Current projects of the Mueller lab
Using attenuated malaria parasites as a tool to understand liver stage biology and immunology
Among the various strategies proposed for antimalarial vaccination, live-attenuated Plasmodium parasites remain the gold standard because they confer long-lasting protection against natural malaria transmission (1); protective immunity elicited by RAS is multi-factorial, involving interferon-g (IFN-g) producing CD8+ and CD4+ T cells and neutralising antibodies. Attenuation of sporozoites by irradiation likely induces multiple random mutations in the sporozoite genome that ultimately lead to a block in liver-stage (LS) development.
Therefore, immunised individuals harbour a heterogeneous population of attenuated yet genetically undefined sporozoites which are likely to develop to different extents, to express different combinations of LS- and early erythrocytic stage antigens and to elicit different immune responses. In addition, if under-irradiated or over-irradiated, these sporozoites fail to induce complete sterile protection (2). Despite recent efforts to scale up the production for larger clinical trials (1), it is unlikely that a vaccine based on radiation-attenuated sporozoites (RAS) will be licensed for wide-scale use in humans. Nevertheless, RAS have proved to be an invaluable tool to dissect the immune mechanisms that result in protection against natural malaria transmission (3) and the challenge now is to use what we have learned from RAS to develop a new generation of attenuated or subunit vaccines.
A major area of interest in our laboratory is the characterisation of protective immune responses to malaria induced by genetically attenuated parasites (GAP) and to compare these responses to those induced by RAS and wild-type (WT) sporozoites. In a separate study, we recently generated and characterised genetically attenuated uis3 (-) and uis4 (-) parasites (GAP) that constitute a reproducible and standardised source of potent live-attenuated parasites. Immunisation with GAP elicits sterilising immunity, but so far the antigenic specificity and the effector mechanisms of this protective immune response have not been carefully characterised. In our laboratory we are combining our molecular and cell biological research on GAP with studies aimed at understanding the immunological correlates of protection elicited by GAP.
In strong collaborations with labs experienced in malaria immunology and cell biology, we have recently started working on the characterisation of effector and memory responses following immunisation with GAP and further on the identification of mechanisms involved in sterilising protection. We found that immunisation with GAP does not cause breakthrough infection in T and B-cell-deficient mice. Intriguingly, protection was abolished in these animals, suggesting a crucial role for adaptive immune responses and interferon-gamma. These GAPs provide us with a new tool to further understand the immunology of malaria liver stages in much more detail. We have already begun with an attempt to identify antigens, which are uniquely expressed or expressed at higher levels in GAP-infected livers. These might represent important targets of GAP-induced protective immunity.
Additionally, recent work in our laboratory deals with the characterisation of the kinetics, phenotype and function of induced CD4+ and CD8+ T cell responses upon immunisation with GAP. First data raise the intriguing possibility that immunisation with GAP induces a differential re-programming of T cells. As a prerequisite for all this work, we have established mosquito transmission for both rodent malaria (including P.yoelii and P.berghei) and at the same time we are establishing Anopheles transmission for P.falciparum parasites.
Funding is greatly acknowledged from the following institutions: German Ministry of Economy and Technology (BMWi through Exist-Forschungstransfer), German Ministry of Education and Science (BMBF through Deutsche Zentrum für Infektionsforschung), German Research Foundation (DFG through SPP 1580 and SFB 1129), European Molecular Biology Organization (EMBO long-term and short-term fellowships to Ann-Kristin Mueller and to Florian Kohlhepp, respectively), University of Heidelberg Frontier programme, University Hospital Heidelberg, Landesgraduiertenförderung (Priyanka Fernandes), HBIGS Heidelberg (Klara Vochyanova), MWK Baden-Württemberg (Kooperierendes Promotionskolleg: "Krankheitsmodelle und Wirkstoffe"), Rahel-Goitein-Strauss-Programm of the University Hospital Heidelberg (Dr. Julia Sattler), GSK travel grant (Matthew Lewis, Franziska Hentzschel), and the Minerva-Arches prize from 2012 in collaboration with Prof. Ron Dzikowski (Hebrew University Jerusalem).