Requirement of Autophagy Proteins Beclin-1 and Map1-LC3b in the Induction of Innate and Adaptive Immune Responses to Respiratory Syncytial Virus Infection.

Abstract

Respiratory syncytial virus (RSV) is a ubiquitous human pathogen that produces severe lower respiratory disease in vulnerable individuals, characterized by bronchiolitis, excessive mucus production, and immune-mediated lung damage. Severe RSV bronchiolitis in infancy is strongly correlated with development of recurrent wheezing later in childhood, suggesting persistent alteration of the lung immune environment long after successful viral clearance. Within the lung environment, dendritic cells (DCs) direct innate and adaptive immune responses to viral pathogens through secretion of pro-inflammatory cytokines and type I interferon (IFN), as well as through activation of antiviral T cell responses. Signaling through the RNA-sensing intracellular toll-like receptors (TLR) -3 and -7 is required for robust production of type I IFN and antigen presentation function by DCs during viral infection. The intracellular process of macroautophagy (autophagy), or the sequestration of intracellular contents in a double-walled membrane for degradation by lysosomes, promotes TLR-dependent innate cytokine production and antigen-presenting cell function by RSV-infected DCs. The autophagic pathway further limits inflammation by degrading assembled inflammasome platforms and their substrate pro-IL-1β, suggesting that autophagy may also block excessive pro-inflammatory signaling during infection. However, the requirement for autophagy in promoting antiviral responses and limiting lung pathology during in vivo RSV infection has yet to be investigated. Using a mouse model of lower respiratory tract infection, we found that deficiency in the autophagy proteins Beclin-1 (Becn1) or Map1-LC3b (LC3b) led to increased lung immunopathology upon infection, characterized by delayed viral clearance, increased mucus secretion, and altered T cell cytokine production. Becn1+/- and LC3b-/- DCs were impaired in antiviral cytokine production, and elicited Th2- or Th17- skewed responses from co-cultured CD4+ T cells, respectively. Furthermore, studies using bone-marrow chimeric mice revealed a critical role for LC3b in restricting ER stress-dependent, inflammasome-mediated production of IL-1β by infected airway epithelial cells. Finally, neutralization of IL-17 or IL-1 receptor signaling in vivo ameliorated the development of lung pathology in LC3b-/- mice. Overall, these studies shed light on the importance of autophagy to the induction of antiviral immune responses and control of inflammation during RSV infection, and establish the autophagy pathway as a potential target for future therapeutic interventions.