Imaging and improvement of immune protection against malaria parasites

Abstract

About half of the world population is at risk of malaria, the deadliest human parasitic disease, responsible for >200 millions of clinical cases and ~700,000 deaths per year. The absence of an effective malaria vaccine is a major hurdle towards the control and elimination of this devastating disease. However, the development of a malaria vaccine is encouraged by the possibility of protecting humans, known since the 70's, through vaccination using multiple and high doses of live atlenuated sporozoites, the motile and infectious parasite-stage delivered by mosquitoes. Unfortunately, this method is inappropriate for use in large-scale vaccination protocols due to major operational and economical impediments related to the production, storage and delivery of large numbers of live attenuated parasites in tropical countries. Conversely pre-erythrocytic (PE) sub-unit vaccines, which target sporozoites and liver stages, are more suitable to be used in mass vaccination but only moderately protect humans against mala ria as observed for RTS, S, the most prominent PE vaccine candidate based on the circumsporozoite protein (CSP), the major surface protein of sporozoites. Therefore our international collaborative project proposes to improve protection induced by PE vaccines using an innovative double biological-vaccination approach that combines our complementary expertises on targeting antigens to dendritic cells, on lentiviral vectorology and on studying the biology of PE malaria parasites using in vivo imaging. The strategy of our project is based on the use of a relevant pre-clinical rodent malaria model, where mice are extremely susceptible to sporozoite infection. In this model, CSP-based immunizations are not capable of inducing complete protection against a stringent challenge of sporozoites, like observed in humans. Using this model, we preliminary developed a protocol where anti-CSP antibodies completely protected animais challenged with a stringent dose of 5000 sporozoites. We first propose to elucidate the mechanisms of this antibody-mediated sterilizing protection using in vivo imaging and a library of anti-CSP monoclonal antibodies to determine when, where, how and what kind of antibody is eliminating the parasite in the skin and liver of protected hosts. Next, we propose to ameliorate this humoral protective protocol by the addition of a strong CD8+ T cell response elicited by the targeting of the CD8 epitope of CSP to dendritic cells using anti-DEC205/DCIR hybrid antibodies and lentiviral vectors. The third levei of protection will be added by the incorporation of novel protective PE antigens coming from a functional screening using lentiviral vectors, where the hundred most abundant genes of PE stages are being screened based on their protective activity. In summary, using this approach we intend: i) to characterize sterilizing effector activities at a clonallevel using innovative in vivo imaging methods, ii) optimize the CD8+ T cell response via new means of targeting antigens to dendritic cells, and iii) test multi-antigenic formulations to broaden the protective responses against the parasite. We thus propose a functional dissection of effectors of protection, a first step toward devising better vaccines on a rational basis, and a stepwise strategy to improve humoral and cellular immune protection centered on the CSP, using the protein alone or in combination with novel protective PE antigens. (AU)