Parasite Immunology , 2001: 23: 587±597 Susceptibility of the different developmental stages of the asexual (schizogonic) erythrocyte cycle of Plasmodium chabaudi chabaudi to hyperimmune serum, immunoglobulin (Ig)G1, IgG2a and F(ab 0 ) 2 fragments REGIANE A.CAVINATO 1,2 , KARINA R.B.BASTOS 2 , LUIZ R.SARDINHA 2 , ROSA M.ELIAS 2 , JOSE Â M.ALVAREZ 2 & MARIA REGINA D'IMPE Â RIO LIMA 2 Departments of 1 Parasitology and 2 Immunology, Instituto de Cie Ãncias Biome Âdicas, Universidade de Sa Äo Paulo, Sa Äo Paulo, SP, Brazil SUMMARY The mechanisms by which antibodies interfere with Plasmodium growth are still under debate. Characterizing the asexual erythrocyte stages susceptible to antibodies from hyperimmune individuals is therefore a relevant contribution to vaccine research. In this study, using a virulent and synchronous murine malaria parasite, Plas- modium chabaudi chabaudi AJ, we have shown that trophozoites and circulating schizonts are not the main targets for antibodies from hyperimmune serum. In drug- cured mice challenged with a high inoculum of ring- infected erythrocytes, parasitemias do not decline until the moment of erythrocyte rupture, suggesting that effector mechanisms operate immediately prior to reinvasion. Confirming these findings, treatment of primary-infected mice with hyperimmune serum inhibited the generation of new ring forms, but did not alter the numbers of schizont- infected erythrocytes, despite the fact that these cells were recognized by immunoglobulin (Ig)G antibodies. When these mice were treated with IgG1 or IgG2a purified from hyperimmune serum, both subclasses limited reinvasion, but IgG2a showed a stronger protective activity. The fact that Fc digestion decreases but does not abrogate protection suggests that both Fc-dependent and indepen- dent mechanisms participate in this process. Treatment with cobra venom factor did not interfere with the antibody- mediated protection, ruling out the participation of the complement system in both lysis and phagocytosis of merozoites or infected erythrocytes. Therefore, in mice suffering from P. c. chabaudi AJ malaria, merozoite neutralization seems to be a major mechanism of protection conferred by hyperimmune serum antibodies. However, FcgR-mediated interactions, or other mechanisms not yet defined, may also contribute to inhibit erythrocyte reinvasion. Keywords malaria, Plasmodium chabaudi, antibody treatment, passive immunization INTRODUCTION Malaria is one of the most prevalent parasitic diseases in tropical countries, with a report of 500 million clinical cases yearly, resulting in one million deaths, mostly children. The pathology of malaria is caused by the asexual blood stages of the parasite and therefore it has been object of many studies focused on the effector mechanisms underlying protective immunity. In human infection by Plasmodium falciparum, the efficacy of treatment of nonimmune infected patients with immunoglobulin (Ig)G from protected individuals has been confirmed, providing evidence of the protective role of the antibodies (1±3). Nevertheless, the mechanisms by which antibodies interfere with Plasmodium growth are still under debate. Several possibilities have been suggested, such as inhibition of merozoite dispersal or invasion, blockage of intraerythro- cytic development of the parasite, inhibition of cyto- adherence or rosetting of schizont infected cells and opsonization or cytotoxicity of merozoites or parasitized erythrocytes (4). q 2001 Blackwell Science Ltd 587 Correspondence: Maria Regina d'Impe Ârio Lima, Departamento de Imunologia, ICB, Av. Prof. Lineu Prestes, 1730, Universidade de Sa Äo Paulo, Sa Äo Paulo, SP, CEP-05508-900, Brazil (e-mail: relima@usp.br). Received: 15 December 2000 Accepted for publication: 2 August 2001