Protection of human myeloid dendritic cell subsets against influenza A virus infection is differentially regulated upon TLR stimulation.

The proinflammatory microenvironment in the respiratory airway induces maturation of both resident and infiltrating dendritic cells (DCs) upon influenza A virus (IAV) infection. This results in upregulation of antiviral pathways as well as modulation of endocytic processes, which affect the susceptibility of DCs to IAV infection. Therefore, it is highly relevant to understand how IAV interacts with and infects mature DCs. To investigate how different subsets of human myeloid DCs (MDCs) involved in tissue inflammation are affected by inflammatory stimulation during IAV infection, we stimulated primary blood MDCs and inflammatory monocyte-derived DCs (MDDCs) with TLR ligands, resulting in maturation. Interestingly, MDDCs but not MDCs were protected against IAV infection after LPS (TLR4) stimulation. In contrast, stimulation with TLR7/8 ligand protected MDCs but not MDDCs from IAV infection. The reduced susceptibility to IAV infection correlated with induction of type I IFNs. We found that differential expression of TLR4, TRIF, and MyD88 in the two MDC subsets regulated the ability of the cells to enter an antiviral state upon maturation. This difference was functionally confirmed using small interfering RNA and inhibitors. Our data show that different human MDC subsets may play distinct roles during IAV infection, as their capacity to induce type I IFNs is dependent on TLR-specific maturation, resulting in differential susceptibility to IAV infection.