Plasmodium falciparum sexual stage antigens: immunogenicity and cell-mediated responses.

Antibody and cell-mediated immune responses to the transmission-blocking target antigens of Plasmodium falciparum, Pfs 48/45, were determined in infected non-immune patients and in immune individuals from an endemic area. Characterization of the B cell epitopes with monoclonal antibodies showed that there were five regions identifiable but there could be interactions between them causing either competitive or enhancing effects. Sera from infected non-immune patients contained antibodies that would compete with one or more of the mAbs to the different epitopes. Immune responsiveness to purified Pfs 48/45 in P. falciparum-immune adults measured as lymphoproliferation, production of interferon-gamma, or as Pfs 48/45-specific antibody was very limited. This did not appear to be due to MHC class II restriction, to diversity in structure of the parasite antigens or to a failure of immunological memory. The antibody-response data were more consistent with down-regulation of immunity as a result of prolonged exposure to infection.

Failure of malaria vaccination in mice born to immune mothers. II. Induction of specific suppressor cells by maternal IgG.

Female BALB/c mice were vaccinated against blood stage P. yoelii malaria, infected 2 weeks later and after recovery mated to C57B1/6 males. When their offspring were subsequently vaccinated, the effectiveness of vaccination, as assessed by survival after infection, was significantly impaired until 8 weeks of age. Cell and serum transfer experiments indicated that specific maternally derived IgG interferes with protection in two distinct ways: (1) directly by prevention of priming by the vaccine and (2) indirectly by the induction and maintenance of specific suppressor T cells (TS) which act to inhibit the generation of memory T helper cells involved in IgG production, as measured by the response to TNP-P. yoelii. Furthermore, it is shown that the maternal IgG and the TS cells act in synergy to abolish the protective effect of vaccination.

Monoclonal anti-gamete antibodies prevent transmission of murine malaria.

Three monoclonal antibodies prepared using spleen cells from mice immunized with microgametes of Plasmodium yoelii nigeriensis were tested for their ability to block transmission of the infection. Two of them agglutinated microgametes and blocked transmission, this effect being antibody-dose dependent. The third monoclonal used alone was ineffective in both these assays although it stained gametocytes and microgametes by immunofluorescence in the same way as the protective monoclonals. However, when it was administered in combination with one of the protective monoclonals the transmission blocking immunity was enhanced significantly, indicating a synergistic effect of the two antibodies.

Role of T cells in preventing transmission of rodent malaria.

Transmission blocking immunity induced by microgamete vaccination is fully effective for at least 12 months. Passive transfer of immune T cells reduced transmission of a subsequent infection by 95%, the effect being partly due to a significant reduction in numbers of circulating gametocytes during the infection. This immunity was apparently independent of specific antibody, though these were produced within a few days after challenge infection and was mediated by a T cell of the GK1.5+, Ly 2.2 phenotype. Immune serum and immune T cells, administered together, showed a strong additive effect and blocked transmission completely.

Specific monoclonal IgM is a potent adjuvant in murine malaria vaccination.

Recent experiments in the murine system have indicated that the passive acquisition by offspring of maternal anti-malarial IgG antibodies while conferring some degree of immunity against a primary infection, paradoxically prevents the generation of acquired immunity through vaccination. Therefore, in view of earlier findings concerning the competitive effects of specific IgM and IgG antibodies, we investigated whether specific monoclonal IgM antibodies could be used to potentiate the response to a blood-stage murine malaria vaccine. We now report that small amounts of purified monoclonal anti-parasite IgM can specifically potentiate both priming and memory cell generation in response to vaccination as evidenced by survival after infection, and that the magnitude of this effect is greater than that found with a more conventional nonspecific adjuvant (Bordetella pertussis). Additionally, in offspring of immune mothers, where vaccination is ineffective for up to 8 weeks due to the presence of maternal IgG, we have found that IgM when administered with the vaccine can completely overcome this inhibition by its adjuvant effect.

Vaccination with purified microgamete antigens prevents transmission of rodent malaria.

Malaria vaccination with preparations of microgametes has been shown to inhibit transmission of Plasmodium spp. to the mosquito vectors of avian, rodent and simian parasites. This transmission-blocking immunity results from the induction of microgamete-agglutinating antibodies in the vaccinated host which, when ingested in a mosquito blood meal, react with exflagellating microgametes in the midgut to prevent fertilization and oocyst development. Here we have passively transferred the immunity with gamete-specific monoclonal antibodies raised against the rodent malaria parasite Plasmodium yoelii nigeriensis, and an IgG2a isotype monoclonal antibody from a hybridoma cell line, PYG-1, has been used to identify the target antigens on the gametes and to affinity-purify sufficient quantities of specific gamete antigen to facilitate vaccination studies. This transmission-blocking monoclonal antibody immunoprecipitated microgamete antigens of apparent relative molecular mass (Mr), 67K (67,000), 59K, 57K, 42K and 35K. Immunization of mice with these proteins before infection and mosquito feeding led to a 85-99.7% reduction in transmission to the mosquito vector; vaccination via intravenous or intramuscular routes was equally effective and did not require an adjuvant.

Failure of malaria vaccination in mice born to immune mothers.

Female BALB/c mice were vaccinated against blood stage P. yoelii (17XL strain), infected 2 weeks later and after recovery mated to normal C57B1/6 males. Control matings were with normal BALB/c females. The (C57B1/6 x BALB/c)F1 progeny were vaccinated at 4, 6, 8 or 10 weeks of age and infected 2 weeks later with lethal P. yoelii. All control mice were fully protected, but in the offspring of immune mothers mortality was 100, 87, 50, and 0% respectively. Mice in which the protective effect of vaccination had been abolished showed greatly reduced specific IgG and delayed hypersensitivity (DH) responses to challenge with parasite antigen. Results indicate that this failure of vaccination is due to the transmission of maternal IgG to the offspring which acts to suppress both priming by the vaccine and the generation of specific T helper cells involved in IgG production, as measured by the response to TNP-P. yoelii.

Maternal inhibition of malaria vaccination in mice can be overcome by giving a second dose of vaccine.

A single dose of a formalin-fixed malaria vaccine which normally protects mice against challenge with the live parasite, is ineffective in mice born to immune mothers. This inhibition of protection, which is due to maternally-derived IgG, can be overcome if a second dose of vaccine is given 10 or more days after the first. We show that this is related to the production of specific IgM antibody in response to the first dose of vaccine, which competitively blocks the inhibitory effect of the IgG. The implications of this finding in relation to immune regulation and immunization regimes are discussed.