Circumsporozoite Antigen Research Articles

The malaria vaccine – Status quo 2013

It has been 40 years since David Clyde's landmark induction of sterile immunity against deadly falciparum malaria through immunization by exposure to 1000 irradiated mosquitoes, and the first recombinant Plasmodium falciparum vaccine, RTS,S/AS01, is now in Phase III testing. Interim reports from this largest ever Phase III pediatric trial in Africa show the malaria vaccine decreased clinical and severe disease by 56% and 47% respectively in 5-17 month olds, and by 31% and 26% respectively in infants participating in the Expanded Programme on Immunization. Final data in 2014 will more fully describe the efficacy of RTS,S/AS01 over time against all falciparum malaria cases under a variety of transmission conditions, results essential for decisions on licensure and deployment. Meanwhile, candidate components of a second-generation malaria vaccine are emerging. A field trial of the polymorphic blood stage vaccine AMA-1/AS02 demonstrated no overall efficacy (ve = 17%, P = 0.18), yet a sieve analysis revealed allele-specific efficacy (ve = 64%, P = 0.03) against the vaccine strain, suggesting AMA-1 antigens could be part of a multicomponent vaccine. Initial trials of new antigens include the highly conserved pre-erythrocytic candidate PfCelTOS, a synthetic Plasmodium vivax circumsporozoite antigen VMP-001, and sexual stage vaccines containing antigens from both P. falciparum (Pfs25) and P. vivax (Pvs25) intended to interrupt transmission. Targets for a vaccine to protect against placental malaria, the leading remediable cause of low birth weight infants in Africa, have been identified. Lastly, renewed efforts are underway to develop a practical attenuated-sporozoite vaccine to recapture the promise of David Clyde's experiment.

Seasonal prevalence of malaria vectors and entomological inoculation rates in the rubber cultivated area of Niete, South Region of Cameroon

BackgroundDevelopment of large scale agro-industries are subject to serious environmental modifications. In malaria endemic areas this would greatly impact on the transmission paradigm. Two cross-sectional entomological surveys to characterize the Anopheles fauna and their entomological inoculation rates were conducted during May 2010 (peak rainy season) and December 2010 (peak dry season) in the intense rubber cultivated area of Niete in southern forested Cameroon.MethodsMosquitoes were sampled by night collections on human volunteers, identified morphologically and members of the Anopheles gambiae complex further identified to species and molecular form. Parity status was determined following the dissection of the ovaries. Plasmodium falciparum circumsporozoite antigen indices were estimated after the identification of CS antigen by ELISA and the average entomological inoculation rates determined.ResultsA total of 1187 Anopheles was collected, 419 (35.3%) in the rainy season and 768 (64.7%) in the dry season. Species found were the M molecular form of An. gambiae s.s (66.8%), An. ziemanni (28.3%), An. paludis (4.7%), An. smithii (0.2%). An. gambiae M-form was the principal species in the dry (56.2%) and wet (86.2%) seasons. Average overall entomological inoculation rate for the malaria vectors varied between the dry season (1.09 ib/p/n) and the rainy season (2.30 ib/p/n).ConclusionsMalaria transmission in Niete occurs both in the dry and rainy season with the intensities peaking in the dry season. This is unlike previous studies in other areas of southern forested Cameroon where transmission generally peaks in the rainy season. Environmental modifications due to agro-industrial activities might have influenced vector distribution and the dynamics of malaria transmission in this area. This necessitates the possible implementation of control strategies that are related to the eco-geography of the area.

Evaluation of host humoral antibody production against plasmodium falciparum recombinant circumsporozoite antigen in nigerian children

Evaluation of host humoral antibody production against plasmodium falciparum recombinant circumsporozoite antigen in nigerian children

Spatial variability in the density, distribution and vectorial capacity of anopheline species in Rufiji district, south-eastern Tanzania.

Malaria transmission varies from one area to another and there are also local difference in time and space. The objective of the study was to determine the local variability of entomological parameters namely, mosquito abundance, human biting rate (HBR), sporozoite rate for Plasmodiumfalciparum and entomological inoculation rate (EIR). The study was carried out in Rufiji District south eastern Tanzania from October 2001 and September 2004. Adult mosquitoes were collected indoors by CDC light traps. PCR was employed to identify the species within the Anopheles gambiae complex. ELISA was used to determine the sporozoite rate. Over a three year sampling period a total of 64,875 female mosquitoes were caught using light-traps, and of these 28% were Anopheles gambiae complex, 25% An. funestus Giles, 1% An. pharoensis Theobald, 46% Culex species and the rest were Mansonia uniformis Theobald. Mosquito abundance and species composition varied seasonally, spatially and between years. Using PCR, three members of the Anopheles gambiae complex namely An. gambiae s.s. Giles (69%), An. arabiensis Paton (23%) and An. merus D6nitz (7%) were confirmed to occur in the study area. Plasmodiumfalciparum circumsporozoite antigen (CSA) rates were 3.5% for An. gambiae complex and 2.3% for An. funestus. The mean EIR ranged from 28-275 infective bites/person/year. Transmission indices varied over short distances, seasonally and between years. In conclusion, malaria transmission indices in the study area are one of the highest in Tanzania; and there is high variability of entomological parameters over a small geographical area.

A High-Affinity T-Helper Epitope Enhances Peptide-Pulsed Dendritic Cell–Based Vaccine

The NV epitope, a dominant helper determinant from the circumsporozoite antigen of Plasmodium falciparum, is strongly immunogenic and can provide help for cytotoxic T-lymphocyte (CTL) activation. In this study, we evaluated whether the addition of NV peptide can augment the efficacy of peptide-pulsed dendritic cell (DC) immunization in vivo. Using B16 melanoma as tumor model, we demonstrated that DCs pulsed with both NV and gp100 (a melanoma-specific antigen) peptide enhanced immune priming and protection from tumor challenge in vivo. Further, we showed the mechanisms of the NV epitope that help CTL activation; MHC-II-restricted NV peptide induced dramatically more effective helper cells, with a higher level of CD40L expression and IFN-γ production, which, in turn, more effectively conditioned DCs for CTL activation. The improved helper cells also induced greater IL-12 production by DCs, accounting for the reciprocal T-helper polarization to Th1, and increased the expression of costimulatory molecules. Collectively, these findings demonstrate that NV peptide in addition to tumor antigen-pulsed DC immunizations augment helper cell activation, which in turn promotes maturation of DC, and enhance in vivo antitumor activity.

First attempt to validate the gSG6-P1 salivary peptide as an immuno-epidemiological tool for evaluating human exposure to Anopheles funestus bites

The development of a biomarker of exposure based on the evaluation of the human antibody response specific to Anopheles salivary proteins seems promising in improving malaria control. The IgG response specific to the gSG6-P1 peptide has already been validated as a biomarker of An. gambiae exposure. This study represents a first attempt to validate the gSG6-P1 peptide as an epidemiological tool evaluating exposure to An. funestus bites, the second main malaria vector in sub-Saharan Africa. A multi-disciplinary survey was performed in a Senegalese village where An. funestus represents the principal anopheline species. The IgG antibody level specific to gSG6-P1 was evaluated and compared in the same children before, at the peak and after the rainy season. Two-thirds of the children developed a specific IgG response to gSG6-P1 during the study period and--more interestingly--before the rainy season, when An. funestus was the only anopheline species reported. The specific IgG response increased during the An. funestus exposure season, and a positive association between the IgG level and the level of exposure to An. funestus bites was observed. The results suggest that the evaluation of the IgG response specific to gSG6-P1 in children could also represent a biomarker of exposure to An. funestus bites. The availability of such a biomarker evaluating the exposure to both main Plasmodium falciparum vectors in Africa could be particularly relevant as a direct criterion for the evaluation of the efficacy of vector control strategies.

Seasonal variations in antibody response to a Plasmodium falciparum recombinant circumsporozoite antigen in two villages in south western Nigeria

An Enzyme Linked Immunosorbent Assay (ELISA), employing arecombinant peptide capture antigen (R32tet32) was used to detectantibodies against the circumsporozoite protein (CSP) of the malariaparasite, Plasmodium falciparum in 169 serum samples from 16 subjectsfrom two villages, Afefu (FA) and Tobalogbo (TB), in Igbo-OraCommunity of Oyo State, over a period of 12 months. The maximum andmean Ab response for FA was higher than for TB samples (0.511AU±0.170, 0.124±0.045U and 0.250±0.070 AU, 0.090±0.019AUrespectively), with the mean Ab being significantly different (t=2.313;P>0.05). Despite both villages (FA and TB) falling within the samerainfall data zone, the Ab response profile for FA showed a positive(seasonal) relationship with rainfall (r=+0.31, P>0.05) while that of TBwas negatively correlated (r=-0.32; P>0.05). Habits and the environmentcould be prime contributing factors alongside the less controllableimmunogenetic factors. Data obtained here would serve as baseline andwe suggest other expanded sample size studies to include data ontemperature and other climatic factors to help establish sub-populationsat risk and better empower malariologists in planning and execution ofcontrol programmes (Afr. J. Biomed. Res. 9: 37 – 43, 2006)Keywords: Antibodies, Enzyme Linked Immunosorbent Assay (ELISA),Circumsporozoite Protein,, Antigen

Evaluation of a prime-boost vaccine schedule with distinct adenovirus vectors against malaria in rhesus monkeys

A vaccine that elicits both specific antibodies and IFN-γ-producing T cells is required to protect against pre-erythrocytic malaria. Among the most promising approaches to induce such complex immunity are heterologous prime-boost vaccination regimens, in particular ones containing live viral vector. We have demonstrated previously that adenovectors serotype 35 (Ads35) encoding the circumsporozoite (CS) antigen or liver-stage antigen-1 (LSA-1) are highly effective in improving the T-cell responses induced by immunizations with protein-based vaccines in a heterologous prime-boost schedule. Here we evaluated the potential of a heterologous prime-boost vaccination that combines the Ad35.CS vector with the serologically distinct adenovector Ad5.CS, in rhesus macaques, after establishing the potency in mice. We show that the heterologous Ad35.CS/Ad5.CS prime-boost regimen elicits both antibody responses and robust IFN-γ-producing CD8 + T-cell responses against the CS antigen. Analysis of the quality of the antibody responses in rhesus macaques, using indirect immunofluorescence assay (IFA) with Plasmodium falciparum-coated slides, demonstrated that this heterologous prime-boost regimen elicits a high titer of antibodies that are able to bind to P. falciparum sporozoites. Level of the IFA response was superior to the response measured with sera of an adult human population living in endemic malaria region. In conclusion, the combination of Ad35.CS, a vaccine based on a rare serotype adenovirus, with Ad5.CS or possibly another adenovector of a distinct serotype, induces a complex immune response that is required for protection against malaria, and is thus a highly promising approach for pediatric vaccination.

A Hopeful Beginning for Malaria Vaccines

An effective human malaria vaccine has been sought for over 70 years, with little success.1 A successful malaria vaccine used in conjunction with other control interventions would help reduce and eventually eliminate the considerable global disease burden caused by malaria. Many different antigens have been identified as potential targets for malaria-vaccine development. One of these, the repetitive sequence of four amino acids in the circumsporozoite antigen on the surface of the sporozoite of Plasmodium falciparum, arguably the most important of the human malarias, is the basis for the RTS,S vaccine.2 This vaccine was subjected to extensive studies involving human volunteers, . . .

<I>Anopheles kleini</I>, <I>Anopheles pullus</I>, and <I>Anopheles sinensis</I>: Potential Vectors of <I>Plasmodium vivax</I> in the Republic of Korea

Anopheles sinensis Wiedemann (63.3%) was the most abundant Anopheles mosquito captured at cowshed resting collections in malaria high-risk areas (northern Gyeonggi Province) near the demilitarized zone (DMZ) in Korea during 2005, followed by Anopheles kleini Rueda (24.7%) and Anopheles pullus M. Yamada (8.7%). At cowshed resting collections in malaria low-risk areas (Jeonnam and Gyeongnam provinces), An. sinensis accounted for 96.8% of all Anopheles spp. collected, followed by An. kleini Rueda (2.7%), whereas no An. pullus were collected. Three species, An. kleini (50.9%), An. pullus (29.0%), and An. sinensis (13.8%), accounted for nearly all of the 224 Anopheles spp. captured by New Jersey light trap near the DMZ. In addition, An. pullus and An. kleini captured by New Jersey light trap near the DMZ and assayed by enzyme linked immunosorbent assay for Plasmodium vivax circumsporozoite antigen concentrations were higher than An. sinensis sensu stricto (s.s.), indicating higher levels of sporozoites. In laboratory studies of four concurrent artificial membrane feedings on malaria-infected blood from patients, F1 progeny of An. kleini and An. pullus had higher infection rates (8.8 and 7.5%, respectively) than An. sinensis s.s. (4.2%). These data suggest that An. kleini and An. pullus and An. sinensis are vectors of malaria in Korea. Further studies are required to determine the role of these species in the transmission of P. vivax in the Republic of Korea.

Anopheles kleini,Anopheles pullus, andAnopheles sinensis: Potential Vectors ofPlasmodium vivaxin the Republic of Korea

Anopheles sinensis Wiedemann (63.3%) was the most abundant Anopheles mosquito captured at cowshed resting collections in malaria high-risk areas (northern Gyeonggi Province) near the demilitarized zone (DMZ) in Korea during 2005, followed by Anopheles kleini Rueda (24.7%) and Anopheles pullus M. Yamada (8.7%). At cowshed resting collections in malaria low-risk areas (Jeonnam and Gyeongnam provinces), An. sinensis accounted for 96.8% of all Anopheles spp. collected, followed by An. kleini Rueda (2.7%), whereas no An. pullus were collected. Three species, An. kleini (50.9%), An. pullus (29.0%), and An. sinensis (13.8%), accounted for nearly all of the 224 Anopheles spp. captured by New Jersey light trap near the DMZ. In addition, An. pullus and An. kleini captured by New Jersey light trap near the DMZ and assayed by enzyme linked immunosorbent assay for Plasmodium vivax circumsporozoite antigen concentrations were higher than An. sinensis sensu stricto (s.s.), indicating higher levels of sporozoites. In laboratory studies of four concurrent artificial membrane feedings on malaria-infected blood from patients, F1 progeny of An. kleini and An. pullus had higher infection rates (8.8 and 7.5%, respectively) than An. sinensis s.s. (4.2%). These data suggest that An. kleini and An. pullus and An. sinensis are vectors of malaria in Korea. Further studies are required to determine the role of these species in the transmission of P. vivax in the Republic of Korea.

Baculovirus-based Vaccination Vectors Allow for Efficient Induction of Immune Responses Against Plasmodium falciparum Circumsporozoite Protein

Baculovirus vectors are able to transduce a large variety of mammalian cell types and express transgenes placed under the control of heterologous promoters. In this study, we evaluated the potential of baculovirus vectors for malaria vaccination. To induce efficient CD4(+) and CD8(+) T-cell responses, we produced a series of vectors that display the Plasmodium falciparum circumsporozoite (CS) protein in the virion envelope and/or allow for CS expression upon transduction of mammalian cells. We found that baculovirus vectors can transduce professional antigen-presenting cells and trigger their maturation, which is a prerequisite for efficient antigen presentation. Upon intramuscular injection into mice, the vector that both displayed and expressed CS induced higher anti-CS antibody titers (of the immunoglobulin (IgG)1 and IgG2a type) and a higher frequency of interferon-gamma-producing T cells specific to CS, than the vectors which either only displayed or only expressed CS. The baculovirus CS display/expression vector was also superior in inducing CS-specific CD4(+) and CD8(+) T-cell responses in vitro using human peripheral blood mononuclear cells from naive donors. This, together with the absence of pre-existing immunity to baculoviruses in humans, the absence of viral gene expression in mammalian cells, and the relative low immunogenicity of baculovirus virions, makes these vectors promising tools for vaccination. Furthermore, the ability to produce large amounts in serum-free medium at a low cost adds a further advantage to this vector system.

Blood‐feeding behaviour of the malarial mosquitoAnopheles arabiensis: implications for vector control

Feeding behaviour of the malaria vector Anopheles arabiensis Patton (Diptera: Culicidae) was monitored for 12 months (March 2003-February 2004) in the Konso District of southern Ethiopia (5 degrees 15'N, 37 degrees 28'E). More than 45 000 An. arabiensis females were collected by host-baited sampling methods (light-traps, human landing catches, cattle-baited traps) and from resting sites (huts and pit shelters). In the village of Fuchucha, where the ratio of cattle : humans was 0.6 : 1, 51% of outdoor-resting mosquitoes and 66% of those collected indoors had fed on humans, human baits outdoors caught > 2.5 times more mosquitoes than those indoors and the mean catch of mosquitoes from pit shelters was about five times that from huts. Overall, the vast majority of feeding and resting occurred outdoors. In the cattle camps of Konso, where humans slept outdoors close to their cattle, approximately 46% of resting mosquitoes collected outdoors had fed on humans despite the high cattle : human ratio (17 : 1). In both places, relatively high proportions of bloodmeals were mixed cow + human: 22-25% at Fuchucha and 37% in the cattle camps. Anthropophily was also gauged experimentally by comparing the numbers of mosquitoes caught in odour-baited entry traps baited with either human or cattle odour. The human-baited trap caught about five times as many mosquitoes as the cattle-baited one. Notwithstanding the potential pitfalls of using standard sampling devices to analyse mosquito behaviour, the results suggest that the An. arabiensis population is inherently anthropophagic, but this is counterbalanced by exophagic and postprandial exophilic tendencies. Consequently, the population feeds sufficiently on humans to transmit malaria (sporozoite rates: 0.3% for Plasmodium falciparum and 0.5% for P. vivax, by detection of circumsporozoite antigen) but also takes a high proportion of meals from non-human hosts, with 59-91% of resting mosquitoes containing blood from cattle. Hence, classical zooprophylaxis is unlikely to have a significant impact on the malaria vectorial capacity of An. arabiensis in Konso, whereas treating cattle with insecticide might do.

Alternating vector immunizations encoding pre‐erythrocytic malaria antigens enhance memory responses in a malaria endemic area

A heterologous prime-boost strategy has been developed to potently induce T cell responses to pre-erythrocytic malaria antigens. Efficacy in the field is likely to depend on both peak immunogenicity and the durability of responses. To improve both immunogenicity and durability of responses, 54 adult males from a malaria endemic area were immunized with different vaccination regimens, systematically varying antigenic insert and the number and sequence of component vaccinations. The component vaccinations were recombinant attenuated viruses, either fowlpox (FP) 9 or modified vaccinia virus Ankara (MVA). These were recombinant for either of two pre-erythrocytic malaria antigens (multiple epitope-thrombospondin-related adhesion protein, ME-TRAP, or circumsporozoite antigen (CS). ELISPOT assays were used to measure the effector and resting memory T cell responses. Sequence, antigen insert and number of vaccinations influenced immunogenicity, but the novel alternating vector immunizations generated the largest resting memory T cell populations. Effector responses were maintained at 84% of the peak response after 270 days. This durability of response is unprecedented. Classical prime-boost vaccination responses were at 5% of the peak after 270 days. Vaccines administered by heterologous prime-boost regimes are being developed for diverse pathogens and cancer. These data suggest these vaccines should also be administered by alternating vector regimens in clinical development.

980. Specific Immune Response Against the Plasmodium falciparum CS Protein Mediated by Baculovirus Vectors

Top of pageAbstract Malaria is the most common tropical disease worldwide. More than 300 million people are infected and about 2-3 million die annually. The most pathogenic form, Malaria tropica, is caused by Plasmodium falciparum. This enormous world-health problem is compounded by parasites becoming resistant to drugs which are commonly used for treatment and also the complete lack of an effective vaccine. Recombinant baculovirus vectors derived from the Autographa californica nuclear polyhedrosis virus (AcNPV) are commonly used as a tool for high-level expression of recombinant proteins of interest in insect cells. Baculovirus vectors can mediate an immune response against an antigen when it is displayed on the viral surface, or when it is expressed from the baculovirus backbone using promoters which are active in mammalian cells. Their ability to accommodate very large inserts of foreign DNA, the lack of a preexisting/neutralizing immunity to baculoviruses in humans and the easy generation of high titer virus stocks are further clear advantages of this vector system for the delivery of a vaccine. In order to use baculoviruses for an induction of a specific immune response we inserted a P. falciparum circumsporozoite (CS) protein expression cassette, driven by the strong CMV promoter, into the baculoviral genome (AcNPV-CSFVI). An expression of the CS protein in antigen presenting cells (APC) should activate CD8+ T cells via MHC I presentation. The display of a second, recombinant CS peptide in the viral envelope was achieved by fusion of the CS gene to the main baculovirus surface protein gp64 (AcNPV-CS/gp64). This envelope-modified baculovirus mimics the parasite to induce CD4+ T cells and CS specific antibodies following uptake into APC and antigen presentation by MHC II. A third vector (AcNPV-CSFVI-CS/gp64) was constructed to combine expression and presentation of CS antigens in mammalian cells. Expression of the CS protein upon infection of a mouse muscle cell line with AcNPV-CSFVI or AcNPV-CSFVI-CS/gp64 was confirmed by Western blotting with CS specific antibodies. For vaccination studies, BALB/c mice were injected intramuscularly with 5|[times]|108 pfu of AcNPV-CSFVI, AcNPV-CS/gp64, AcNPV-CSFVI-CS/gp64 or 10 ug recombinant CS protein and boosted after 3 weeks with the same dose. Data of IgG1/IgG2a serum ELISAs showed high CS specific antibody titers for mice injected with AcNPV-CS/gp64. Whereas infection with AcNPV-CSFVI could induce only low levels of antibodies, the combined expression and presentation of CS antigens (AcNPV-CSFVI-CS/gp64) could elicit an even higher IgG2a response. Frequencies of CS specific CD4+ and CD8+ T cells will be analyzed by ELIspot and data will be presented. Our current efforts are aimed towards targeting baculoviruses infection to dendritic cells by incorporation of specific receptor ligands into the viral envelope.

Enhanced protective immunity against malaria by vaccination with a recombinant adenovirus encoding the circumsporozoite protein of Plasmodium lacking the GPI-anchoring motif

A major malaria vaccine candidate, the circumsporozoite (CS) protein of Plasmodium, is a pre-erythrocytic stage antigen that is attached to the surface of the sporozoites through a glycosylphosphatidylinositol (GPI) anchor. However, here we show that the motif that signals for glycosylphosphatidylinositol anchor addition interferes with the immunogenicity of this protein and reduces protection in mice upon immunization with a recombinant adenovirus. The presence of the glycosylphosphatidylinositol-anchoring motif sequentially affected total circumsporozoite protein production, cellular distribution, antigen processing and secretion, leading to less effective antigen presentation. Consistently, vaccination with an adenovirus recombinant carrying the anchoring motif-disrupted circumsporozoite gene, resulted in significant increase of the number of interferon-γ (IFN-γ) producing T cells and specific IgG2a isotype antibodies, ensuing more effective vaccination. Given that the anchoring motif is highly conserved among different species of Plasmodium, anti-malaria subunit vaccines encoded by recombinant vectors that aim at the induction of strong cellular immunity could maximize immunogenicity by removing anchoring motifs.

Quantitative determination of sporozoites and circumsporozoite antigen in mosquitoes infected with Plasmodium falciparum or P. vivax.

It is essential for malariologists and researchers to have simple and accurate means of assessing the threat of Plasmodium parasites. An attempt was therefore made to re-standardize one of the circumsporozoite (CS) ELISA that can be used to detect and quantify the circumsporozoite antigens of P. falciparum and P. vivax. A two-site, 'sandwich' ELISA based on a monoclonal antibody was used to test for the CS antigen and sporozoites of each Plasmodium species simultaneously. Using the resultant optical-density values, standard curves, that permit the number of sporozoites in an infected mosquito to be estimated from the quantification of the CS antigen, were constructed. Using these plots and the CS ELISA, the presence of just 12.5 sporozoites (i.e. 0.8 pg CS antigen) of P. falciparum, four sporozoites (3.2 pg antigen) of P. vivax-210 or 12.5 sporozoites (32.0 pg antigen) of P. vivax-247 could be demonstrated.

Dynamics of the malaria-vector populations in coastal Lagos, south-western Nigeria.

An entomological study was carried out in coastal Lagos, south-western Nigeria, to explore the role of Anopheles gambiae s.l. and An. moucheti in the transmission of Plasmodium falciparum in this holo-endemic area. Mosquitoes were caught, on human bait at night and by pyrethrum-spray catches during the day, twice a month throughout 2000. Of the 1812 Anopheles mosquitoes collected, An. gambiae s.l., the predominant vector species, represented 78.7% while the other 21.3% were An. moucheti. The results of a PCR-based test identified 56.8% of the mosquitoes of the An. gambiae complex collected as An. gambiae s.s., 36.9% as An. melas and 6.3% as An. arabiensis. Anopheles gambiae s.s. was predominantly recorded in the wet season, biting females being collected from May to October, with a peak in July. Anopheles melas and An. moucheti were present throughout the year-long study whereas An. arabiensis was mainly found in the dry season. The results of ELISA-based analyses of bloodmeals indicated that An. gambiae s.s., An. melas and An. moucheti were predominantly anthropophagic whereas An. arabiensis was largely zoophagic. Among all of the females investigated, 3.6% of the An. gambiae s.s., 1.9% of the An. melas, 1.8% of the An. moucheti and 0% of the An. arabiensis were found to be infected with P. falciparum (i.e. carrying the parasite's circumsporozoite antigen). The corresponding proportions for the females collected during the dry season were 1.3%, 2.3%, 2.7% and 0%. The entomological inoculation rates for An. melas and An. moucheti were significantly higher during the dry season than at other times of the year. Taken together, these results indicate that An. melas and An. moucheti maintain transmission of P. falciparum during the dry season, while the biting population ofAn. gambiae s.s. is relatively small.

Lack of association between maternal antibody and protection of African infants from malaria infection.

Maternally derived antibodies are believed to protect infants against infection, but there is little direct evidence for a protective role of passively acquired antibodies against malaria. A longitudinal study of malaria infection in 143 infants was conducted in a region of southern Ghana where Plasmodium falciparum is endemic. Infants born in the high-transmission season were less likely to become infected in the first 20 weeks of life than children born in the low-transmission season. Plasma, obtained at birth, was tested for immunoglobulin G (IgG) and IgG subclasses to P. falciparum schizonts and recombinant circumsporozoite antigen, MSP-1(19), MSP-2, AMA-1, and Pf155 (also called ring-infected erythrocyte surface antigen). Antibody levels at birth were not associated with resistance to malaria infection. On the contrary, antibodies at birth were positively associated with infection, indicating that high levels of maternally derived antibodies represent a marker for intensity of exposure to malaria infection in infants. However, all five children who experienced high-density infections (>100 parasites/microl of blood) were seronegative for MSP-1(19) at the time of infection.

Sequence variation in the circumsporozoite protein gene of [formula omitted] isolated from an endemic area in South Korea

We have employed polymerase chain reaction-based techniques to examine the transcript encoding the circumsporozoite (CS) antigen, the immunodominant coat protein of the infectious stage of the murine parasite Plasmodium berghei. Earlier studies suggested that the 3′ terminus of the CS message might be determined by transcription termination rather than by cleavage and polyadenylation, as in most eukaryotes. Here we report that a subset of CS messages are polyadenylated. Moreover, the poly(A) tails are added at multiple sites clustered within a short region 300 bp downstream from the stop codon. Whether 3′ end heterogeneity is peculiar to the CS message or a common feature of plasmodial transcripts remains to be determined.