Institut Pasteur

Biomedical Parasitology Unit, Prof. Pierre Druilhe

The Biomedical Parasitology Unit has been focusing on the understanding of immune host/parasite interactions between P. falciparum in human beings both at the level of sporozoite and liver stage, and that of erythrocytic stages. This work has focused primarily on the understanding of defense mechanisms taking place in the human host against the parasite at the different steps of those two schizogonic cycles.

In the field of pre-erythrocytic stage the group has, for instance, brought evidence that what was called in the past an “anti-sporozoite immunity” was in fact related to the arrested development of young intra-hepatic trophozoites and, in this manner, stressed the importance of the liver forms in the development of protective immunity. It has contributed to the identification of numerous new molecules expressed in either the liver stage alone, or both the sporozoite and liver stages and demonstrated their role in protection in immunized primates against a P.falciparum challenge.

For the erythrocytic stages, the group has focused on an understanding of the immunological basis of the type of acquired immunity that takes place in adults from endemic areas and protects them against malaria attacks. It has initially observed that naturally occurring antibodies had little, if any, merozoite inhibition of invasion activity and little antibody dependent specific inhibition of growth. Conversely, it focused on the role of antibodies in collaboration with various cell types, including blood and tissue cells derived from the monocyte-macrophage lineage. This led first to the identification of an opsonisation promoting activity of antibodies from protected individuals and soon after, to the identification of distinct types of activity where antibodies trigger the release from monocytes of parasite inhibitory molecules in a monocyte-dependent antibody-mediated fashion that was called ADCI. The value of this mechanism was fully reassessed in details by performing passive immunisation experiments where the protective IgG from immune African adults was transferred into non immune P. falciparum infected Thai recipients and showed in vivo a major anti-parasite activity leading to the reduction by more than 3 orders of magnitude of the parasitaemia. The material from this experiment was used to reassess all effector mechanisms described in the literature and by our group. The material derived from the passive transfer experiments in humans was critical to rule out the major role of invasion inhibitory antibodies from immune African adults, as well as the major role of antibodies inhibiting cyto-adherence. In contrast, it very strongly confirmed the mechanism of monocyte dependent antibody-mediated parasite killing which was found with the transferred antibodies against each of the recipients’ strains. However, this was not found with antibodies from immune not protected recipients, and with serum from individuals passively transferred with IgGs from African donors.

This mechanism was also used to screen the whole genome of P. falciparum for molecules targeted by the antibodies involved in that mechanism. This led to the identification of a new merozoite surface molecule, Merozoite Surface Protein 3 (MSP-3) which is considered a major target of this type of antibodies. Thereafter, further targets of the ADCI mechanism were identified mainly the Glutamate-Rich Protein (GLURP) and the P126 Serine Rich Protein (SERP-SERA).

The group has acquired, over the last two decades, a vast experience in the handling of antibodies and monocytes in ADCI which has become a routine standard assay in our group. The importance of this mechanism was re-confirmed under in vivo conditions by developing an immunocompromised mouse model, which besides lacking B and T lymphocytes, was further manipulated for nonadaptive innate immunity and led to the design of a mouse model able to harbour P. falciparum parasitaemia with engrafted human red blood cells. In this model, the absence of effect of total antibodies as well as antigen specific antibodies was found, in addition to the cooperative effect between human monocytes and these antibodies.

There are several versions of the ADCI assay that can be performed. In this project, we will use one that ensures both high-throughput and a non-subjective automated type of measurement using a 2-step type approach. Monocytes are activated by antibodies and bursting merozoites from mature schizonts. Supernatant containing the monokines is transferred onto a normal P. falciparum culture where parasite growth is monitored by incorporation of tritiated hypoxanthin. Well-established positive and negative controls will be used.

Role and main tasks: In OPTIMALVAC, the role of UPBM (IP) will be to improve the reliability, rreproducibility, ease of manipulation of the ADCI through the plan of work in order to develop harmonised procedures, harmonise by blind performed experiments in other labs the reproducibily by following the SOPs, and evaluate the relevance to clinical protection against malaria. The other is to participate in the harmonization of T-cell assays . A third is to produce harmonised reagents.