specific for certain immunodominant peptides. In contrast, of 40 mostly BCG-vaccinated, United Kingdom-resident healthy adults, none responded to either antigen. This study suggests an 80% prevalence of latent M. tuberculosis infection in urban India.

Human CD8+ CTL specific for the mycobacterial major secreted antigen 85A

The role of CD8(+) CTL in protection against tuberculosis in human disease is unclear. In this study, we stimulated the peripheral blood mononuclear cells of bacillus Calmette-Guérin (BCG)-vaccinated individuals with live Mycobacterium bovis BCG bacilli to establish short-term cell lines and then purified the CD8(+) T cells. A highly sensitive enzyme-linked immunospot (ELISPOT) assay for single cell IFN-gamma release was used to screen CD8(+) T cells with overlapping peptides spanning the mycobacterial major secreted protein, Ag85A. Three peptides consistently induced a high frequency of IFN-gamma responsive CD8(+) T cells, and two HLA-A*0201 binding motifs, P(48-56) and P(242-250), were revealed within the core sequences. CD8(+) T cells responding to the 9-mer epitopes were visualized within fresh blood by ELISPOT using free peptide or by binding of HLA-A*0201 tetrameric complexes. The class I-restricted CD8(+) T cells were potent CTL effector cells that efficiently lysed an HLA-A2-matched monocyte cell line pulsed with peptide as well as autologous macrophages infected with Mycobacterium tuberculosis or recombinant vaccinia virus expressing the whole Ag85A protein. Tetramer assays revealed a 6-fold higher frequency of peptide-specific T cells than IFN-gamma ELISPOT assays, indicating functional heterogeneity within the CD8(+) T cell population. These results demonstrate a previously unrecognized, MHC class I-restricted, CD8(+) CTL response to a major secreted Ag of mycobacteria and supports the use of Ag85A as a candidate vaccine against tuberculosis.

Allelic association between the NRAMP1 gene and susceptibility to tuberculosis in Guinea-Conakry

Forty four families from Guinea-Conakry were analysed to test for association between NRAMP1 (Natural Resistance Associated Macrophage Protein 1) polymorphisms and tuberculosis. Each family included at least one affected sib and one parent. Healthy sibs were also analysed and on average the families included four members. A total of 160 individuals were included in the final dataset. The analysis of association was performed using an extended TDT test, TRANSMlT, to allow for missing information in the parental genotypes. Three polymorphisms in the NRAMP1 gene were typed: a microsatellite (CA) repeat, a 4 bp deletion in the 3' untranslated region and a single nucleotide change in intron 4. The single base change in intron 4 was significantly associated (p = 0.036) with tuberculosis. Our results therefore confirm, using a family-based approach on a newly studied population, the previously reported association between this polymorphism and tuberculosis in a population-based study of West Africans.

High frequencies of circulating IFN-gamma-secreting CD8 cytotoxic T cells specific for a novel MHC class I-restricted Mycobacterium tuberculosis epitope in M. tuberculosis-infected subjects without disease

MHC class I-restricted CD8 cytotoxic T lymphocytes (CTL) are essential for protective immunity to Mycobacterium tuberculosis in animal models but their role in humans remains unclear. We therefore studied subjects who had successfully contained M. tuberculosis infection in vivo, i.e. exposed healthy household contacts and individuals with inactive self-healed pulmonary tuberculosis. Using the ELISPOT assay for IFN-gamma, we screened peptides from ESAT-6, a secreted antigen that is highly specific for M. tuberculosis. We identified a novel nonamer epitope: unstimulated peripheral blood-derived CD8 T cells displayed peptide-specific IFN-gamma release ex vivo while CD8 T cell lines and clones exhibited HLA-A68.02-restricted cytolytic activity and recognized endogenously processed antigen. The frequency of CD8 CTL specific for this single M. tuberculosis epitope, 1/2500 peripheral blood lymphocytes, was equivalent to the combined frequency of all IFN-gamma-secreting purified protein derivative-reactive T cells ex vivo. This highly focused CTL response was maintained in an asymptomatic contact over 2 years and is the most potent antigen-specific antimycobacterial CD8 CTL response hitherto described. Thus, human M. tuberculosis-specific CD8 CTL are not necessarily associated with active disease per se. Rather, our results are consistent with a protective role for these ESAT-6-specific CD8 T cells in the long-term control of M. tuberculosis in vivo in humans.

Improved method for distinguishing the human source of mosquito blood meals between close family members

We have developed a simple and relatively cheap method to distinguish the origin of mosquito blood meals between close family members, effective for both laboratory and field samples. Each blood meal was squashed on to filter paper and eluted overnight with 0.5 mL phosphate-buffered saline. Deoxyribonucleic acid (DNA) was extracted using a chemical matrix (Insta-gene) which bound to everything from the blood meal except DNA, which remained in the supernatant. Following extractions, reference DNA samples taken directly from finger-prick blood of human subjects and those from blood meals of unknown origin were amplified with human microsatellite markers using a thermal cycler. Polymerase chain reaction products were then run on an ABI gel (Automated Biosystems) to obtain a genotype for each sample. The DNA from each mosquito blood meal was then matched to an individual host. With laboratory samples, human DNA which had been extracted from mosquito blood meals up to 12 h after feeding could be used. One important application of this method will be to identify which members of a community are most at risk from vector-borne diseases. It also has numerous potential applications in studies of insect biting behaviour in both human and veterinary science.

Vaccines against intracellular infections requiring cellular immunity

Vaccines against a variety of infectious diseases represent one of the great triumphs of medicine. The immune correlates of protection induced by most current vaccines seem to be mediated by long-lived humoral immune responses. By contrast, there are no currently available vaccines that are uniformly effective for diseases such as HIV, malaria and tuberculosis, in which the cellular immune response might be crucial in mediating protection. Here we examine the mechanisms by which long-lived cellular immune responses are generated and maintained in vivo. We then discuss current approaches for vaccination against diseases in which cellular immune responses are important for protection.

The immunology of malaria infection

As global malaria mortality increases the urgency for vaccine development, analysis of immune responses in naturally exposed populations is providing clues to the nature of protective immunity. Recently, sophisticated immune evasion strategies adopted by the parasite have been analysed at the molecular level. More immunogenic vaccination strategies have been identified, providing renewed optimism that effective malaria control through vaccination should be feasible.

Genetic susceptibility to tuberculosis in Africans: a genome-wide scan

Human genetic variation is an important determinant of the outcome of infection with Mycobacterium tuberculosis. We have conducted a two-stage genome-wide linkage study to search for regions of the human genome containing tuberculosis-susceptibility genes. This approach uses sibpair families that contain two full siblings who have both been affected by clinical tuberculosis. For any chromosomal region containing a major tuberculosis-susceptibility gene, affected sibpairs inherit the same parental alleles more often than expected by chance. In the first round of the screen, 299 highly informative genetic markers, spanning the entire human genome, were typed in 92 sibpairs from The Gambia and South Africa. Seven chromosomal regions that showed provisional evidence of coinheritance with clinical tuberculosis were identified. To identify whether any of these regions contained a potential tuberculosis-susceptibility gene, 22 markers from these regions were genotyped in a second set of 81 sibpairs from the same countries. Markers on chromosomes 15q and Xq showed suggestive evidence of linkage (lod = 2.00 and 1.77, respectively) to tuberculosis. The potential identification of susceptibility loci on both chromosomes 15q and Xq was supported by an independent analysis designated common ancestry using microsatellite mapping. These results indicate that genome-wide linkage analysis can contribute to the mapping and identification of major genes for multifactorial infectious diseases of humans. An X chromosome susceptibility gene may contribute to the excess of males with tuberculosis observed in many different populations.

Comparison of tests for association and linkage in incomplete families

To analyze incomplete families, the following statistical tests can be used: LRAT-a simple likelihood-based association test, TRANSMIT, SIBASSOC/STDT, and RCTDT. We compared these four tests, for the diallelic case, on simulated data sets. The comparisons focused on the power to detect linkage and association when different familial structures, resistance to population stratification, resistance to misclassification of the disease status of the healthy sib, and the effect of nonpaternity were considered. The simulations lead to the following conclusions. The type I errors of TRANSMIT, SIBASSOC/STDT, and RCTDT were not affected by population stratification. LRAT showed bias under strong population stratification. High nonpaternity rates can lead to inflated type I errors, highlighting the importance of identification of half sibs. Under different homogeneous models, the power of TRANSMIT was very similar to that of LRAT, and, similarly, no difference in power was observed between SIBASSOC/STDT and RCTDT. Under various recessive and additive models, TRANSMIT was slightly more powerful than SIBASSOC/STDT when monoparental families with one affected and one unaffected sib were analyzed. Under various dominant models, SIBASSOC/STDT was slightly more powerful than TRANSMIT. Misclassification of the disease status of healthy sibs, as well as the discarding of incomplete families, resulted in a consistent loss of power.

In vivo antigen challenge in celiac disease identifies a single transglutaminase-modified peptide as the dominant A-gliadin T-cell epitope

Celiac disease (CD) is an increasingly diagnosed enteropathy (prevalence, 1:200-1:300) that is induced by dietary exposure to wheat gliadins (as well as related proteins in rye and barley) and is strongly associated with HLA-DQ2 (alpha1*0501, beta1*0201), which is present in over 90% of CD patients. Because a variety of gliadin peptides have been identified as epitopes for gliadin-specific T-cell clones and as bioactive sequences in feeding studies and in ex vivo CD intestinal biopsy challenge, it has been unclear whether a 'dominant' T-cell epitope is associated with CD. Here, we used fresh peripheral blood lymphocytes from individual subjects undergoing short-term antigen challenge and tissue transglutaminase-treated, overlapping synthetic peptides spanning A-gliadin to demonstrate a transient, disease-specific, DQ2-restricted, CD4 T-cell response to a single dominant epitope. Optimal gamma interferon release in an ELISPOT assay was elicited by a 17-amino-acid peptide corresponding to the partially deamidated peptide of A-gliadin amino acids 57-73 (Q65E). Consistent with earlier reports indicating that host tissue transglutaminase modification of gliadin enhances gliadin-specific CD T-cell responses, tissue transglutaminase specifically deamidated Q65 in the peptide of A-gliadin amino acids 56-75. Discovery of this dominant epitope may allow development of antigen-specific immunotherapy for CD.

Secretor polymorphism and human immunodeficiency virus infection in Senegalese women

The FUT2 gene encodes the enzyme alpha (1,2) fucosyltransferase, which determines expression of blood-group antigens on mucosal epithelial cell surfaces and in secretions. Homozygotes for a specific stop mutation in FUT2 (nonsecretors) cannot produce this enzyme and thus are unable to express blood group antigens. Nonsecretor status is associated with a decreased risk of several respiratory viral infections. By use of molecular genotyping, 2 populations of Senegalese women were examined for polymorphisms of the FUT2 gene. Among Senegalese commercial sex workers, absence of FUT2 (nonsecretor genotype) was associated with reduced risk of human immunodeficiency virus (HIV) type 1 infection (odds ratio [OR] adjusted for cervical and vaginal infection, 0.18; 95% confidence interval [CI], 0.04-0.90) and HIV-2 infection (adjusted OR, 0.43; 95% CI, 0.13-1.39), although the latter was not statistically significant. Modification of cell surface carbohydrates at mucosal surfaces determined by the FUT2 gene may underlie the protective association against heterosexual HIV infection.

Cytotoxic T-lymphocyte epitopes for HLA-B53 and other HLA types in the malaria vaccine candidate liver-stage antigen 3

The development of an effective preerythrocytic vaccine against Plasmodium falciparum malaria is likely to require inclusion of components from several preerythrocytic antigens. The association of HLA-B53 with resistance to severe malaria in West Africa provided evidence that HLA class I-restricted CD8(+) T-cell responses play a role in protective immunity in African children, supporting data from rodent models of malaria. Previously, a single epitope from liver-stage-specific antigen 1 (LSA-1) has been shown to be recognized by HLA-B53-specific cytotoxic T lymphocytes (CTL), but HLA-B53 epitopes were not found in four other antigens. In this study we measured CTL responses to peptides from the recently sequenced antigen liver-stage antigen 3 (LSA-3) and identified in it a new epitope restricted by HLA-B53. Several CTL epitopes restricted by other class I types were also identified within LSA-3 in studies in The Gambia and Tanzania. CTL were also identified to an additional P. falciparum antigen, exported protein 1 (Exp-1), the homologue of which is a protective antigen in a rodent model of malaria. These findings emphasize the diversity of P. falciparum antigens recognized by CD8(+) T cells in humans and support the inclusion of components from several antigens in new CTL-inducing vaccines against malaria.

Gene gun intradermal DNA immunization followed by boosting with modified vaccinia virus Ankara: enhanced CD8+ T cell immunogenicity and protective efficacy in the influenza and malaria models

In influenza and malaria, CD8+ T cells play an important role in protective immunity in mice. An immunization strategy consisting of DNA priming followed by boosting with recombinant modified vaccinia virus Ankara (MVA) induces complete protection, associated with high levels of CD8+ T cells, against Plasmodium berghei sporozoite challenge in mice. Intradermal delivery of DNA with a gene gun requires smaller amounts of DNA than intramuscular injection, in order to induce similar levels of immune responses. The present study compares both routes for the induction of specific CD8+ T cell responses and protection using different prime-boost immunization regimes in the influenza and the malaria models. In the DNA/MVA regime, equally high CD8+ T cell responses and levels of protection are achieved using ten times less DNA when delivered with a gene gun compared to intramuscular injection.

Potent induction of focused Th1-type cellular and humoral immune responses by RTS,S/SBAS2, a recombinant Plasmodium falciparum malaria vaccine

The RTS,S/SBAS2 vaccine confers sterile protection against Plasmodium falciparum sporozoite challenge. The mechanisms underlying this are of great interest, yet little is known about the immune effector mechanisms induced by this vaccine. The immune responses induced by RTS,S/SBAS2 were characterized in 10 malaria-naive volunteers. Several epitopes in the circumsporozoite protein (CSP) were identified as targets of cultured interferon (IFN)-gamma-secreting CD4+ T cells. RTS,S-specific IFN-gamma-secreting effector T cells were induced in 8 subjects; this ex vivo response mapped to a single peptide in Th2R. CSP-specific CD8+ cytotoxic T lymphocytes were not detected. RTS, S-specific IFN-gamma production was universal, whereas interleukin-4 and -5 production was rare. RTS,S-specific lymphoproliferative responses and antibodies to CSP were strongly induced in all volunteers. Responses waned with time but were boostable. Thus, RTS, S/SBAS2 is a potent inducer of Th1-type cellular and humoral immunity. These results highlight possible immune mechanisms of protection and have important implications for vaccine design in general.

Induction of CD8+ T cells using heterologous prime-boost immunisation strategies

One of the current challenges in vaccine design is the development of antigen delivery systems or vaccination strategies that induce high protective levels of CD8+ T cells. These cells are crucial for protection against certain tumours and intracellular pathogens such as the liver-stage parasite of malaria. A liver-stage malaria vaccine should therefore include CD8+ T-cell-inducing components. This review provides an overview of prime-boost immunisation strategies that result in protective CD8+ T-cell responses against malaria with an emphasis on work from our laboratory. Possible mechanisms explaining why heterologous prime-boost strategies, in particular boosting with replication-impaired recombinant poxviruses, are so effective are discussed.

The immunogenetics of resistance to malaria

The genetic basis of susceptibility to malaria has been studied extensively using a variety of approaches. The protective role of several erythrocytic variants is now well established. More recently, there has been growing evidence that genes determining a variety of immune responses influence susceptibility to malaria. Some of these genes may specifically affect susceptibility to particular strains of malaria parasite. The recent adoption of genetic linkage approaches supplements the established strategy of assessing candidate gene polymorphisms in case-control studies. Immunogenetic associations with severe malaria have already suggested new approaches for intervention, and the highly polygenic nature of susceptibility to this disease suggests that the identification and analysis of new susceptibility and resistance loci should be worthwhile.

Interleukin 10-mediated immunosuppression by a variant CD4 T cell epitope of Plasmodium falciparum

The immunodominant CD4 T cell epitope region, Th2R, of the circumsporozoite protein of Plasmodium falciparum is highly polymorphic. Such variation might be utilized by the parasite to escape from or interfere with CD4 T cell effector functions. Here, we show that costimulation with naturally occurring altered peptide ligands (APL) can induce a rapid change from IFNgamma production to the immunosuppressive mediator interleukin 10 (IL-10). This mechanism may contribute to the low levels of T cell responses observed to this pathogen in malaria-endemic areas.

A polymorphism that affects OCT-1 binding to the TNF promoter region is associated with severe malaria

Genetic variation in cytokine promoter regions is postulated to influence susceptibility to infection, but the molecular mechanisms by which such polymorphisms might affect gene regulation are unknown. Through systematic DNA footprinting of the TNF (encoding tumour necrosis factor, TNF) promoter region, we have identified a single nucleotide polymorphism (SNP) that causes the helix-turn-helix transcription factor OCT-1 to bind to a novel region of complex protein-DNA interactions and alters gene expression in human monocytes. The OCT-1-binding genotype, found in approximately 5% of Africans, is associated with fourfold increased susceptibility to cerebral malaria in large case-control studies of West African and East African populations, after correction for other known TNF polymorphisms and linked HLA alleles.

Broadly distributed T cell reactivity, with no immunodominant loci, to the pre-erythrocytic antigen thrombospondin-related adhesive protein of Plasmodium falciparum in West Africans

Protective immunity to malaria has been achieved in human volunteers utilizing the pre-erythrocytic Plasmodium falciparum antigen, the circumsporozoite protein (CS). However, T cell reactivity to CS is focused on several highly polymorphic T cell epitope regions, potentially limiting the efficacy of any vaccine to specific malaria strains. Another important pre-erythrocytic malaria antigen, the thrombospondin-related adhesive protein (TRAP), can induce protection in animal models of malaria, but knowledge of human T cell responses is limited to the identification of CD8 T cell epitopes, with no CD4 epitopes identified to date. This comprehensive study assessed reactivity to overlapping peptides spanning almost the whole of P. falciparum TRAP (PfTRAP), as well as peptides selected on the basis of HLA class II-binding motifs. A total of 50 naturally exposed Gambian adults were assessed to define 26 T cell epitopes in PfTRAP capable of inducing rapid IFN-gamma or IL-4 production, as assessed by enzyme-linked immunospot assays. In contrast to the CS protein, this reactivity was broadly distributed along the length of TRAP. Moreover, of the 26 epitopes identified, 10 were found to be conserved in West Africa.

Altered peptide ligands narrow the repertoire of cellular immune responses by interfering with T-cell priming

Variation in epitopes of infectious pathogens inhibits various effector functions of T lymphocytes through antagonism of the T-cell receptor. However, a more powerful strategy for immune evasion would be to prevent the induction of T-cell responses. We report here mutual 'interference' with the priming of human T-cell responses by a pair of naturally occurring variants of a malaria cytotoxic T-cell epitope. Interference with priming also occurs in vivo for a murine malaria T-cell epitope. Reshaping of the T-cell repertoire by such immune interference during naive T-cell induction may provide a general mechanism for observed patterns of immunodominance and persistence by many polymorphic pathogens.

Ty virus-like particles, DNA vaccines and Modified Vaccinia Virus Ankara; comparisons and combinations

Three types of vaccine, all expressing the same antigen from Plasmodium berghei, or a CD8+ T cell epitope from that antigen, were compared for their ability to induce CD8+ T cell responses in mice. Higher levels of lysis and numbers of IFN-gamma secreting T cells were primed with Ty virus-like particles and Modified Vaccinia Virus Ankara (MVA) than with DNA vaccines, but none of the vaccines were able to protect immunised mice from infectious challenge even after repeated doses. However, when the immune response was primed with one type of vaccine (Ty-VLPs or DNA) and boosted with another (MVA) complete protection against infection was achieved. Protection correlated with very high levels of IFN-gamma secreting T cells and lysis. This method of vaccination uses delivery systems and routes that can be used in humans and could provide a generally applicable regime for the induction of high levels of CD8+ T cells.