Duffy-null promoter heterozygosity reduces DARC expression and abrogates adhesion of the P. vivax ligand required for blood-stage infection

The Black Box of Cellular and Molecular Events of Plasmodium vivax Merozoite Invasion into Reticulocytes

Plasmodium vivax is the most widely distributed malaria parasite affecting humans worldwide, causing ~5 million cases yearly. Despite the disease's extensive burden, there are gaps in the knowledge of the pathophysiological mechanisms by which P. vivax invades reticulocytes. In contrast, this crucial step is better understood for P. falciparum, the less widely distributed but more often fatal malaria parasite. This discrepancy is due to the difficulty of studying P. vivax's exclusive invasion of reticulocytes, which represent 1-2% of circulating cells. Its accurate targeting mechanism has not yet been clarified, hindering the establishment of long-term continuous in vitro culture systems. So far, only three reticulocyte invasion pathways have been characterised based on parasite interactions with DARC, TfR1 and CD98 host proteins. However, exposing the parasite's alternative invasion mechanisms is currently being considered, opening up a large field for exploring the entry receptors used by P. vivax for invading host cells. New methods must be developed to ensure better understanding of the parasite to control malarial transmission and to eradicate the disease. Here, we review the current state of knowledge on cellular and molecular mechanisms of P. vivax's merozoite invasion to contribute to a better understanding of the parasite's biology, pathogenesis and epidemiology.

Serological and Molecular Study of the Duffy Blood Group among Malarial Endemic Region Residents in Brazil

Background:

The atypical chemokine receptor 1 (ACKR1) gene encodes the Duffy blood group antigens in two allelic forms: FY*A (FY*01) and FY*B (FY*02), which define the Fy(a+b-), Fy(a-b+), and Fy(a+b+) phenotypes. FY*BES (FY*02N.01) is a single T to C substitution at nucleotide -67 that prevents the FY*B from being expressed in red blood cells (RBCs).

Methods:

We evaluated 250 residents from a Brazilian malarial endemic region (RsMR). All individuals were phenotyped for Fy^a and Fy^b antigens and genotyped for FY*A, FY*B, FY*B^SE , and FY*B^weak alleles.

Results:

Among the 250 individuals, 209 (83.6%) reported previous malaria infection, and 41 (16.4%) did not. The Fy(a+b+) phenotype was present in 97/250 (38.8%), while the Fy(a-b-) was present in 7/250 (2.8%). The FY*A/FY*B was found in 130/250 (52%) and the FY*A/FY*A in 45/250 (18%). The c.1-67>TC was present, in homozygosity, in 11/250 (4.4%). Among 34 individuals with the Fy(a+b-) and FYA*/FYB* mutations, 4/34 (11.8%) had homozygosity for the c.1-67T>C. One individual presented the Fy(a+b-), FY*A/FY*B, and c.1-67T>C in homozygosis, whereas the other presented the Fy(a+b-), FY*A/FY*A, and c.1-67T>C in heterozygosis.

Conclusions:

We reported a low prevalence of the Fy(a-b-) in persons who had previously been infected with Plasmodium vivax (67.5%). We observed that 102/141 (72.3%) individuals expressing the Fy^b antigen had a P. vivax infection, indicating the importance of the Fy^b antigen, silenced by a c.1-67T>C mutation in homozygosis, in preventing the P. vivax infection. We showed that the c.1-67T>C mutation in the FY*A did not silence the FY*A expression on RBCs.

Vivax malaria in Duffy-negative patients shows invariably low asexual parasitaemia: implication towards malaria control in Ethiopia

BACKGROUND

The increase in detections of Plasmodium vivax infection in Duffy-negative individuals in Africa has challenged the dogma establishing the unique P. vivax Duffy Binding Protein-Duffy antigen receptor for chemokines (PvDBP-DARC) pathway used by P. vivax merozoites to invade reticulocytes. Information on the impact of Duffy antigen polymorphisms on the epidemiology of P. vivax malaria remains elusive. The objective of this study was to determine the distribution of asexual parasitaemia of P. vivax according to the Duffy antigen polymorphisms in Ethiopia.

METHODS

DNA was extracted from dried blood spots (DBS) collected from prospectively recruited 138 P. vivax-infected patients from health centres. The identification and estimation of P. vivax asexual parasitaemia were performed by microscopic examination and quantitative real-time polymerase chain reaction (PCR). Duffy genotyping was conducted by DNA sequencing in a total of 138 P.vivax infected samples.

RESULTS

The proportion of Duffy-negatives (FY*BES/FY*BES) in P. vivax infected patients was 2.9% (4/138). Duffy genotype FY*B/FY*BES (48.6%) was the most common, followed by FY*A/FY*BES genotype (25.4%). In one patient, the FY*02 W.01/FY*02 N.01 genotype conferring a weak expression of the Fyb antigen was observed. All P.vivax infected Duffy-negative patients showed low asexual parasitaemia (≤ 110 parasites/µL). The median P. vivax parasitaemia in Duffy-negative patients (53 parasites/µL) was significantly lower than those found in homozygous and heterozygous individuals (P < 0.0001).

CONCLUSION

Plasmodium vivax in Duffy-negative patients shows invariably low asexual parasitaemia. This finding suggests that the pathway used by P. vivax to invade Duffy-negative reticulocytes is much less efficient than that used in Duffy-positives. Moreover, the low asexual parasitaemia observed in Duffy-negative individuals could constitute an 'undetected silent reservoir', thus likely delaying the elimination of vivax malaria in Ethiopia.

A bioinformatic search for correspondence between differentially expressed genes of domestic versus wild animals and orthologous human genes altering reproductive potential

One of the greatest achievements of genetics in the 20th century is D.K. Belyaev’s discovery of destabilizing selection during the domestication of animals and that this selection affects only gene expression regulation (not gene structure) and inf luences systems of neuroendocrine control of ontogenesis in a stressful environment. Among the experimental data generalized by Belyaev’s discovery, there are also f indings about accelerated extinc tion of testes’ hormonal function and disrupted seasonality of reproduction of domesticated foxes in comparison with their wild congeners. To date, Belyaev’s discovery has already been repeatedly conf irmed, for example, by independent observations during deer domestication, during the use of rats as laboratory animals, after the reintroduction of endangered species such as Przewalski’s horse, and during the creation of a Siberian reserve population of the Siberian grouse when it had reached an endangered status in natural habitats. A genome-wide comparison among humans, several domestic animals, and some of their wild congeners has given rise to the concept of self-domestication syndrome, which includes autism spectrum disorders. In our previous study, we created a bioinformatic model of human self-domestication syndrome using differentially expressed genes (DEGs; of domestic animals versus their wild congeners) orthologous to the human genes (mainly, nervous-system genes) whose changes in expression affect reproductive potential, i. e., growth of the number of humans in the absence of restrictions caused by limiting factors. Here, we applied this model to 68 human genes whose changes in expression alter the reproductive health of women and men and to 3080 DEGs of domestic versus wild animals. As a result, in domestic animals, we identif ied 16 and 4 DEGs, the expression changes of which are codirected with changes in the expression of the human orthologous genes decreasing and increasing human reproductive potential, respectively. The wild animals had 9 and 11 such DEGs, respectively. This difference between domestic and wild animals was signif icant according to Pearson’s χ2 test (p < 0.05) and Fisher’s exact test (p < 0.05). We discuss the results from the standpoint of restoration of endangered animal species whose natural habitats are subject to an anthropogenic impact.

Epidemiological and entomological studies of malaria transmission in Tibati, Adamawa region of Cameroon 6 years following the introduction of long-lasting insecticide nets

BACKGROUND

Malaria remains a serious public health problem in Cameroon. Implementation of control interventions requires prior knowledge of the local epidemiological situation. Here we report the results of epidemiological and entomological surveys carried out in Tibati, Adamawa Region, Cameroon, an area where malaria transmission is seasonal, 6 years after the introduction of long-lasting insecticidal bed nets.

METHODS

Cross-sectional studies were carried out in July 2015 and 2017 in Tibati. Thick blood smears and dried blood spots were collected from asymptomatic and symptomatic individuals in the community and at health centers, respectively, and used for the molecular diagnosis of Plasmodium species. Adult mosquitoes were collected by indoor residual spraying and identified morphologically and molecularly. The infection status of Plasmodium spp. was determined by quantitative PCR, and positivity of PCR-positive samples was confirmed by Sanger sequencing.

RESULTS

Overall malaria prevalence in our study population was 55.0% (752/1367) and Plasmodium falciparum was the most prevalent parasite species (94.3%), followed by P. malariae (17.7%) and P. ovale (0.8%); 92 (12.7%) infections were mixed infections. Infection parameters varied according to clinical status (symptomatic/asymptomatic) and age of the sampled population and the collection sites. Infection prevalence was higher in asymptomatic carriers (60.8%), but asexual and sexual parasite densities were lower. Prevalence and intensity of infection decreased with age in both the symptomatic and asymptomatic groups. Heterogeneity in infections was observed at the neighborhood level, revealing hotspots of transmission. Among the 592 Anopheles mosquitoes collected, 212 (35.8%) were An. gambiae, 172 (29.1%) were An. coluzzii and 208 (35.1%) were An. funestus (s.s.). A total of 26 (4.39%) mosquito specimens were infected by Plasmodium sp. and the three Anopheles mosquitoes transmitted Plasmodium at equal efficiency. Surprisingly, we found an An. coluzzii specimen infected by Plasmodium vivax, which confirms circulation of this species in Cameroon. The positivity of all 26 PCR-positive Plasmodium-infected mosquitoes was successively confirmed by sequencing analysis.

CONCLUSION

Our study presents the baseline malaria parasite burden in Tibati, Adamawa Region, Cameroon. Our results highlight the high malaria endemicity in the area, and hotspots of disease transmission are identified. Parasitological indices suggest low bednet usage and that implementation of control interventions in the area is needed to reduce malaria burden. We also report for the first time a mosquito vector with naturally acquired P. vivax infection in Cameroon.

Zoonotic Transmissions and Host Switches of Malaria Parasites

Malaria is a deadly disease that affects the health of hundreds of millions of people annually. There are five Plasmodium parasite species that can naturally infect humans, including Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi. Some of the parasites can also infect various non-human primates. Parasites mainly infecting monkeys such as Plasmodium cynomolgi (in fact P. knowlesi was considered as a parasite of monkeys for years) can also be transmitted to human hosts. Recently, many new Plasmodium species were discovered in African apes, and it is possible that some of the parasites can be transmitted to humans in the future. Here, we searched PubMed and the internet via Google and selected articles concerning zoonotic transmission and evolution of selected malaria parasite species. We reviewed the current advances in the relevant topics emphasizing on transmissions of malaria parasites between humans and non-human primates. We also briefly discuss the transmissions of some avian malaria parasites between wild birds and domestic fowls. Zoonotic malaria transmissions are widespread, which poses a threat to public health. More studies on parasite species identification in non-human primates, transmission, and evolution are needed to reduce or prevent transmission of malaria parasites from non-human primates to humans.

Candidate SNP Markers of Atherogenesis Significantly Shifting the Affinity of TATA-Binding Protein for Human Gene Promoters show stabilizing Natural Selection as a Sum of Neutral Drift Accelerating Atherogenesis and Directional Natural Selection Slowing It

(1) Background: The World Health Organization (WHO) regards atherosclerosis-related myocardial infarction and stroke as the main causes of death in humans. Susceptibility to atherogenesis-associated diseases is caused by single-nucleotide polymorphisms (SNPs). (2) Methods: Using our previously developed public web-service SNP_TATA_Comparator, we estimated statistical significance of the SNP-caused alterations in TATA-binding protein (TBP) binding affinity for 70 bp proximal promoter regions of the human genes clinically associated with diseases syntonic or dystonic with atherogenesis. Additionally, we did the same for several genes related to the maintenance of mitochondrial genome integrity, according to present-day active research aimed at retarding atherogenesis. (3) Results: In dbSNP, we found 1186 SNPs altering such affinity to the same extent as clinical SNP markers do (as estimated). Particularly, clinical SNP marker rs2276109 can prevent autoimmune diseases via reduced TBP affinity for the human MMP12 gene promoter and therefore macrophage elastase deficiency, which is a well-known physiological marker of accelerated atherogenesis that could be retarded nutritionally using dairy fermented by lactobacilli. (4) Conclusions: Our results uncovered SNPs near clinical SNP markers as the basis of neutral drift accelerating atherogenesis and SNPs of genes encoding proteins related to mitochondrial genome integrity and microRNA genes associated with instability of the atherosclerotic plaque as a basis of directional natural selection slowing atherogenesis. Their sum may be stabilizing the natural selection that sets the normal level of atherogenesis.

The enigmatic mechanisms by which Plasmodium vivax infects Duffy-negative individuals

The absence of the Duffy protein at the surface of erythrocytes was considered for decades to confer full protection against Plasmodium vivax as this blood group is the receptor for the key parasite ligand P. vivax Duffy binding protein (PvDBP). However, it is now clear that the parasite is able to break through this protection and induce clinical malaria in Duffy-negative people, although the underlying mechanisms are still not understood. Here, we briefly review the evidence of Duffy-negative infections by P. vivax and summarize the current hypothesis at the basis of this invasion process. We discuss those in the perspective of malaria-elimination challenges, notably in African countries.

Сandidate SNP-markers of rheumatoid arthritis that can significantly alter the affinity of the TATA-binding protein for human gene promoters

Rheumatoid polyarthritis (RA) is an autoimmune disease with autoantibodies, including antibodies to citrullant antigens and proinflammatory cytokines, such as TNF-α and IL-6, which are involved in the induction of chronic synovitis, bone erosion, followed by deformity. Immunopathogenesis is based on the mechanisms of the breakdown of immune tolerance to its own antigens, which is characterized by an increase in the activity of T-effector cells, causing RA symptomatology. At the same time, against the background of such increased activity of effector lymphocytes, a decrease in the activity of a number of regulatory cells, including regulatory T-cells (Treg) and myeloid suppressor cells, is recorded. There is reason to say that it is the change in the activity of suppressor cells that is the leading element in RA pathogenesis. That is why only periods of weakening (remission) of RA are spoken of. According to the more powerful female immune system compared to the male one, the risk of developing RA in women is thrice as high, this risk decreases during breastfeeding and grows during pregnancy as well as after menopause in proportion to the level of sex hormones. It is believed that 50 % of the risk of developing RA depends on the conditions and lifestyle, while the remaining 50 % is dependent on genetic predisposition. That is why, RA fits the main idea of postgenomic predictive-preventive personalized medicine that is to give a chance to those who would like to reduce his/her risk of diseases by bringing his/her conditions and lifestyle in line with the data on his/her genome sequenced. This is very important, since doctors consider RA as one of the most frequent causes of disability. Using the Web service SNP_TATA_Z-tester (http://beehive.bionet.nsc.ru/cgi-bin/mgs/tatascan_fox/start.pl), 227 variants of single nucleotide polymorphism (SNP) of the human gene promoters were studied. As a result, 43 candidate SNP markers for RA that can alter the affinity of the TATA-binding protein (TBP) for the promoters of these genes were predicted.

Cancer-malaria: hidden connections

Cancer and malaria exemplify two maladies historically assigned to separated research spaces. Cancer, on the one hand, ranks among the top priorities in the research agenda of developed countries. Its rise is mostly explained by the ageing of these populations and linked to environment and lifestyle. Malaria, on the other hand, represents a major health burden for developing countries in the Southern Hemisphere. These two diseases also belong to separate fields of medicine: non-communicable diseases for cancer and communicable diseases for malaria.

Susceptibility to Plasmodium vivax malaria associated with DARC (Duffy antigen) polymorphisms is influenced by the time of exposure to malaria

Malaria has provided a major selective pressure and has modulated the genetic diversity of the human genome. The variants of the Duffy Antigen/Receptor for Chemokines (DARC) gene have probably been selected by malaria parasites, particularly the FY*O allele, which is fixed in sub-Saharan Africa and confers resistance to Plasmodium vivax infection. Here, we showed the influence of genomic ancestry on the distribution of DARC genotypes in a highly admixed Brazilian population and confirmed the decreased susceptibility of the FY*A/FY*O genotype to clinical P. vivax malaria. FY*B/FY*O individuals were associated with a greater risk of developing clinical malaria. A remarkable difference among DARC variants concerning the susceptibility to clinical malaria was more evident for individuals who were less exposed to malaria, as measured by the time of residence in the endemic area. Additionally, we found that DARC-negative and FY*A/FY*O individuals had a greater chance of acquiring high levels of antibodies against the 19-kDa C-terminal region of the P. vivax merozoite surface protein-1. Altogether, our results provide evidence that DARC polymorphisms modulate the susceptibility to clinical P. vivax malaria and influence the naturally-acquired humoral immune response to malaria blood antigens, which may interfere with the efficacy of a future vaccine against malaria.

Engagement Rules That Underpin DBL-DARC Interactions for Ingress of Plasmodium knowlesi and Plasmodium vivax Into Human Erythrocytes

Malaria parasite erythrocytic stages comprise of repeated bursts of parasites via cyclical invasion of host erythrocytes using dedicated receptor-ligand interactions. A family of erythrocyte-binding proteins from Plasmodium knowlesi (Pk) and Plasmodium vivax (Pv) attach to human Duffy antigen receptor for chemokines (DARC) via their Duffy binding-like domains (DBLs) for invasion. Here we provide a novel, testable and overarching interaction model that rationalizes even contradictory pieces of evidence that have so far existed in the literature on Pk/Pv-DBL/DARC binding determinants. We further address the conundrum of how parasite-encoded Pk/Pv-DBLs recognize human DARC and collate evidence for two distinct DARC integration sites on Pk/Pv-DBLs.

Candidate SNP markers of reproductive potential are predicted by a significant change in the affinity of TATA-binding protein for human gene promoters

BACKGROUND

The progress of medicine, science, technology, education, and culture improves, year by year, quality of life and life expectancy of the populace. The modern human has a chance to further improve the quality and duration of his/her life and the lives of his/her loved ones by bringing their lifestyle in line with their sequenced individual genomes. With this in mind, one of genome-based developments at the junction of personalized medicine and bioinformatics will be considered in this work, where we used two Web services: (i) SNP_TATA_Comparator to search for alleles with a single nucleotide polymorphism (SNP) that alters the affinity of TATA-binding protein (TBP) for the TATA boxes of human gene promoters and (ii) PubMed to look for retrospective clinical reviews on changes in physiological indicators of reproductive potential in carriers of these alleles.

RESULTS

A total of 126 SNP markers of female reproductive potential, capable of altering the affinity of TBP for gene promoters, were found using the two above-mentioned Web services. For example, 10 candidate SNP markers of thrombosis (e.g., rs563763767) can cause overproduction of coagulation inducers. In pregnant women, Hughes syndrome provokes thrombosis with a fatal outcome although this syndrome can be diagnosed and eliminated even at the earliest stages of its development. Thus, in women carrying any of the above SNPs, preventive treatment of this syndrome before a planned pregnancy can reduce the risk of death. Similarly, seven SNP markers predicted here (e.g., rs774688955) can elevate the risk of myocardial infarction. In line with Bowles' lifespan theory, women carrying any of these SNPs may modify their lifestyle to improve their longevity if they can take under advisement that risks of myocardial infarction increase with age of the mother, total number of pregnancies, in multiple pregnancies, pregnancies under the age of 20, hypertension, preeclampsia, menstrual cycle irregularity, and in women smokers.

CONCLUSIONS

According to Bowles' lifespan theory-which links reproductive potential, quality of life, and life expectancy-the above information was compiled for those who would like to reduce risks of diseases corresponding to alleles in own sequenced genomes. Candidate SNP markers can focus the clinical analysis of unannotated SNPs, after which they may become useful for people who would like to bring their lifestyle in line with their sequenced individual genomes.

What Is Known about the Immune Response Induced by Plasmodium vivax Malaria Vaccine Candidates?

Malaria caused by Plasmodium vivax continues being one of the most important infectious diseases around the world; P. vivax is the second most prevalent species and has the greatest geographic distribution. Developing an effective antimalarial vaccine is considered a relevant control strategy in the search for means of preventing the disease. Studying parasite-expressed proteins, which are essential in host cell invasion, has led to identifying the regions recognized by individuals who are naturally exposed to infection. Furthermore, immunogenicity studies have revealed that such regions can trigger a robust immune response that can inhibit sporozoite (hepatic stage) or merozoite (erythrocyte stage) invasion of a host cell and induce protection. This review provides a synthesis of the most important studies to date concerning the antigenicity and immunogenicity of both synthetic peptide and recombinant protein candidates for a vaccine against malaria produced by P. vivax.

Molecular basis of the Duffy blood group system

ACKR1, located on chromosome 1q23.2, is the gene that encodes a glycoprotein expressing the Duffy blood group antigens. This gene is transcribed in two mRNA variants yielding two isoforms, encoding proteins with 338 and 336 amino acids. This review provides a general overview of the Duffy blood group to characterise and elucidate the genetic basis of this system. The Fy^a and Fy^b antigens are encoded by co-dominant FY*A (FY*01) and FY*B (FY*02) alleles, which differ by c.125G>A (rs12075), defining the Fy(a+b-), Fy(a-b+) and Fy(a+b+) phenotypes. The Fy(a-b-) phenotype that occurs in Africans provides an explanation for the apparent absence of Plasmodium vivax in this region: this phenotype arises from homozygosity for the FY*B allele carrying a point mutation c.1-67T>C (rs2814778), which prevents Fy^b antigen expression only in red blood cells. The same mutation has also been found on the FY*A allele, but it is very rare. The Fy(a-b-) phenotype in Europeans and Asians arises from mutations in the coding region of the FY*A or FY*B allele, preventing Duffy antigen expression on any cell in the body and thus are true Duffy null phenotypes. According to the International Society for Blood Transfusion, ten alleles are associated with the null expression of the Fy antigens. Furthermore, different allelic forms of FY*B modify Fy^b antigen expression, which may result in very weak or equivocal serology results. The mostly common found variants, c.265C>T (rs34599082) and c.298G>A (rs13962) -previously defined in combination only with the FY*B allele - have already been observed in the FY*A allele. Thus, six alleles have been recognised and associated with weak expression of the Fy antigens. Considering the importance of the Duffy blood group system in clinical medicine, additional studies via molecular biology approaches must be performed to resolve and clarify the discrepant results that are present in the erythrocyte phenotyping.

The role of the human Duffy antigen receptor for chemokines in malaria susceptibility: current opinions and future treatment prospects

The Duffy antigen receptor for chemokine (DARC) is a nonspecific receptor for several proinflammatory cytokines. It is homologous to the G-protein chemokine receptor superfamily, which is suggested to function as a scavenger in many inflammatory-and proinflammatory-related diseases. G-protein chemokine receptors are also known to play a critical role in infectious diseases; they are commonly used as entry vehicles by infectious agents. A typical example is the chemokine receptor CCR5 or CXCR4 used by HIV for infecting target cells. In malaria, DARC is considered an essential receptor that mediates the entry of the human and zoonotic malaria parasites Plasmodium vivax and Plasmodium knowlesi into human reticulocytes and erythrocytes, respectively. This process is mediated through interaction with the parasite ligand known as the Duffy binding protein (DBP). Most therapeutic strategies have been focused on blocking the interaction between DBP and DARC by targeting the parasite ligand, while strategies targeting the receptor, DARC, have not been intensively investigated. The rapid increase in drug resistance and the lack of new effective drugs or a vaccine for malaria constitute a major threat and a need for novel therapeutics to combat disease. This review explores strategies that can be used to target the receptor. Inhibitors of DARC, which block DBP-DARC interaction, can potentially provide an effective strategy for preventing malaria caused by P. vivax.

Strain-Transcending Inhibitory Antibodies against Homologous and Heterologous Strains of Duffy Binding Protein region II

Duffy binding protein region II (DBPII) is a promising vaccine candidate against vivax malaria. However, polymorphisms of DBPII are the major obstacle to designing a successful vaccine. Here, we examined whether anti-DBPII antibodies from individual P. vivax exposures provide strain-transcending immunity and whether their presence is associated with DBPII haplotypes found in patients with acute P. vivax. The ability of antibodies to inhibit DBL-TH-erythrocyte binding was tested by COS7 erythrocyte binding inhibition assay. Seven samples of high responders (HR) were identified from screening anti-DBPII levels. HR no.3 and HR no.6 highly inhibited all DBL-TH binding to erythrocytes, by >80%. Antibodies from these two patients’ plasma had the potential to be broadly inhibitory against DBL-TH1, -TH2, -TH6, -TH7, -TH8 and -TH9 haplotypes when plasma was serially diluted from 1:500 to 1:2000. To further examine the association of DBPII haplotypes and the ability of antibodies to broadly inhibit DBL-TH variants, the individual samples underwent sequencing analysis and the inhibitory function of the anti-DBPII antibodies was tested. The patterns of DBPII polymorphisms in acute patients were classified into two groups, DBPII Sal I (55%) and DBL-TH variants (45%). Plasma from Sal I and DBPII-TH patients who had the highest inhibition against Sal I or DBL-TH4 and -TH5 was serially diluted from 1:500 to 1:2000 and their inhibitory capacity was tested against a panel of DBL-TH haplotypes. Results provided evidence of both strain-transcending inhibition as well as strain-specific inhibition by antibodies that blocked erythrocyte binding against some DBL-TH variants and against homologous alleles. This study demonstrated broad inhibition by anti-DBPII antibodies against DBL-TH haplotypes in natural P. vivax exposed individuals. The identification of conserved epitopes among DBL-TH may have implications for vaccine development of a DBPII-based vaccine against diverse P. vivax infections.

Candidate SNP Markers of Gender-Biased Autoimmune Complications of Monogenic Diseases Are Predicted by a Significant Change in the Affinity of TATA-Binding Protein for Human Gene Promoters

Some variations of human genome [for example, single nucleotide polymorphisms (SNPs)] are markers of hereditary diseases and drug responses. Analysis of them can help to improve treatment. Computer-based analysis of millions of SNPs in the 1000 Genomes project makes a search for SNP markers more targeted. Here, we combined two computer-based approaches: DNA sequence analysis and keyword search in databases. In the binding sites for TATA-binding protein (TBP) in human gene promoters, we found candidate SNP markers of gender-biased autoimmune diseases, including rs1143627 [cachexia in rheumatoid arthritis (double prevalence among women)]; rs11557611 [demyelinating diseases (thrice more prevalent among young white women than among non-white individuals)]; rs17231520 and rs569033466 [both: atherosclerosis comorbid with related diseases (double prevalence among women)]; rs563763767 [Hughes syndrome-related thrombosis (lethal during pregnancy)]; rs2814778 [autoimmune diseases (excluding multiple sclerosis and rheumatoid arthritis) underlying hypergammaglobulinemia in women]; rs72661131 and rs562962093 (both: preterm delivery in pregnant diabetic women); and rs35518301, rs34166473, rs34500389, rs33981098, rs33980857, rs397509430, rs34598529, rs33931746, rs281864525, and rs63750953 (all: autoimmune diseases underlying hypergammaglobulinemia in women). Validation of these predicted candidate SNP markers using the clinical standards may advance personalized medicine.

Immunogenicity of a synthetic vaccine based on Plasmodium vivax Duffy binding protein region II

Molecules that play a role in Plasmodium merozoite invasion of host red blood cells represent attractive targets for blood-stage vaccine development against malaria. In Plasmodium vivax, merozoite invasion of reticulocytes is mediated by the Duffy binding protein (DBP), which interacts with its cognate receptor, the Duffy antigen receptor for chemokines, on the surface of reticulocytes. The DBP ligand domain, known as region II (DBPII), contains the critical residues for receptor recognition, making it a prime target for vaccine development against blood-stage vivax malaria. In natural infections, DBP is weakly immunogenic and DBPII allelic variation is associated with strain-specific immunity, which may compromise vaccine efficacy. In a previous study, a synthetic vaccine termed DEKnull that lacked an immunodominant variant epitope in DBPII induced functional antibodies to shared neutralizing epitopes on the native Sal1 allele. Anti-DEKnull antibody titers were lower than anti-Sal1 titers but produced more consistent, strain-transcending anti-DBPII inhibitory responses. In this study, we further characterized the immunogenicity of DEKnull, finding that immunization with recombinant DEKnull produced an immune response comparable to that obtained with native recombinant DBP alleles. Further investigation of DEKnull is necessary to enhance its immunogenicity and broaden its specificity.

Duffy antigen receptor for chemokine (DARC) polymorphisms and its involvement in acquisition of inhibitory anti-duffy binding protein II (DBPII) immunity

The Plasmodium vivax Duffy binding protein (PvDBP) and its erythrocytic receptor, the Duffy antigen receptor for chemokines (DARC), are involved in the major P. vivax erythrocyte invasion pathway. An open cohort study to analyze DARC genotypes and their relationship to PvDBP immune responses was carried out in 620 volunteers in an agricultural settlement of the Brazilian Amazon. Three cross-sectional surveys were conducted at 6-month intervals, comprising 395, 410, and 407 subjects, respectively. The incidence rates of P. vivax infection was 2.32 malaria episodes per 100 person-months under survey (95% confidence interval [CI] of 1.92-2.80/100 person-month) and, of P. falciparum, 0.04 per 100 person-months (95% CI of 0.007-0.14/100 person-month). The distribution of DARC genotypes was consistent with the heterogeneous ethnic origins of the Amazon population, with a predominance of non-silent DARC alleles: FY*A > FY*B. The 12-month follow-up study demonstrated no association between DARC genotypes and total IgG antibodies as measured by ELISA targeting PvDBP (region II, DBPII or regions II-IV, DBPII-IV). The naturally acquired DBPII specific binding inhibitory antibodies (BIAbs) tended to be more frequent in heterozygous individuals carrying a DARC-silent allele (FY*BES). These results provide evidence that DARC polymorphisms may influence the naturally acquired inhibitory anti-Duffy binding protein II immunity.

Duffy blood group and malaria

Very important progress has been made over the last years in understanding the Duffy blood group system and its complexity. The Duffy blood group antigen serves not only as blood group antigen, but also as a receptor for a family of proinflammatory cytokines termed chemokines, and as a receptor for Plasmodium vivax malaria parasites. The Duffy antigen has been termed the "Duffy Antigen Receptor for Chemokines" (DARC) or the Duffy chemokine receptor. DARC might play a role as a scanvenger on the red blood cell surface to eliminate excess of toxic chemokines produced in some pathologic situations [48]. Plasmodium vivax (P. vivax) causes approximately between 70 and 80 million cases of malaria per year and is the most amply distributed human malaria in the world [51]. Individuals with the Duffy-negative phenotype are resistant to P. vivax invasion, and the molecular mechanism that gives rise to the phenotype Fy(a - b - ) in black individuals has been associated with a point mutation - 33TC expressed in homozigosity in the FYB allele [5]. Despite P. vivax be widespread throughout the tropical and subtropical world, it is absent from West Africa, where more than 95% of the population is Duffy negative. Recently, this point mutation has been described in heterozigosity in the FYA allele in others malaria endemic regions [7, 8], and until now we do not know if it confers a certain degree of protection against P. vivax infection.

Immunogenicity of single versus mixed allele vaccines of Plasmodium vivax Duffy binding protein region II

The Duffy binding protein (DBP) of Plasmodium vivax is vital for host erythrocyte invasion. DBP region II (DBPII) contains critical residues for receptor recognition and anti-DBPII antibodies have been shown to inhibit erythrocyte binding and invasion, thereby making the molecule an attractive vaccine candidate against P. vivax blood stages. Similar to other blood-stage antigens, allelic variation within the DBPII and associated strain-specific immunity is a major challenge for development of a broadly effective vaccine against P. vivax malaria. We hypothesized that immunization with a vaccine composed of multiple DBP alleles or a modified epitope DBP (DEKnull) will be more effective in producing a broadly reactive and inhibitory antibody response to diverse DBPII alleles than a single allele vaccine. In this study, we compared single, naturally occurring DBPII allele immunizations (Sal1, 7.18, P) and DEKnull with a combination of (Sal1, 7.18, P) alleles. Quantitative analysis by ELISA demonstrated that the multiple allele vaccine tend to be more immunogenic than any of the single allele vaccines when tested for reactivity against a panel of DBPII allelic variants whereas DEKnull was less immunogenic than the mixed-allele vaccine but similar in reactivity to the single allele vaccines. Further analysis for functional efficacy by in vitro erythrocyte-binding inhibition assays demonstrated that the multiple allele immunization produced a stronger strain-neutralizing response than the other vaccination strategies even though inhibition remained biased toward some alleles. Overall, there was no correlation between antibody titer and functional inhibition. These data suggest that a multiple allele vaccine may enhance immunogenicity of a DBPII vaccine but further investigation is required to optimize this vaccine strategy to achieve broader coverage against global P. vivax strains.

Red blood cell polymorphism and susceptibility to Plasmodium vivax

Resistance to Plasmodium vivax blood-stage infection has been widely recognised to result from absence of the Duffy (Fy) blood group from the surface of red blood cells (RBCs) in individuals of African descent. Interestingly, recent studies from different malaria-endemic regions have begun to reveal new perspectives on the association between Duffy gene polymorphism and P. vivax malaria. In Papua New Guinea and the Americas, heterozygous carriers of a Duffy-negative allele are less susceptible to P. vivax infection than Duffy-positive homozygotes. In Brazil, studies show that the Fy(a) antigen, compared to Fy(b), is associated with lower binding to the P. vivax Duffy-binding protein and reduced susceptibility to vivax malaria. Additionally, it is interesting that numerous studies have now shown that P. vivax can infect RBCs and cause clinical disease in Duffy-negative people. This suggests that the relationship between P. vivax and the Duffy antigen is more complex than customarily described. Evidence of P. vivax Duffy-independent red cell invasion indicates that the parasite must be evolving alternative red cell invasion pathways. In this chapter, we review the evidence for P. vivax Duffy-dependent and Duffy-independent red cell invasion. We also consider the influence of further host gene polymorphism associated with malaria endemicity on susceptibility to vivax malaria. The interaction between the parasite and the RBC has significant potential to influence the effectiveness of P. vivax-specific vaccines and drug treatments. Ultimately, the relationships between red cell polymorphisms and P. vivax blood-stage infection will influence our estimates on the population at risk and efforts to eliminate vivax malaria.

Conserved and variant epitopes of Plasmodium vivax Duffy binding protein as targets of inhibitory monoclonal antibodies

The Duffy binding protein (DBP) is a vital ligand for Plasmodium vivax blood-stage merozoite invasion, making the molecule an attractive vaccine candidate against vivax malaria. Similar to other blood-stage vaccine candidates, DBP allelic variation eliciting a strain-specific immunity may be a major challenge for development of a broadly effective vaccine against vivax malaria. To understand whether conserved epitopes can be the target of neutralizing anti-DBP inhibition, we generated a set of monoclonal antibodies to DBP and functionally analyzed their reactivity to a panel of allelic variants. Quantitative analysis by enzyme-linked immunosorbent assay (ELISA) determined that some monoclonal antibodies reacted strongly with epitopes conserved on all DBP variants tested, while reactivity of others was allele specific. Qualitative analysis characterized by anti-DBP functional inhibition using an in vitro erythrocyte binding inhibition assay indicated that there was no consistent correlation between the endpoint titers and functional inhibition. Some monoclonal antibodies were broadly inhibitory while inhibition of others varied significantly by target allele. These data demonstrate a potential for vaccine-elicited immunization to target conserved epitopes but optimization of DBP epitope target specificity and immunogenicity may be necessary for protection against diverse P. vivax strains.

Design and immunogenicity of a novel synthetic antigen based on the ligand domain of the Plasmodium vivax duffy binding protein

The Duffy binding protein is considered a leading vaccine candidate against asexual blood-stage Plasmodium vivax. The interaction of P. vivax merozoites with human reticulocytes through Duffy binding protein (DBP) and its cognate receptor is vital for parasite infection. The ligand domain of DBP (DBPII) is polymorphic, showing a diversity characteristic of selective immune pressure that tends to compromise vaccine efficacy associated with strain-specific immunity. A previous study resolved that a polymorphic region of DBPII was a dominant B-cell epitope target of human inhibitory anti-DBP antibodies, which we refer to as the DEK epitope for the amino acids in the SalI allele. We hypothesized that the polymorphic residues, which are not functionally important for erythrocyte binding but flank the receptor binding motif of DBPII, comprise variant epitopes that tend to divert the immune response away from more conserved epitopes. In this study, we designed, expressed, and evaluated the immunogenicity of a novel artificial DBPII allele, termed DEKnull, having nonpolar amino acids in the naturally occurring polymorphic charged residues of the DEK epitope. The DEKnull antigen retained erythrocyte-binding activity and elicited antibodies to shared epitopes of SalI DBPII from which it was derived. Our results confirmed that removal of the dominant variant epitope in the DEKnull vaccine lowered immunogenicity of DBPII, but inhibitory anti-DBPII antibodies were elicited against shared neutralizing epitopes on SalI. Focusing immune responses toward more conserved DBP epitopes may avoid development of a strain-specific immunity and enhance functional inhibition against broader range of DBPII variants.

Is there a genetic basis for health disparities in human immunodeficiency virus disease?

The highest global prevalence rates for human immunodeficiency virus and acquired immune deficiency syndrome have been recorded in southern Africa; in the United States, individuals of African descent are disproportionately affected by human immunodeficiency virus infection. Human immunodeficiency virus-infected individuals with African ancestry are also estimated to have a 17-fold or greater risk for developing human immunodeficiency virus-associated nephropathy in comparison with their counterparts of non-African descent. Several recent studies have implicated genetic alleles that are more frequent in populations of African descent and increase the risk of human immunodeficiency virus infection and the risk of human immunodeficiency virus-associated neuropathy (HIVAN). The supposition that persons of African descent are more susceptible to human immunodeficiency virus infection because of an underlying genetic predisposition is not supported by available evidence. However, strong, replicated data show that the increased risk for human immunodeficiency virus-associated nephropathy, as well as other major forms of kidney disease in individuals of African descent, is due in part to MYH9 (myosin, heavy chain 9, non-muscle) renal disease susceptibility alleles that are very frequent throughout sub-Saharan Africa but are infrequent or absent in non-Africans. Selection, drift, and demographic events shape the allelic architecture of the human genome: it is expected that these events will be reflected in geographic-specific differentiation in allele frequencies for a small subset of alleles that may be associated with either increased or reduced risk for complex and infectious diseases.

Determination of the molecular basis for a limited dimorphism, N417K, in the Plasmodium vivax Duffy-binding protein

Invasion of human red blood cells by Plasmodium merozoites is vital for replication and survival of the parasite and, as such, is an attractive target for therapeutic intervention. Merozoite invasion is mediated by specific interactions between parasite ligands and host erythrocyte receptors. The P. vivax Duffy-binding protein (PvDBP) is heavily dependent on the interaction with the human Duffy blood group antigen/receptor for chemokines (DARC) for invasion. Region II of PvDBP contains many allelic polymorphisms likely to have arisen by host immune selection. Successful vaccine development necessitates a deeper understanding of the role of these polymorphisms in both parasite function and evasion of host immunity. A 3D structure of the homologous P. knowlesi DBP predicts that most variant residues are surface-exposed, including N417K, which is a dimorphic residue change that has previously been shown to be part of a linked haplotype that alters DBP sensitivity to inhibitory antibody. In natural isolates only two residues are found at this site, asparagine (N) and lysine (K). Site-directed mutagenesis of residue 417 was used to create a panel of 20 amino acid variants that were then examined for their binding phenotype and response to immune sera. Our results suggest that the observed dimorphism likely arose due to both structural requirements and immune selection pressure. To our knowledge, this is the first exhaustive examination of this kind of the role of a single amino acid residue in antigenic character and binding ability. Our results demonstrate that a single amino acid substitution can dramatically alter both the ability of the PvDBP to bind to human erythrocytes and its antigenic character.

The role played by natural selection on Mendelian traits in humans

Detecting whether and how natural selection has targeted regions of the human genome represents a complementary strategy for identifying functionally important loci and variants involved in disease resistance and adaptation to the environment. In contrast with most complex diseases or traits, the genetic architecture of most Mendelian traits is relatively well established. Most mutations associated with Mendelian disease-related traits are highly penetrant and kept at low population frequencies because of the effects of purifying selection. However, this is not always the case. Here, we review several examples of Mendelian mutations-associated with various disease conditions or other traits of anthropological interest-that have increased in frequency in the human population as a result of past positive selection. These examples clearly illustrate the value of a population genetics approach to unravel the biological mechanisms that have been central to our past and present survival against the selective pressures imposed by diseases and other environmental factors.

Population genetics of GYPB and association study between GYPB*S/s polymorphism and susceptibility to P. falciparum infection in the Brazilian Amazon

BACKGROUND

Merozoites of Plasmodium falciparum invade through several pathways using different RBC receptors. Field isolates appear to use a greater variability of these receptors than laboratory isolates. Brazilian field isolates were shown to mostly utilize glycophorin A-independent invasion pathways via glycophorin B (GPB) and/or other receptors. The Brazilian population exhibits extensive polymorphism in blood group antigens, however, no studies have been done to relate the prevalence of the antigens that function as receptors for P. falciparum and the ability of the parasite to invade. Our study aimed to establish whether variation in the GYPB*S/s alleles influences susceptibility to infection with P. falciparum in the admixed population of Brazil.

METHODS

Two groups of Brazilian Amazonians from Porto Velho were studied: P. falciparum infected individuals (cases); and uninfected individuals who were born and/or have lived in the same endemic region for over ten years, were exposed to infection but have not had malaria over the study period (controls). The GPB Ss phenotype and GYPB*S/s alleles were determined by standard methods. Sixty two Ancestry Informative Markers were genotyped on each individual to estimate admixture and control its potential effect on the association between frequency of GYPB*S and malaria infection.

RESULTS

GYPB*S is associated with host susceptibility to infection with P. falciparum; GYPB*S/GYPB*S and GYPB*S/GYPB*s were significantly more prevalent in the in the P. falciparum infected individuals than in the controls (69.87% vs. 49.75%; P<0.02). Moreover, population genetics tests applied on the GYPB exon sequencing data suggest that natural selection shaped the observed pattern of nucleotide diversity.

CONCLUSION

Epidemiological and evolutionary approaches suggest an important role for the GPB receptor in RBC invasion by P. falciparum in Brazilian Amazons. Moreover, an increased susceptibility to infection by this parasite is associated with the GPB S+ variant in this population.

Mechanisms of genetically-based resistance to malaria

Malaria remains one of the most prevalent parasitoses worldwide. About 350 to 500 million febrile episodes are observed yearly in African children alone and more than 1 million people die because of malaria each year. Multiple factors have hampered the effective control of this disease, some of which include the complex biology of the Plasmodium parasites, their high polymorphism and their increasingly high resistance to antimalarial drugs, mainly in endemic regions. The ancient interaction between malarial parasites and humans has led to the fixation in the population of several inherited alterations conferring protection against malaria. Some of the mechanisms underlying protection against this disease are described in this review for hemoglobin-inherited disorders (thalassemia, sickle-cell trait, HbC and HbE), erythrocyte polymorphisms (ovalocytosis and Duffy blood group), enzymopathies (G6PD deficiency and PK deficiency) and immunogenetic variants (HLA alleles, complement receptor 1, NOS2, tumor necrosis factor-α promoter and chromosome 5q31-q33 polymorphisms).

Acquired antibody responses against Plasmodium vivax infection vary with host genotype for duffy antigen receptor for chemokines (DARC)

Background

Polymorphism of the Duffy Antigen Receptor for Chemokines (DARC) is associated with susceptibility to and the severity of Plasmodium vivax malaria in humans. P. vivax uses DARC to invade erythrocytes. Individuals lacking DARC are ‘resistant’ to P. vivax erythrocytic infection. However, susceptibility to P. vivax in DARC+ individuals is reported to vary between specific DARC genotypes. We hypothesized that the natural acquisition of antibodies to P. vivax blood stages may vary with the host genotype and the level of DARC expression. Furthermore, high parasitemia has been reported to effect the acquisition of immunity against pre-erythrocytic parasites. We investigated the correlation between host DARC genotypes and the frequency and magnitude of antibodies against P. vivax erythrocytic stage antigens.

Methodology/Findings

We assessed the frequencies and magnitudes of antibody responses against P. vivax and P. falciparum sporozoite and erythrocytic antigens in Colombian donors from malaria-endemic regions. The frequency and level of naturally-acquired antibodies against the P. vivax erythrocytic antigens merozoite surface protein 1 (PvMSP1) and Duffy binding protein (PvDBP) varied with the host DARC genotypes. Donors with one negative allele (FY*B/FY*Bnull and FY*A/FY*Bnull) were more likely to have anti-PvMSP1 and anti-PvDBP antibodies than those with two positive alleles (FY*B/FY*B and FY*A/FY*B). The lower IgG3 and IgG1 components of the total IgG response may account for the decreased responses to P. vivax erythrocytic antigens with FY*A/FY*B and FY*B/FY*B genotypes. No such association was detected with P. falciparum erythrocytic antigens, which does not use DARC for erythrocyte invasion.

Conclusion/Significance

Individuals with higher DARC expression, which is associated with higher susceptibility to P. vivax infection, exhibited low frequencies and magnitudes of P. vivax blood-stage specific antibody responses. This may indicate that one of the primary mechanisms by which P. vivax evades host immunity is through DARC indirectly down-regulating humoral responses against erythrocytic invasion and development.

FY polymorphisms and vivax malaria in inhabitants of Amazonas State, Brazil

Although the importance of glycoprotein Duffy in the human red cells invasion process by Plasmodium vivax merozoites has been demonstrated, little is known about the associations of FY polymorphisms with malaria vivax parasitic density. In this study, we investigated the associations of the SNPs 125 G>A, 265 C>T, and 298 G>A on FY gene and the SNP -33T>C on GATA box with the vivax malaria parasitic density in inhabitants of Amazon State, Brazil. Verifications of P. vivax, as well as the definition of parasitism, were determined by standard screening tests in 497 patients. FY phenotyping was performed in all samples by hemagglutination using gel cards. Molecular analysis for FY/GATA polymorphisms were performed by polymerase chain reaction-restriction fragment length polymorphism. Our data showed that in this population, FY*A/FY*B-33 and FY*B/FY*B-33 genotypes may be a selective advantage, reducing the frequency of P. vivax infection in the studied area. FY*A/FY*B and FY*A/FY*A genotypes showed to be associated with the rise of the frequency of P. vivax infection, and FY*B/FY*X and FY*A/FY*X showed to be associated with the low levels of parasitism. These results suggest that natural adaptations, in malaria-endemic regions, could be leading to the arising of partial defense mechanisms against P. vivax, which is different from the previously described in African descents, as well as adaptations that could be increasing the susceptibility of human to this kind of malaria.

Duffy blood group gene polymorphisms among malaria vivax patients in four areas of the Brazilian Amazon region

Background

Duffy blood group polymorphisms are important in areas where Plasmodium vivax predominates, because this molecule acts as a receptor for this protozoan. In the present study, Duffy blood group genotyping in P. vivax malaria patients from four different Brazilian endemic areas is reported, exploring significant associations between blood group variants and susceptibility or resistance to malaria.

Methods

The P. vivax identification was determined by non-genotypic and genotypic screening tests. The Duffy blood group was genotyped by PCR/RFLP in 330 blood donors and 312 malaria patients from four Brazilian Amazon areas. In order to assess the variables significance and to obtain independence among the proportions, the Fisher's exact test was used.

Results

The data show a high frequency of the FYA/FYB genotype, followed by FYB/FYB, FYA/FYA, FYA/FYB-33 and FYB/FYB-33. Low frequencies were detected for the FYA/FY^X, FYB/FY^X, FYX/FY^X and FYB-33/FYB-33 genotypes. Negative Duffy genotype (FYB-33/FYB-33) was found in both groups: individuals infected and non-infected (blood donors). No individual carried the FY^X/FYB-33 genotype. Some of the Duffy genotypes frequencies showed significant differences between donors and malaria patients.

Conclusion

The obtained data suggest that individuals with the FYA/FYB genotype have higher susceptibility to malaria. The presence of the FYB-33 allele may be a selective advantage in the population, reducing the rate of infection by P. vivax in this region. Additional efforts may contribute to better elucidate the physiopathologic differences in this parasite/host relationship in regions endemic for P. vivax malaria, in particular the Brazilian Amazon region.

Real-time multiplex allele-specific polymerase chain reaction for genotyping of the Duffy antigen, the Plasmodium vivax invasion receptor

BACKGROUND AND OBJECTIVES

Duffy blood group is of major interest in clinical medicine as it is not only involved in blood-transfusion risks and occasionally in neonatal haemolytic disease, but it is also the receptor for the human malaria parasite Plasmodium vivax in the erythrocyte invasion. The aim of this study was to develop a rapid and inexpensive approach for high-throughput Duffy genotyping.

MATERIALS AND METHODS

This paper reported the development of a Duffy genotyping assay based on multiplex real-time polymerase chain reaction (PCR) using SYBR Green I fluorescent dye.

RESULTS

By using this approach for Duffy genotyping we obtained the same results as that for the conventional allele-specific PCR, however, in a high-throughput assay. The Duffy genotyping of field samples demonstrated that P. vivax-infected individuals showed a significantly higher prevalence of two functional alleles than Plasmodium falciparum-infected and non-infected individuals. This finding corroborates the hypothesis that the presence of two functional alleles increases the risk of P. vivax infection.

CONCLUSION

This methodology may be suitable for epidemiological studies, particularly for exploring the relationship between Duffy alleles and malaria susceptibility, and also for identification of transfusional incompatibility in blood banks.

Reduced Plasmodium vivax erythrocyte infection in PNG Duffy-negative heterozygotes

Background

Erythrocyte Duffy blood group negativity reaches fixation in African populations where Plasmodium vivax (Pv) is uncommon. While it is known that Duffy-negative individuals are highly resistant to Pv erythrocyte infection, little is known regarding Pv susceptibility among heterozygous carriers of a Duffy-negative allele (+/−). Our limited knowledge of the selective advantages or disadvantages associated with this genotype constrains our understanding of the effect that interventions against Pv may have on the health of people living in malaria-endemic regions.

Methods and Findings

We conducted cross-sectional malaria prevalence surveys in Papua New Guinea (PNG), where we have previously identified a new Duffy-negative allele among individuals living in a region endemic for all four human malaria parasite species. We evaluated infection status by conventional blood smear light microscopy and semi-quantitative PCR-based strategies. Analysis of a longitudinal cohort constructed from our surveys showed that Duffy heterozygous (+/−) individuals were protected from Pv erythrocyte infection compared to those homozygous for wild-type alleles (+/+) (log-rank tests: LM, p = 0.049; PCR, p = 0.065). Evaluation of Pv parasitemia, determined by semi-quantitative PCR-based methods, was significantly lower in Duffy +/− vs. +/+ individuals (Mann-Whitney U: p = 0.023). Overall, we observed no association between susceptibility to P. falciparum erythrocyte infection and Duffy genotype.

Conclusions

Our findings provide the first evidence that Duffy-negative heterozygosity reduces erythrocyte susceptibility to Pv infection. As this reduction was not associated with greater susceptibility to Pf malaria, our in vivo observations provide evidence that Pv-targeted control measures can be developed safely.

Plasmodium vivax in Africa: hidden in plain sight?

People who live in tropical Africa, south of the Sahara, are predominantly negative for the Duffy blood-group antigen, which mediates invasion of reticulocytes by Plasmodium vivax. Recent reports of a parasite that was molecularly diagnosed as P. vivax from populations who are suspected, or known, to be Duffy negative confound a large body of evidence that states that invasion of P. vivax requires the Duffy antigen. If confirmed, one of several possible explanations is that P. vivax, which originated in Asia, is now evolving to exploit alternate invasion receptors in Africa.

Genetic susceptibility to infectious diseases: big is beautiful, but will bigger be even better?

Genetic epidemiology, including twin studies, provides robust evidence that genetic variation in human populations contributes to susceptibility to infectious disease. One of the major limitations of studies that attempt to identify the genes and mechanisms that underlie this susceptibility has been lack of power caused by small sample size. With the development of novel technologies, burgeoning information on the human genome, the HapMap project, and human genetic diversity, we are at the beginning of a new era in the study of the genetics of complex diseases. This review looks afresh at the epidemiological evidence that supports a role for genetics in susceptibility to infectious disease, examines the somewhat limited achievements to date, and discusses current advances in methodology and technology that will potentially lead to translational data in the future.

CCR5: no longer a ‘good for nothing’ gene – chemokine control of West Nile virus infection

The chemokine receptor CCR5 was identified in 1996 as a crucial host factor exploited by HIV for cell entry. CCR5 presumably functions normally in antimicrobial host defense because it generally mediates leukocyte chemotactic responses; however, evidence of antimicrobial functions for CCR5 in humans has been elusive. Recently, genetic analyses in mice and humans have provided strong evidence for the CCR5 control of infection by West Nile virus (WNV), a re-emerging pathogen capable of causing fatal encephalitis. Thus, the same receptor can benefit or harm the host, depending on the virus. Although CCR5 might be a logical target for new drug development in HIV/AIDS, the benefits of blocking CCR5 could carry the cost of an increased risk of WNV disease in co-infected patients.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

How malaria has affected the human genome and what human genetics can teach us about malaria

Malaria is a major killer of children worldwide and the strongest known force for evolutionary selection in the recent history of the human genome. The past decade has seen growing evidence of ethnic differences in susceptibility to malaria and of the diverse genetic adaptations to malaria that have arisen in different populations: epidemiological confirmation of the hypotheses that G6PD deficiency, alpha+ thalassemia, and hemoglobin C protect against malaria mortality; the application of novel haplotype-based techniques demonstrating that malaria-protective genes have been subject to recent positive selection; the first genetic linkage maps of resistance to malaria in experimental murine models; and a growing number of reported associations with resistance and susceptibility to human malaria, particularly in genes involved in immunity, inflammation, and cell adhesion. The challenge for the next decade is to build the global epidemiological infrastructure required for statistically robust genomewide association analysis, as a way of discovering novel mechanisms of protective immunity that can be used in the development of an effective malaria vaccine.

Conserved residues in the Plasmodium vivax Duffy-binding protein ligand domain are critical for erythrocyte receptor recognition

Malaria merozoite invasion of human erythrocytes depends on recognition of specific erythrocyte surface receptors by parasite ligands. Plasmodium vivax merozoite invasion is totally dependent on the recognition of the Duffy blood group antigen by the parasite ligand Duffy-binding protein (DBP). Receptor recognition by P. vivax relies on a cysteine-rich domain, the DBL domain or region II, at the N terminus of the extracellular portion of DBP. The minimal region of the DBP implicated for receptor recognition lies between cysteines 4 and 8 of the DBL domain, which is a region that also has the highest rate of allelic polymorphisms among parasite isolates. We previously found that allelic polymorphisms in this region altered the P. vivax DBL domain antigenic character, which contrasts with changes in receptor specificity attributed to polymorphisms in some homologous ligands of Plasmodium falciparum. To further investigate the relative importance of conserved and polymorphic residues within this DBL central region, we identified residues critical for receptor recognition by site-directed mutagenesis. Seventy-seven surface-predicted residues of the Sal-1 DBL domain were substituted with alanine and assayed for erythrocyte binding activity by expression of the mutant proteins on the surface of transiently transfected COS cells. The functional effect of alanine substitution varied from nil to complete loss of DBL erythrocyte-binding activity. Mutations that caused loss of ligand function mostly occurred in discontinuous clusters of conserved residues, whereas nearly all mutations in polymorphic residues did not affect erythrocyte binding. These data delineate DBL domain residues essential for receptor recognition.

Erythrocyte-binding activity of Plasmodium yoelii apical membrane antigen-1 expressed on the surface of transfected COS-7 cells

Malaria merozoite surface and apical organellar molecules facilitate invasion into the host erythrocyte. The underlying molecular mechanisms of invasion are poorly understood, and there are few data to delineate roles for individual merozoite proteins. Apical membrane antigen-1 (AMA-1) is a conserved apicomplexan protein present in the apical organelle complex and at times on the surface of Plasmodium and Toxoplasma zoites. AMA-1 domains 1/2 are conserved between Plasmodium and Toxoplasma and have similarity to the defined ligand domains of MAEBL, an erythrocyte-binding protein identified from Plasmodium yoelii. We expressed selected portions of the AMA-1 extracellular domain on the surface of COS-7 cells to assay for erythrocyte-binding activity. The P. yoelii AMA-1 domains 1/2 mediated adhesion to mouse and rat erythrocytes, but not to human erythrocytes. Adhesion to rodent erythrocytes was sensitive to trypsin and chymotrypsin, but not to neuraminidase. Other parts of the AMA-1 ectodomain, including the full-length extracellular domain, mediated significantly less erythrocyte adhesion activity than the contiguous domains 1/2. The results support the role of AMA-1 as an adhesion molecule during merozoite invasion of erythrocytes and identify highly conserved domains 1/2 as the principal ligand of the Plasmodium AMA-1 and possibly the Toxoplasma AMA-1. Identification of the AMA-1 ligand domains involved in interaction between the parasite and host cell should help target the development of new therapies to block growth of the blood-stage malaria parasites.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.