The immune recognition of malaria antigens

Identification and characterization of Neospora caninum cyclophilin that elicits gamma interferon production

Gamma interferon (IFN-gamma) response is essential to the development of a host protective immunity in response to infections by intracellular parasites. Neosporosis, an infection caused by the intracellular protozoan parasite Neospora caninum, is fatal when there is a complete lack of IFN-gamma in the infected host. However, the mechanism by which IFN-gamma is elicited by the invading parasite is unclear. This study has identified a microbial protein in the N. caninum tachyzoite N. caninum cyclophilin (NcCyP) as a major component of the parasite responsible for the induction of IFN-gamma production by bovine peripheral blood mononuclear cells (PBMC) and antigen-specific CD4(+) T cells. NcCyP has high sequence homology (86%) with Toxoplasma gondii 18-kDa CyP with a calculated molecular mass of 19.4 kDa. NcCyP is a secretory protein with a predicted signal peptide of 17 amino acids. Abundant NcCyP was detected in whole-cell N. caninum tachyzoite lysate antigen (NcAg) and N. caninum tachyzoite culture supernatant. In N. caninum tachyzoite culture supernatant, three NcCyP bands of 19, 22, and 24 kDa were identified. NcAg stimulated high levels of IFN-gamma production by PBMC and CD4(+) T cells. The IFN-gamma-inducing effect of NcAg was blocked by cyclosporine, a specific ligand for CyP, in a dose-dependent manner. Furthermore, cyclosporine abolished IFN-gamma production by PBMC from naïve cows as well as PBMC and CD4(+) T cells from infected/immunized cows. These results indicate that the N. caninum tachyzoite naturally produces a potent IFN-gamma-inducing protein, NcCyP, which may be important for parasite survival as well as host protection.

Regulation of immunity to malaria: valuable lessons learned from murine models

A major advance in immunology has been the establishment of a framework for analysing how certain immune responses dominate following exposure to a particular pathogen or antigen. CD4(+) T helper (Th) cells can be separated into two major subsets which mediate qualitatively distinct cell-mediated (Th1) and humoral (Th2) immune responses. Immunity to most pathogens can be broadly categorized into a predominant protective response of either type. A characteristic of murine malarias is that primary infections with asexual erythrocytic parasites (the pathogenic stage of the malaria life cycle) generate a host protective immune response with a broad spectrum of Th1- and Th2-type CD4(+) T-cell involvement and so can be examined as models of the interaction of Th1 and Th2 cells during an immune response to an infectious agent. Andrew Taylor-Robinson here describes recent events in the dissection of the mechanisms responsible for the generation of protective immunity to Plasmodium chabaudi chabaudi and other experimental malarias in mice.

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.

Heritability and segregation analysis of immune responses to specific malaria antigens in Papua New Guinea

Familial patterns of inheritance of immune responses to specific Plasmodium falciparum antigens were studied in 214 adults in an area of Papua New Guinea highly endemic for malaria. Preliminary variance component analysis indicated familial aggregation in both humoral and cellular immune responses against the ring-infected erythrocyte surface antigen (RESA) and the FC27 allele of the Merozoite surface antigen 2 (MSA-2). Including a term for sharing houses in the models affected only the antibody response to RESA. Segregation analysis of the antibody responses against RESA indicated inheritance via a multifactorial model and analysis of the proliferation response suggested a possible recessive major gene. The best fitting models for the immune responses against MSA-2 (FC27) postulated dominant major gene inheritance. We found no significant associations between HLA class I or II alleles and these two antigens in this population. Although there was evidence of familial aggregation of antibody responses to MSA-2 (3D7), the segregation analysis failed to identify a mode of inheritance. There was little or no heritability of either humoral or cellular immune responses against the NANP repeats of the Circumsporozoite protein (NANP), the synthetic malaria vaccine SPf66, or a preparation of MSA-2 (3D7) from which the repetitive part was deleted (MSA-2 (d3D7)). Although it is often difficult to separate genetic effects from the effects of living in the same environment, it appears that some immune responses against certain malaria antigens may be partly influenced by genetic factors.

Identification of a novel antigenic domain of Plasmodium falciparum merozoite surface protein-1 that specifically binds to human erythrocytes and inhibits parasite invasion, in vitro

Merozoite surface protein 1 (MSP-1) of Plasmodium falciparum is a promising candidate for vaccine development against malaria. Identification of protective epitopes within MSP-1 is an important step towards the elucidation of mechanisms of parasitic invasion and for the creation of a multi-subunit vaccine. In this study, we show that a 115 amino acid region (p115MSP-1) within the p38 domain of MSP-1 can: (i) specifically bind to human erythrocytes, independent of glycophorin A; (ii) inhibit parasite invasion at significant levels, in vitro; and (iii) be recognized by human sera of individuals from malaria-endemic regions of Africa. More importantly, we also show that polyclonal antibodies specific to this region prevent parasite invasion at levels approaching 90%, in vitro. Our data illustrate that not only is p115MSP-1 involved in parasite recognition/invasion of human erythrocytes, but that this region is highly antigenic, producing high titer antibodies. The delineation of the role of MSP-1 in parasite invasion is an important component of the development of a multi-subunit malaria vaccine, and this study identifies a candidate antigen in this context.

Genetic epidemiology of host predisposition microfilaraemia in human loiasis

Evidence is accumulating from experimental and human studies that genetic factors are involved both in the control of infectious diseases and in the regulation of infection levels and clinical presentation. So far few studies have investigated the role of these genetic factors in human infection by the filarial parasite Loa loa. We present a segregation analysis on 74 nuclear families who live in the tropical rainforest of southern Cameroun and are exposed to homogeneous loiasis transmission. The results indicate that there is a genetic predisposition to be microfilaraemic and that predisposed subjects might be genetically unable to mount an efficient immune response against loiasis antigens. This individual susceptibility could explain at least in part why the prevalence of infection (microfilaraemic individuals) does not usually exceed 30% of the exposed population in hyperendemic regions. Further genetic studies, based on linkage analysis using both familial information and genetic markers, will help to identify the nature of the genetic factors predisposing to microfilaraemia.

Naturally acquired cellular immune responses to the synthetic malarial peptide SPf66 in children in Papua New Guinea

A prospective longitudinal study to examine the relationship between cellular immune responses to the synthetic malarial peptide SPf66 and malaria infection and morbidity was carried out in 187 children aged 0.5-15 years in the Wosera area of Papua New Guinea. Cellular responses were assessed by proliferation and stimulation of cytokines representing the Th1 and Th2 cell subsets (interferon gamma [IFN gamma] and interleukin-4 [IL-4]. Most children (66%) did not respond to SPf66 by any measure. Among the responders, the highest response was obtained for IL-4 (19%) followed by IFN gamma (10%), and the least for proliferation (5%). Analyses of the relation of T cell response to malaria infection showed that the IFN gamma response to SPf66 was positively correlated with parasite density (r = 0.27, P = 0.001). There was no association between the cellular response to SPf66 and concurrent or subsequent malaria morbidity, whichever clinical definition was used. Thus none of these cellular immune responses predicted efficacy of SPf66 in this highly endemic area.

Infectious diseases and immunity: special reference to major histocompatibility complex

Human leukocyte antigens (HLAs) are an inherent system of alloantigens, which are the products of genes of the major histocompatibility complex (MHC). These genes span a region of approximately 4 centimorgans on the short arm of human chromosome 6 at band p 21.3 and encode the HLA class I and class II antigens, which play a central role in cell-to-cell interaction in the immune system. These antigens interact with the antigen-specific cell surface receptors of T lymphocytes (TCR) thus causing activation of the lymphocytes and the resulting immune response. Class I antigens restrict cytotoxic T-cell (CD8+) function thus killing viral infected targets, while class II antigens are involved in presentation of exogenous antigens to T-helper cells (CD4+) by antigen presenting cells (APC). The APC processes the antigens, and the immunogenic peptide is then presented at the cell surface along with the MHC molecule for recognition by the TCR. Since the MHC molecules play a central role in regulating the immune response, they may have an important role in controlling resistance and susceptibility to diseases. In this review we have highlighted studies conducted to look for an association between HLA and infectious diseases; such studies have had a variable degree of success because the pathogenesis of different diseases varies widely, and most diseases have a polygenic etiology.

Efficacy trial of malaria vaccine SPf66 in Gambian infants

SPf66 malaria vaccine is a synthetic protein with aminoacid sequences derived from pre-erythrocytic and asexual blood-stage proteins of Plasmodium falciparum. SPf66 was found to have a 31% protective efficacy in an area of intensive malaria transmission in Tanzanian children, 1-5 years old. We report a randomised, double-blind, placebo-controlled trial of SPf66 against clinical P falciparum malaria in Gambian infants. 630 children, aged 6-11 months at time of the first dose, received three doses of SPf66 or injected polio vaccine (IPV). Morbidity was monitored during the following rainy season by means of active and passive case detection. Cross-sectional surveys were carried out at the beginning and at the end of the rainy season. An episode of clinical malaria was defined as fever (> or = 37.5 degrees C) and a parasite density of 6000/microL or more. Analysis of efficacy was done on 547 children (316 SPf66/231 IPV). No differences in mortality or in health centre admissions were found between the two groups of children. 347 clinical episodes of malaria were detected during the three and a half months of surveillance. SPf66 vaccine was associated with a protective efficacy against the first or only clinical episode of 8% (95% CI -18 to 29, p = 0.50) and against the overall incidence of clinical episodes of malaria of 3% (95% CI -24 to 24, p = 0.81). No significant differences in parasite rates or in any other index of malaria were found between the two groups of children. The findings of this study differ from previous reports on SPf66 efficacy from South America and from Tanzania. In The Gambia, protection against clinical attacks of malaria during the rainy season after immunisation in children 6-11 months old at time of the first dose was not achieved.

Epidemiology of canine leishmaniasis: a comparative serological study of dogs and foxes in Amazon Brazil

The paper describes a longitudinal field study of canine leishmaniasis in sympatric domestic dog and crab-eating fox (Cerdocyon thous) populations. Dogs were studied in house-to-house surveys, and foxes were studied by live-trapping and radio-telemetry. Because serological tests (IFAT in this case) for leishmaniasis are often of uncertain sensitivity and specificity, we draw conclusions comparatively. Both cross-sectional (age-prevalence) and longitudinal analyses indicate that incidence in dogs was highest in the dry season. Seasonal changes in the age-prevalence relationship for dogs suggest that serological conversion and recovery rates decline with prior exposure to infection, where 'recovery' may be due to loss of a positive antibody titre or death from leishmaniasis. The mean incidence in dogs was higher in the rural than in the urban population and higher in hunting dogs than pet dogs. Cross-sectional and longitudinal analyses of fox serological data suggest that foxes remain positive for longer than dogs on average, either because detectable antibody is more persistent or because they experience a lower mortality rate due to leishmaniasis.

Antibody response against Plasmodium falciparum exoantigens and somatic antigens: a longitudinal survey in a rural community in Rondônia, western Brazilian Amazon

Three clinical and sero-epidemiological cross-sectional surveys involving 50 subjects were performed at six-month intervals in Urupá, a rural community characterized by unstable malaria transmission, situated in Rondônia State, Western Brazilian Amazon. Between the surveys, a clinically and parasitologically passive surveillance was established in this community and 48 malaria attacks (28 due to Plasmodium falciparum and 20 due to Plasmodium vivax) were recorded in this cohort of 50 subjects. Serum samples were collected at each survey and tested by enzyme immunoassay (ELISA) for IgG, IgG subclass and IgM antibodies against P. falciparum exoantigens isolated from culture supernatants and detergent-soluble somatic antigens. As expected, both anti-malarial IgG and IgM antibody titres were shown to rise after a malaria outbreak observed during the follow-up period. Nevertheless, in marked contrast with the profile of anti-malarial IgG subclasses described for semi-immune Africans, in this Amazonian community IgG2 antibodies (that are non-cytophilic) against both antigens were shown to predominate over other IgG subclasses. Such overall predominance of IgG2 subclass titres was statistically significant concerning exoantigens, but was of borderline significance in relation to IgG1 antibodies against somatic antigens (p = 0.052). Moreover, highly variable patterns of boosting were observed in antibody responses against both antigens among the patients who suffered P. falciparum malaria attack during the study.

The major histocompatibility complex and peptide vaccines in domestic animals

Three hypotheses are suggested to explain the phenomenon of low responsiveness in domestic animals after injection of peptide vaccines. The first hypothesis proposes involvement of MHC haplotype and the special case in livestock breeding, where inheritance of the sire's haplotype can be closely examined by injection of antigen into a large number of paternal half-sib progeny. The second hypothesis examines the effect of repeated antigen injections in overcoming age and MHC haplotype effects and distinguishing these effects from those caused by deficiencies in the T cell repertoire. The third hypothesis concerns non-MHC effects that influence the expression of MHC haplotype effects and enable the host to mount an effective immune response. It is suggested that the antigen recognition signal from T cell receptor/MHC interaction is amplified to a varying extent in animal populations. Deficiency in this amplification through myeloid cell or cytokine responses may be yet another factor limiting immune responsiveness.

Th1 and Th2 CD4+ T cell clones specific for Plasmodium chabaudi but not for an unrelated antigen protect against blood stage P. chabaudi infection

The host protective immune response to blood stage malaria infection was studied using Plasmodium chabaudi chabaudi (P.chabaudi) in NIH mice. It has been shown previously that CD4+ cells are critically required for protection against erythrocytic infection. Mice lacking a functional CD4+ cell compartment suffer unremitting patent primary parasitemias for at least 60 days after infection. Here, we report that the adoptive transfer of eight P. chabaudi-specific CD4+ T cell clones of either the Th1 or Th2 type to mice rendered CD4-depleted by adult thymectomy and anti-CD4 monoclonal antibody therapy fully restored the ability of recipients to control challenge infection. Control Th1 and Th2 clones specific for an unrelated antigen, ovalbumin, were unable to confer a comparable level of protection in CD4-depleted mice, even though they received regular doses of the antigen. These data demonstrate that protective immunity to asexual P. chabaudi parasites can be mediated through immune CD4+ T cell clones of either the Th1 or the Th2 subset.

High and low responsiveness to vaccines in farm animals

Low responsiveness in some farm animals is emerging as a problem in the application of newly developed vaccines which operate at skin surfaces and mucous membranes. Breeding for resistance to specific diseases seems to be associated with breeding for specific immune responsiveness in farm animals and very likely this involves selection for major histocompatibility complex (MHC) haplotype. However, other factors contribute to low responsiveness and these include poor nutrition, sire effects, antigenic competition and inadequate effector mechanisms. The future of newly developed vaccines will rest on the solution to the low responder problem, and once solved, the application of these vaccines will be fully utilized for disease control in farm animals.

Exploiting structural differences among heteroduplex molecules to simplify genotyping the DQA1 and DQB1 alleles in human lymphocyte typing

A novel approach to DNA probe hybridization and heteroduplex analysis, termed directed heteroduplex analysis (DHDA) is presented here to illustrate its utility in simplification of human lymphocyte antigen (HLA)-typing. By strategic labeling of single-stranded probe sequences, DHDA allows the identification of specific heteroduplex structures that contribute to the differentiation of DQA1 and DQB1 alleles. Because of the high degree of polymorphism among major histocompatibility complex class II second exon sequences, this analysis of 50 different heteroduplex molecules provides evidence of the importance of unpaired bases and mismatched base pairs and their effect on heteroduplex electrophoretic-mobility differences. This strategy is further used to genotype accurately a family for DQA1 which was previously analyzed by sequence specific oligonucleotide (SSO) probe hybridization. To differentiate by SSO-typing among the DQA1 and DQB1 alleles analyzed in this study requires the use of 23 different probes. Equivalent results are obtained by DHDA using only three probes. Therefore, this study suggests that accurate HLA-typing can be simplified by DHDA. Additionally, DHDA may be useful for differentiation of DNA sequence polymorphisms in other genetic systems.

Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia

Preclinical and clinical studies have established the safety and immunogenicity of the chemically synthesised SPf66 malaria vaccine. The present study is a phase III randomised, double-blind, placebo-controlled, efficacy trial completed in La Tola, Colombia. 1548 volunteers over one year of age received three doses of either the vaccine (n = 738) or placebo (n = 810). Active and passive case detection methods were used to document clinical episodes of malaria among the study population. The follow-up period began one month after the third dose and lasted for one year. 168 and 297 episodes of Plasmodium falciparum malaria were documented in the SPf66 group and the placebo group, respectively; this corresponds to a crude protective efficacy of 38.8%. Incidence rates for first or only P falciparum malarial episodes were 22.3% per annum among the vaccinee group and 33.5% among the placebo group (RR = 1.5; 95% Cl 1.23, 1.84). Therefore, the protective efficacy of SPf66 against first or only episodes was 33.6% (95% Cl 18.8, 45.7), being highest in children aged 1-4 years (77%) and adults older than 45 years (67%). The estimated protective efficacy against second episodes was 50.5% (95% Cl 12.9-71.9). Our study shows that the chemically synthesised SPf66 malaria vaccine is safe, immunogenic, and protective against P falciparum malaria in semi-immune populations subject to natural challenge.

In vitro lymphoproliferative responses to malaria antigens: a prospective study of residents of a holoendemic area with perennial malaria transmission

A longitudinal, prospective study to examine the relationship between the outcome of infection with Plasmodium falciparum parasites and in vitro T-cell proliferative responses to a P. falciparum schizont extract (PfSE) was conducted in a village in south-eastern Gabon, an area where malaria is holoendemic and transmission is intense and perennial. The donor's age was found to have a strong independent influence on all malariometric indices. At the community level, the in vitro lymphoproliferative response to PfSE was bimodal with 30% of the villagers studied showing persistently low responses. The frequency of low or high responders within the study population did not show any consistent relationship with the community parasite rates or the number of either patent parasitaemic episodes or clinical malarial attacks per individual. At the individual donor level, the response was negatively correlated with P. falciparum parasite density in those donors who were parasitaemic at the time of sampling. High in vitro lymphoproliferative responses to PfSE were predictive of resistance to clinical malaria. The PfSE-induced in vitro lymphoproliferative response was dependent on antigen presenting cells, CD4+ T-cells and UCHL-1+ cells.

In vitro immune recognition of synthetic peptides from the Plasmodium falciparum CS protein by individuals naturally exposed to different sporozoite challenge

The impact of duration and intensity of sporozoite challenge on the in vitro cell immune response to synthetic peptides of the circumsporozoite (CS) protein of Plasmodium falciparum was investigated in residents of a malaria endemic area in Burkina Faso (West Africa). Lymphocyte proliferation and interferon-gamma (IFN-gamma) production were used to assess immune recognition of synthetic peptides corresponding to the polymorphic Th2R and Th3R regions, to the conserved CS.T3 sequence and to NANP and degenerate NVDP repeats. Immune responses were measured in adults and children from a village where they received more than 100 sporozoite inoculations per year and in adults living in a town, exposed to a 10-100 times lower challenge. A lifetime intense exposure apparently increased the ability to proliferate in response to most peptides in the rural adults, who all produced antibodies to NANP repeats. Surprisingly, cell cultures from these subjects seldom contained appreciable levels of IFN-gamma. In the urban adults, possibly due to the moderate challenge they are exposed to, significant differences in the proliferative potentials of the peptides could be detected. The highest stimulation indices were obtained with the genetically unrestricted CS.T3 peptide. Remarkably, proliferative responses to Th2R and Th3R appeared to be correlated with the humoral response to the CS protein, indicating a T helper significance of the epitopes. The differing proliferative potential of the polymorphic epitopes in the urban adults suggests that polymorphism might delay the development of immune responsiveness under conditions of sporadic transmission. The children from the highly malarious village displayed the lowest proliferative scores, accompanied by a high prevalence of antibodies to NANP repeats. On the basis of these findings, the hypothesis is proposed that a pure B cell reactivity to NANP repeats could ontogenetically precede the mounting of a conventional T-B cooperative immune response.

T-cell responses in malaria

Malaria is caused by infection with protozoan parasites of the genus Plasmodium. It remains one of the most severe health problems in tropical regions of the world, and the rapid spread of resistance to drugs and insecticides has stimulated intensive research aimed at the development of a malaria vaccine. Despite this, no efficient operative vaccine is currently available. A large amount of information on T-cell responses to malaria antigens has been accumulated, concerning antigens derived from all stages of the parasite life cycle. The present review summarizes some of that information, and discusses factors affecting the responses of T cells to malaria antigens.

HLA-DR and -DQ gene polymorphism in West Africans is twice as extensive as in north European Caucasians: evolutionary implications

The HLA genes are the most polymorphic coding loci known in humans. DRB-DQA-DQB gene polymorphism was investigated by Taq I restriction fragment length polymorphism analysis in more than 700 West Africans and found to be almost twice as extensive in West Africans as in North European Caucasians. This finding indicates that Africans comprise the oldest and genetically most diverse human population and supports the hypothesis of the occurrence of a population bottleneck in the emergence of the White race. As in Caucasians, less than one-third of possible cis-encoded DQA-DQB combinations were encountered, indicating constraints on the pairing of DQ alpha and beta polypeptides. Heterozygote advantage (i.e., positive selection) was found for DRB, DQA, and DQB alleles as well as for DQA-DQB combinations. However, in West Africans as well as in North Europeans the observed frequencies of DRB-DQA-DQB homozygotes were close to neutrality expectations. Although the hypothesis that HLA polymorphism is maintained by parasite-driven overdominant selection is attractive, there is little evidence to support that view. We propose instead that one of the forces maintaining a low frequency of HLA homozygotes might be a decreased likelihood of potentially autoreactive T-cell clones escaping thymic selection in HLA heterozygotes. This would be consistent with the central role of HLA molecules as self/non-self discriminators.

Common west African HLA antigens are associated with protection from severe malaria

A large case-control study of malaria in West African children shows that a human leucocyte class I antigen (HLA-Bw53) and an HLA class II haplotype (DRB1*1302-DQB1*0501), common in West Africans but rare in other racial groups, are independently associated with protection from severe malaria. In this population they account for as great a reduction in disease incidence as the sickle-cell haemoglobin variant. These data support the hypothesis that the extraordinary polymorphism of major histocompatibility complex genes has evolved primarily through natural selection by infectious pathogens.