Human respiratory syncytial virus (RSV) is the most common cause of viral bronchiolitis and pneumonia in infants and children worldwide. Inflammation induced by RSV infection is responsible for its hallmark manifestation of bronchiolitis and pneumonia. The cellular debris created through lytic cell death of infected cells is a potent initiator of this inflammation. Macrophages are known to play a pivotal role in the early innate immune and inflammatory response to viral pathogens. However, the lytic cell death mechanisms associated with RSV infection in macrophages remains unknown. Two distinct mechanisms involved in lytic cell death are pyroptosis and necroptosis. Our studies revealed that RSV induces lytic cell death in macrophages via both of these mechanisms, specifically through the ASC (Apoptosis-associated speck like protein containing a caspase recruitment domain)-NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3) inflammasome activation of both caspase-1 dependent pyroptosis and receptor-interacting serine/threonine-protein kinase 3 (RIPK3), as well as a mixed lineage kinase domain like pseudokinase (MLKL)-dependent necroptosis. In addition, we demonstrated an important role of reactive oxygen species (ROS) during lytic cell death of RSV-infected macrophages.

Human respiratory syncytial virus (hRSV) is the most common cause of viral pneumonia in infants and children worldwide and creates significant airway disease in immunocompromised people and the elderly. Inflammation induced by hRSV infection is responsible for its hallmark manifestations of bronchiolitis and pneumonia. The cellular debris created secondary to lytic cell death of infected cells is a potent initiator of this inflammation. Macrophages are known to play a pivotal role in the early innate immune and inflammatory response to viral pathogens. While previous studies have investigated the mechanisms responsible for lytic cell death in Influenza A-infected macrophages and hRSV-infected neutrophils, the lytic cell death mechanisms associated with hRSV infection in macrophages remain unknown. In order to address this knowledge gap, we treated an hRSV-infected human THP-1 macrophage cell line with various inhibitors that block two lytic cell death pathways: pyroptosis and necroptosis. In addition, we used pyroptosis-deficient human macrophages to further clarify the lytic cell death mechanisms occurring during infection. Our studies revealed that hRSV induces lytic cell death in macrophages via both pyroptosis and necroptosis, specifically through caspase-1 and inflammasome-dependent pyroptotic mechanisms and through RIPK1-RIPK3-MLKL necroptotic mechanisms of cell death.