Plasmodium berghei HAP2 induces strong malaria transmission-blocking immunity in vivo and in vitro

Fertilization in Plasmodium is a complex process that occurs in the gut of the female Anopheles mosquito upon uptake of a bloodmeal. It requires the emergence of the gametocyte from the RBC and release of eight flagellate male gametes from each male cell, and subsequent fertilization of a similarly emerged immotile extracellular female macrogamete. Previous studies have demonstrated that antibodies against male gamete surface proteins ingested from the blood of an infected and immunized host inhibit parasite transmission. Gene disruption studies in Plasmodium berghei 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. Genetic disruption of the HAP2 locus revealed that parasite fertilization is prevented, yet hap2 KO male gametes still retained the ability to form tight pre-fusion membrane attachments with females.We demonstrate that heterologous expression of the P. berghei HAP2 protein in Escherichia coli, and subsequent immunization of rabbits, has produced anti-sera that react specifically with recombinant HAP2, and with the native protein on the male gamete. Additionally, anti-HAP2 sera 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%. Inhibition is dose dependent. These results indicate that HAP2 should be considered as a potential target for any future anti-malarial transmission-blocking vaccine.