Preliminary Study On Regulable DNA Vaccines Against Plasmodium Falciparum

There have been more than 200 years since vaccines were recognized and used, vaccines were currently calssfied into four categoried:live attenuated microorganisms.whole killed microorganisms,subunit vaccines and DNA vaccines.After Wolff and Tang reporting plasmid DNA can induce immune response to the encoded protein,DNA immunization developed fast and was thought to be one important discovery in vaccinology.In the special conference of Geneva in 1994,DNA immunization is defined as DNA vaccine.The emergence and development of DNA vaccine is considered to be the third revolution in the history of vaccine.Plasmids are the common vectors for DNA vaccines.By using promoter and terminator on the plasmids and the host cells'transcriptional and translational machinery,the synthetic proteins has natural conformation comparing to the prokaryotic systems.DNA immunization can induce humoral and cellular immune responses against the encoded immunogenic proteins in a very broad of hosts.Furthermore,these responses can be boosted by repeated injection of DNA vaccines and are long lasting. Phase I and Phase II clinical trials with DNA vaccines have been conducted for HIV, HBV, HCV, HSV, tuberculosis, and malaria. Clinical trials are also in handfor cancer and the treatment of allergies. This new approach of DNA vaccination offers new hope because of their low cost and manufacturing stability at ambient temperature. However, there are still several safety concerns about the use of a DNA vaccine, which include the possibility of integration into the host genome, adverse immunopathology, and anti-DNA autoantibody induction.Attempting to solve some of these problems, eukaryotic expression plasmids pTL-8/ AMA-1 (tTA) and pTL-8/ AMA-1( rtTA ) which express trans-activator(tTA) or reverse trans-activator(rtTA), respectively, and AMA-1 gene of Plasmodium falciparum were constructed by using tetracycline regulable system. BALB/c mice were immunized with these plasmids and doxycycline (dox) was administered to regulate the expression of AMA-1. For some mice immunized with pTL-8/AMA-1 (rtTA) ,pUHS6-1, a plasmid containing trans-silencer (tTS) to suppress basal expression of AMA-1 from pTL-8/AMA-l (rtTA) ,was coinjected into these mice with pTL-8/AMA-l (rtTA) .The sera of the mice were isolated at 2,4,6 and 8 weeks post-immunization and the specific antibodies to AMA-1 were measured by ELISA.The results showed that pTL-8/AMA-l and pTL-8/AMA-l(rtTA) were constructed successfully . The mice immunized by pTL-8/AMA-l(tTA) with dox or by pTL-8/AMA-l(rtTA) without dox (at these conditions, AMA1 was expressed at basal level)developed significant antibodies against AMA-1. Mice coimmunized by pTL-8/AMA-l (rtTA) and pUHS6-l without dox did not develop significantly antibodies against AMA-1. In contrast, the mice coimmunized by pTL-8/AMA-l (rtTA) and pUHS6-l with dox produced high level of antibodies. Therefore,pTL-8/AMA-l(rtTA) combined with pUHS6-l is a good regulable DNA vaccine candidate against Plasmodium falciparum. The constructionof this system lays the foundation for future research to explore the mechanism of DNA immunizaiton and regulate the antigen expression to reduce side effects of DNA vaccines.