Cellular and Molecular Mechanisms of Plasmodium Parasite Sensing and Immune Activation in Severe Malaria

Abstract

Malaria is a significant global health burden, causing death in half a million people every year. High levels of inflammatory cytokines are thought to be one important contributor to severe disease, yet few studies have precisely examined the molecular and cellular mechanisms of parasite recognition and early immune response in vivo that undoubtedly contribute to downstream pathogenic immune responses. In this work, we hypothesized that an early immune sensing event is essential for orchestrating immune response outcomes to Plasmodium infections in vivo.We characterized high inflammatory cytokines and immune cell activation during malaria infection in a cohort of patients with severe disease in Malawi, and using a surrogate mouse model of severe malaria, Plasmodium yoelii 17XYM (PyYM). We noted high levels of type I interferon (IFN-I) and upregulation of IFN-I responsive activation markers, and based on these results, hypothesized that IFN-I orchestrates immune activation leading to severe disease. Consistent with our hypothesis, mice lacking IFN-I signaling (IFNalphabetaR -/-) were mostly protected from lethal PyYM infection (>80%), exhibited reduced inflammatory cytokines, and diminished recruitment and activation of innate cells. We next sought to determine the cellular source of IFN-I production, and the signaling pathways and underlying mechanisms. Using IFN(3-reporter mice, we found that pDCs are the primary source of systemic IFN-I. Through genetic depletion of pDCs, we also demonstrated that pDC-derived IFN-I orchestrates innate immune cell activation. pDCs secreted IFN-I via cell-intrinsic TLR7 sensing, but not TLR9, and cell-extrinsic STING signaling pathways. pDCs also require cell-intrinsic IFN-I signaling for activation, and genetic depletion of CD169+ macrophages largely prevented pDCs from producing IFN-I. Therefore, we postulated that STING inside CD169+ macrophages is necessary for pDCs to produce IFN-I. Intravital imaging of the BM showed that pDCs indeed closely interact with CD169+ macrophages in PyYM infected mice. In summary, this work has identified systemic IFN-I as an essential orchestrator of the early innate immune response in severe malaria, and clarify the in vivo relevance of multiple, cell type-specific sensing pathways (TLR7/MyD88 and STING). This work highlights potentially important targets for therapies aimed at preventing or treating severe malaria.