Identification of malaria antigens for incorporation in to blood-stage and liver-stage vaccines
Plasmodium species, the infectious agent responsible for malaria, have a complex life cycle that involves two replicative stages in mammalian hosts. After introduction into the skin by a mosquito during a blood meal, sporozoites migrate to the liver where they undergo replication in hepatocytes. Their second stage of replication occurs when merozoites are released from the liver into the blood to cause cyclic infection of erythrocytes. Both stages of infection are susceptible to control by the immune system. For the liver stage, tissue-resident CD8 T cells are critical for killing infected hepatocytes, while for blood-stage infection, antibody is the main effector mechanism. Immunity to both stages depends on CD4 T cells, which provide help for responses by CD8 T cells and B cells but also have inherent capacity to control blood-stage infection, most likely through cytokine production. To study the role of CD4 T cells in malaria immunity, we have generated a CD4 T cell receptor (TCR) transgenic line with specificity for an antigen expressed throughout the Plasmodium life-cycle. CD4 T cells from this line are able to help both CD8 T cells and B cells and show capacity to control infection independent of these additional cell types. Our recent discovery of the antigen recognised by this TCR specificity provides the basis for assessment of its potential in vaccination. In this project, we aim to monitor immunity to this novel antigen during Plasmodium infection and test its immunogenicity in various vaccination strategies designed to generate immunity to either the liver or blood-stages of the Plasmodium life-cycle.
Increasing Plasmodium immunogenicity during liver stage infection to induce protective CTL immunity against Malaria
With over 580.000 deaths each year, mostly of children, malaria remains a major scourge to public health worldwide (WHO report 2014). Intriguingly, the seemingly ideal preventative measure already exists, i.e. a vaccine established using small animal models that was recently shown to be protective in humans (Seder et al. 2013). Intravenous injections of live γ- irradiated plasmodium parasites (sporozoites) achieve sterilising immunity mediated by cytotoxic T-cells, thus controlling the infection already during the initial liver stage. Unfortunately, obtaining the currently required amounts of sporozoites from mosquito salivary glands is too cumbersome for routine use in places where the vaccine is most needed. Therefore, alternative protocols aimed at increasing sporozoite immunogenicity during T-cell priming and subsequently during expansion and differentiation in the liver are warranted.
The Barchet lab focuses on unravelling innate immune pathways of pathogen detection, and contributed to the discovery that the cytosolic helicase MDA-5 is a key immune receptor in plasmodium infected hepatocytes. Moreover, they recently showed that oxidative damage increases cGAMP synthase (cGAS) and a STING mediated immune response to pathogen DNA. Based on these findings, they developed novel agonists of these pathways as vaccine adjuvants.
The Heath laboratory combines leading expertise in T cell and DC biology , with a long-standing interest in studying the protective T cell responses to malaria infection . The group has generated CD4+ and CD8+ T cell hybridomas and T-cell transgenic mice specific for Plasmodium antigen, which can facilitate study of the immune response to this pathogen.
Based on the complementary tools established in both laboratories, and by modulating the attenuation protocol (e.g. oxidative, chemical and/or genetic modification of parasite DNA), as well as the vaccine adjuvants employed, we will study (A) the identity and the maturation state of the myeloid cell type presenting sporozoite antigen during T cell priming in lymphoid tissue, (B) the immunogenicity of sporozoite-induced hepatocyte cell death, and parasite antigen presentation by myeloid cells taking up the dying hepatocytes, and approaches of liver-targeted boost vaccine delivery to (C) attract, expand and (D) differentiate sporozoite antigen specific T cells. Given recent understanding that tissue-resident memory T cells (TRM) represent the first line on defence in many tissues, our goal will be to direct immunity towards their formation. The long-term goal is to (E) extrapolate from these approaches a lymphoid tissue directed prime, and a liver APC targeting boost vaccination protocol as a proof-of-concept in mice that requires fewer sporozoites to achieve TRM mediated protective immunity against malaria.