DNA-based vaccines for malaria: a heterologous prime-boost immunisation strategy

A generic approach to inducing high level CD8+ T cell responses would be of value for prophylactic and therapeutic immunisation against several infectious diseases. However, it has been very difficult to achieve such immune responses using available vaccination strategies. Malaria is one of several diseases against which a new generation of better CD8+ T cell-inducing vaccines might be useful and is unusual in that it allows assessment of vaccine efficacy in small numbers of volunteers in carefully controlled challenge studies. Here we review the identification of a heterologous prime-boost regime using DNA priming and recombinant modified vaccinia Ankara (MVA) boosting that induces high level T cell responses in both mice and non-human primates. Clinical trials to determine whether this prime-boost approach is immunogenic in humans are in progress.

The genomics and genetics of human infectious disease susceptibility

A genetic basis for interindividual variation in susceptibility to human infectious diseases has been indicated by twin, adoptee, pedigree, and candidate gene studies. This has led to the identification of a small number of strong genetic associations with common variants for malaria, HIV infection, and infectious prion diseases. Numerous other genes have shown less strong associations with these and some other infectious diseases, such as tuberculosis, leprosy, and persistent hepatitis viral infections. Many immunogenetic loci influence susceptibility to several infectious pathogens. Recent genetic linkage analyses of measures of infection as well as of infectious disease, including some genome-wide scans, have found convincing evidence of genetic linkage to chromosomal regions wherein susceptibility genes have yet to be identified. These studies indicate a highly polygenic basis for susceptibility to many common infectious diseases, with some emerging examples of interaction between variants of specific polymorphic host and pathogen genes.

Strong HLA class I--restricted T cell responses in dengue hemorrhagic fever: a double-edged sword?

Dengue is an increasingly important cause of morbidity and mortality in the tropics, but vaccine development has been impeded by a poor understanding of disease pathogenesis and, in particular, of immunologic enhancement. In a large case-control study of Vietnamese patients with dengue hemorrhagic fever (DHF), variation at the HLA-A locus was significantly associated with susceptibility to DHF (P=.02), and specific HLA-A susceptibility and resistance alleles were identified. HLA-A-specific epitopes were predicted from binding motifs, and ELISPOT analyses of patients with DHF revealed high frequencies of circulating CD8 T lymphocytes that recognized both serotype-specific and -cross-reactive dengue virus epitopes. Thus, strong CD8 T cell responses are induced by natural dengue virus infection, and HLA class I genetic variation is a risk factor for DHF. These genetic and immunologic data support both protective and pathogenic roles for dengue virus-specific CD8 T cell responses in severe disease. The potentially pathogenic role of serotype-cross-reactive CD8 T cells poses yet another obstacle to successful dengue vaccine development.

Induction of CD8+ T-lymphocyte responses to a secreted antigen of Mycobacterium tuberculosis by an attenuated vaccinia virus

Protective immunity against Mycobacterium tuberculosis infection requires the activation of mycobacterium-specific CD8+ T cells, as well as CD4+ T cells. Therefore, optimizing strategies that stimulate CD8+ T cells recognizing dominant mycobacterial antigens, including secreted proteins, may lead to the development of more effective vaccines against tuberculosis. To generate a viral vaccine that is safe in humans, the early secreted protein, MPT64, was expressed in the attenuated vaccinia virus (VV) strain, modified vaccinia virus Ankara (MVA-64). The immunogenicity of MVA-64 was compared with that of the Western Reserve strain of VV (VVWR-64). The replication-defective MVA-64 was as efficient as VVWR-64 in inducing specific antibodies and cytolytic T-cell responses to a defined H-2-Db-restricted epitope on MTP-64. In addition, priming with MPT64-expressing plasmid DNA (DNA-64), and boosting with either MVA-64 or VVWR-64, markedly enhanced MPT64-specific cytolytic and IFN-gamma-producing CD8+ T-cell responses. These findings suggest that MVA may be a suitable vaccine carrier for stimulating mycobacterium-specific CD8+ T-cell responses and may be particularly relevant for developing vaccines for use in regions endemic for tuberculosis and HIV infection.

Association of NRAMP1 polymorphism with leprosy type but not susceptibility to leprosy per se in west Africans

Twin and family studies indicate that host genetic factors influence susceptibility to leprosy and, possibly, leprosy type. Murine studies have suggested a role for the natural resistance-associated macrophage protein 1 (Nramp1) gene, which can influence cellular immune responses to intracellular pathogens. We evaluated a variation in the human homolog, NRAMP1, recently associated with tuberculosis susceptibility in West Africa. A total of 273 patients with leprosy and 201 controls from Mali were genotyped for NRAMP1 polymorphisms previously associated with tuberculosis. No association was found with leprosy per se (P = 0.83), but the NRAMP1 3'-untranslated region 4-bp insertion/deletion polymorphism was associated with leprosy type (P = 0.007). Heterozygotes were more frequent among multibacillary than paucibacillary leprosy cases. Thus, variation in or near the NRAMP1 gene may exert an influence on the clinical presentation of leprosy, possibly by influencing cellular immune response type.

Genetics of susceptibility to human infectious disease

Before Robert Koch's work in the late nineteenth century, diseases such as tuberculosis and leprosy were widely believed to be inherited disorders. Heritability of susceptibility to several infectious diseases has been confirmed by studies in the twentieth century. Infectious diseases, old and new, continue to be an important cause of mortality worldwide. A greater understanding of disease processes is needed if more effective therapies and more useful vaccines are to be produced. As part of this effort, developments in genetics have allowed a more systematic study of the impact that the human genome and infectious disease have on each other.

Direct ex vivo analysis of antigen-specific IFN-gamma-secreting CD4 T cells in Mycobacterium tuberculosis-infected individuals: associations with clinical disease state and effect of treatment

The wide spectrum of clinical outcomes following infection with Mycobacterium tuberculosis is largely determined by the host immune response; therefore, we studied several clinically defined groups of individuals (n = 120) that differ in their ability to contain the bacillus. To quantitate M. tuberculosis-specific T cells directly ex vivo, we enumerated IFN-gamma-secreting CD4 T cells specific for ESAT-6, a secreted Ag that is highly specific for M. tuberculosis, and a target of protective immune responses in animal models. We found that frequencies of circulating ESAT-6 peptide-specific IFN-gamma-secreting CD4 T cells were higher in latently infected healthy contacts and subjects with minimal disease and low bacterial burdens than in patients with culture-positive active pulmonary tuberculosis (p = 0.009 and p = 0.002, respectively). Importantly, the frequency of these Ag-specific CD4 T cells fell progressively in all groups with treatment (p = 0.005), suggesting that the lower responses in patients with more extensive disease were not due to tuberculosis-induced immune suppression. This population of M. tuberculosis Ag-specific Th1-type CD4 T cells appears to correlate with clinical phenotype and declines during successful therapy; these features are consistent with a role for these T cells in the containment of M. tuberculosis in vivo. Such findings may assist in the design and evaluation of novel tuberculosis vaccine candidates.

Unique T cell effector functions elicited by Plasmodium falciparum epitopes in malaria-exposed Africans tested by three T cell assays

Natural immunity to malaria is characterized by low level CD4 T cell reactivity detected by either lymphoproliferation or IFN-gamma secretion. Here we show a doubling in the detection rate of responders to the carboxyl terminus of circumsporozoite protein (CS) of Plasmodium falciparum by employing three T cell assays simultaneously: rapid IFN-gamma secretion (ex vivo ELISPOT), IFN-gamma secretion after reactivation of memory T cells and expansion in vitro (cultured ELISPOT), and lymphoproliferation. Remarkably, for no individual peptide did a positive response for one T cell effector function correlate with any other. Thus these CS epitopes elicited unique T cell response patterns in malaria-exposed donors. Novel or important epitope responses may therefore be missed if only one T cell assay is employed. A borderline correlation was found between anti-CS Ab levels and proliferative responses, but no correlation was found with ex vivo or cultured IFN-gamma responses. This suggested that the proliferating population, but not the IFN-gamma-secreting cells, contained cells that provide help for Ab production. The data suggest that natural immunity to malaria is a complex function of T cell subgroups with different effector functions and has important implications for future studies of natural T cell immunity.

A prime-boost immunisation regimen using DNA followed by recombinant modified vaccinia virus Ankara induces strong cellular immune responses against the Plasmodium falciparum TRAP antigen in chimpanzees

Two chimpanzees were vaccinated intramuscularly against malaria using plasmid DNA expressing the pre-erythrocytic antigens thrombospondin related adhesion protein (PfTRAP) and liver stage specific antigen-1 (PfLSA-1) of Plasmodium falciparum together with GM-CSF protein. A recombinant modified vaccinia virus Ankara (MVA) expressing PfTRAP was injected intramuscularly 6 weeks later to boost the immune response. This sequence of antigen delivery induced a specific and long-lasting T cell and antibody response to PfTRAP as detected by ELISPOT assay and ELISA. Antibody responses were detected after four DNA injections, and were boosted by injection of recombinant MVA expressing PfTRAP. Interferon-gamma secreting antigen-specific T cells were detected in both animals, but only after boosting with recombinant MVA. By screening a panel of PfTRAP-derived peptides, an epitope was identified that was recognized by cytotoxic T lymphocytes in one of the chimpanzees studied. T cells specific for this epitope were present in PBMCs and liver-infiltrating lymphocytes at a frequency of between 1 in 200 and 1 in 500. The high immunogenicity of this prime-boost regimen in chimpanzees supports further assessment of this delivery strategy for the induction of protection against P. falciparum malaria in humans.