Biomedical subjects

G V Brown

Publications and source records attributed to G V Brown.

At least 19 recordsLinked to original sources

PubMed1992

Adherence of infected erythrocytes to venular endothelium selects for antigenic variants of Plasmodium falciparum.

Erythrocytes (E) infected with asexual forms of malaria parasites exhibit surface antigenic variation. In Plasmodium falciparum infections, the variant Ag is the P. falciparum E membrane protein 1 (PfEMP1). This molecule may also mediate the adherence of infected E to host venular endothelium. We show here that parasite lines selected for increased adherence to endothelial cells have undergone antigenic variation. Three adherent lines selected from the same P. falciparum clone reacted with the same agglutinating antiserum that failed to agglutinate the parental clone. Immunoprecipitation experiments with the agglutinating anti-serum demonstrated that the selected lines expressed cross-reactive forms of PfEMP1 that were of higher m.w. and antigenically distinct from PfEMP1 of the parental clone. When one of the adherent lines was cloned in the absence of selection, a range of variant antigenic types emerged with differing cytoadherence phenotypes. These findings show that selection for cytoadherence in vitro favors the emergence of antigenic variants of P. falciparum and suggest that the requirement for cytoadherence in vivo may restrict the range of antigenic variants of P. falciparum in natural infections.

Animals↗

PubMed1992

Acute intramedullary spinal cord abscess: case report.

We report the first case of an acute pyogenic intramedullary cervical spinal cord abscess, brain abscesses and meningitis due to an unusual anerobe, Bacteroides disiens. The importance of spinal magnetic resonance imaging for establishing the diagnosis is emphasized.

Abscess↗

PubMed1992

A chromosome 9 deletion in Plasmodium falciparum results in loss of cytoadherence.

Many lines of Plasmodium falciparum undergo a deletion of the right end of chromosome 9 during in vitro culture accompanied by loss of cytoadherence and gametocytogenesis. Selection of cytoadherent cells from a mixed population co-selects for those with an undeleted chromosome 9 and the selected cells produce gametocytes. The deletion also results in loss of expression of PfEMP1, the putative cytoadherence ligand, suggesting that PfEMP1 or a regulatory gene controlling PfEMP1 expression and gametocytogenesis may be encoded in this region. We have isolated several markers for the deleted region and are currently using a YAC-P. falciparum library to investigate this region of the genome in detail.

Animals↗

PubMed1991

Antigenic variation in Plasmodium falciparum.

Antigenic variation of infectious organisms is a major factor in evasion of the host immune response. However, there has been no definitive demonstration of this phenomenon in the malaria parasite Plasmodium falciparum. In this study, cloned parasites were examined serologically and biochemically for the expression of erythrocyte surface antigens. A cloned line of P. falciparum gave rise to progeny that expressed antigenically distinct forms of an erythrocyte surface antigen but were otherwise identical. This demonstrates that antigenic differences on the surface of P. falciparum-infected erythrocytes can arise by antigenic variation of clonal parasite populations. The antigenic differences were shown to result from antigenic variation of the parasite-encoded protein, the P. falciparum erythrocyte membrane protein 1.

Agglutination↗

PubMed1991

Immunization of owl monkeys with the ring-infected erythrocyte surface antigen of Plasmodium falciparum.

Aotus nancymai were immunized with the 4-mer, 8-mer, and 11-mer repeat peptides of the ring-infected erythrocyte surface antigen molecule of Plasmodium falciparum conjugated to diphtheria toxoid with muramyl dipeptide (MDP) as adjuvant. Immunization failed to induce protective immunity against the Uganda Palo Alto strain of P. falciparum as judged by maximum levels of parasitemia of immunized monkeys relative to those of controls. The fused polypeptide FPAg632, when combined with MDP, also failed to induce protective immunity. However, the maximum level of parasitemia and serologic response to the 11-mer peptide were inversely correlated. The safety of the use of MDP was evident.

Acetylmuramyl-Alanyl-Isoglutamine↗

PubMed1990

Knob-independent cytoadherence of Plasmodium falciparum to the leukocyte differentiation antigen CD36.

The survival of Plasmodium falciparum-infected erythrocytes is enhanced by the sequestration of mature trophozoites and schizonts from the peripheral circulation. Cytoadherence of infected erythrocytes in vivo is associated with the presence of knobs on the erythrocyte surface, but we and others have shown recently that cytoadherence to C32 melanoma cells may occur in vitro in the absence of knobs. We show here that a knobless clone of P. falciparum adheres to the leukocyte differentiation antigen, CD36, suggesting that binding to CD36 is independent of the presence of knobs on the surface of the infected erythrocyte. This clone showed little cytoadherence to immobilized thrombospondin or to endothelial cells expressing the intercellular adhesion molecule 1. Furthermore, an Mr approximately 300-kD trypsin-sensitive protein doublet was immunoprecipitated from knobless trophozoite-infected erythrocytes. Finding a P. falciparum erythrocyte membrane protein 1 (PfEMP1)-like molecule on these infected erythrocytes is consistent with a role for PfEMP1 in cytoadherence to CD36 and C32 melanoma cells.

Animals↗

PubMed1990

The mature-parasite-infected erythrocyte surface antigen (MESA) of Plasmodium falciparum associates with the erythrocyte membrane skeletal protein, band 4.1.

Several proteins synthesized by mature asexual stages of Plasmodium falciparum interact with the erythrocyte membrane skeleton. One of these is the mature-parasite-infected erythrocyte surface antigen (MESA; also called PfEMP2), a phosphoprotein of 250-300 kDa, which is found on the internal face of the erythrocyte membrane. When MESA is precipitated with anti-MESA antibodies, another phosphoprotein of 80 kDa is co-precipitated. This 80-kDa phosphoprotein was identified by peptide mapping as the erythrocyte membrane component band 4.1. Thus, MESA is apparently anchored at the erythrocyte membrane through an association with band 4.1. Band 4.1 is more intensely phosphorylated in infected erythrocytes and is increased in relative molecular mass in erythrocytes infected by isolates of P. falciparum that cytoadhere.

Animals↗

PubMed1990

Chromosome 9 from independent clones and isolates of Plasmodium falciparum undergoes subtelomeric deletions with similar breakpoints in vitro.

We show that chromosome 9 in all isolates and clones of Plasmodium falciparum examined so far exists as one of two distinctly different forms, a large form about 1.9 megabases long or a smaller form about 25% shorter. Physical maps of chromosome 9 from independent clones with large and small forms of chromosome 9, and from an isolate with the large form and 3 derived clones with the small form reveal the underlying structural basis of this size polymorphism. The small form differs from the large only in that there are subtelomeric deletions at each end, one of these deletions involving about 0.45 megabases. Remarkably, the breakpoints map within about +/- 1% of the total chromosome length for each of these populations. We discuss some possible mechanisms for this.

Animals↗

PubMed1990

Malaria: advice to travellers.

Malaria continues to be a major international public health problem but many Australians who travel overseas are ill-informed of the risk of infection and have inadequate protection against this potentially fatal disease. Management is complicated by the resurgence of malaria in areas that have previously had satisfactory control programmes, and the emergence of resistance to chloroquine and other drugs. This article focuses on prevention of malaria in travellers. The more important issue of malaria control for populations residing in endemic areas is beyond the scope of this review.

Adult↗

PubMed1989

Plasmodium falciparum: cytoadherence of a knobless clone.

Sequestration of Plasmodium falciparum-infected erythrocytes is crucial to parasite survival as it prevents destruction in the liver and spleen. Knobs have been considered necessary but not sufficient for cytoadherence to vascular endothelial cells in vivo and to melanoma or umbilical vein endothelial cells in vitro. We describe here a knobless clone that cytoadheres strongly to C32 melanoma cells. This clone cannot express the knob-associated histidine-rich protein (KAHRP) due to the deletion of the KAHRP gene. Our results raise the possibility of an alternative mechanism for in vitro cytoadherence and suggest that the use of long term cultured isolates and melanoma cells as a model for cytoadherence in vivo may be misleading.

Animals↗

PubMed1989

Subtelomeric chromosome deletions in field isolates of Plasmodium falciparum and their relationship to loss of cytoadherence in vitro.

Subtelomeric deletions are responsible for the loss of expression of several Plasmodium falciparum antigens, including the knob-associated histidine-rich protein (KAHRP). Such deletions are detectable by two-dimensional pulsed-field gradient electrophoresis (PFGE) in which the chromosomes separated in dimension 1 are cleaved with Apa I, and the sizes of telomeric fragments are determined in dimension 2. This sensitive technique has enabled us to examine the role of subtelomeric deletions in two aspects of the biology of Plasmodium falciparum. First, we show that similar subtelomeric deletions to those that occur in vitro also occur in field isolates. Second, we demonstrate a correlation between subtelomeric deletions and loss of the phenotype of "cytoadherence" in cultured isolates. Subclones were generated from the cytoadherent cloned isolate ItG2F6, and their phenotypes were examined with respect to cytoadherence, the expression of "knobs," and agglutination of infected erythrocytes with rabbit antiserum. The only chromosomal change detectable by two-dimensional PFGE among subclones that differ from wild type in each of these three characteristics is a deletion of approximately 100 kilobases at one end of chromosome 2. This deletion includes the gene coding for KAHRP and the subtelomeric repeat designated rep20.

Agglutination↗

PubMed1989

Diversity of antigens expressed on the surface of erythrocytes infected with mature Plasmodium falciparum parasites in Papua New Guinea.

Antigens were detected on the surface of erythrocytes from children with acute falciparum malaria in Madang, Papua New Guinea. These parasite-induced erythrocyte surface antigens (PIESA) were serotyped with convalescent sera from children and hyperimmune sera from adults in parasite infected cell agglutination assays (PICAs) and by inhibition of binding of infected cells to melanoma cells. Extensive serological diversity of PIESA was demonstrated. A significant correlation between serotypes defined by reactivity of immune sera in PICA and inhibition of melanoma cell binding (MCB) was observed. This suggests that both assays measure antibody responses to the same antigen(s). Increased recognition of different PIESA specificities with age is consistent with the hypothesis that repeated exposure to malaria confers immunity against a range of PIESA serotypes and parallels the development of clinical immunity to malaria in this area of Papua New Guinea.

Adult↗

PubMed1988

Variation in the precursor to the major merozoite surface antigens of Plasmodium falciparum.

The major antigens on the surface of Plasmodium falciparum merozoites are derived from a single high molecular weight polypeptide. The precursor to the major merozoite surface antigen (PMMSA) has conserved and variable antigenic determinants and varies in size in different isolates. Since the protein is a candidate for a malaria vaccine, it is important to understand the molecular basis of this variation. We present the complete sequence of the PMMSA of the Papua New Guinea isolate FC27 and the partial sequence of the West African isolate NF7. The gene consists of blocks of sequence which are either conserved or variable between different isolates. Variable sequences fall into two distinct types, indicating that the PMMSA is encoded by dimorphic alleles that undergo recombination within conserved blocks at the 5' end of the gene. Genetic exchange is not apparent within the other two-thirds of the gene in 12 isolates, suggesting strong selection against such recombinants. The most variable block located near the 5' end contains repeats that are different in independent cloned isolates. This variation presumably accounts for much of the size and antigenic variability.

Alleles↗

PubMed1988

A component of an antigenic rhoptry complex of Plasmodium falciparum is modified after merozoite invasion.

Human antibodies affinity purified on an adsorbent prepared from a cDNA clone (Ag44) expressing a portion of a rhoptry antigen were used to characterize the synthesis and fate of the antigen in the asexual blood stages of Plasmodium falciparum. The rhoptry antigen is synthesized in the mature trophozoite-stage parasites as a 103 kDa polypeptide, is present in the schizonts and merozoites as a 105 kDa polypeptide, is discharged from the rhoptries and found in the newly invaded red cells as a 110 kDa polypeptide. Anti-Ag44 antibodies immunoprecipitate the antigen and two additional polypeptides of 135 and 150 kDa from lysates of infected cells and from culture supernatants. The three polypeptides are associated in a non-covalent complex that persists in the newly invaded red cells. All the components of the high molecular weight rhoptry complex are antigenic and can be precipitated with immune human serum. The 135 kDa polypeptide is identical to a 140 kDa rhoptry antigen previously identified by a monoclonal antibody.

Animals↗

PubMed1988

Identification of two integral membrane proteins of Plasmodium falciparum.

We describe the isolation and cloning of two integral membrane protein antigens of Plasmodium falciparum. The antigens were isolated by Triton X-114 temperature-dependent phase separation, electrophoretically transferred to nitrocellulose, and used to affinity-purify monospecific human antibodies. These antibodies were used to isolate the corresponding cDNA clones from a phage lambda gt11-Amp3 cDNA expression library. Clone Ag512 corresponds to a Mr 55,000 merozoite rhoptry antigen, and clone Ag513 corresponds to a Mr 45,000 merozoite surface antigen. Both proteins can be biosynthetically labeled with [3H]glucosamine and [3H]myristic acid, suggesting that they may be anchored in membranes via a glycosylphosphatidylinositol moiety. Similarities in the C-terminal sequences of the Mr 45,000 merozoite surface antigen and the Trypanosoma brucei variant surface glycoproteins provides further evidence that this antigen has a glycosylphosphatidylinositol anchor.

Amino Acid Sequence↗

PubMed1988

A second antigenic heat shock protein of Plasmodium falciparum.

We describe here an antigen of Plasmodium falciparum, defined by a cDNA clone designated Ag361. The antigen is a soluble cytoplasmic 70-kD polypeptide present in all isolates analyzed and in all stages of asexual development in the blood. The antigen is a natural immunogen, although it lacks repeating epitopes of many P. falciparum antigens. Ag361 shares extensive sequence homology with the hsp70 proteins of Xenopus laevis, Drosophila melanogaster, Escherichia coli, and man, as well as a previously isolated P. falciparum hsp70 protein. The genome of P. falciparum contains at least five hsp70-like genes, located on at lest four different chromosomes.

Amino Acid Sequence↗

PubMed1988

Patterns of antigen expression in asexual blood stages and gametocytes of Plasmodium falciparum.

The stage specificity and localization of 12 Plasmodium falciparum antigens were determined by immunofluorescence using acetone-fixed parasites reacted with monospecific antibodies against cloned antigens. Antibodies were prepared by immunization of rabbits with recombinant proteins or by affinity purification of human plasma against cloned antigen adsorbents. Most of the antigens occurred predominantly in mature asexual parasites, two were abundant in ring stages and three were absent in rings. Four of the 12 antigens were detected in asexual stages but not in gametocytes. Grouping of antigens by localization within blood stages was difficult because of the complexity of fluorescence patterns observed. With some antibodies, fluorescence was apparently distributed evenly over the parasites, but in other cases label was concentrated within discrete compartments or organelles. Extraparasitic intraerythrocytic fluorescence was also observed.

Animals↗

PubMed1988

Immunization trials with the ring-infected erythrocyte surface antigen of Plasmodium falciparum in owl monkeys (Aotus vociferans).

A protocol was developed for the testing of blood stage vaccines against Plasmodium falciparum using Peruvian Aotus vociferans and the Indochina I/CDC strain of the parasite. Three different fused polypeptide vaccines containing elements of the ring-infected erythrocyte surface antigen molecule combined with Freund's complete and Freund's incomplete adjuvants were tested to determine their ability to protect against overwhelming infection following challenge with this highly virulent strain of P. falciparum, and to invoke antibody responses as measured by a standard indirect immunofluorescence technique. Nine of 14 immunized animals exhibited some protection. Presented are the test procedures developed for the conduct of such trials with New World monkeys and the analysis of results that led to the identification of variables selected for study in future trials.

Animals↗