If we are to contemplate control or elimination of malaria we must attack Plasmodium directly on two fronts; we must reduce the impact of disease upon the infected individual, and at the population level we must reduce the number of new infections. To reduce new infections, potentially the most effective point to attack the parasite is during its transmission through the mosquito vector, a process that in the field, commonly results in infection of less than 5 parasites per mosquito. It is now unquestionable that transmission of Plasmodium to the mosquito can be reduced by transmission-blocking drugs (e.g. mefloquine, primaquine, ACTs and atovaquone); and by transmission-blocking vaccines and antibodies, targeting parasite stages that establish infection within the mosquito (e.g. gametocytes, gametes, zygotes and ookinetes).
Identification of novel transmission blocking targets
The development of a transmission-blocking vaccine (TBV) against the sexual stages of the Plasmodium has been a longstanding goal. TBVs have a number of unique biological advantages over vaccines targeting other stages of the Plasmodium lifecycle; these include low target population numbers, a long vulnerable extracellular period, and reduced antigenic polymorphism. For these reasons, the development of a TBV remains both a logical and attractive proposition. In the laboratory, mosquito feeds supplemented with transmission-blocking antibodies can result in profound and often total reductions in parasite transmission. However, the pool of potential TBV immunogens for which compelling evidence exists is disappointingly small (p48/45, p230, p25/28). As a result, there is a real need to identify novel potent targets to enable the development of an effective anti-malarial TBV. By data-mining proteomic data sets, or examining relevant homologues in non-plasmodial species, we can identify molecules that are located on the surface of the gametocyte, gamete or ookinete stages of the parasite, and are therefore accessible for targeting by the host immune system to initiate a potent transmission-blocking response. Subsequent generation and analysis of transgenic parasites, and production of antibodies, can be used to accurately assess transmission-blocking potency.
As an example, gene disruption studies in Plasmodium and complimentary studies on the green alga Chlamydomonas have shown that a conserved male gamete sterility gene, HAP2, is essential for fusion of male and female gametes. We have demonstrated that heterologous expression of the P. berghei HAP2 protein in Escherichia coli, and subsequent antibody production, has produced anti-sera that reduces in vitro formation of ookinetes by up to 81%, and, using standard membrane feeding assays, reduces oocyst burden within the mosquito host by up to 81.1%, and prevalence of in vivo infection by up to 34%. A range of complimentary studies to maximise the impact of our findings, and to identify and characterise other novel transmission-blocking vaccine targets are currently ongoing. Our research goal is to characterise the efficacy of these antigens both in vitro and in vivo, and provide the malaria community with a raft of new candidates to aid the development of novel transmission-clocking vaccines.
Funded by: Transmalbloc (EU FP7), BBSRC, MRC
New options for the optimal delivery of TBVs are currently undergoing study, from the immunisation of recombinant protein with a variety of adjuvants (e.g. Freund’s adjuvant, aluminium hydroxide and cholera toxin), to the viral delivery of immunogens. Additionally, to improve the safety and efficacy of current TBV candidates, new vaccine vehicles and/or delivery systems (e.g., needle- and adjuvant-free, long-lasting and cost-effective) should be considered.
To examine these options further, in collaboration with researchers at Kanazawa University, Japan, we have produced a baculovirus expression system to facilitate the study of immunogens capable of inducing humoral and cellular immune responses without the need for extraneous adjuvants, and demonstrated its ability to act as a novel laboratory tool for the development of anti-malarial vaccines. In this manner, we aim to establish a cheap, robust and quick model for the in vivo assessment of TBV-induced functional immune responses - prior to human clinical trials. We additionally collaborate with the Jenner Institute (Oxford, UK) to assess the potency of a simian adenovirus (ChAd63) and poxvirus (MVA) viral vectored vaccine platform to screen potential TBV antigens for potent antibody induction and increased vaccine efficacy. In conjunction with our partners at the Fraunhofer, (USA) we also assess the TB potency of numerous vaccine candidates produced in a plant virus-based transient expression system in transgenic parasite/animal models.
Funded by: Fraunhofer Institute (USA), JSPS
Impact of TBIs on populations, and implications for elimination
TBIs aim to reduce the prevalence of infection in endemic communities by targeting Plasmodium within the insect host. Whilst ‘traditional’ studies and assays have reported the successful reduction of infection in the mosquito vector, direct evidence that there is an onward reduction in infection in the vertebrate host is lacking. We have developed a novel population transmission-based study to assess the impact of a TBI upon both insect and vertebrate populations over multiple transmission cycles. We have demonstrated that a TBI which inhibits transmission from vertebrate-to-insect by only 32%, reduces the basic reproduction number of the parasite by 20%, and in our model system, can eliminate Plasmodium from mosquito and mouse populations at low transmission intensities. These findings clearly experimentally demonstrate for the first time that use of TBIs alone can eliminate Plasmodium from a vertebrate population, and have significant implications for the future design and implementation of TBIs within the field. Complementary studies are being used to examine the impact on transmission of a range of different drugs and vaccines before the most promising candidates are taken forward into human trials.
Funded by: Medicines for Malaria Venture, MVI-PATH