Endocytosis-free DNA sensing by cell surface TLR9 in neutrophils: Rapid defense with autoimmune risks

TLR9 senses microbial DNA, but may also respond to self-DNA. To prevent the initiation of innate immune responses to self-DNA, TLR9 is thought to sense microbial DNA in endolysosomes, and not at the cell surface. A report by Lindau et al. in this issue of the European Journal of Immunology [Eur. J. Immunol. 2013. 43: 2101-2113] shows that TLR9 is expressed on the surface of human and mouse neutrophils and, furthermore, shows that cell surface TLR9, instead of endosomal TLR9, senses DNA in neutrophils. These findings demonstrate that DNA sensing by TLR9 in neutrophils is quite distinct from that in DCs or macrophages. The unique DNA sensing by cell surface TLR9 in neutrophils may reflect their role in inducing rapid inflammation by degranulation with a minimal role in engulfing microbial products for antigen presentation.