Plasmacytoid dendritic cells, often abbreviated pDCs, detect viral RNA in living organisms primarily by delivering nucleic acids into specialized endosomal receptors that trigger a rapid interferon response. This sensing is central to early antiviral defense because pDCs are uniquely equipped to secrete large amounts of type I interferon within hours of exposure, shaping innate and adaptive responses while also carrying risks for inflammatory pathology when dysregulated.
Molecular sensor and signaling
The dominant molecular sensor for single-stranded viral RNA in these cells is TLR7, an endosomal Toll-like receptor that signals through the adaptor MyD88 to activate transcription factors including IRF7, producing robust type I interferons. Marco Colonna at Washington University in St. Louis has characterized pDC specialization, noting features such as high baseline IRF7 expression that enable unusually rapid interferon production. The MyD88–IRF7 axis distinguishes pDC responses from other antigen-presenting cells and explains their outsized contribution to the systemic antiviral cytokine milieu.
How viral RNA reaches endosomes in vivo
In physiological settings viral RNA reaches endosomal TLR7 by several routes. Whole virions can be endocytosed directly; antibody-coated viruses form immune complexes that human pDCs internalize via Fc receptors, delivering RNA into endosomes. Cell-to-cell transfer is also important in tissues: Akiko Iwasaki at Yale School of Medicine described mechanisms by which pDCs form close contacts with infected cells and acquire viral material through an interferogenic synapse, concentrating viral RNA into endosomal compartments for sensing. These pathways operate differently across tissues; mucosal and skin microenvironments modulate uptake and signaling thresholds, and Michel Gilliet at University of Lausanne has linked pDC-driven nucleic acid sensing to skin inflammation in conditions such as psoriasis.
The biological consequences are double-edged. Rapid type I interferon production limits viral replication and coordinates adaptive immunity, but excessive or misplaced pDC activation contributes to autoimmunity and tissue damage when self-RNA becomes available in immune complexes. Clinically relevant human genetics underline the pathway’s importance: Jean-Laurent Casanova at Rockefeller University reported that defects affecting TLR7 or type I interferon signaling predispose to severe viral disease, illustrating how variation in pDC sensing can determine disease outcome. Understanding the balance between protective and pathogenic pDC responses remains crucial for antiviral therapies and for managing inflammatory diseases tied to nucleic acid sensing.