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

Importance of CD8 T cell-mediated immune response during intracellular parasitic infections and its implications for the development of effective vaccines

Obligatory intracellular parasites such as Plasmodium sp, Trypanosoma cruzi, Toxoplasma gondii and Leishmania sp are responsible for the infection of hundreds of millions of individuals every year. These parasites can deliver antigens to the host cell cytoplasm that are presented through MHC class I molecules to protective CD8 T cells. The in vivo priming conditions of specific CD8 T cells during natural infection are largely unknown and remain as an area that has been poorly explored. The antiparasitic mechanisms mediated by CD8 T cells include both interferon-gamma-dependent and -independent pathways. The fact that CD8 T cells are potent inhibitors of parasitic development prompted many investigators to explore whether induction of these T cells can be a feasible strategy for the development of effective subunit vaccines against these parasitic diseases. Studies performed on experimental models supported the hypothesis that CD8 T cells induced by recombinant viral vectors or DNA vaccines could serve as the basis for human vaccination. Regimens of immunization consisting of two different vectors (heterologous prime-boost) are much more efficient in terms of expansion of protective CD8 T lymphocytes than immunization with a single vector. The results obtained using experimental models have led to clinical vaccination trials that are currently underway.

Parasitic exploitation as an engine of diversity

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant-pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Parasites and immune responses: memory illusion?

Immunological memory responses to intracellular protozoa and extracellular helminths govern host resistance and susceptibility to reinfection. Humans and livestock living in parasitic disease endemic regions face continuous exposure from a very early age that often leads to asymptomatic chronic infection over their entire lifespan. Fundamental immunological studies suggest that the generation of T-cell memory is driven by tightly coordinated innate and adaptive cellular immune responses rapidly triggered following initial host infection. A key distinguishing feature of immune memory maintenance between the majority of parasitic diseases and most bacterial or viral diseases is long-term antigen persistence. Consequently, functional parasite immune memory is in a continuous, dynamic flux between activation and deactivation producing functional parasite killing or functional memory cell death. In this sense, T-cell immune memory can be regarded as "memory illusion." Furthermore, due to the finite capacity of memory lymphocytes to proliferate, continuous parasite antigen stimulation may exceed a threshold level at some point in the chronically infected host. This may result in suboptimal effector immune memory leading to host susceptibility to reinfection, or immune dysregulation yielding disease reactivation or immune pathology. The goal of this review is to highlight, through numerous examples, what is currently known about T-cell immune memory to parasites and to provide compelling hypotheses on the survival and maintenance of parasite "memory illusion." These novel concepts are discussed in the context of rationale parasite vaccine design strategies.

Peptide-based analysis of the amino acid sequence important to the immunoregulatory function of Trypanosoma cruzi Tc52 virulence factor

The intracellular protozoan parasite Trypanosoma cruzi is the aetiological agent of Chagas' disease. We have previously identified a T. cruzi-released protein called Tc52, which is crucial for parasite survival and virulence. In the present study, we attempted to define the Tc52 epitope(s) responsible for its immunoregulatory function. A naturally occurring major peptide fragment of molecular mass 28 kDa (Tc28k) was identified, which was localized in the C-terminal portion of Tc52 and was inhibitory for T-cell activation. Synthetic peptides corresponding to amino acid sequences in Tc52 were evaluated for their ability to modulate T-cell proliferation and cytokine production. Results obtained using five peptides spanning the N-terminal or C-terminal domain of the Tc52 protein indicated that the activity mapped to Tc52 residues 432-445. Moreover, it was found that the peptide, when coupled to a carrier protein (ovalbumin), exhibited increased inhibitory activity on T-lymphocyte activation. Incubation with 8 nm ovalbumin-coupled peptide 432-445 resulted in approximately the same levels (>75%) of inhibition of T-cell proliferation as 5 micro g/ml Tc28k. Furthermore, we showed that the coupled peptide significantly down-regulated the secretion of interferon-gamma (IFN-gamma) and interleukin-2 (IL-2). Likewise, in immunized mice, the coupled peptide 432-445 was a very poor B- and T-cell antigen compared with the other Tc52-derived peptides. These results suggest that the immunomodulatory portion of the T. cruzi Tc52 virulent factor may reside, at least in part, in a conserved sequence within its C-terminal domain, which could minimize its antigenicity.

Measuring immune selection

Immune responses that kill pathogens or reduce their reproductive rate are generally important in protecting hosts from infection and disease. Pathogens that escape the full impact of such responses will survive, and any heritable genetic basis of this evasion will be selected. Due to the memory component of vertebrate immune responses, pathogens with rare alleles of a target antigen can have an advantage over those with common alleles, leading to the maintenance of a polymorphism. At the genetic level, there ought to be detectable signatures of balancing selection in the genes encoding these antigens. Here, methods for identifying these selective signatures are reviewed. Their practical utility for identifying which antigens are targets of protective immune responses is discussed.

Could TCR antagonism explain associations between MHC genes and disease?

Alleles of major histocompatibility complex (MHC) loci are associated with certain types of diseases, including those of infectious and autoimmune origin. MHC products can promote susceptibility or resistance to disease by stimulating or inhibiting immune responses. Recent evidence suggests that MHC-associated peptides derived from self-proteins can act as antagonists of T-cell activation, thereby inhibiting immune responses to antigens. We suggest that self-peptide-promoted antagonism might explain some associations between MHC alleles and particular chronic diseases.

Island and taxon effects in parasitism revisited: avian malaria in the Lesser Antilles

We identify and describe the distribution of 12 genetically distinct malaria parasite lineages over islands and hosts in four common passerine birds in the Lesser Antilles. Combined parasite prevalence demonstrates strong host effects, little or no island effect, and a significant host-times-island interaction, indicating independent outcomes of host-parasite infections among island populations of the same host species. Host- and/or island-specific parasite lineages do not explain these host-parasite associations; rather, individual lineages themselves demonstrate the same type of independent interactions. Unlike overall prevalence, individual parasite lineages show considerable geographic structure (i.e., island effects) as well as species effects indicating that parasite lineages are constrained in their ability to move between hosts and locations. Together, our results suggest an upper limit to the number of host individuals that malaria parasites, as a community, can infect. Within this limit, however, the relative frequency of the different lineages varies reflecting fine scale interactions between host and parasite populations. Patterns of host-parasite associations within this system suggest both historical co-evolution and ecologically dynamic and independent host-parasite interactions.

The association of HLA -A, -B, and -DRB1 genotypes with Graves' disease in Taiwanese people

Graves' disease has been associated with different human leukocyte antigen (HLA) genes in different races. To evaluate the association of HLA type in Taiwanese with Graves' disease, the HLA-A, -B, and -DRB1 alleles in a total of 236 Taiwanese adults with Graves' disease and 533 racially matched normal control subjects were examined using the PCR-SSOP (sequence specific oligonucleotide probe) technique. The prevalence of HLA-A*0207, -B*2704, -B*4601, and -DRB1*0901 among patients with Graves' disease was found to be increased, with odds ratios (OR) of 2.21, 3.82, 1.76 and 1.62, respectively. However, after correction for multiple comparisons, the relative risk of HLA-A*0207 susceptibility to Graves' disease remained statistically significant and the haplotype HLA-A*3303 -B*5801 -DRB1*0301 had a significantly protective effect. None of the other 2- or 3-locus haplotypes showed any significantly increased risk. Although HLA-DRB1*1405 showed an increased relative risk in patients with GO (Graves' opthalmopathy) (OR 4.61) when compared with patients without GO, the relative risk after adjusting for the number of comparisons was not significant. Taiwanese patients with Graves' disease have HLA-associated susceptibility genes which are similar to those found in Chinese patients in Hong Kong and Singapore. However, the finding in this study of a higher frequency of HLA-A*0207 in Taiwanese with Graves' disease has not been documented in any other ethnic group.

T cells as mediators of protective immunity against liver stages of Plasmodium

T cells from different subsets play a major role in protective immunity against pre-erythrocytic stages of malaria parasites. Exposure of humans and animals to malaria sporozoites induces (alphabeta CD8(+) and CD4(+) T cells specific for antigens expressed in pre-erythrocytic stages of Plasmodium. These T cells inhibit parasite development in the liver, and immunization with subunit vaccines expressing the respective antigenic moieties confers protection against sporozoite challenge. gammadelta and natural killer T cells can also play a role in protective immunity. Recent studies with mice transgenic for the alphabeta T-cell receptor have revealed the existence of complex mechanisms regulating the induction and development of these responses.

Malaria blood stage suppression of liver stage immunity by dendritic cells

Malaria starts with Plasmodium sporozoites infection of the host's liver, where development into blood stage parasites occurs. It is not clear why natural infections do not induce protection against the initial liver stage and generate low CD8+ T cell responses. Using a rodent malaria model, we show that Plasmodium blood stage infection suppresses CD8+ T cell immune responses that were induced against the initial liver stage. Blood stage Plasmodium affects dendritic cell (DC) functions, inhibiting maturation and the capacity to initiate immune responses and inverting the interleukin (IL)-12/IL-10 secretion pattern. The interaction of blood stage parasites with DCs induces the secretion of soluble factors that inhibit the activation of CD8+ T cells in vitro and the suppression of protective CD8+ T cell responses against the liver stage in vivo. We propose that blood stage infection induces DCs to suppress CD8+ T cell responses in natural malaria infections. This evasion mechanism leaves the host unprotected against reinfection by inhibiting the immune response against the initial liver stage of the disease.

Plasmodium falciparum multiple infections in Mozambique, its relation to other malariological indices and to prospective risk of malaria morbidity

We describe the frequency of Plasmodium falciparum clones infecting individuals living in a rural area of southern Mozambique and analyse the relationship between multiplicity of infection, age and other malariometric indices, including prospective risk of clinical malaria. The genotyping was based on the use of restriction fragment length polymorphism-polymerase chain reaction (RFLP-PCR) analysis of P. falciparum merozoite surface protein 2 (msp2). We analysed 826 samples collected during five cross-sectional surveys from residents of Manhiça ranging in age from 4 months to 83 years. We also determined the multiplicity of infection in samples obtained from 6-month-old infants (n = 79) and children <10 years (n = 158) who were then treated and followed prospectively for 1 year or 75 weeks, respectively. Multiplicity of infection did not vary significantly during the first year of life, but increased thereafter, and decreased during adulthood to the levels found in infants. With increasing multiplicity of infection, there was a statistically significant decrease in the risk of submicroscopic infections. There was also a significant correlation between multiplicity of infection and parasite density in infants, children <4 years of age and adults, suggesting that high densities increase the probability of discriminating more clones in complex infections. We found that the relationship between multiple infections and malaria morbidity is age-dependent. In infants, the risk of subsequent episodes of clinical malaria was related to the parasite density but not to baseline multiplicity of infection. In older children, however, the more clones a child carried, the more likely they were to have a clinical malaria episode, and this was true after adjusting for parasite densities. This change in the association between multiplicity and risk of clinical malaria may indicate a shift in the host response to P. falciparum.

The molecular epidemiology of malaria

This review discusses how the use of molecular genetic techniques such as the polymerase chain reaction are helping in the management and prevention of malaria.

Biological and biomedical implications of the co-evolution of pathogens and their hosts

Co-evolution between host and pathogen is, in principle, a powerful determinant of the biology and genetics of infection and disease. Yet co-evolution has proven difficult to demonstrate rigorously in practice, and co-evolutionary thinking is only just beginning to inform medical or veterinary research in any meaningful way, even though it can have a major influence on how genetic variation in biomedically important traits is interpreted. Improving our understanding of the biomedical significance of co-evolution will require changing the way in which we look for it, complementing the phenomenological approach traditionally favored by evolutionary biologists with the exploitation of the extensive data becoming available on the molecular biology and molecular genetics of host-pathogen interactions.

Short-term antigen presentation and single clonal burst limit the magnitude of the CD8(+) T cell responses to malaria liver stages

Malaria sporozoites induce swift activation of antigen-specific CD8(+) T cells that inhibit the intracellular development of liver-stage parasites. The length of time of functional in vivo antigen presentation, estimated by monitoring the activation of antigen-specific CD8(+) T cells, is of short duration, with maximum T cell activation occurring within the first 8 h after immunization and lasting approximately 48 h. Although the magnitude of the CD8(+) T cell response closely correlates with the number of parasites used for immunization, increasing the time of antigen presentation by daily immunizations does not enhance the magnitude of this response. Thus, once a primary clonal burst is established, the CD8(+) T cell response becomes refractory or unresponsive to further antigenic stimulation. These findings strongly suggest that the most efficient strategy for the induction of primary CD8(+) T cell responses is the delivery of a maximal amount of antigen in a single dose, thereby ensuring a clonal burst that involves the largest number of precursors to become memory cells.

Identification of two cerebral malaria resistance loci using an inbred wild-derived mouse strain

Malaria is a complex infectious disease in which the host/parasite interaction is strongly influenced by host genetic factors. The consequences of plasmodial infections range from asymptomatic to severe complications like the neurological syndrome cerebral malaria induced by Plasmodium falciparum in humans and Plasmodium berghei ANKA in rodents. Mice infected with P. berghei ANKA show marked differences in disease manifestation and either die from experimental cerebral malaria (ECM) or from hemolytic anemia caused by hyperparasitemia (HP). A majority of laboratory mouse strains so far investigated are susceptible to ECM; however, a number of wild-derived inbred strains show resistance. To evaluate the genetic basis of this difference, we crossed a uniquely ECM-resistant, wild-derived inbred strain (WLA) with an ECM susceptible laboratory strain (C57BL/6J). All of the (WLA x C57BL/6J) F(1) and 97% of the F(2) progeny displayed ECM resistance similar to the WLA strain. To screen for loci contributing to ECM resistance, we analyzed a cohort of mice backcrossed to the C57BL/6J parental strain. A genome wide screening of this cohort provided significant linkage of ECM resistance to marker loci in two genetic regions on chromosome 1 (chi(2) = 18.98, P = 1.3 x 10(-5)) and on chromosome 11 (chi(2) = 16.51, P = 4.8 x 10(-5)), being designated Berr1 and Berr2, respectively. These data provide the first evidence of loci associated with resistance to murine cerebral malaria, which may have important implications for the search for genetic factors controlling cerebral malaria in humans.

In vitro correlates of Ld-restricted resistance to toxoplasmic encephalitis and their critical dependence on parasite strain

Resistance to murine toxoplasmic encephalitis has been precisely and definitively mapped to the L(d) class I gene. Consistent with this, CD8(+) T cells can adoptively transfer resistance to toxoplasmic encephalitis. However, cytotoxic CD8(+) T cells, capable of killing class I-matched, infected target cells, are generated during the course of Toxoplasma gondii infection even in mice lacking the L(d) gene. L(d)-restricted killing could not be demonstrated, and the functional correlate of the L(d) gene has therefore remained elusive. Herein, L(d)-restricted killing of T. gondii-infected target cells is demonstrated for the first time. L(d)-restricted killing is critically dependent on the strain of T. gondii and is observed with all the derivatives of type II strains tested, but not with a type I strain. These results have important implications for vaccine development.

Molecular variation of Bothriocephalus acheilognathi Yamaguti, 1934 (Cestoda: Pseudophyllidea) in different fish host species based on ITS rDNA sequences

The molecular variation in Bothriocephalus acheilognathi Yamaguti, 1934 from 11 species of freshwater fish collected in Australia, China, the Czech Republic, England and Hawaii was investigated by determining the nucleotide sequences of the internal transcribed spacer region. The length of the first and second internal transcribed spacer sequences of multiple individuals ranged from 553 to 571 bp and 553 to 615 bp, and the G + C content from 53.1 to 53.5%. The percentage sequence divergence varied between 0 and 0.9% in the ITS1 and 0 and 6.6% in the ITS2, respectively, indicating the occurrence of intraspecific variation. It is demonstrated that the fragment length variation resulted primarily from microsatellite polymorphisms present in the ITS region, especially in the ITS2 region. Phylogenetic analyses revealed that B. acheilognathi examined in this study consisted of three closely related genotypes with certain degrees of host-specificity, and the genotype representing isolates from Cyprinus carpio L. was the most common and diverse form within the species B. acheilognathi.

Susceptibility to experimental cerebral malaria induced by Plasmodium berghei ANKA in inbred mouse strains recently derived from wild stock

The neurological syndrome caused by Plasmodium berghei ANKA in rodents partially mimics the human disease. Several rodent models of cerebral malaria (CM) exist for the study of the mechanisms that cause the disease. However, since common laboratory mouse strains have limited gene pools, the role of their phenotypic variations causing CM is restricted. This constitutes an obstacle for efficient genetic analysis relating to the pathogenesis of malaria. Most common laboratory mouse strains are susceptible to CM, and the same major histocompatibility complex (MHC) haplotype may exhibit different levels of susceptibility. We analyzed the influence of the MHC haplotype on overcoming CM by using MHC congenic mice with C57BL/10 and C3H backgrounds. No correlation was found between MHC molecules and the development of CM. New wild-derived mouse strains with wide genetic polymorphisms were then used to find new models of resistance to CM. Six of the twelve strains tested were resistant to CM. For two of them, F(1) progeny and backcrosses performed with the reference strain C57BL/6 showed a high level of heterogeneity in the number and characteristics of the genetic factors associated with resistance to CM.

Why should parasite resistance be costly?

Parasite resistance is sometimes associated with fitness costs. Costs of resistance are fundamentally important in epidemiology, and in the ecology and evolution of host-parasite interactions. The cost of resistance is often envisioned as the cost of re-allocating limiting resources to resistance machinery from other traits. This popular paradigm has resulted in a spate of research that assumes a fitness cost to resistance. We comment on this trend and propose a working framework of various resistance means and mechanisms. Within these means and mechanisms, we suggest that many are not likely to incur significant fitness costs.

General T-cell receptor antagonists to immunomodulate HLA-A2-restricted minor histocompatibility antigen HA-1-specific T-cell responses

T-cell receptors (TCRs) of a series of minor histocompatibility antigen (mHag) HA-1-specific cytotoxic T-cell (CTL) clones isolated from 3 unrelated patients have been shown to use the same BV6S4A2 segment with conserved amino acids in the CDR3Vbeta region. This suggests that different HA-1-specific TCRs interact similarly to the HA-1 antigen presented by the HLA-A2 molecule. The mHag HA-1 forms an immunogenic complex with HLA-A2 and induces strong alloimmune responses after stem cell transplantation (SCT). It was questioned, therefore, whether clonal and polyclonal HA-1-specific CTL responses can be antagonized by a single TCR antagonistic peptide. Functional analysis and molecular modeling of single and double amino acid substitutions of TCR contact residues, adjacent residues, and HLA-A2 binding residues resulted in 4 peptides with high affinity for HLA-A2 and with the capacity to inhibit the lysis of endogenously HA-1-expressing EBV-BLCL by 3 different HA-1-specific CTL clones. These peptides also efficiently antagonized HA-1-specific polyclonal CTL lines derived from 3 patients and significantly reduced the number of interferon-gamma-producing HA-1-specific CTL of a patient with graft-versus-host disease after HA-1-mismatched SCT. These data show that general TCR antagonists can be developed that inhibit HLA-A2-restricted HA-1-specific CTL responses on the clonal and the polyclonal level and that TCR antagonists may modulate the immunodominant mHag HA-1 responses.

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.

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.

Recombination in the ompA gene but not the omcB gene of Chlamydia contributes to serovar-specific differences in tissue tropism, immune surveillance, and persistence of the organism

Sequences of the major outer membrane protein (MOMP) gene (ompA) and the outer membrane complex B protein gene (omcB) from Chlamydia trachomatis, Chlamydia pneumoniae, and Chlamydia psittaci were analyzed for evidence of intragenic recombination and for linkage equilibrium. The Sawyer runs test, compatibility matrices, and index of association analyses provided substantial evidence that there has been a history of intragenic recombination at ompA including one instance of interspecies recombination between the C. trachomatis mouse pneumonitis strain and the C. pneumoniae horse N16 strain. Although none of these methods detected intragenic recombination within omcB, differences in divergence reported in earlier studies suggested that there has been intergenic recombination involving omcB, and the analyses presented in this study are consistent with this. For C. trachomatis, index-of-association analyses suggested a higher degree of recombination for C class than for B class strains and a higher degree of recombination in the downstream half of ompA. In concordance with these findings, many significant breakpoints were found in variable segments 3 and 4 of MOMP for the recombinant strains D/B120, G/UW-57, E/Bour, and LGV-98 identified in this study. We provide examples of how genetic diversity generated by repeated recombination in these regions may be associated with evasion of immune surveillance, serovar-specific differences in tissue tropism, and persistence.

SEASONALITY, PARASITE DIVERSITY, AND LOCAL EXTINCTIONS IN PLASMODIUM FALCIPARUM MALARIA

We incorporate stochastic, density-dependent seasonal recruitment in adult Anopheles mosquito populations in a discrete-event model of Plasmodium falciparum malaria transmission and find the probabilities of parasite extinction higher than with perennial transmission. Seasonal fluctuations in vector populations act to synchronize the dynamics of infection and immunity in host populations, leading to fluctuations in parasite prevalence greater than expected solely on the basis of high- and low-season vector densities. This synchronization also biases frequencies of infection with multiple parasite phenotypes or genotypes.

Meiotic recombination, cross-reactivity, and persistence in Plasmodium falciparum

We incorporate a representation of Plasmodium falciparum recombination within a discrete-event model of malaria transmission. We simulate the introduction of a new parasite genotype into a human population in which another genotype has reached equilibrium prevalence and compare the emergence and persistence of the novel recombinant forms under differing cross-reactivity relationships between the genotypes. Cross-reactivity between the parental (initial and introduced) genotypes reduces the frequency of appearance of recombinants within three years of introduction from 100% to 14%, and delays their appearance by more than a year, on average. Cross-reactivity between parental and recombinant genotypes reduces the frequency of appearance to 36% and increases the probability of recombinant extinction following appearance from 0% to 83%. When a recombinant is cross-reactive with its parental types, its probability of extinction is influenced by cross-reactivity between the parental types in the opposite manner; that is, its probability of extinction after appearance decreases. Frequencies of P. falciparum outcrossing are mediated by frequencies of mixed-genotype infections in the host population, which are in turn mediated by the structure of cross-reactivity between parasite genotypes. The three leading hypotheses about how meiosis relates to oocyst production lead to quantitative, but no qualitative, differences in these results.

Immunogenetics and genomics

Immunogenetic analysis of disease susceptibility has been encouraged by the identification of strong HLA associations with several diseases of uncertain cause. Weaker HLA associations exist with a large number of infectious and non-infectious diseases and the mechanisms of these effects are beginning to be uncovered. Extensive analyses of non-HLA immunogenetic variants have also been undertaken and associations with a variety of genes identified. Genetic linkage analysis of multicase families has recently identified new major susceptibility loci for a few immunologically determined common diseases. However, the greatest potential for the future lies in genome-wide searches for susceptibility genes that individually might have quite modest effects but cumulatively have a large impact on individual risk. This new era of immunogenomics promises to provide key insights into disease pathogenesis and identify multiple molecular targets for intervention strategies.

Temporal patterns of genetic variation for resistance and infectivity in a Daphnia-microparasite system

Theoretical studies have indicated that the population genetics of host-parasite interactions may be highly dynamic. with parasites perpetually adapting to common host genotypes and hosts evolving resistance to common parasite genotypes. The present study examined temporal variation in resistance of hosts and infectivity of parasites within three populations of Daphnia magna infected with the sterilizing bacterium Pasteuria ramosa. Parasite isolates and host clones were collected in each of two years (1997, 1998) from one population; in two other populations, hosts were collected from both years, but parasites from only the first year. We then performed infection experiments (separately for each population) that exposed hosts to parasites from the same year or made combinations involving hosts and parasites from different years. In two populations, patterns were consistent with the evolution of host resistance: either infectivity or the speed with which parasites sterilized hosts declined from 1997 to 1998. In another population, infectivity, virulence, and parasite spore production did not vary among host-year or parasite-year. For this population, we also detected strong within-population genetic variation for resistance. Thus, in this case, genetic variability for fitness-related traits apparently did not translate into evolutionary change. We discuss a number of reasons why genetic change may not occur as expected in parasite-host systems, including negative correlations between resistance and other traits, gene flow, or that the dynamic process itself may obscure the detection of gene frequency changes.

Fixed, epitope-specific, cytophilic antibody response to the polymorphic block 2 domain of the Plasmodium falciparum merozoite surface antigen MSP-1 in humans living in a malaria-endemic area

The MSP-1 merozoite surface antigen of the human malaria parasite Plasmodium falciparum is a major target of immune response. The domain called block 2 shows extensive allelic diversity, with more than 50 alleles identified, grouped into three allelic families. Presence of anti-block 2 antibodies has been associated with reduced risk for clinical malaria, but whether or not allele-specific antibodies are implicated remains unclear. To study the fine specificity of the human antibody response, we have used a series of 82 overlapping, N-biotinylated, 15-mer peptides scanning reference alleles and including numerous sequence variants. Peptide antigenicity was validated using sera from mice immunized with recombinant proteins. A cross-sectional survey conducted in a Senegalese village with intense malaria transmission indicated an overall 56 % seroprevalence. The response was specific for individuals and unrelated to the HLA type. Each responder reacted to a few peptides, unrelated to the infecting parasite genotype(s). Seroprevalence of each individual peptide was low, with no identifiable immunodominant epitope. Anti-block 2 antibodies were mostly of the IgG3 isotype, consistent with an involvement in cytophilic antibody-mediated merozoite clearance. The number of responders increased with age, but there was no accumulation of novel specificities with age and hence with exposure to an increasingly large number of alleles. A 15-month longitudinal follow up outlined a remarkably fixed response, with identical reactivity profiles, independent of the past or current parasite types, a pattern reminiscent of clonal imprinting. The implications of the characteristics of the anti-block 2 antibody response in parasite clearance are discussed.

Non-conservative substitutions distinguish previously uncharacterized HLA-A molecules

The extent of class I HLA polymorphism is not yet realized, and to provide a glimpse of the HLA-A polymorphism which remains undetected, we have analyzed approximately 3,700 National Marrow Donor Program (NMDP) Donor/Recipient Pair Retrospective Study Samples with HLA-A DNA sequence-based typing (SBT). Seventeen new HLA-A alleles were detected, with a total of 19 nucleotide substitutions distinguishing these new alleles from their closest HLA-A relatives. Nearly all of the new alleles differ by single nucleotide substitutions; a majority of these substitutions can be explained by gene conversion events but 6 alleles likely originated by point mutation. Fifteen of the 19 nucleotide substitutions translate into amino acid differences in the molecule. Structurally, the inferred amino acid alterations were non-conservative in terms of chemical property, and most substitutions were positioned in 1 or more of the specificity pockets which determine peptide binding. Although these new alleles were identified in a primarily Caucasian sample population, 9 of the 17 new HLA-A alleles were found in samples of non-Caucasoid origin. A new allele detection rate of 1 in approximately 200 individuals in our data set would, therefore, be higher in a non-Caucasoid sample population. In summary, the single nucleotide substitutions that distinguish undetected HLA-A alleles translate into functionally distinct HLA-A molecules. Further studies of the role of HLA-A in transplantation, in disease association, and in evolution must therefore accommodate the discovery of new alleles differing by single nucleotides.

Major hematologic diseases in the developing world- new aspects of diagnosis and management of thalassemia, malarial anemia, and acute leukemia

The three presentations in this session encompass clinical, pathophysiological and therapeutic aspects of hematologic diseases which impact most heavily on developing world countries. Dr. Victor Gordeuk discusses new insights regarding the multi-faceted pathogenesis of anemia in the complicated malaria occurring in Africa. He describes recent investigations indicating the possible contribution of immune dysregulation to this serious complication and the implications of these findings for disease management. Dr. Surapol Issaragrisil and colleagues describe epidemiologic and clinical characteristics of the thalassemic syndromes. In addition to being considered a major health problem in Southeast Asia, the migration throughout the world of people from this region has caused the disease to have global impact. A unique thalassemia variant, Hb Ebeta-thalassemia, with distinctive clinical features, has particular relevance for this demographic issue. Special focus will be reported regarding recent prenatal molecular screening methods in Thailand which have proven useful for early disease detection and disease control strategies. Dr. Raul Ribeiro describes a clinical model for providing effective treatment for a complex malignancy (childhood acute lymphoblastic leukemia) in countries with limited resources. With the multidisciplinary approach in Central American of the joint venture between St. Jude Children's Research Hospital International Outreach Program and indigenous health care personnel, major therapeutic advances for this disease have been achieved. Given the major demographic population shifts occurring worldwide, these illnesses also have important clinical implications globally. These contributions demonstrate that lessons learned within countries of disease prevalence aid our understanding and management of a number of disorders prominently seen in developed countries. They will show how effective partnerships between hematologists in more and less developed nations may work together to produce important advances for treating major hematologic diseases in less developed regions. A major focus relates to the socio-economic and medical burden of these diseases in developing countries with limited resources. As such, these problems provide a challenge and an opportunity for collaborative interaction between hematologists and policy makers worldwide.

Antagonism of direct alloreactivity of an HLA-B27-specific CTL clone by altered peptide ligands of its natural epitope

Antagonism of allospecific CTL by altered MHC ligands is a potential approach to specific immunomodulation of allogeneic T cell responses in acute graft rejection and graft-vs-host disease. In this study we have analyzed the capacity of peptide analogs of a natural HLA-B27-allospecific CTL epitope to antagonize direct alloreactivity. Alanine scanning demonstrated that positions 4, 5, and 7 of the peptide epitope were critical for allorecognition. A number of relatively conservative substitutions at each of these positions were then tested for their effect on allorecognition and antagonism. All substitutions at position 5 abrogated cytotoxicity. In contrast, a few changes at positions 4 and 7 were tolerated, indicating a limited flexibility of the allospecific CTL in recognition of peptide epitope variants. Most of the substitutions impairing cytotoxicity actually induced antagonism. However, whereas epitope variants with changes at positions 4 and 7 behaved as weak or intermediate antagonists, some of the variants with changes at position 5 antagonized CTL alloreactivity almost completely. The results in this study demonstrate for the first time that antagonism of direct class I-mediated alloreactivity can be achieved by variants of a natural allospecific peptide epitope.

Generation and characterization of malaria-specific human CD8(+) lymphocyte clones: effect of natural polymorphism on T cell recognition and endogenous cognate antigen presentationby liver cells

CD8(+) cytolytic T lymphocytes (CTL) play a fundamental role in the clearance of malaria parasites from the liver in mouse models. In humans, however, only low levels of parasite-specific CD8(+) T lymphocytes have been observed in individuals living in endemic areas. In the present study, we identified high levels of circulating CD8(+) T lymphocytes specific for a previously described HLA-A2-restricted CTL epitope of the circumsporozoite (CS) protein of Plasmodium falciparum in an adult living in Burkina Faso, as evidenced by IFN-gamma ELISPOT assay and MHC-tetramer technology. After cloning by limiting dilution culture, T cell recognition of natural CS variants of P. falciparum was studied. The results demonstrate that naturally occurring variations drastically affect residues critical for T cell recognition as only two out of nine sequences analyzed were efficiently recognized by the CTL clones. These clones were also used to analyze T cell recognition of the endogenously presented cognate antigen. We observed efficient antigen recognition of both HLA-A*0201-transfected murine antigen presenting cells and liver cells from HLA-A*0201/K(b)-transgenic mice upon infection with recombinant vaccinia virus encoding the CS protein (WR-CS). More importantly, we demonstrate for the first time efficient recognition of WR-CS-infected human liver cells.

The cause of parasitic infection in natural populations of Daphnia (Crustacea: Cladocera): the role of host genetics

Disease patterns in nature may be determined by genetic variation for resistance or by factors, genetic or environmental, which influence the host-parasite encounter rate. Elucidating the cause of natural infection patterns has been a major pursuit of parasitologists, but it also matters for evolutionary biologists because host resistance genes must influence the expression of disease if parasite-mediated selection is to occur. We used a model system in order to disentangle the strict genetic component from other causes of infection in the wild. Using the crustacean Daphnia magna and its sterilizing bacterial parasite Pasteuria ramosa, we tested whether genetic variation for resistance, as determined under controlled conditions, accounted for the distribution of infections within natural populations. Specifically, we compared whether the clonally produced great-granddaughters of those individuals that were infected in field samples (but were subsequently 'cured' with antibiotics) were more susceptible than were the great-granddaughters of those individuals that were healthy in field samples. High doses of parasite spores led to increased infection in all four study populations, indicating the importance of encounter rate. Host genetics appeared to be irrelevant to natural infection patterns in one population. However, in three other populations hosts that were healthy in the field had greater genetic-based resistance than hosts that were infected in the field, unambiguously showing the effect of host genetic factors on the expression of disease in the wild.

Influence of age and HLA type on interferon-gamma (IFN-gamma) responses to a naturally occurring polymorphic epitope of Plasmodium falciparum liver stage antigen-1 (LSA-1)

Antigenic polymorphism and HLA restriction may limit the immunogenicity of a subunit vaccine against liver-stage Plasmodium falciparum. We examined 59 clinical isolates and five laboratory clones of P. falciparum for polymorphism in the N- and C-terminal regions of LSA-1, evaluated binding of the corresponding peptides to selected HLA class I alleles, and measured IFN-gamma responses in residents of a malaria-endemic area of Papua New Guinea where HLA-A*1101, -24, -B13, and -B40 are the most common class I alleles. LSA-1 polymorphism was limited to a single non-synonymous mutation encoding serine (S), proline (P), or threonine (T) at amino acid 85. Nine-mer 84-92 peptides with S, T, or P at the primary anchor position bound differentially to HLA-A11, -A2, and -B7. IFN-gamma ELISPOT responses increased with age in malaria-exposed subjects: 14-16% and 30-36% of 2-5- and 6-54-year-olds, respectively, had > or =10 IFN-gamma-secreting cells/106 peripheral blood mononuclear cells when stimulated with at least one peptide variant (P<0.05). IFN-gamma responses to all three peptides were also greater for older than younger individuals. No children < 3 years old had lymphocytes that responded to all three 84-92 peptides, whereas 45% of adults (mean age 48 years) had aggregated IFN-gamma responses. These data support the notion that age-related cumulative exposure to P. falciparum increases the frequency of IFN-gamma responses to polymorphic epitopes of liver-stage antigens such as LSA-1.

Toxoplasma gondii: identification of a developmentally regulated family of genes related to SAG2

Previous studies have shown the surface of Toxoplasma gondii to be dominated by a family of proteins closely related to SAG1. In this study, we report the existence of a second family of genes defined by homology to SAG2. The predicted amino acid sequences of these three new proteins suggests that they are all glycosylphosphatidylinositol-linked surface antigens. All three also contain N-linked glycosylation sites, although their use has yet to be demonstrated. One of these SAG2-related antigens, SAG2B, is expressed in tachyzoites with an apparent size of 23 kDa. It is distinct, however, from the previously identified P23. In contrast to SAG2B, SAG2C and SAG2D appear to be expressed exclusively on the surface of bradyzoites. Analysis of the SAG2 family shows it to have weak but significant homology to the SAG1 family. Thus, all of the sequenced surface antigens of tachyzoites and many of those of bradyzoites fall into one large superfamily that can be divided into two subgroups defined by the prototypic and highly immunogenic SAG1 and SAG2, respectively.

Do infectious diseases drive MHC diversity?

The primary function of the major histocompatibility complex (MHC) is to allow the immune system to identify infectious pathogens and eliminate them. Infectious diseases are now thought to be the main selection force that drives and maintains the extraordinary diversity of the MHC.