Patterns of parasitaemia, antibodies, complement and circulating immune complexes in drug-suppressed simian Plasmodium knowlesi malaria.

Rhesus monkeys were inoculated intravenously with 1 x 10(4) P. knowlesi infected erythrocytes. After about three days prepatency, peripheral smears were found positive and the animals were cured with chloroquine phosphate when parasitaemia reached about 15-25 per cent. The monkeys were repeatedly exposed with three bouts of infection. The first and second bouts were cured but after the third challenge, all 10 monkeys showed a longer prepatent period, lower parasitaemia and then self-recovery. Sera were collected in different phases of infection to assess immune responses. Antimalarial IgG and IgM responses were measured by ELISA. The presence of IgM antibody was associated with every bout of infection. With repeated infections until self-recovery, a substantial amount of IgG was found in circulation. A significant level of schizont-infected cell agglutination antibody was also detected in the animals after survival from challenge infection. Antigen-specific circulating immune complexes, both of IgG and IgM types, appeared in various phases of infection, but their appearance did not coincide with the acquired immune responses of the animals. During the self-recovery phase, almost all monkeys had an elevated level of serum C3 and C4.