APMIS zyxwvutsrqp 99: zyxwvutsr 21-29, 1991 Isolation and characterization of a soluble antigen complex of zyx Plasmodium falciparum with pyrogenic properties P. H. JAKOBSEN', L. HVIID', T. G. THEANDER', E. M. RILEY3, P. GRELLIER4,L. S. BRUUN', K. DALSGAARD6, J. SCHREVEL4and S. JEPSEN' 'Malaria Research Laboratory, Department of Treponematoses, Statens Seruminstitut, Copenhagen, Denmark, 'Lymphocyte Laboratory, Department of Infectious Diseases, Rigshospitalet, Copenhagen, Denmark, 'Medical Research Council Laboratory, Fajara, The Gambia, 4Laboratoriede Biologie Cellulaire, Universite de Poitiers, Poitiers Cedex, France, 'Animal Department, Statens Seruminstitut, Copenhagen, Denmark, %ate Veterinary Institute for Virus Research, Lindholm, Kalvehave, Denmark, and 'WHO National Malaria Diagnostic Centre, Statens Seruminstitut, Copenhagen, Denmark Jakobsen, P. H., Hviid, L., Theander, T. G., Riley, E. M., Grellier, P., Bruun, L. S., Dalsgaard, K., Schrevel, J. zyxwvutsrq & Jepsen, S. Isolation and characterization of a soluble antigen complex of Plasmodium faleiparum with pyrogenic properties. APMIS YY; 21-29, 1991. A soluble antigen complex, previously designated antigen no. 7 (Ag7) on the basis of the pattern obtained by crossed immunoelectrophoresis of culture supernatants of P. faleiparum, was isolated by affinity chromatography. It was shown to be synthesized at the schizont stage of the parasite growth cycle and to be located on the surface of the schizonts. Antibodies to Ag7 did not inhibit the growth of the parasite in vitro. Ag7 is recognized by immune human sera from many parts of the world and it stimulated the production of specific antibody in mice when incorporated into immune-stimulating complex (ISCOM) structures. It also specificallystimulated zyxwvu in vitro proliferation of lymphocytes from clinically immune adults. That it induced the secretion of interleukin 1 by human monocytes and was pyrogenic in rabbits was of particular interest. Thus Ag7 has endotoxin-like properties which make it a possible candidate for an antitoxic malaria vaccine. Key words: Malaria; Plasmodium faleiparum ; soluble antigens; pyrogenicity. Palle H0y Jakobsen, Malaria Research Laboratory, Department of Treponematoses, Statens Seruminstitut, Amager Boulevard 80, DK-2300 Copenhagen S, Denmark. A number of soluble antigens of P. falciparum are known to be released during parasite multiplica- tion; in vivo they can be found in the sera of patients with malaria, and in vitro they are released into the culture medium. When such antigens are isolated from culture supernatants by affinity pu- rification using as ligand IgG from immune donors, and are subjected to crossed immunoelec- trophoresis (CIE) using IgG from immune donors, at least seven different precipitates can be seen (Jakobsen et al. 1987). Received March 22, 1990. Accepted May 28, 1990. We reported previously that some of these antigens, which are clearly of parasite origin, possess endotoxin-like properties, as measured by their reactivity with Limulus Amoebocyte Lysate (LAL) and polymyxin B sulphate (Jakobsen et al. 1988), and that such antigens might be responsible for the positive LAL tests obtained with plasma from patients with malaria (Tubbs 1980, Usawat- tanakul et al. 1985). Since it has been suggested that endotoxin-like substances may play a role in malaria, both in pathology and protection, through the induction of cytokines (Clark 1987, Bate et al. 1988, Bate et al. 1989), we have isolated and further character- 21