No abstract

Rosetting, the binding of parasitized erythrocytes to 2 or more uninfected erythrocytes, is an in vitro correlate of disease severity in Plasmodium falciparum malaria. Although cell ligands and receptors have been identified and a role for immunoglobulin M has been suggested, the molecular mechanisms of rosette formation are unknown. The authors demonstrate unequivocally that rosette formation by P falciparum-infected erythrocytes is specifically dependent on human serum, and they propose that serum components act as bridging molecules between the cell populations. Using heparin treatment and Percoll density gradient centrifugation, they have developed an assay in which parasitized erythrocytes grown in serum-containing medium and optimally forming rosettes are stripped of serum components. These infected cells were no longer able to form rosettes when mixed with erythrocytes and incubated in serum-free medium. Rosette formation was restored by the addition of serum or certain serum fractions obtained by concanavalin A (conA) affinity, anti-IgM affinity, anion exchange, and gel filtration chromatography. The authors clearly demonstrate that multiple serum components—IgM and at least 2 others—are involved in rosette formation. Those others consist of 1 or more acidic components of high-molecular mass that binds to conA (but that is not thrombospondin, fibronectin, or von Willebrand's factor) and of at least 1 more basic, smaller component that does not bind to conA. Data on the size and number of rosettes formed support the authors' hypothesis that multiple bridges are involved in this complex cellular interaction. These findings have important implications for the understanding of pathogenic adhesive interactions of P falciparum and host susceptibility to severe malaria.

The 19 kDa carboxy-terminal domain of Plasmodium yoelii merozoite surface protein-I (MSPI,,) was expressed in Salmonella vaccine strains as a carboxyterminal fusion to fragment C of tetanus toxin (TetC). This study demonstrates that antibodies that recognize disulphide-dependent conformational epitopes in native MSPI react with the TetC-MSPI,, fusion protein expressed in Salmonella. The proper folding of MSPI,, polypeptide is dependent on both the Salmonella host strain and the protein to which the MSPI,, polypeptide is fused. Serum from mice immunized with Salmonella fyphimurium C5aroD expressing TetC-MSPI ,, recognized native MSPI as shown by immunofluorescence with P. yoelii-infected erythrocytes. Antibody levels to MSPI,, were highest in out-bred mice immunized with S-typhimurium C5aroD carrying pTECH2-MSPI 19 and antibody was mostly directed against reductionsensitive conformational epitopes. However, antibody levels were lower than in BALBlc mice immunized with a glutathione S-transferase (GST)-MSPI ,, fusion protein in Freund's adjuvant, and which were protected against P. yoelii challenge infection. In challenge experiments with P. yoelii the Salmonellaimmunized mice were not protected, probably reflecting the magnitude of the antibody response. The results of this study have important implications in the design of live multivalent bacterial vaccines against eukaryotic pathogens.

[3‐14C]3‐Hydroxy‐3‐methyl‐glutaric acid (HMG) was incorporated (0.9%) into pseudomonic acid A when administered to a shaken flask fermentation of Pseudomonas fluorescens 4, 9 and 14 h after inoculation. [2‐14C]‐Mevalonic acid was not incorporated into pseudomonic acid. Experimental evidence supports the previous deduction that pseudomonic acid biosynthesis might directly involve HMG, an apparently unusual intermediary metabolite in prokaryotes.