Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 31. (Budapest, 1998)

The SIII fraction contained high activity of acid protease (specific activity 5.573± 0.091 ) and acid phosphatase (specific activity 13.840+0.090). In addition to host cell haemoglobin degradation the proteases from Plasmodium (Goldberg et al. 1990; Bailly et al. 1991) are also involved in the invasion of erythrocytes by merozoites (Banyai et al. 1981 and Hadley et al. 1983). It appears that in vivo protection exhibited by SIII may be mediated by the humoral antibodies induced by proteases present in the SIII as its immune serum exhibited an invasion-blocking effect in vitro (about 89% inhibition). In our laboratory, we have observed that mice immunized with Sephadex G-200 purified acid protease from P. berghei were partially protected upon challenge and antiprotease antibodies inhibited in vitro invasion of mouse erythrocytes by P. berghei (Makkar et al. 1995). Antiprotease antibodies have also been indicated to inhibit propagation oîP berghei (Bernard and Schrevel 1987). Our studies with SIII have shown that the antigenic component isolated from this by immunoadsorption contained 66 kDa and 79 kDa proteins and exhibited proteolytic activity, and mice immunized with this component were protected upon challenge (Upma and Banyai, manuscript in preparation). Further, the mice immunized with SIII containing 30 pg of saponin were protected upon challenge (Fig. 2). The maximum parasitaemia in these mice was less as compared to SIII-FCA immunized mice, showing that saponin appeared to be a more potent adjuvant of the two (Freeman and Holder 1983). The cellular response induced by SIII is indicated by the presence of a higher level of IL-1 in the prechallenge sera. Cytokines play an important role in malaria immunity. Exogenous administration of 80 ng of IL-1 protected the mice against cerebral malaria and also delayed the onset of parasitaemia in P. berghei infection (Curfs et al. 1990). In the present study, an enhanced level of IL-1 (305 pg/ml) was observed in the sera of prechallenged mice as compared to the controls. These mice were protected upon challenge with live parasite and showed very low parasitaemia. This indicates that the SIII induced the production of IL-1 in immunized animals and the protection achieved might have also been mediated by this cytokine as the level of IL-1 fell considerably in postchallenge sera. The present study thus indicates that SIII of P. berghei, having high activity of lysosomal marker enzymes, induces a protective immune response in mice. The host red cell contamination, if any, becomes irrelevant as the antibodies to red cells in mice would have resulted in agglutination of normal red cells in vitro invasion assay; however, no agglutination of the normal red cells was observed and in vivo also the antibodies to host red cell antigens may not have protected the mice against malaria infection as immunity to malaria is species as well as stage specific (Jeffery 1966; Newbold 1984). A component of SIII isolated by immunoadsorption, i.e. 66 kDa and 79 kDa protein, is protective against P. berghei infection (Upma and Banyai, manuscript in preparation), while the other constituents of the parasite, - in this case SII - did not induce antibodies in mice or protected the animals. Thus, SIII represents a promising target as protective antigenic material in malaria infection. Further elucidations are required as to whether this sediment containing high acid phosphatase activity (a marker of lysosomes) is enriched with lysosomal organelles/food vacuoles or certain other components of P. berghei which act as protective antigens. ACKNOWLEDGEMENTS Dr. Upma is thankful to U. G. C, New Delhi, for the financial assistance.