Exit of Plasmodium Sporozoites from Oocysts Is an Active Process That Involves the Circumsporozoite Protein

Plasmodium sporozoites develop within oocysts residing in the mosquito midgut. Mature sporozoites exit the oocysts, enter the hemolymph, and invade the salivary glands. The circumsporozoite (CS) protein is the major surface protein of salivary gland and oocyst sporozoites. It is also found on the oocyst plasma membrane and on the inner surface of the oocyst capsule. CS protein contains a conserved motif of positively charged amino acids: region II-plus, which has been implicated in the initial stages of sporozoite invasion of hepatocytes. We investigated the function of region II-plus by generating mutant parasites in which the region had been substituted with alanines. Mutant parasites produced normal numbers of sporozoites in the oocysts, but the sporozoites were unable to exit the oocysts. In in vitro as well, there was a profound delay, upon trypsin treatment, in the release of mutant sporozoites from oocysts. We conclude that the exit of sporozoites from oocysts is an active process that involves the region II-plus of CS protein. In addition, the mutant sporozoites were not infective to young rats. These findings provide a new target for developing reagents that interfere with the transmission of malaria

COMPLEMENT-DEPENDENT RELEASE OF IMMUNE COMPLEXES FROM THE LYMPHOCYTE MEMBRANE

Soluble antigen-antibody-complement complexes bound to mouse B lymphocytes are rapidly released from the cell membrane in the presence of normal serum from several mammalian species. The release is not the result of antigen-antibody dissociation or extensive degradation of the complexes. However, the released complexes have been altered because they will no longer bind to fresh lymphocytes. The release is not the result of lymphocyte damage mediated by complement. It is complement-dependent, and is generated either preferentially or exclusively via the alternate pathway, since it occurs in C4-deficient serum, is Mg++ but not Ca++ dependent, and requires C3 proactivator. C3 inactivator is not involved. The release activity of the serum, once generated, is unstable at 37°C. The release of complexes from the lymphocyte membrane by serum provides a convenient assay for the functioning of the alternate pathway in the mouse and in other species

Yellow fever 17D as a vaccine vector for microbial CTL epitopes: protection in a rodent malaria model

The yellow fever vaccine 17D (17D) is safe, and after a single immunizing dose, elicits long-lasting, perhaps lifelong protective immunity. One of the major challenges facing delivery of human vaccines in underdeveloped countries is the need for multiple injections to achieve full efficacy. To examine 17D as a vector for microbial T cell epitopes, we inserted the H-2Kd–restricted CTL epitope of the circumsporozoite protein (CS) of Plasmodium yoelii between 17D nonstructural proteins NS2B and NS3. The recombinant virus, 17D-Py, was replication competent and stable in vitro and in vivo. A single subcutaneous injection of 105 PFU diminished the parasite burden in the liver by ∼70%. The high level of protection lasted between 4 and 8 wk after immunization, but a significant effect was documented even 24 wk afterwards. Thus, the immunogenicity of a foreign T cell epitope inserted into 17D mimics some of the remarkable properties of the human vaccine. Priming with 17D-Py followed by boosting with irradiated sporozoites conferred sterile immunity to 90% of the mice. This finding indicates that the immune response of vaccine-primed individuals living in endemic areas could be sustained and magnified by the bite of infected mosquitoes