Macrophages and Plasmacytoid Dendritic Cells Initiate Early Immune Response To Severe Malaria In The Bone Marrow

Malaria still remains a significant global health problem worldwide with approximately 200 million people infected, and half a million deaths every year. Most of these fatalities (80%) occur in children under the age of five, and the latest RTS, S, preventative vaccine trial only exhibited low protective efficacy (below 30%). The blood stage of Plasmodium infection accounts for the clinical symptoms of malaria, yet the immune mechanisms underlying disease outcomes are poorly understood. Here we have analyzed a cohort of severely infected Malawian children and revealed dramatic levels of inflammatory cytokines and extremely robust blood leukocyte activation, notably for CD14 + CD16 lo CCR2 + inflammatory monocytes, plasmacytoid DCs (pDCs), NK and T cells. We could recapitulate these observations using the Plasmodium yoelii (Py) 17X YM surrogate mouse model of lethal blood stage malaria. Using this experimental system, we demonstrate that type I interferon (IFN) signals as a key cytokine controlling lethal outcomes and immune cell activation. Plasmacytoid dendritic cells represented the main cellular source of type I IFN in the bone marrow and the blood of infected mice. Most interestingly, the activation of pDCs required CD169 + macrophages, and both cell types exhibited prolonged interactions in the bone marrow of infected mice by intravital microscopy. We will present additional data uncovering the molecular sensing pathways controlling these processes. Altogether our study establishes essential molecular pathways and cellular interactions that occur during this significant human parasitic infection, and suggest novel potential therapeutic targets involved in severe malaria.