ZBP1 signaling in RSV triggered host defense and pathogenesis - ABSTRACT Respiratory syncytial virus (RSV) is a clinically important virus causing mortality and morbidity among infants, children, and immune-compromised individuals. RSV-driven immuno-pathogenesis during host-defense is characterized by rampant lung inflammation and pulmonary injury leading to development of bronchiolitis and pneumonia. RSV infection also in early life can also predispose individuals to chronic respiratory diseases such as bronchiolitis and asthma. As part of immune host-defense, RSV infected cells undergo lytic cell death comprising of necroptosis in infected non-myeloid stromal cells, and pyroptosis in myeloid cells. Unchecked necrotic cell death can lead to lung injury. How RSV triggers cell death, and the role of such cell death in host defense, immunity, and pulmonary injury is poorly understood. Our preliminary studies have found that RSV activates the immune sensor protein ZBP1, which triggers both necroptosis and pyroptosis following infection. We have also found that RSV produces Z-RNA (left-handed dsRNA duplexes) and propose that such Z-RNAs are activating ligands for ZBP1. Surprisingly, our preliminary results also suggest that the RSV non-structural protein NS2 functions as a ZBP1 activator, possibly in conjunction with Z-RNA, and ZBP1 is involved in triggering exaggerated inflammation and exacerbated airway disease during RSV infection of mice. These pioneering observations implicate ZBP1 as the long-sought sensor of RSV in the pathway leading to lytic cell death in the respiratory tract, implicate virus-produced Z-RNAs as essential ligands for triggering ZBP1 activation, and identify the viral NS2 protein as key co-factor in this process. Once ZBP1 is activated, both necroptosis (in stromal cells) and pyroptosis (in myeloid cells) ensue. Necroptosis is driven by RIPK3-MLKL signaling, and pyroptosis by engagement of the inflammasome, downstream of RIPK3/MLKL activation. Based on these intriguing new data, we put forward the central hypotheses that (1) RSV Z-RNAs, perhaps in tandem with RSV NS2 protein, are activating ligands for ZBP1; (2) ZBP1 activation drives pyroptosis and necroptosis in infected cells, both of which are RIPK3/MLKL dependent; (3) these ZBP1-triggered lytic pathways of cell death drive RSV-induced lung injury in RSV-triggered respiratory diseases; and (4) since RIPK3 , a lytic cell death activating kinase is downstream of ZBP1, a new RIPK3 kinase inhibitor just developed by us will be a novel therapeutic for RSV-induced lung injury and viral pneumonia, for which there is no current effective treatment. To test these hypotheses, we outline three Aims. In aim-1 we will investigate the mechanism by which RSV activate ZBP1 for necroptosis and pyroptosis. In aim 2 we will study the role of ZBP1 mediated cell death in immunity, host defense, and lung injury. In aim 3 we will examine the therapeutic activity of a novel and potent RIPK3 kinase inhibitor developed in our laboratory in limiting RSV-associated respiratory disease. Significance: Our proposed studies on mechanisms regulating RSV-associated host immune response could lead to development of effective therapy to combat severe RSV disease, currently a major unmet medical need.
