Regulation of lung dendritic cell function during influenza virus infection in vivo

A timely immune response is crucial for the effective control of a respiratory influenza virus infection. Influenza virus codes for an antagonistic protein known as non-structural protein 1 (NS1), that blocks innate responses from infected cells by suppressing type I interferon (IFN), and pro-inflammatory cytokine production. All our knowledge about influenza virus antagonism comes from in vitro studies and the significance of such events for adaptive immunity in vivo remains largely unknown.;In our first study, we systematically investigated the early events occurring in the lungs and draining lymph nodes upon infection with influenza virus. Strikingly, no sign of innate immunity was detected in the lungs for almost two days after infection when a sudden inflammatory burst including IFNs, cytokines, and chemokines occurred. This burst preceded the robust dendritic cell (DC) migration and T cell activation in the lymph nodes. In stark contrast, NS1-deficient virus triggered rapid inflammation in the lungs. Thus we demonstrate that in vivo, influenza virus utilizes the NS1 protein to replicate for almost two days after infection prior to detection by the immune system. We named this privileged time of virus replication the "stealth phase" that has additional implications in understanding the regulation of systemic antiviral responses and transmission of respiratory viruses.;Data from our first study indicated that lung DCs were loaded with viral Ag and migrated in the midst of intense type I IFN production at the end of the stealth phase. In a second set of experiments we focused on whether lung DCs were infected in vivo, how this process was regulated or not by type I IFN signaling through its receptor (IFNAR) and what were the biological consequences of such a phenomenon for innate and adaptive immunity.;We investigated the two major migratory lung DC subsets, CD103 + DCs and CD11bhigh DCs. Lung DCs were found to migrate with infectious virus to the MLNs in vivo causing localized virus replication at this site. The CD103+ DC subset was the major cell type responsible for virus translocation to the MLN. Strikingly, viral mRNA levels were significantly higher in CD103+ DCs than CD11bhigh DCs, and CD103+ DCs could promote virus replication ex vivo. CD11bhigh DCs compared to CD103 + DCs showed higher expression levels of genes involved in IFNAR signaling, correlating with stronger induction of IFN stimulated genes (ISGs) in the MLNs. Ablation of IFNAR signaling enhanced viral mRNA expression in CD11b high DCs and these cells gained the ability to sustain virus replication ex vivo. We next asked whether enhanced virus replication by CD11b high DCs may be an important determinant in Ag presentation by this cell type. Recent studies and unpublished observations by our group, had shown that CD103+ DCs are superior at presenting Ag to naive CD8+ T cells compared to CD11bhigh DCs during influenza infection, but the mechanism underlying this difference remained unknown. Here we demonstrate that CD11bhigh DCs isolated from infected IFNAR-/- mice primed CD8+ T cells equally well to CD103+ DC. Our findings show that type I IFN exquisitely regulates CD11bhigh DCs and CD103 + DCs have evolved a resistance to this cytokine. The implications of these findings open new avenues into understanding basic mechanisms of innate and adaptive immunity to respiratory viruses.