Hematological profiles of malaria-infected patients in an endemic area of Peru

OBJECTIVES.

To evaluate the variation of hematological profiles of patients infected with uncomplicated Plasmodium vivax (Pv) and P. falciparum (Pf) malaria before, during and after treatment in a population of the Loreto region.

MATERIALS AND METHODS.

This study was conducted between 2010 and 2012, in Zungarococha (Iquitos). The 425 participants had three visits (visit 1-day 0-before treatment, visit 2-day 7-during treatment, visit 3-day 28-after treatment), complete blood count, microscopic and molecular diagnosis (PCR).

RESULTS.

At the first visit, 93 (21.9%) participants were found positive for Pv and 34 (8.0%) for Pf. All positives showed a reduction in hematocrit, white blood cell count (WBC), ablated and segmented neutrophils, eosinophils and platelets (p<0.001) compared to the negative group. A higher percentage of ablated neutrophils was found in Pf and segmented neutrophils in Pv compared to the negative group. Variations in hematological profiles were observed after treatment for both species; ablated neutrophils decreased, platelets increased, eosinophils increased at day 7 and declined at day 28, hematocrit and segmented neutrophils decreased at day 7 and normalized at day 28. Interspecies differences over time showed a bigger daily decrease in ablated neutrophils in Pv-infected when compared to Pf.

CONCLUSIONS.

The hematological profile in uncomplicated malaria-positive patients varies over time during and after treatment. These are indicators of disease progression and help in the therapeutic surveillance of Plasmodium-infected patients.

Walls talk: Microbial biogeography of homes spanning urbanization

Westernization has propelled changes in urbanization and architecture, altering our exposure to the outdoor environment from that experienced during most of human evolution. These changes might affect the developmental exposure of infants to bacteria, immune development, and human microbiome diversity. Contemporary urban humans spend most of their time indoors, and little is known about the microbes associated with different designs of the built environment and their interaction with the human immune system. This study addresses the associations between architectural design and the microbial biogeography of households across a gradient of urbanization in South America. Urbanization was associated with households' increased isolation from outdoor environments, with additional indoor space isolation by walls. Microbes from house walls and floors segregate by location, and urban indoor walls contain human bacterial markers of space use. Urbanized spaces uniquely increase the content of human-associated microbes-which could increase transmission of potential pathogens-and decrease exposure to the environmental microbes with which humans have coevolved.

Population Structure Shapes Copy Number Variation in Malaria Parasites

If copy number variants (CNVs) are predominantly deleterious, we would expect them to be more efficiently purged from populations with a large effective population size (Ne) than from populations with a small Ne. Malaria parasites (Plasmodium falciparum) provide an excellent organism to examine this prediction, because this protozoan shows a broad spectrum of population structures within a single species, with large, stable, outbred populations in Africa, small unstable inbred populations in South America and with intermediate population characteristics in South East Asia. We characterized 122 single-clone parasites, without prior laboratory culture, from malaria-infected patients in seven countries in Africa, South East Asia and South America using a high-density single-nucleotide polymorphism/CNV microarray. We scored 134 high-confidence CNVs across the parasite exome, including 33 deletions and 102 amplifications, which ranged in size from <500 bp to 59 kb, as well as 10,107 flanking, biallelic single-nucleotide polymorphisms. Overall, CNVs were rare, small, and skewed toward low frequency variants, consistent with the deleterious model. Relative to African and South East Asian populations, CNVs were significantly more common in South America, showed significantly less skew in allele frequencies, and were significantly larger. On this background of low frequency CNV, we also identified several high-frequency CNVs under putative positive selection using an FST outlier analysis. These included known adaptive CNVs containing rh2b and pfmdr1, and several other CNVs (e.g., DNA helicase and three conserved proteins) that require further investigation. Our data are consistent with a significant impact of genetic structure on CNV burden in an important human pathogen.

Genetic diversity in merozoite surface protein-1 and 2 among Plasmodium falciparum isolates from malarious districts of tribal dominant state of Jharkhand, India

INTRODUCTION

The genetic make-up of malaria parasite is potent for understanding the parasite virulence, designing antimalarial vaccine and evaluating the impact of malaria control measures. There is a paucity of information on genetic structure of Plasmodium falciparum in Jharkhand, India where malaria is rampant and this study aimed to establish molecular characterization of P. falciparum field isolates from Jharkhand measured with two highly polymorphic genetic markers, i.e. the merozoite surface proteins (MSPs) 1 and 2.

METHODS

The genetic diversity of P. falciparum population from low transmission area, Ranchi, Bokaro and Hazaribagh and highly malarious area, Latehar and Palamau districts of Jharkhand were evaluated by polymerase chain reaction-sequencing analyzing msp-1 and msp-2 genes to explore the genetic structure of parasite from this understudied region.

RESULTS

A total of 134 P. falciparum isolates were analyzed by polymorphic regions of msp-1 and msp-2 and classified according to prevalence of allelic families. The majority of patients from all the five sites had mean monoclonal infections of 67·1 and 60·4% of P. falciparum for msp-1 and msp-2, respectively, whereas, mean multiple genotypes of 32·8 and 39·5% for msp-1 and msp-2, respectively. Interestingly, we observed higher multiclonal infection in low transmission area as compared to highly malarious area in the case of msp-1 genotypes, whereas in msp-2 higher multiclonal infection was observed in highly malarious area compared to low transmission area. The overall multiplicities of infection of msp-1 and msp-2 were 1·38 and 1·39, respectively.

CONCLUSION

This is the first report on molecular characterization of P. falciparum field isolates from Jharkhand. The genetic diversity and allelic distribution found in this study is somewhat similar to other reports from India and Southeast Asian countries. However, P. falciparum infection can be highly complex and diverse in these disease-endemic regions of Jharkhand, suggesting continual genetic mixing that could have significant implications for the use of antimalarial drugs and vaccines.

Antibodies directed against merozoite surface protein-6 are induced by natural exposure to Plasmodium falciparum in a low transmission environment

Malaria caused by Plasmodium falciparum is a major cause of global infant mortality, and there is currently no licensed vaccine that provides protection against infection or disease. Several P. falciparum vaccine targets have undergone early testing, but many more candidates remain with little data to support their development. Plasmodium falciparum Merozoite Surface Protein 6 (PfMSP6) is a candidate of particular interest because it is a member of the PfMSP3 multi-gene family, raising the possibility that vaccine-induced immune responses could cross-react across multiple family members. However, few immunoepidemiological studies of PfMSP6 have been carried out to measure domain-specific anti-PfMSP6 responses. This study investigated anti-PfMSP6 responses in P. falciparum-infected individuals from the Peruvian Amazon, using two different PfMSP6 N-terminal allele antigens and a single C-terminal domain antigen, and compared the responses with both PfMSP6 genotyping data and anti-PfMSP3 response data that had been previously generated for the same samples. Anti-PfMSP6 responses were detected despite the low transmission setting, but were less frequent and of considerably lower intensity than anti-PfMSP3 responses. There was a positive correlation between anti-PfMSP3 and PfMSP6 responses, suggesting that the possibility that PfMSP3 family antigens could induce cross-reactive responses requires further detailed investigation.

Placental histopathologic changes associated with subclinical malaria infection and its impact on the fetal environment

Microscopic examination of placental tissue can provide an accurate assessment of malaria infection during pregnancy. In this cross-sectional study of 193 women in Iquitos, Peru, 1.0% and 6.6% had parasites in the peripheral blood as detected by microscopy and polymerase chain reaction, respectively. However, 22% had placental malaria pigment indicating past, subclinical infections. Placental tissues with pigment from 24 cases were matched by gravidity and month of delivery to 24 controls and histopathologically examined. Cases had significantly higher number of monocytes in the intervillous space (44.7 versus 25.5; P = 0.012). Pigmented monocytes in fetal vessels were present in 33.3% of cases. This study demonstrated that subclinical malarial infection occurred frequently in pregnant women and is associated with increased presence of monocytes in the placenta. Pigmented monocytes in fetal vessels suggest parasites can breach the placental barrier and enter the fetal circulation.

Plasmodium falciparum genetic diversity maintained and amplified over 5 years of a low transmission endemic in the Peruvian Amazon

Plasmodium falciparum entered into the Peruvian Amazon in 1994, sparking an epidemic between 1995 and 1998. Since 2000, there has been sustained low P. falciparum transmission. The Malaria Immunology and Genetics in the Amazon project has longitudinally followed members of the community of Zungarococha (N = 1,945, 4 villages) with active household and health center-based visits each year since 2003. We examined parasite population structure and traced the parasite genetic diversity temporally and spatially. We genotyped infections over 5 years (2003–2007) using 14 microsatellite (MS) markers scattered across ten different chromosomes. Despite low transmission, there was considerable genetic diversity, which we compared with other geographic regions. We detected 182 different haplotypes from 302 parasites in 217 infections. Structure v2.2 identified five clusters (subpopulations) of phylogenetically related clones. To consider genetic diversity on a more detailed level, we defined haplotype families (hapfams) by grouping haplotypes with three or less loci differences. We identified 34 different hapfams identified. The F_st statistic and heterozygosity analysis showed the five clusters were maintained in each village throughout this time. A minimum spanning network (MSN), stratified by the year of detection, showed that haplotypes within hapfams had allele differences and haplotypes within a cluster definition were more separated in the later years (2006–2007). We modeled hapfam detection and loss, accounting for sample size and stochastic fluctuations in frequencies overtime. Principle component analysis of genetic variation revealed patterns of genetic structure with time rather than village. The population structure, genetic diversity, appearance/disappearance of the different haplotypes from 2003 to 2007 provides a genome-wide “real-time” perspective of P. falciparum parasites in a low transmission region.

Limited variation in vaccine candidate Plasmodium falciparum Merozoite Surface Protein-6 over multiple transmission seasons

BACKGROUND

Plasmodium falciparum Merozoite Surface Protein-6 (PfMSP6) is a component of the complex proteinacious coat that surrounds P. falciparum merozoites. This location, and the presence of anti-PfMSP6 antibodies in P. falciparum-exposed individuals, makes PfMSP6 a potential blood stage vaccine target. However, genetic diversity has proven to be a major hurdle for vaccines targeting other blood stage P. falciparum antigens, and few endemic field studies assessing PfMSP6 gene diversity have been conducted. This study follows PfMSP6 diversity in the Peruvian Amazon from 2003 to 2006 and is the first longitudinal assessment of PfMSP6 sequence dynamics.

METHODS

Parasite DNA was extracted from 506 distinct P. falciparum infections spanning the transmission seasons from 2003 to 2006 as part of the Malaria Immunology and Genetics in the Amazon (MIGIA) cohort study near Iquitos, Peru. PfMSP6 was amplified from each sample using a nested PCR protocol, genotyped for allele class by agarose gel electrophoresis, and sequenced to detect diversity. Allele frequencies were analysed using JMP v.8.0.1.0 and correlated with clinical and epidemiological data collected as part of the MIGIA project.

RESULTS

Both PfMSP6 allele classes, K1-like and 3D7-like, were detected at the study site, confirming that both are globally distributed. Allele frequencies varied significantly between transmission seasons, with 3D7-class alleles dominating and K1-class alleles nearly disappearing in 2005 and 2006. There was a significant association between allele class and village location (p-value = 0.0008), but no statistically significant association between allele class and age, sex, or symptom status. No intra-allele class sequence diversity was detected.

CONCLUSIONS

Both PfMSP6 allele classes are globally distributed, and this study shows that allele frequencies can fluctuate significantly between communities separated by only a few kilometres, and over time in the same community. By contrast, PfMSP6 was highly stable at the sequence level, with no SNPs detected in the 506 samples analysed. This limited diversity supports further investigation of PfMSP6 as a blood stage vaccine candidate, with the clear caveat that any such vaccine must either contain both alleles or generate cross-protective responses that react against both allele classes. Detailed immunoepidemiology studies are needed to establish the viability of these approaches before PfMSP6 advances further down the vaccine development pipeline.

Plasmodium falciparum and Plasmodium vivax infections in the Peruvian Amazon: propagation of complex, multiple allele-type infections without super-infection

Outcrossing potential between Plasmodium parasites is defined by the population-level diversity (PLD) and complexity of infection (COI). There have been few studies of PLD and COI in low transmission regions. Since the 1995-1998 Peruvian Amazon epidemic, there has been sustained transmission with < 0.5 P. falciparum and < 1.6 P. vivax infections/person/year. Using weekly active case detection, we described PLD by heterozygosity (H(e)) and COI using P. falciparum Pfmsp1-B2 and P. vivax Pvmsp3alpha. Not being homologous genes, we limited comparisons to within species. P. falciparum (N = 293) had low (H(e) = 0.581) and P. vivax (N = 186) had high (H(e) = 0.845) PLD. A total of 9.5% P. falciparum infections and 26.3% P. vivax infections had COI > 1. Certain allele types were in more mixed infections than expected by chance. The few appearances of new alleles could be explained by stochastic polymerase chain reaction detection or synchronization/sequestration. The results suggest propagation of mixed infections by multiple inocula, not super-infection, implying decade-long opportunity for outcrossing in these mixed infections.

Prevalence and risk of Plasmodium falciparum and P. vivax malaria among pregnant women living in the hypoendemic communities of the Peruvian Amazon

The Amazon region of Iquitos, Peru is hypoendemic for Plasmodium vivax and P. falciparum. There is limited information regarding the epidemiology of malaria during pregnancy in this region. Passive surveillance for clinical malaria among pregnant women was conducted in eight health posts in 2004 and 2005. Community-based active surveillance was conducted to determine the incidence of malarial infection among pregnant women in the community of Zungarococha in 2004 and 2005. Passive surveillance demonstrated that pregnant women had a prevalence of clinical malaria of 7.5% in 2004 and 6.6% in 2005 compared with 20.6% and 22.4% of the total population. Active surveillance showed that pregnant women were 2.3 (95% confidence interval = 1.32-3.95, P = 0.004) times more likely to have a P. falciparum infection compared with non-pregnant women. This study demonstrated that because of detection bias, passive surveillance underestimates the burden of malarial infection during pregnancy, and that subclinical malarial infections may occur frequently among pregnant women in this region. Furthermore, pregnant women in this low-transmission and P. vivax-dominant setting, experience an increased risk for P. falciparum infection, but not P. vivax infection.

Genetic diversity in the Block 2 region of the merozoite surface protein 1 (MSP-1) of Plasmodium falciparum: additional complexity and selection and convergence in fragment size polymorphism

Fragment size in the Block 2 repetitive region of merozoite surface protein 1 (MSP1) has commonly been used as a molecular marker in studies of malaria transmission dynamics and host immunity in Plasmodium falciparum malaria. In this study, we further explore the genetic variation in MSP-1 Block 2 underlying potential problems faced while studying the immune responses elicited by this vaccine target and while using it as a molecular marker in epidemiologic investigations. We describe the distribution of a new Block 2 recombinant allele family in samples collected from western Kenya and other malarious regions of the world and provide evidence that this allele family is found worldwide and that all MR alleles most likely originated from a single recombination event. We test whether the number of tandem repeats (i.e. fragment size) can be considered neutral in an area of high transmission in western Kenya. In addition, we investigate the validity of the assumption that Block 2 alleles of the same size and allele family are identical by examining MSP1 Block 2 amino acid sequences obtained from full-length MSP-1 clones generated from infected Kenyan children and find that this assumption does not hold. We conclude that the worldwide presence of a new allele family, the effect of positive natural selection, and the lack of conserved amino acid motifs within alleles of the same size suggest a higher level of complexity that may hamper our ability to elucidate allele family specific immune responses elicited by this vaccine target and its overall use as genetic marker in other types of epidemiologic investigations.

Chromosome-wide SNPs reveal an ancient origin for Plasmodium falciparum

The Malaria's Eve hypothesis, proposing a severe recent population bottleneck (about 3,000-5,000 years ago) of the human malaria parasite Plasmodium falciparum, has prompted a debate about the origin and evolution of the parasite. The hypothesis implies that the parasite population is relatively homogeneous, favouring malaria control measures. Other studies, however, suggested an ancient origin and large effective population size. To test the hypothesis, we analysed single nucleotide polymorphisms (SNPs) from 204 genes on chromosome 3 of P. falciparum. We have identified 403 polymorphic sites, including 238 SNPs and 165 microsatellites, from five parasite clones, establishing chromosome-wide haplotypes and a dense map with one polymorphic marker per approximately 2.3 kilobases. On the basis of synonymous SNPs and non-coding SNPs, we estimate the time to the most recent common ancestor to be approximately 100,000-180,000 years, significantly older than the proposed bottleneck. Our estimated divergence time coincides approximately with the start of human population expansion, and is consistent with a genetically complex organism able to evade host immunity and other antimalarial efforts.

Antibody responses to repetitive epitopes of the circumsporozoite protein, liver stage antigen-1, and merozoite surface protein-2 in infants residing in a Plasmodium falciparum-hyperendemic area of western Kenya. XIII. Asembo Bay Cohort Project

The present study was initiated to characterize antibody responses to repetitive epitopes of the circumsporozoite protein (CSP), liver stage antigen-1 (LSA-1), and merozoite surface protein-2 (MSP-2) of Plasmodium falciparum in infants residing in a P. falciparum-hyperendemic area of western Kenya. In this study, development and maintenance of these antibody responses in 28 infants were studied longitudinally by use of monthly serum samples collected from birth to age 1 year. Mother plasma and infant umbilical cord plasma were also tested to assess the transplacental transfer of maternal antibodies. Results showed that antibodies passively transferred from mothers were detectable for CSP, LSA-1, and MSP-2 repeat epitopes. Infants were able to mount and maintain a strong antibody response against LSA-1 in their first year of life. Infants often responded to CSP repeats, but with a much lower antibody titer. Antibody responses in infants against Fc27 and 3D7 repeats of MSP-2 were low throughout their first year. In addition, 51 infants whose first detected infection occurred at > 4 months of age were selected to determine antibody responses to the antigens tested upon their first and second detected infections. Antibody responses to LSA-1 and, to a lesser degree, CSP increased in positivity rates and titer upon second infection. Antibody responses to Fc27-type and 3D7-type repeats of MSP-2 were low upon both infections. There was no association between maternally transferred anti-LSA-1, anti-CSP, or anti-MSP-2 antibodies and an infant's first detected infection. No significant correlation was found between an infant's antibody responses to the 4 antigen repetitive epitopes and protection against malarial parasitemia during the first year of life.

Plasmodium falciparum genotypes, low complexity of infection, and resistance to subsequent malaria in participants in the Asembo Bay Cohort Project

To assess the relationship between the within-host diversity of malaria infections and the susceptibility of the host to subsequent infection, we genotyped 60 children's successive infections from birth through 3 years of life. MSP-1 Block2 genotypes were used to estimate the complexity of infection (COI). Malaria transmission and age were positively associated with the number of K1 and Mad20 alleles detected (COI(KM)) (P < 0.003). Controlling for previous parasitemia, transmission, drug treatment, parasite density, sickle cell, and age, COI(KM) was negatively correlated with resistance to parasitemia of > 500/microl (P < 0.0001). Parasitemias with the RO-genotype were more resistant than those without this genotype (P < 0.0000). The resistance in low COI(KM) infections was not genotype specific. We discuss the impact of genotype-transcending immunity to conserved antigenic determinants. We also propose a diversity-driven immunomodulation hypothesis that may explain the delayed development of natural immunity in the first few years of life and suggest that interventions that decrease the COI(KM) could facilitate the development of protective immunity.

Anti-merozoite surface protein-1 19-kDa IgG in mother-infant pairs naturally exposed to Plasmodium falciparum: subclass analysis with age, exposure to asexual parasitemia, and protection against malaria. V. The Asembo Bay Cohort Project

The anti-merozoite surface protein-1 19-kDa IgG (anti-MSP119KD) IgG responses of 33 parasitemic infants, aged 6-14 months, were compared with those of their mothers at the time of the infant's delivery and at the time the infants were sampled; the antimalaria protection associated with these responses was also compared. IgG1 and IgG3 were the predominant subclasses. Infants <300 days old and pregnant mothers had the lowest cytophilic-to-noncytophilic IgG ratio. By 300 days of age, the infants had IgG subclass compositions and levels similar to those of their mothers at the same date. Among infants, older infants with only 1 or 2 detected asexual parasitemias had the highest cytophilic-to-noncytophilic IgG ratio and IgG1 levels. IgG1 level was negatively correlated with protection. The findings suggest that the MSP119KD antibody response develops with age, not with multiple experiences with parasitemia, and, thus, that an antimalaria vaccine strategy for pregnant mothers could delay infants' first parasitemias until they are more capable of mounting a favorable anti-MSP119KD response.

A longitudinal investigation of IgG and IgM antibody responses to the merozoite surface protein-1 19-kiloDalton domain of Plasmodium falciparum in pregnant women and infants: associations with febrile illness, parasitemia, and anemia

This study was aimed at delineating characteristics of naturally acquired immunity against the merozoite surface antigen-1 (MSP-1) of Plasmodium falciparum, a candidate malaria vaccine antigen. A case/control study was performed on 75 case/control pairs of infants with febrile illness at the time of the first detected infection indicating a clinical case. The presence and level of antibodies at one month prior to the first infection and at the time of the first infection in the afebrile group was significantly higher than in the febrile group. Decreased parasite density and decreased infection-related loss of hemoglobin was seen in infants with anti-MSP-1(19kD) IgG antibodies. In addition, mothers who were positive for the presence of these antibodies conferred protection against placental infection and infection in their infants. In this study, development of anti-MSP-1(19kD) antibody responses in 24 infants were studied longitudinally using monthly serum samples collected from birth until approximately one year of age. In addition, umbilical cord blood sera and respective mothers' sera were analyzed. Longitudinal studies of antibody responses revealed several short-lived IgG and IgM peaks throughout an infant's first year that correlated with detection of parasitemia. The protection against parasitemia and febrile illness was observed in infants when anti-MSP-1(19kD) antibodies were present; when infants were negative for IgG, they had a 10-times greater risk of becoming parasitemic. These data from a longitudinal and prospective study of malaria suggest a protective role for anti-MSP-1(19kD) antibodies in infants and pregnant women.

Identification of T and B cell epitopes recognized by humans in the C-terminal 42-kDa domain of the Plasmodium falciparum merozoite surface protein (MSP)-1

The 42-kDa, C-terminal region of the merozoite surface protein-1 (MSP-1) of Plasmodium falciparum is a putative malaria vaccine candidate Ag. Nine synthetic peptides corresponding to predicted T cell sites of MSP-1 in blocks 15 and 16 and eight overlapping peptides representing the conserved block 17 were used to identify naturally immunogenic epitopes. These peptides were tested for their ability to induce proliferation of PBMC from residents in western Kenya, where malaria transmission is holoendemic. Six peptides (PL145, PL146, PL147, PL148, PL149, and PL150) from blocks 15 and 16 induced a positive proliferative response in &gt; 30% of the individuals tested, and three peptides (PL151, PL152, and PL153) induced a proliferative response in &lt; 25% of the donors. Among these peptides, PL146 was from the highly conserved region, PL150 was from a polymorphic region, and all other peptides were from a dimorphic region of blocks 15 and 16. In block 17, only three peptides, PL99, PL100, and PL103, induced proliferation in 30 to 37% of the volunteers. The rest of the peptides induced a proliferative response in approximately 13 to 25% of the donors. The plasma from these donors widely reacted with different allelic forms of 19-kDa recombinant proteins representing block 17 and recognized at least two linear B epitopes, PL104 and PL97. In summary, this study revealed that a majority of immunodominant T and B epitopes are localized in the conserved or dimorphic regions that are nonpolymorphic in the 42-kDa protein of MSP-1. This study suggests that incorporation of T epitopes from the dimorphic blocks 15 and 16 in a vaccine construct may be useful to ensure Ag-specific memory responses.

Identification of T and B cell epitopes recognized by humans in the C-terminal 42-kDa domain of the Plasmodium falciparum merozoite surface protein (MSP)-1

The 42-kDa, C-terminal region of the merozoite surface protein-1 (MSP-1) of Plasmodium falciparum is a putative malaria vaccine candidate Ag. Nine synthetic peptides corresponding to predicted T cell sites of MSP-1 in blocks 15 and 16 and eight overlapping peptides representing the conserved block 17 were used to identify naturally immunogenic epitopes. These peptides were tested for their ability to induce proliferation of PBMC from residents in western Kenya, where malaria transmission is holoendemic. Six peptides (PL145, PL146, PL147, PL148, PL149, and PL150) from blocks 15 and 16 induced a positive proliferative response in > 30% of the individuals tested, and three peptides (PL151, PL152, and PL153) induced a proliferative response in < 25% of the donors. Among these peptides, PL146 was from the highly conserved region, PL150 was from a polymorphic region, and all other peptides were from a dimorphic region of blocks 15 and 16. In block 17, only three peptides, PL99, PL100, and PL103, induced proliferation in 30 to 37% of the volunteers. The rest of the peptides induced a proliferative response in approximately 13 to 25% of the donors. The plasma from these donors widely reacted with different allelic forms of 19-kDa recombinant proteins representing block 17 and recognized at least two linear B epitopes, PL104 and PL97. In summary, this study revealed that a majority of immunodominant T and B epitopes are localized in the conserved or dimorphic regions that are nonpolymorphic in the 42-kDa protein of MSP-1. This study suggests that incorporation of T epitopes from the dimorphic blocks 15 and 16 in a vaccine construct may be useful to ensure Ag-specific memory responses.