Falciparum Challenge Research Articles

Protection against malaria in Aotus monkeys immunized with a recombinant blood-stage antigen fused to a universal T-cell epitope: correlation of serum gamma interferon levels with protection.

The major surface antigen p190 of the human malaria parasite Plasmodium falciparum contains nonpolymorphic, immunogenic stretches of amino acids which are attractive components for a subunit vaccine against malaria. One such polypeptide, termed 190L, is contained in the 80-kDa processing product of p190, which constitutes the major coat component of mature merozoites. We report here that immunization of Aotus monkeys with 190L gives only poor protection against P. falciparum challenge. However, addition by genetic engineering of a universal T-cell epitope (CS.T3) to 190L improved immunity, and as a result three of four monkeys were protected following challenge infection with blood-stage parasites. Neither antibody against the immunizing antigens or against blood-stage parasites nor the capacity of the monkeys' sera to inhibit in vitro parasite invasion correlated with protection. However, in contrast to sera from nonprotected monkeys, sera from protected animals contained elevated levels of gamma interferon. These results suggest that gamma interferon is directly or indirectly involved in the process of asexual parasite control in vivo.

Long-term protection of squirrel monkeys (Saimiri sciureus) against Plasmodium falciparum challenge inoculations after various time intervals.

When used as an experimental host for Plasmodium falciparum, the squirrel monkey Saimiri sciureus, intact or splenectomized, develops a significant, long-lasting, protective immunity against the blood stages of this parasite. In the present study, this length of protection after a single initial infection was determined by test infections carried out at varying intervals of time in 26 splenectomized animals. An initial period of about 7 months was observed, which was characterized by a marked resistance to trial reinfestations by a homologous as well as a heterologous strain. A second period was observed during which the primates became sensitive to the parasite in varying degrees. These observations single out the squirrel monkey in comparison with other experimental hosts tested under similar conditions and underline the value of research on antimalarial vaccinations, because when confronted with parasite antigens, the organism can produce the immunological response necessary to control a test infection.

Human lymphocyte responses to Plasmodium falciparum merozoite antigens. A functional assay of protective immunity?

Merozoites obtained from cutaneous cultures of Plasmodium falciparum were used as antigen for an in vitro lymphocyte assay. Antigen specific proliferative responses were observed with lymphocytes from individuals with long-standing immunity to P. falciparum. Donors whose last P. falciparum challenge occurred within the year preceding the assay exhibited lymphocyte responses significantly higher than those from donors whose infection was more remote. This suggests that a lymphocyte dependent assay may relate to the protective status of the donor.

The use of immunoadjuvant in vaccination studies against human malaria

After several infections and drug cure, Aotus spp. monkeys are protected against homologous Plasmodium falciparum challenge. Sera from these monkeys passively transfers protection (Diggs CL, Hines F, Wellde BT. Exp Parasitol 1995;80:291–6). Antibodies in these sera agglutinate merozoites emerging from ruptured schizonts forming immune clusters and thereby presumably prevent merozoite invasion of erythroyctes (Lyon JA, Haynes JD, Diggs CL, Chulay JD, Pratt-Rossiter JM. J Immunol 1986;136:2252–8). It is reasoned that antibodies eluted from the immune clusters of merozoites must recognize exposed epitopes with functional relevance to the survival of the live, invasive merozoites. The erythrocyte binding protein, EBA-175 was used as a model to study this hypothesis. We expressed and purified nine overlapping fragments of EBA-175 as recombinant proteins and probed them in immunoblots with antibodies eluted from the immune clusters. Only the two regions of EBA-175 previously shown to be functionally relevant were recognized by the eluted antibodies. One was region II which includes the erythrocyte binding domain of this parasite ligand which binds to its receptor glycophorin A. The other was the region containing EBA-peptide 4, antibodies to which block binding of EBA-175 to erythrocytes and inhibit invasion of merozoites into erythrocytes. Immunization of mice with the region II recombinant protein induced antibodies that recognize merozoites in immunofluorescence assays, identified a 175 kDa band on unreduced immunoblots, blocked binding of EBA-175 to erythrocytes and inhibited merozoite invasion of erythrocytes. These findings corroborate the importance of EBA-175 region II and EBA-peptide 4, and suggest that antibodies eluted from immune clusters can be used to identify protective epitopes exposed on invasive merozoites from other P. falciparum proteins.