No abstract

). Blockage of NS3 protease activity therefore is expected to inhibit HCV replication by both direct suppression of viral protein production as well as by restoring host responsiveness to IFN. Using structure-assisted design, a ketoamide inhibitor, SCH 503034, was generated which demonstrated potent (overall inhibition constant, 14 nM) time-dependent inhibition of the NS3 protease in cell-free enzyme assays as well as robust in vitro activity in the HCV replicon system, as monitored by immunofluorescence and real-time PCR analysis. Continuous exposure of repliconbearing cell lines to six times the 90% effective concentration of SCH 503034 for 15 days resulted in a greater than 4-log reduction in replicon RNA. The combination of SCH 503034 with IFN was more effective in suppressing replicon synthesis than either compound alone, supporting the suggestion of Foy and coworkers that combinations of IFN with protease inhibitors would lead to enhanced therapeutic efficacy.

Plasmodium falciparum-infected erythrocytes (IE) specifically adhere to vascular endothelium in vivo and to human endothelial cells, some human melanoma cell lines, and human monocytes in vitro. The tissue cell receptor for a ligand on the surface of the infected erythrocytes is an Mr 88,000 glycoprotein (GP88) recognized by the MAb OKM5, which also blocks cytoadherence of IE. Isolated, affinity-purified GP88 (CD36) competitively blocks cytoadherence and when absorbed to plastic surfaces, specifically binds P. falciparum IE. Additionally, monoclonal and polyclonal antibodies to GP88 block cytoadherence to both target cells and immobilized GP88. Binding to GP88 by IE is unaffected by the absence of calcium or the absence of thrombospondin, a putative mediator for cytoadherence of P. falciparum IE. Thus, GP88 (CD36), which has been demonstrated to be the same as platelet glycoprotein IV, interacts directly with P. falciparum IE, presumably via a parasite-induced ligand exposed on the surface of the infected erythrocytes. CD36 is shown to be present on brain endothelium in both individuals without malaria and individuals with cerebral malaria. This would suggest that factors other than just cerebral sequestration of IE play an initiating role in the genesis of cerebral malaria.

Two related Plasmodium falciparum genes and their encoded proteins have been identified by comparative analyses with Plasmodium vivax reticulocyte binding protein 2 (PvRBP-2). The P. falciparum genes have a structure which suggests that they may be the result of an evolutionary duplication event, as they share more than 8 kb of closely related nucleotide sequence but then have quite divergent unique 3 ends. Between these shared and unique regions is a complex set of repeats, the nature and number of which differs between the two genes, as well as between different P. falciparum strains. Both genes encode large hydrophilic proteins, which are concentrated at the invasive apical end of the merozoite and are predicted to be more than 350 kDa, with an N-terminal signal sequence and a single transmembrane domain near their C termini. Importantly, they also share gene structure and amino acid homology with the Plasmodium yoelii 235-kDa rhoptry protein family, which is also related to PvRBP-2. Together these Plasmodium proteins define an extended family of proteins that appear to function in erythrocyte selection and invasion. As such, they may prove to be essential components of malaria vaccine preparations. P arasites of the genus Plasmodium are estimated to cause between 300 and 500 million cases of malaria, the majority of which are caused by Plasmodium vivax and Plasmodium falciparum (1). Plasmodium parasites have a complex life cycle involving a series of developmental stages in both mosquitoes and mammals, but the clinical manifestations of malaria are all caused by the asexual blood stage. Merozoites, ovoid cells with an apical prominence at one end, invade red blood cells (RBCs), wherein they undergo a growth and multiplication phase (schizogony). The resulting schizont eventually ruptures the RBC, releasing newly formed merozoites for subsequent rounds of invasion.How merozoites identify and invade RBCs has long been a focus of research (2, 3). The merozoite first attaches to a RBC at any point on its surface, and then reorients to bring its apical end into contact with the RBC. The initial attachment stages are reversible, and merozoites can disassociate and attach to a new potential target cell. The subsequent steps are irreversible, and involve the formation of an electron-dense adhesion zone between the apical end of the merozoite and the RBC. This zone then moves around the merozoite toward its posterior end, with a concurrent invagination of the RBC membrane and entry of the merozoite. This cascade of molecular events also involves release of proteins from the rhoptries and micronemes, specialized apical organelles central to the invasion process.The molecular adhesion details behind this tantalizing outline are sketchy. The merozoite surface proteins (MSPs), several of which have been described in a number of species of Plasmodium, together make up a structurally complex coat around the outer membrane of the merozoite and may have a role in the initial reversible adhesive interaction between the merozo...