Malaria circumsporozoite protein binds to heparan sulfate proteoglycans associated with the surface membrane of hepatocytes

Phase I safety and immunogenicity trial of Plasmodium vivax CS derived long synthetic peptides adjuvanted with montanide ISA 720 or montanide ISA 51

We assessed the safety, tolerability, and immunogenicity of a mixture of three synthetic peptides derived from the Plasmodium vivax circumsporozoite protein formulated in Montanide ISA 720 or Montanide ISA 51. Forty healthy malaria-naive volunteers were allocated to five experimental groups (A-E): four groups (A-D) were immunized intramuscularly with 50 and 100 μg/dose injections of a mixture of N, R, and C peptides formulated in the two different adjuvants at 0, 2, and 4 months and one group was administered placebo. Vaccines were immunogenic, safe, well tolerated, and no serious adverse events related to the vaccine occurred. Seroconversion occurred in > 90% of the vaccines and antibodies recognized the sporozoite protein on immunofluorescent antibody test. Vaccines in Montanide ISA 51 showed a higher sporozoite protein recognition and interferon production. Results encourage further testing of the vaccine protective efficacy.

Heparan Sulfate Proteoglycans in Infection

To cause infections, microbial pathogens elaborate a multitude of factors that interact with host components. Using these host–pathogen interactions to their advantage, pathogens attach, invade, disseminate, and evade host defense mechanisms to promote their survival in the hostile host environment. Many viruses, bacteria, and parasites express adhesins that bind to cell surface heparan sulfate proteoglycans (HSPGs) to facilitate their initial attachment and subsequent cellular entry. Some pathogens also secrete virulence factors that modify HSPG expression. HSPGs are ubiquitously expressed on the cell surface of adherent cells and in the extracellular matrix. HSPGs are composed of one or several heparan sulfate (HS) glycosaminoglycan chains attached covalently to specific core proteins. For most intracellular pathogens, cell surface HSPGs serve as a scaffold that facilitates the interaction of microbes with secondary receptors that mediate host cell entry. Consistent with this mechanism, addition of HS or its pharmaceutical functional mimic, heparin, inhibits microbial attachment and entry into cultured host cells, and HS-binding pathogens can no longer attach or enter cultured host cells whose HS expression has been reduced by enzymatic treatment or chemical mutagenesis. In pathogens where the specific HS adhesin has been identified, mutant strains lacking HS adhesins are viable and show normal growth rates, suggesting that the capacity to interact with HSPGs is strictly a virulence activity. The goal of this chapter is to provide a mechanistic overview of our current understanding of how certain microbial pathogens subvert HSPGs to promote their infection, using specific HSPG–pathogen interactions as representative examples.

Invasion of mosquito salivary glands by malaria parasites: prerequisites and defense strategies

The interplay between vector and pathogen is essential for vector-borne disease transmission. Dissecting the molecular basis of refractoriness of some vectors may pave the way to novel disease control mechanisms. A pathogen often needs to overcome several physical barriers, such as the peritrophic matrix, midgut epithelium and salivary glands. Additionally, the arthropod vector elicites immune responses that can severely limit transmission success. One important step in the transmission of most vector-borne diseases is the entry of the disease agent into the salivary glands of its arthropod vector. The salivary glands of blood-feeding arthropods produce a complex mixture of molecules that facilitate blood feeding by inhibition of the host haemostasis, inflammation and immune reactions. Pathogen entry into salivary glands is a receptor-mediated process, which requires molecules on the surface of the pathogen and salivary gland. In most cases, the nature of these molecules remains unknown. Recent advances in our understanding of malaria parasite entry into mosquito salivary glands strongly suggests that specific carbohydrate molecules on the salivary gland surface function as docking receptors for malaria parasites.

Binding patterns of peptide-containing liposomes in liver and spleen of developing mice: comparison with heparan sulfate immunoreactivity

BACKGROUND

Liposomes incorporating peptide from the Plasmodium circumsporozoite protein (CSP) accumulate rapidly and selectively in adult mouse liver.

PURPOSE

The development of the liposome-binding pattern in liver and spleen was studied in relationship to the development of extracellular matrix molecules.

METHODS

Liposomes were administered to mice intravascularly or applied to the surface of liver and spleen slices in vitro. Slices were analyzed immunocytochemically.

RESULTS

Liposomes were found along sinusoidal borders of liver, including the basolateral border of hepatocytes. The pattern was detected in the youngest animals studied (newborn). Intensity of heparan sulfate immunoreactivity increased until adult levels were reached at 20 days. Immunoreactivity for heparan sulfate proteoglycan, but not other proteoglycans, was detected in the youngest animals, and mimicked the pattern of liposome binding. The pattern of liposome binding in the spleen, concentrated in marginal zones, was similar to the pattern of heparan sulfate immunoreactivity, and also similar to the distribution of macrophage immunoreactivity.

CONCLUSION

The postnatal development of liposome binding parallels the development of heparan sulfate immunoreactivity, supporting the suggestion that peptide-containing liposomes target liver by binding to heparan sulfate proteoglycans. Specific delivery of liposomes by targeting heparan sulfate proteoglycans is an effective strategy even at early time periods.

Innate inflammatory response to the malarial pigment hemozoin

Malaria is an infectious disease caused by parasites of the genus Plasmodium. This intraerythrocytic protozoan produces hemozoin (HZ), an insoluble crystalline metabolite resulting from the heme detoxification mechanism. This review will focus on HZ biosynthesis and synthetic preparation, but in particular on its effect on host's innate inflammatory responses.

Immunogenicity and protective efficacy of a recombinant yellow fever vaccine against the murine malarial parasite Plasmodium yoelii

The live-attenuated yellow fever vaccine (YF17D) is one of the safest and most effective vaccines available today. Here, YF17D was genetically altered to express the circumsporozoite protein (CSP) from the murine malarial parasite Plasmodium yoelii. Reconstituted recombinant virus was viable and exhibited robust CSP expression. Immunization of naïve mice resulted in extensive proliferation of adoptively transferred CSP-specific transgenic CD8(+) T-cells. A single immunization of naïve mice with recombinant YF17D resulted in robust production of IFN-gamma by CD8(+) T-cells and IFN-gamma and IL-2 by CD4(+) T-cells. A prime-boost regimen consisting of recombinant virus followed by a low-dose of irradiated sporozoites conferred protection against challenge with P. yoelii. Taken together, these results show that recombinant YF17D can efficiently express CSP in culture, and prime a protective immune response in vivo.

The crystal structures of macrophage migration inhibitory factor from Plasmodium falciparum and Plasmodium berghei

Malaria, caused by Plasmodium falciparum and related parasites, is responsible for millions of deaths each year, mainly from complications arising from the blood stages of its life cycle. Macrophage migration inhibitory factor (MIF), a protein expressed by the parasite during these stages, has been characterized in mammals as a cytokine involved in a broad spectrum of immune responses. It also possesses two catalytic activities, a tautomerase and an oxidoreductase, though the physiological significance of neither reaction is known. Here, we have determined the crystal structure of MIF from two malaria parasites, Plasmodium falciparum and Plasmodium berghei at 2.2 Å and 1.8 Å, respectively. The structures have an α/β fold and each reveals a trimer, in agreement with the results of analytical ultracentrifugation. We observed open and closed active sites, these being distinguished by movements of proline-1, the catalytic base in the tautomerase reaction. These states correlate with the covalent modification of cysteine 2 to form a mercaptoethanol adduct, an observation confirmed by mass spectrometry. The Plasmodium MIFs have a different pattern of conserved cysteine residues to the mammalian MIFs and the side chain of Cys58, which is implicated in the oxidoreductase activity, is buried. This observation and the evident redox reactivity of Cys2 suggest quite different oxidoreductase characteristics. Finally, we show in pull-down assays that Plasmodium MIF binds to the cell surface receptor CD74, a known mammalian MIF receptor implying that parasite MIF has the ability to interfere with, or modulate, host MIF activity through a competitive binding mechanism.

Phosphothioate oligodeoxynucleotides inhibit Plasmodium sporozoite gliding motility

Plasmodium sporozoites, transmitted to the mammalian host through a mosquito bite, travel to the liver, where they invade hepatocytes, and develop into a form that is then able to infect red blood cells. In spite of the importance of innate immunity in controlling microbial infections, almost nothing is known about its role during the liver stage of a malaria infection. Here, we tested whether synthetic CpG phosphothioate (PS) oligodeoxynucleotides (ODNs), which bind to Toll-like receptor 9 (Tlr9), could have a protective effect on Plasmodium berghei infection in hepatocytes. Surprisingly, CpG PS-ODNs potently impair P. berghei infection in hepatoma cell lines independently of Tlr9 activation. Indeed, not only CpG but also non-CpG PS-ODNs, which do not activate Tlr9, decreased parasite infection. Moreover, the ability of PS-ODNs to impair infection was not due to an effect on the host but rather on the parasite itself. In fact, CpG PS-ODNs, as well as non-CpG PS-ODNs, impair parasite gliding motility. Furthermore, our analysis reveals that PS-ODNs inhibit parasite migration and invasion due to their negative charge, whereas development inside hepatocytes is undisturbed. Altogether, PS-ODNs might represent a new class of prophylactic anti-malaria agents, which hamper hepatocyte entry by Plasmodium sporozoites.

Induction of mucosal and systemic humoral immune responses in murine system by intranasal immunization with peptide antigens of P. vivax and CpG oligodeoxynucleotide (ODN) in microparticle delivery

In the present study we have investigated the immunomodulatory effects of two adjuvants, CpG 1826 (two copies of CpG motifs) and CpG 2006 (three copies of CpG motifs) to the five peptide antigens of Plasmodium vivax derived from circumsporozoite protein (CSP), merozoite surface protein-1 (MSP1#1, MSP1#23), apical membrane antigen-1 (AMA-1) and gametocyte surface antigen (Pvs24) in alum and microparticle formulations, using intramuscular and intranasal routes of immunization. Alum formulation without CpG ODN generated low serum IgG and IgA antibody titers and the predominant IgG isotypes were IgG1 but with the addition of CpG ODN (1826 or 2006), the antibody titers were increased by four fold with the predominance of IgG2a/2b isotypes. The SIgA peak titers in lung and intestinal washes were significantly increased with the intranasal mode of administration. Specific activity measurement was done to calculate for the accurate amounts of total serum IgG, IgA and SIgA in washes and showed direct correlation between antibody titer and its concentration. High titer anti-Pvs24 antibodies have significant inhibitory effects on parasite development in the mosquito midgut when tested in membrane feeding assays. The immunofluorescence results show that the peptide specific antisera reacted with the air-dried parasite antigens isolated from P. vivax patients. The present study demonstrates that intranasal route of immunization appears to be an alternate mode of inducing protective immunity in P. vivax malaria.

Calcium-dependent signaling and kinases in apicomplexan parasites

Calcium controls many critical events in the complex life cycles of apicomplexan parasites including protein secretion, motility, and development. Calcium levels are normally tightly regulated and rapid release of calcium into the cytosol activates a family of calcium-dependent protein kinases (CDPKs), which are normally characteristic of plants. CDPKs present in apicomplexans have acquired a number of unique domain structures likely reflecting their diverse functions. Calcium regulation in parasites is closely linked to signaling by cyclic nucleotides and their associated kinases. This Review summarizes the pivotal roles that calcium- and cyclic nucleotide-dependent kinases play in unique aspects of parasite biology.

T-cell epitope polymorphisms of the Plasmodium falciparum circumsporozoite protein among field isolates from Sierra Leone: age-dependent haplotype distribution?

Background

In the context of the development of a successful malaria vaccine, understanding the polymorphisms exhibited by malaria antigens in natural parasite populations is crucial for proper vaccine design. Recent observations have indicated that sequence polymorphisms in the C-terminal T-cell epitopes of the Plasmodium falciparum circumsporozoite protein (Pfcsp) are rather low and apparently stable in low endemic areas. This study sought to assess the pattern in a malaria endemic setting in Africa, using samples from Freetown, Sierra Leone.

Methods

Filter-paper blood samples were collected from subjects at a teaching hospital in Freetown during September–October 2006 and in April–May 2007. The C-terminal portion of the Pfcsp gene spanning the Th2R and Th3R epitopes was amplified and directly sequenced; sequences were analysed with subject parameters and polymorphism patterns in Freetown were compared to that in other malaria endemic areas.

Results and Discussion

Overall, the genetic diversity in Freetown was high. From a total of 99 sequences, 42 haplotypes were identified with at least three accounting for 44.4% (44/99): the 3D7-type (19.2%), a novel type, P-01 (17.2%), and E12 (8.1%). Interestingly, all were unique to the African sub-region and there appeared to be predilection for certain haplotypes to distribute in certain age-groups: the 3D7 type was detected mainly in hospitalized children under 15 years of age, while the P-01 type was common in adult antenatal females (Pearson Chi-square = 48.750, degrees of freedom = 34, P = 0.049). In contrast, the single-haplotype predominance (proportion > 50%) pattern previously identified in Asia was not detected in Freetown.

Conclusion

Haplotype distribution of the T-cell epitopes of Pfcsp in Freetown appeared to vary with age in the study population, and the polymorphism patterns were similar to that observed in neighbouring Gambia, but differed significantly at the sequence level from that observed in Asia. The findings further emphasize the role of local factors in generating polymorphisms in the T-cell epitopes of the P. falciparum circumsporozoite protein.

The association of CD81 with tetraspanin-enriched microdomains is not essential for Hepatitis C virus entry

BACKGROUND

Three percent of the world's population is chronically infected with hepatitis C virus (HCV) and thus at risk of developing liver cancer. Although precise mechanisms regulating HCV entry into hepatic cells are still unknown, several cell surface proteins have been identified as entry factors for this virus. Among these molecules, the tetraspanin CD81 is essential for HCV entry. Interestingly, CD81 is also required for Plasmodium infection. A major characteristic of tetraspanins is their ability to interact with each other and other transmembrane proteins to build tetraspanin-enriched microdomains (TEM).

RESULTS

In our study, we describe a human hepatoma Huh-7 cell clone (Huh-7w7) which has lost CD81 expression and can be infected by HCV when human CD81 (hCD81) or mouse CD81 (mCD81) is ectopically expressed. We took advantage of these permissive cells expressing mCD81 and the previously described MT81/MT81w mAbs to analyze the role of TEM-associated CD81 in HCV infection. Importantly, MT81w antibody, which only recognizes TEM-associated mCD81, did not strongly affect HCV infection. Furthermore, cholesterol depletion, which inhibits HCV infection and reduces total cell surface expression of CD81, did not affect TEM-associated CD81 levels. In addition, sphingomyelinase treatment, which also reduces HCV infection and cell surface expression of total CD81, raised TEM-associated CD81 levels.

CONCLUSION

In contrast to Plasmodium infection, our data show that association of CD81 with TEM is not essential for the early steps of HCV life cycle, indicating that these two pathogens, while using the same molecules, invade their host by different mechanisms.

Genetic diversity of the malaria vaccine candidate Plasmodium falciparum merozoite surface protein-3 in a hypoendemic transmission environment

The N-terminal domain of Plasmodium falciparum merozoite surface protein-3 (PfMSP3) has been excluded from malaria vaccine development largely because of genetic diversity concerns. However, no study to date has followed N-terminal diversity over time. This study describes PfMSP3 variation in a hypoendemic longitudinal cohort in the Peruvian Amazon over the 2003-2006 transmission seasons. Polymerase chain reaction was used to amplify the N-terminal domain in 630 distinct P. falciparum infections, which were allele-typed by size and also screened for sequence variation using a new high-throughput technique, denaturing high performance liquid chromatography. PfMSP3 allele frequencies fluctuated significantly over the 4-year period, but sequence variation was very limited, with only 10 mutations being identified of 630 infections screened. The sequence of the PfMSP3 N-terminal domain is relatively stable over time in this setting, and further studies of its status as a vaccine candidate are therefore warranted.

Malaria parasite development in the mosquito and infection of the mammalian host

Plasmodium sporozoites are the product of a complex developmental process in the mosquito vector and are destined to infect the mammalian liver. Attention has been drawn to the mosquito stages and pre-erythrocytic stages owing to recognition that these are bottlenecks in the parasite life cycle and that intervention at these stages can block transmission and prevent infection. Parasite progression in the Anopheles mosquito, sporozoite transmission to the mammalian host by mosquito bite, and subsequent infection of the liver are characterized by extensive migration of invasive stages, cell invasion, and developmental changes. Preparation for the liver phase in the mammalian host begins in the mosquito with an extensive reprogramming of the sporozoite to support efficient infection and survival. Here, we discuss what is known about the molecular and cellular basis of the developmental progression of parasites and their interactions with host tissues in the mosquito and during the early phase of mammalian infection.

Plasmodium sporozoite passage across the sinusoidal cell layer

Malaria sporozoites must cross at least two cell barriers to reach their initial site of replication in the mammalian host. After transmission into the skin by an infected mosquito, they migrate towards small dermal capillaries, traverse the vascular endothelial layer, and rapidly home to the liver. To infect hepatocytes, the parasites must cross the sinusoidal cell layer, composed of specialized highly fenestrated sinusoidal endothelia and Kupffer cells, the resident macrophages of the liver (Fig. 1). The exact route Plasmodium sporozoites take to hepatocytes has been subject of controversial discussions for many years. Recent cell biological, microscopic, and genetic approaches have considerably enhanced our understanding of the initial events leading to the establishment of a malaria infection in the liver.

Liposomes incorporating a Plasmodium amino acid sequence target heparan sulfate binding sites in liver

Previous studies demonstrated that intravenously administered liposomes, incorporating a peptide from the Plasmodium circumsporozoite protein, accumulate rapidly and selectively in mouse liver. The present investigation was designed to determine the molecular components in liver responsible for liposome targeting. Studies of liver tissue slices demonstrated that immunoreactivity for heparan sulfate proteoglycan (HSPG), but not other tested proteoglycans, was distributed along sinusoidal borders of liver; this immunoreactivity appeared associated with nonparenchymal cells of the sinusoids and with the basolateral portion of hepatocytes. Peptide-containing liposomes bound to liver tissue in a pattern similar to the distribution of heparan sulfate immunoreactivity, either after intravenous injection of liposomes in vivo or after incubation of liposomes with liver slices in vitro. Control liposomes, without the peptide, displayed very light binding without a pattern. Pretreatment of liver slices with heparinase, but not chondroitinase or hyaluronidase, eliminated peptide-containing liposome binding, but did not affect binding of control liposomes. Coincubation of peptide-containing liposomes with heparin, but not with other glycosaminoglycans, markedly inhibited liposome binding to liver slices. N-desulfated and O-desulfated heparins individually were less effective inhibitors of liposome binding than was heparin. These results indicate that liposomes containing a peptide from Plasmodium target liver tissue by binding to HSPGs in the extracellular matrix.

Interaction of an atypical Plasmodium falciparum ETRAMP with human apolipoproteins

BACKGROUND

In order to establish a successful infection in the human host, the malaria parasite Plasmodium falciparum must establish interactions with a variety of human proteins on the surface of different cell types, as well as with proteins inside the host cells. To better understand this aspect of malaria pathogenesis, a study was conducted with the goal of identifying interactions between proteins of the parasite and those of its human host.

METHODS

A modified yeast two-hybrid methodology that preferentially selects protein fragments that can be expressed in yeast was used to conduct high-throughput screens with P. falciparum protein fragments against human liver and cerebellum libraries. The resulting dataset was analyzed to exclude interactions that are not likely to occur in the human host during infection.

RESULTS

An initial set of 2,200 interactions was curated to remove proteins that are unlikely to play a role in pathogenesis based on their annotation or localization, and proteins that behave promiscuously in the two-hybrid assay, resulting in a final dataset of 456 interactions. A cluster that implicates binding between P. falciparum PFE1590w/ETRAMP5, a putative parasitophorous vacuole membrane protein, and human apolipoproteins ApoA, ApoB and ApoE was selected for further analysis. Different isoforms of ApoE, which are associated with different outcomes of malaria infection, were shown to display differential interactions with PFE1590w.

CONCLUSION

A dataset of interactions between proteins of P. falciparum and those of its human host was generated. The preferential interaction of the P. falciparum PFE1590w protein with the human ApoE epsilon3 and ApoE epsilon4 isoforms, but not the ApoE epsilon2 isoform, supports the hypothesis that ApoE genotype affects risk of malaria infection. The dataset contains other interactions of potential relevance to disease that may identify possible vaccine candidates and drug targets.

Malaria parasite pre-erythrocytic stage infection: gliding and hiding

In malaria, the red blood cell-infectious form of the Plasmodium parasite causes illness and the possible death of infected hosts. The initial infection in the liver caused by the mosquito-borne sporozoite parasite stage, however, causes little pathology and no symptoms. Nevertheless, pre-erythrocytic parasite stages are attracting passionate research efforts not least because they are the most promising targets for malaria vaccine development. Here, we review how the infectious sporozoite makes its way to the liver and subsequently develops within hepatocytes. We discuss the factors, both parasite and host, involved in the interactions that occur during this "silent" phase of infection.

Delivery of siRNA mediated by histidine-containing reducible polycations

Histidine containing reducible polycations based on CH(6)K(3)H(6)C monomers (His6 RPCs), are highly effective DNA transfection agents combining pH buffering endosomal escape mechanisms with rapid unpackaging following reduction in the cytoplasm. We examined their ability to mediate siRNA uptake into cells focusing on hepatocyte delivery. Co-delivery of EGFP siRNA with pEGFP plasmid DNA reduced reporter gene expression by 85%. However while DNA transfection efficiency increased with polymer size, with 162 k His6 RPCs proving the most effective, delivery of siRNA alone to EGFP stably expressing cells was only possible using 36-80 k polymers. Analysis of particle sizes showed that 80 k polymers formed more compact siRNA complexes than 162 k polymers. The reducible nature of the polymer was necessary for siRNA activity, since siRNA combined with non-reducible polylysine showed little activity. Incorporation of a targeting peptide from the Plasmodium falciparum circumsporozoite (CS) protein onto His6 RPCs, significantly improved transfection of plasmid DNA and siRNA activity in hepatocytes, but not in most non-liver cells tested. siRNA targeted to the hepatitis B virus surface antigen delivered by CS-His6 RPC, mediated falls in both mRNA and protein expression, suggesting that this delivery system could be developed for potential therapies for viral hepatitis.

Heme oxygenase-1 is an anti-inflammatory host factor that promotes murine plasmodium liver infection

The clinically silent Plasmodium liver stage is an obligatory step in the establishment of malaria infection and disease. We report here that expression of heme oxygenase-1 (HO-1, encoded by Hmox1) is upregulated in the liver following infection by Plasmodium berghei and Plasmodium yoelii sporozoites. HO-1 overexpression in the liver leads to a proportional increase in parasite liver load, and treatment of mice with carbon monoxide and with biliverdin, each an enzymatic product of HO-1, also increases parasite liver load. Conversely, mice lacking Hmox1 completely resolve the infection. In the absence of HO-1, the levels of inflammatory cytokines involved in the control of liver infection are increased. These findings suggest that, while stimulating inflammation, the liver stage of Plasmodium also induces HO-1 expression, which modulates the host inflammatory response, protecting the infected hepatocytes and promoting the liver stage of infection.

Gene-specific signatures of elevated non-synonymous substitution rates correlate poorly across the Plasmodium genus

BACKGROUND

Comparative genome analyses of parasites allow large scale investigation of selective pressures shaping their evolution. An acute limitation to such analysis of Plasmodium falciparum is that there is only very partial low-coverage genome sequence of the most closely related species, the chimpanzee parasite P. reichenowi. However, if orthologous genes have been under similar selective pressures throughout the Plasmodium genus then positive selection on the P. falciparum lineage might be predicted to some extent by analysis of other lineages.

PRINCIPAL FINDINGS

Here, three independent pairs of closely related species in different sub-generic clades (P. falciparum and P. reichenowi; P. vivax and P. knowlesi; P. yoelii and P. berghei) were compared for a set of 43 candidate ligand genes considered likely to be under positive directional selection and a set of 102 control genes for which there was no selective hypothesis. The ratios of non-synonymous to synonymous substitutions (dN/dS) were significantly elevated in the candidate ligand genes compared to control genes in each of the three clades. However, the rank order correlation of dN/dS ratios for individual candidate genes was very low, less than the correlation for the control genes.

SIGNIFICANCE

The inability to predict positive selection on a gene in one lineage by identifying elevated dN/dS ratios in the orthologue within another lineage needs to be noted, as it reflects that adaptive mutations are generally rare events that lead to fixation in individual lineages. Thus it is essential to complete the genome sequences of particular species of phylogenetic importance, such as P. reichenowi.

Exoerythrocytic development of Plasmodium gallinaceum in the White Leghorn chicken

Plasmodium gallinaceum typically causes sub-clinical disease with low mortality in its primary host, the Indian jungle fowl Gallus sonnerati. Domestic chickens of European origin, however, are highly susceptible to this avian malaria parasite. Here we describe the development of P. gallinaceum in young White Leghorn chicks with emphasis on the primary exoerythrocytic phase of the infection. Using various regimens for infection, we found that P. gallinaceum induced a transient primary exoerythrocytic infection followed by a fulminant lethal erythrocytic phase. Prerequisite for the appearance of secondary exoerythrocytic stages was the development of a certain level of parasitaemia. Once established, secondary exoerythrocytic stages could be propagated from bird to bird for several generations without causing fatalities. Infected brains contained large secondary exoerythrocytic stages in capillary endothelia, while in the liver primary and secondary erythrocytic stages developed primarily in Kupffer cells and remained smaller. At later stages, livers exhibited focal hepatocyte necrosis, Kupffer cell hyperplasia, stellate cell proliferation, inflammatory cell infiltration and granuloma formation. Because P. gallinaceum selectively infected Kupffer cells in the liver and caused a histopathology strikingly similar to mammalian species, this avian Plasmodium species represents an evolutionarily closely related model for studies on the hepatic phase of mammalian malaria.

Role of Plasmodium falciparum thrombospondin-related anonymous protein in host-cell interactions

Background

Thrombospondin-related anonymous protein (TRAP) is essential for sporozoite motility and for liver cell invasion. TRAP is a type 1 membrane protein that possesses multiple adhesive domains in its extracellular region.

Methods

Plasmodium falciparum TRAP (PfTRAP) and its subdomains were expressed in a mammalian expression system, and eleven different mutants generated to study interaction of PfTRAP with liver cells. Binding studies between HepG2 cell extracts and PfTRAP were performed using co-immunoprecipitation protocols.

Results

Five different amino acid residues of PfTRAP that are involved in liver cell binding have been identified. These PfTRAP mutants bound to heparin like the wild type PfTRAP thereby suggesting a non-heparin mediated binding of PfTRAP to liver cells. Three Src family proteins -Lyn, Lck and CrkL which interact with PfTRAP are also identified. Liver cell extracts and immunoprecipitated Src family kinases phosphorylated PfTRAP at multiple sites. An analysis of multiple TRAP sequences revealed Src homology 3 domain (SH3) binding motifs.

Conclusion

Binding of PfTRAP to SH3-domain containing proteins like Src-family kinases and their ability to phosphorylate PfTRAP suggests a novel role for PfTRAP in cell signaling during sporozoite invasion and homing inside the liver cells. These data shed new light on TRAP-liver cell interactions.

Hepatocyte permissiveness to Plasmodium infection is conveyed by a short and structurally conserved region of the CD81 large extracellular domain

Invasion of hepatocytes by Plasmodium sporozoites is a prerequisite for establishment of a malaria infection, and thus represents an attractive target for anti-malarial interventions. Still, the molecular mechanisms underlying sporozoite invasion are largely unknown. We have previously reported that the tetraspanin CD81, a known receptor for the hepatitis C virus (HCV), is required on hepatocytes for infection by sporozoites of several Plasmodium species. Here we have characterized CD81 molecular determinants required for infection of hepatocytic cells by P. yoelii sporozoites. Using CD9/CD81 chimeras, we have identified in CD81 a 21 amino acid stretch located in a domain structurally conserved in the large extracellular loop of tetraspanins, which is sufficient in an otherwise CD9 background to confer susceptibility to P. yoelii infection. By site-directed mutagenesis, we have demonstrated the key role of a solvent-exposed region around residue D137 within this domain. A mAb that requires this region for optimal binding did not block infection, in contrast to other CD81 mAbs. This study has uncovered a new functionally important region of CD81, independent of HCV E2 envelope protein binding domain, and further suggests that CD81 may not interact directly with a parasite ligand during Plasmodium infection, but instead may regulate the function of a yet unknown partner protein.

Minutes after the bite of a female mosquito, the malaria parasite Plasmodium enters the liver where it invades liver-specific cells called hepatocytes and undergoes one round of multiplication. This stage is a prerequisite to the blood stages of the life cycle which cause the malaria symptoms. The invasion of hepatocytes probably requires a series of interaction between the host cell and the parasite, but the exact mechanisms are still elusive. CD81, a protein of the tetraspanin superfamily, is the only hepatocyte surface protein that has been shown to be strictly required for the infection by the malaria parasite. We have here studied the regions of CD81 that are important for infection, by exchanging segments with the corresponding parts of a closely related molecule, or by mutating discrete residues. This study has uncovered a new functionally important region of CD81 and, by comparing the ability of several CD81 antibodies to block infection, has strengthened the hypothesis that CD81 might regulate the function of another molecule present at the hepatocyte surface during Plasmodium infection. The region of CD81 identified here is different from the region involved in the binding of the hepatitis C virus.

Antibodies to pre-erythrocytic Plasmodium falciparum antigens and risk of clinical malaria in Kenyan children

BACKGROUND

IgG antibodies to pre-erythrocytic antigens are involved in prevention of infection and disease in animal models of malaria but have not been associated with protection against disease in human malaria.

METHODS

Levels of IgG antibodies to circumsporozoite protein (CSP), liver-stage antigen type 1 (LSA-1), and thrombospondin-related adhesive protein (TRAP) were measured in 86 children in a malaria-holoendemic area of Kenya. The children were then monitored for episodes of clinical malaria for 52 weeks.

RESULTS

Children with high levels of IgG antibodies to CSP, LSA-1, and TRAP had a decreased risk of clinical malaria (adjusted hazard ratio, 0.29; 95% confidence interval 0.10-0.81; P = .02), a lower incidence of clinical malaria (P=.006), protection from clinical malaria with a parasite level of > or =4000 parasites/microL (P= .03), and a higher hemoglobin level at enrollment (P= .009), compared with children with lower antibody levels. Protection against malaria morbidity was associated primarily with antibodies to CSP and LSA-1.

CONCLUSIONS

Kenyan children with high levels of IgG antibodies to the pre-erythrocytic antigens CSP, LSA-1, and TRAP have a lower risk of developing clinical malaria than children without high levels of these antibodies. The decreased risk of clinical malaria may be mediated in part by prevention of high-density parasitemia.

Release of hepatic Plasmodium yoelii merozoites into the pulmonary microvasculature

Plasmodium undergoes one round of multiplication in the liver prior to invading erythrocytes and initiating the symptomatic blood phase of the malaria infection. Productive hepatocyte infection by sporozoites leads to the generation of thousands of merozoites capable of erythrocyte invasion. Merozoites are released from infected hepatocytes as merosomes, packets of hundreds of parasites surrounded by host cell membrane. Intravital microscopy of green fluorescent protein-expressing P. yoelii parasites showed that the majority of merosomes exit the liver intact, adapt a relatively uniform size of 12-18 microm, and contain 100-200 merozoites. Merosomes survived the subsequent passage through the right heart undamaged and accumulated in the lungs. Merosomes were absent from blood harvested from the left ventricle and from tail vein blood, indicating that the lungs effectively cleared the blood from all large parasite aggregates. Accordingly, merosomes were not detectable in major organs such as brain, kidney, and spleen. The failure of annexin V to label merosomes collected from hepatic effluent indicates that phosphatidylserine is not exposed on the surface of the merosome membrane suggesting the infected hepatocyte did not undergo apoptosis prior to merosome release. Merosomal merozoites continued to express green fluorescent protein and did not incorporate propidium iodide or YO-PRO-1 indicating parasite viability and an intact merosome membrane. Evidence of merosomal merozoite infectivity was provided by hepatic effluent containing merosomes being significantly more infective than blood with an identical low-level parasitemia. Ex vivo analysis showed that merosomes eventually disintegrate inside pulmonary capillaries, thus liberating merozoites into the bloodstream. We conclude that merosome packaging protects hepatic merozoites from phagocytic attack by sinusoidal Kupffer cells, and that release into the lung microvasculature enhances the chance of successful erythrocyte invasion. We believe this previously unknown part of the plasmodial life cycle ensures an effective transition from the liver to the blood phase of the malaria infection.

Malaria - an overview

Malaria is caused by protozoan parasites of the genus Plasmodium and is a major cause of mortality and morbidity worldwide. These parasites have a complex life cycle in their mosquito vector and vertebrate hosts. The primary factors contributing to the resurgence of malaria are the appearance of drug-resistant strains of the parasite, the spread of insecticide-resistant strains of the mosquito and the lack of licensed malaria vaccines of proven efficacy. This minireview includes a summary of the disease, the life cycle of the parasite, information relating to the genome and proteome of the species lethal to humans, Plasmodium falciparum, together with other recent developments in the field.

High affinity interactions between red blood cell receptors and synthetic Plasmodium thrombospondin-related apical merozoite protein (PTRAMP) peptides

Plasmodium falciparum thrombospondin-related apical merozoite protein (PTRAMP) has a thrombospondin related (TSR) domain which in many proteins has been reported as a fragment involved in pathogen-host and cell-interactions. Receptor-ligand studies using eighteen non-overlapping 20-aminoacid-long synthetic peptides from this protein were carried out to determine regions involved in parasite invasion of red blood cells (RBC). Two high activity binding peptides (HABPs) were determined, 33405 (21YISSNDLTSTNLKVRNNWEH40) and 33413 (180LEGPIQFSLGKSSGAFRINY199), presenting high dissociation constants and positive cooperativity. One of the HABPs displayed a modified Plasmodium export element (PEXEL), suggesting that this protein could be involved in the merozoite cytoplasmic reticulum, parasitophorous vacuole, red blood cell (RBC) cytosol, and probably infected RBC (iRBC) membrane transport of some other molecules and nutrients. Enzymatic treatment of RBCs increased HABP 33405 binding to them whilst it decreased HABP 33413 binding. Merozoite invasion assays revealed that HABPs have around 57% ability to inhibit new RBC invasion. Circular dichroism revealed the presence of possible alpha-helical elements in both HABPs structures. RBC binding interaction specificity and the presence of a PEXEL motif make these 2 HABPs good candidates for being included in further studies to develop a new multi-antigenic, multi-stage, subunit-based, chemically-synthesised, anti-malarial vaccine.

Plasmodium falciparum: effect of radiation on levels of gene transcripts in sporozoites

Humans immunized by the bites of irradiated Plasmodium falciparum (Pf) sporozoite-infected mosquitoes are protected against malaria. Radiation attenuates the sporozoites preventing them from fully developing and replicating in hepatocytes, but the effects of radiation on gene expression in sporozoites are unknown. We used RT-PCR (35 cycles of PCR followed by densitometry) to assess the expression of ten genes in Pf sporozoites, and in sporozoites irradiated with 15,000cGy. Irradiation reduced expression substantially (>60%) of two DNA repair genes; moderately (30-60%) of PfUIS3, the Pf orthologue of PbUIS3, a gene up-regulated in Plasmodium berghei sporozoites and of a third DNA repair gene; and minimally (<30%) of the Pf18S ribosomal RNA, PfCSP, PfSSP2/TRAP, and PfCELTOS genes. Irradiation increased expression of PfSPATR minimally. PfLSA1 RNA was not detectable in sporozoites. These results establish that radiation of sporozoites affects gene expression levels and provide the foundation for studies to identify specific genes involved in attenuation and protective immunity.

A long and winding road: the Plasmodium sporozoite's journey in the mammalian host

The Plasmodium sporozoite, the infectious stage of the malaria parasite, makes a remarkable journey in its mammalian host. Here we review our current knowledge of the molecular and cellular basis of this journey, which begins in the skin and ends in the hepatocyte.

Mosquito heparan sulfate and its potential role in malaria infection and transmission

Heparan sulfate has been isolated for the first time from the mosquito Anopheles stephensi, a known vector for Plasmodium parasites, the causative agents of malaria. Chondroitin sulfate, but not dermatan sulfate or hyaluronan, was also present in the mosquito. The glycosaminoglycans were isolated, from salivary glands and midguts of the mosquito in quantities sufficient for disaccharide microanalysis. Both of these organs are invaded at different stages of the Plasmodium life cycle. Mosquito heparan sulfate was found to contain the critical trisulfated disaccharide sequence, -->4)beta-D-GlcNS6S(1-->4)-alpha-L-IdoA2S(1-->, that is commonly found in human liver heparan sulfate, which serves as the receptor for apolipoprotein E and is also believed to be responsible for binding to the circumsporozoite protein found on the surface of the Plasmodium sporozoite. The heparan sulfate isolated from the whole mosquito binds to circumsporozoite protein, suggesting a role within the mosquito for infection and transmission of the Plasmodium parasite.

Structural basis for binding of Plasmodium falciparum erythrocyte membrane protein 1 to chondroitin sulfate and placental tissue and the influence of protein polymorphisms on binding specificity

Chondroitin sulfate (CS) A is a key receptor for adhesion of Plasmodium falciparum-infected erythrocytes (IEs) in the placenta and can also mediate adhesion to microvascular endothelial cells. IEs that adhere to CSA express var2csa-type genes, which encode specific variants of the IE surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1). We report direct binding of native PfEMP1, isolated from IEs and encoded by var2csa, to immobilized CSA. Binding of PfEMP1 was dependent on 4-O-sulfated disaccharides and glucuronic acid rather than iduronic acid, consistent with the specificity of intact IEs. Using immobilized CS oligosaccharides as neoglycolipid probes, the minimum chain length for direct binding of PfEMP1 was eight monosaccharide units. Similarly for IE adhesion to placental tissue there was a requirement for 4-O-sulfated GalNAc and glucuronic acid mixed with non-sulfated disaccharides; 6-O-sulfation interfered with the interaction between placental CSA and IEs. The minimum chain length for maximal inhibition of adhesion was 10 monosaccharide residues. Partially 4-O-sulfated CS oligosaccharides (45-55% sulfation) were highly effective inhibitors of placental adhesion (IC(50), 0.15 microg/ml) and may have potential for therapeutic development. We used defined P. falciparum isolates expressing different variants of var2csa in adhesion assays and found that there were isolate-specific differences in the preferred structural motifs for adhesion to CSA that correlated with polymorphisms in PfEMP1 encoded by var2csa-type genes. This may influence sites of IE sequestration or parasite virulence. These findings have significant implications for understanding the pathogenesis and biology of malaria, particularly during pregnancy, and the development of targeted interventions.

Exposure of Plasmodium sporozoites to the intracellular concentration of potassium enhances infectivity and reduces cell passage activity

Malaria sporozoites migrate through several cells prior to a productive invasion that involves the formation of a parasitophorous vacuole (PV) where sporozoites undergo transformation into Exo-erythorcytic forms (EEFs). The precise mechanism leading to sporozoite activation for invasion is unknown, but prior traversal of host cells is required. During cell migration sporozoites are exposed to large shifts in K(+) concentration. We report here that incubation of sporozoites to the intracellular K(+) concentration enhances 8-10 times the infectivity of Plasmodium berghei and 4-5 times the infectivity of Plasmodium yoelli sporozoites for a hepatocyte cell line, while simultaneously decreasing cell passage activity. The K(+) enhancing effect was time and concentration dependent, and was significantly decreased by K(+) channel inhibitors. Potassium-treated P. berghei sporozoites also showed enhanced numbers of EEFs in non-permissive cell lines. Treated sporozoites had reduced infectivity for mice, but infectivity was enhanced upon Kupffer cell depletion. Transcriptional analysis of K(+) treated and control sporozoites revealed a high degree of correlation in their levels of gene expression, indicating that the observed phenotypic changes are not due to radical changes in gene transcription. Only seven genes were upregulated by more than two-fold in K(+) treated sporozoites. The highest level was noted in PP2C, a phosphatase known to dephosphorylate the AKT potassium channel in plants.

Malaria circumsporozoite protein inhibits the respiratory burst in Kupffer cells

After transmission by infected mosquitoes, malaria sporozoites rapidly travel to the liver. To infect hepatocytes, sporozoites traverse Kupffer cells, but surprisingly, the parasites are not killed by these resident macrophages of the liver. Here we show that Plasmodium sporozoites and recombinant circumsporozoite protein (CSP) suppress the respiratory burst in Kupffer cells. Sporozoites and CSP increased the intracellular concentration of cyclic adenosyl mono-phosphate (cAMP) and inositol 1,4,5-triphosphate in Kupffer cells, but not in hepatocytes or liver endothelia. Preincubation with cAMP analogues or inhibition of phosphodiesterase also inhibited the respiratory burst. By contrast, adenylyl cyclase inhibition abrogated the suppressive effect of sporozoites. Selective protein kinase A (PKA) inhibitors failed to reverse the CSP-mediated blockage and stimulation of the exchange protein directly activated by cAMP (EPAC), but not PKA inhibited the respiratory burst. Both blockage of the low-density lipoprotein receptor-related protein (LRP-1) with receptor-associated protein and elimination of cell surface proteoglycans inhibited the cAMP increase in Kupffer cells. We propose that by binding of CSP to LRP-1 and cell surface proteoglycans, malaria sporozoites induce a cAMP/EPAC-dependent, but PKA-independent signal transduction pathway that suppresses defence mechanisms in Kupffer cells. This allows the sporozoites to safely pass through these professional phagocytes and to develop inside neighbouring hepatocytes.

Plasmodium vivax thrombospondin related adhesion protein: immunogenicity and protective efficacy in rodents and Aotus monkeys

The thrombospondin related adhesion protein (TRAP) is a malaria pre-erythrocytic antigen currently pursued as malaria vaccine candidate to Plasmodium falciparum. In this study, a long synthetic peptide (LSP) representing a P. vivax TRAP fragment involved in hepatocyte invasion was formulated in both Freund and Montanide ISA 720 adjutants and administered by IM and subcutaneous routes to BALB/c mice and Aotus monkeys. We measured specific humoral immune responses in both animal species and performed a sporozoite challenge in Aotus monkeys to assess the protective efficacy of the vaccine. After immunization both mice and Aotus seroconverted as shown by ELISA, and the specific anti-peptide antibodies cross reacted with the parasite in IFAT assays. Only two out of six immunized animals became infected after P. vivax sporozoite challenge as compared with four out of six animals from the control group. These results suggest that this TRAP fragment has protective potential against P. vivax malaria and deserves further studies as vaccine candidate.

Live and let die: manipulation of host hepatocytes by exoerythrocytic Plasmodium parasites

The generation of rodent Plasmodium strains expressing fluorescent proteins in all life cycle stages has had a big impact on malaria research. With this tool in hand, for the first time it was possible to follow in real time by in vivo microscopy the infection route of Plasmodium sporozoites transmitted to the mammalian host by Anopheles mosquitoes. Recently, this work has been extended to the analysis of both hepatocyte infection by Plasmodium sporozoites, as well as liver merozoite transport into blood vessels. The stunning results of these studies have considerably changed our understanding of hepatocyte invasion and parasite liberation. Here, we describe the most important findings of the last years and in addition, we elaborate on the molecular events during the intracellular development of Plasmodium exoerythrocytic forms that give rise to erythrocyte infecting merozoites.

An update on the search for a Plasmodium vivax vaccine

Although Plasmodium falciparum is the leading cause of morbidity and mortality due to malaria worldwide, nearly 2.5 billion people, mostly outside Africa, are also at risk from malaria caused by Plasmodium vivax infection. Currently, almost all efforts to develop a malaria vaccine have focused on P. falciparum. For example, there are 23 P. falciparum vaccine candidates undergoing advanced clinical studies and only two P. vivax vaccine candidates being tested in preliminary (Phase I) clinical trials, with few others being assessed in preclinical studies. More investment and a greater effort toward the development of P. vivax vaccine components for a multi-species vaccine are required. This is mainly because of the wide geographical coexistence of both parasite species but also because of increasing drug resistance, recent observations of severe and lethal P. vivax cases and relapsing parasite behaviour. Availability of the P. vivax genome has contributed to antigen discovery but new means to test vaccines in future trials remain to be designed.

Differential evidence of natural selection on two leading sporozoite stage malaria vaccine candidate antigens

Experimental malaria vaccines based on two sporozoite stage candidate antigens of Plasmodium falciparum, the circumsporozoite protein (CSP) and thrombospondin-related adhesive protein (TRAP), have undergone clinical trials of efficacy. The relevance of naturally existing polymorphism in these molecules remains unknown. Sequence polymorphism in the genes encoding these antigens was studied in a Gambian population (sample of 48 trap and 44 csp gene sequences) to test for signatures of selection that would result from naturally acquired immunity. Allele frequency distributions were analyzed and compared with data from another population (in Thailand). Patterns of non-synonymous and synonymous polymorphism in P. falciparum and in Plasmodium vivax were compared with divergence from related species. Results indicate that polymorphism in TRAP is under strong selection for amino acid sequence diversity and that allele frequencies are under balancing selection within the Gambian P. falciparum population. There was no such evidence for CSP, calling into question the idea that most polymorphisms in this gene are under immune selection. There was a weak trend for regions known to encode T cell epitopes to have slightly higher indices suggesting balancing selection. Overall, the results predict more allele-specific immunity to TRAP than to CSP and should be considered in design and efficacy testing of vaccine candidates based on these antigens.

The circumsporozoite protein is an immunodominant protective antigen in irradiated sporozoites

Malaria infection starts when mosquitoes inject sporozoites into the skin. The parasites enter the blood stream and make their way to the liver where they develop into the exo-erythrocytic forms (EEFs). Immunization with irradiated sporozoites (IrSp) leads to robust protection against malaria infection in rodents, monkeys and humans by eliciting antibodies to circumsporozoite protein (CS) that inhibit sporozoite infectivity, and T cells that destroy the EEFs. To study the role of non-CS antigens in protection, we produced CS transgenic mice that were tolerant to CS T-cell epitopes. Here we show that in the absence of T-cell-dependent immune responses to CS, protection induced by immunization with two doses of IrSp was greatly reduced. Thus, although hundreds of other Plasmodium genes are expressed in sporozoites and EEFs, CS is a dominant protective antigen. Nevertheless, sterile immunity could be obtained by immunization of CS transgenics with three doses of IrSp.

Developmental biology of sporozoite-host interactions in Plasmodium falciparum malaria: implications for vaccine design

The Plasmodium falciparum sporozoite infects different types of cells in a mosquito's salivary glands and human epithelial and Kuppfer cells and hepatocytes. These become differentiated later on, transforming themselves into the invasive red blood cell form, the merozoite. The ability of sporozoites to interact with different types of cells requires a wide variety of mechanisms allowing them to survive in both hosts: mobility, receptor-ligand interactions with different cellular receptors, and transformation and development into other invasive parasite forms, which are vitally important for parasite survival. Sporozoite complexity is reflected in the large quantity of proteins that can be expressed. Some of them have been extensively studied, such as CSP, TRAP, STARP, LSA-1, LSA-3, SALSA, SPECT1, SPECT2, MAEBL, and SPATR, due to their importance in infection and their potential use as vaccines. Our work has been focused on the search for the molecular mechanisms of parasite-host cellular receptor-ligand interactions by identifying amino acid sequences and the critical binding residues from these proteins relevant to parasite invasion. Once such sequences have been identified, it will be possible to modify them to induce a strong immune response against P. falciparum in the experimental Aotus monkey model. This all leads towards developing multistage, multicomponent, subunit-based vaccines that will be effective in eradicating or controlling malaria caused by P. falciparum.

The silent path to thousands of merozoites: the Plasmodium liver stage

Plasmodium sporozoites are deposited in the skin of their vertebrate hosts through the bite of an infected female Anopheles mosquito. Most of these parasites find a blood vessel and travel in the peripheral blood circulation until they reach the liver sinusoids. Once there, the sporozoites cross the sinusoidal wall and migrate through several hepatocytes before they infect a final hepatocyte, with the formation of a parasitophorous vacuole, in which the intrahepatic form of the parasite grows and multiplies. During this period, each sporozoite generates thousands of merozoites. As the development of Plasmodium sporozoites inside hepatocytes is an obligatory step before the onset of disease, understanding the parasite's requirements during this period is crucial for the development of any form of early intervention. This Review summarizes our current knowledge on this stage of the Plasmodium life cycle.

Nomadic or sessile: can Kupffer cells function as portals for malaria sporozoites to the liver?

The initial site of replication for Plasmodium parasites in mammalian hosts are hepatocytes, cells that offer unique advantages for the extensive parasite replication occurring prior to the erythrocytic phase of the life cycle. The liver is the metabolic centre of the body and has an unusual relationship to the immune system. However, to reach hepatocytes, sporozoites must cross the sinusoidal barrier, composed of specialized endothelia and Kupffer cells, the resident macrophages of the liver. Mounting evidence suggests that, instead of taking what would seem a safer route through endothelia, the parasites traverse Kupffer cells yet suffer no harm. Kupffer cells have a broad range of responses towards incoming microorganisms, toxins and antigens which depend on the nature of the intruder, the experimental conditions and the environmental circumstances. Kupffer cells may become activated or remain anergic, produce pro- or anti-inflammatory mediators. Consequently, outcomes are diverse and include development of immunity or tolerance, parenchymal necrosis or regeneration, chronic cirrhotic transformation or acute liver failure. Here we review data concerning the unique structural and functional characteristics of Kupffer cells and their interactions with Plasmodium sporozoites in the context of a model in which these hepatic macrophages function as the sporozoite gate to the liver.

Molecular targets for detection and immunotherapy in Cryptosporidium parvum

Cryptosporidium parvum is an obligate protozoan parasite responsible for the diarrheal illness cryptosporidiosis in humans and animals. Although C. parvum is particularly pathogenic in immunocompromised hosts, the molecular mechanisms by which C. parvum invades the host epithelial cells are not well understood. Characterization of molecular-based antigenic targets of C. parvum is required to improve the specificity of detection, viability assessments, and immunotherapy (treatment). A number of zoite surface (glyco)proteins are known to be expressed during, and believed to be involved in, invasion and infection of host epithelial cells. In the absence of protective treatments for this illness, antibodies targeted against these zoite surface (glyco)proteins offers a rational approach to therapy. Monoclonal, polyclonal and recombinant antibodies represent useful immunotherapeutic means of combating infection, especially when highly immunogenic C. parvum antigens are utilized as targets. Interruption of life cycle stages of this parasite via antibodies that target critical surface-exposed proteins can potentially decrease the severity of disease symptoms and subsequent re-infection of host tissues. In addition, development of vaccines to this parasite based on the same antigens may be a valuable means of preventing infection. This paper describes many of the zoite surface glycoproteins potentially involved in infection, as well as summarizes many of the immunotherapeutic studies completed to date. The identification and characterization of antibodies that bind to C. parvum-specific cell surface antigens of the oocyst and sporozoite will allow researchers to fully realize the potential of molecular-based immunotherapy to this parasite.

A mosquito-specific protein family includes candidate receptors for malaria sporozoite invasion of salivary glands

We describe a previously unrecognized protein family from Aedes and Anopheles mosquitoes, here named SGS proteins. There are no SGS homologues in Drosophila or other eukaryotes, but SGS presence in two mosquito genera suggests that the protein family is widespread among mosquitoes. Ae. aegypti aaSGS1 mRNA and protein are salivary gland specific, and protein is localized in the basal lamina covering the anatomical regions that are preferentially invaded by malaria sporozoites. Anti-aaSGS1 antibodies inhibited sporozoite invasion into the salivary glands in vivo, confirming aaSGS1 as a candidate sporozoite receptor. By homology to aaSGS1 we identified the complete complement of four SGS genes in An. gambiae, which were not recognized in the genome annotation. Two An. gambiae SGS genes display salivary gland specific expression like aaSGS1. Bioinformatic analysis predicts that SGS proteins possess heparin-binding domains, and have among the highest density of tyrosine sulphation sites of all An. gambiae proteins. The major sporozoite surface proteins (CS and TRAP) also bind heparin, and interact with sulphoconjugates during liver cell invasion. Thus, we speculate that sporozoite invasion of mosquito salivary glands and subsequently the vertebrate liver may share similar mechanisms based on sulphation. Phylogenomic analysis suggests that an SGS ancestor was involved in a lateral gene transfer.

Effective targeting of liposomes to liver and hepatocytes in vivo by incorporation of a Plasmodium amino acid sequence

PURPOSE

Several species of the protozoan Plasmodium effectively target mammalian liver during the initial phase of host invasion. The purpose of this study was to demonstrate that a Plasmodium targeting amino acid sequence can be engineered into therapeutic nanoparticle delivery systems.

METHODS

A 19-amino peptide from the circumsporozoite protein of Plasmodium berghei was prepared containing the conserved region I as well as a consensus heparan sulfate proteoglycan binding sequence. This peptide was attached to the distal end of a lipid-polyethylene glycol bioconjugate. The bioconjugate was incorporated into phosphatidylcholine liposomes containing fluorescently labeled lipids to follow blood clearance and organ distribution in vivo.

RESULTS

When administered intravenously into mice, the peptide-containing liposomes were rapidly cleared from the circulation and were recovered almost entirely in the liver. Fluorescence and electron microscopy demonstrated that the liposomes were accumulated both by nonparenchymal cells and hepatocytes, with the majority of the liposomal material associated with hepatocytes. Accumulation of liposomes in the liver was several hundredfold higher compared to heart, lung, and kidney, and more than 10-fold higher compared to spleen. In liver slice experiments, liposome binding was specific to sites sensitive to heparinase.

CONCLUSIONS

Incorporation of amino acid sequences that recognize glycosaminoglycans is an effective strategy for the development of targeted drug delivery systems.

Motility and infectivity of Plasmodium berghei sporozoites expressing avian Plasmodium gallinaceum circumsporozoite protein

Avian and rodent malaria sporozoites selectively invade different vertebrate cell types, namely macrophages and hepatocytes, and develop in distantly related vector species. To investigate the role of the circumsporozoite (CS) protein in determining parasite survival in different vector species and vertebrate host cell types, we replaced the endogenous CS protein gene of the rodent malaria parasite Plasmodium berghei with that of the avian parasite P. gallinaceum and control rodent parasite P. yoelii. In anopheline mosquitoes, P. berghei parasites carrying P. gallinaceum and rodent parasite P. yoelii CS protein gene developed into oocysts and sporozoites. Plasmodium gallinaceum CS expressing transgenic sporozoites, although motile, failed to invade mosquito salivary glands and to infect mice, which suggests that motility alone is not sufficient for invasion. Notably, a percentage of infected Anopheles stephensi mosquitoes showed melanotic encapsulation of late stage oocysts. This was not observed in control infections or in A. gambiae infections. These findings shed new light on the role of the CS protein in the interaction of the parasite with both the mosquito vector and the rodent host.

The dissection of CD8 T cells during liver-stage infection

Multiple injections of gamma-radiation-attenuated Plasmodium sporozoites (gamma-spz) can induce long-lived, sterile immunity against pre-erythrocytic stages of malaria. Malaria antigen (Ag)-specific CD8 T cells that produce IFN-gamma are key effector cells in this model of protection. Although there have been numerous reports dealing with gamma-spz-induced CD8 T cells in the spleen, CD8 T cells most likely confer protection by targeting infected hepatocytes. Consequently, in this chapter we discuss observations and hypotheses concerning CD8 T cell responses that occur in the liver after an encounter with the Plasmodium parasite. Protracted protection against pre-erythrocytic stages requires memory CD8 T cells and we discuss evidence that gamma-spz-induced immunity is indeed accompanied by the presence of intrahepatic CD44hi CD45RBlo CD62lo CD122lo effector memory (EM) CD8 T cells and CD44hi CD45RBhi CD621hi CD122hi central memory (CM) CD8 T cells. In addition, the EM CD8 T cells rapidly release IFN-gamma in response to spz challenge. The possible role of Kupffer cells in the processing of spz Ags and the production of cytokines is also considered. Finally, we discuss evidence that is consistent with a model whereby intrahepatic CM CD8 T cells are maintained by IL-15 mediated-homeostatic proliferation while the EM CD8 T cells are conscripted from the CM pool in response to a persisting depot of liver-stage Ag.