Toll-like receptor 9 (TLR9) promiscuously binds self-and microbial DNA, but only microbial DNA elicits an inflammatory response. How TLR9 discriminates between self-and foreign DNA is unclear, but inappropriate localization of TLR9 permits response to self-DNA, suggesting that TLR9 localization and trafficking are critical components. The molecular mechanisms controlling the movement of TLR9 may provide new insight into the recognition of DNA in normal and in pathological conditions such as autoimmune systemic lupus erythematosus. We have shown earlier that TLR9 is retained in the endoplasmic reticulum (ER) and it moves to endolysosomes to recognize CpG DNA. Other studies have suggested that TLR9 bypasses the Golgi complex to access endolysosomes. Here, we show that TLR9 translocates from ER to endolysosomes through the Golgi complex and that Golgi export is required for optimal TLR9 signaling. In all, 6-13% of TLR9 constitutively exits the ER, moves through the Golgi complex and resides in lysosomal-associated membrane protein-1-positive vesicles. TLR9 bound to CpG DNA had glycan modifications indicative of Golgi processing confirming that TLR9 travels through the Golgi complex to access CpG DNA in endolysosomes. Together, these data support a model where TLR9 uses traditional secretory pathways and does not bypass the Golgi complex. Keywords: brefeldin A; CpG DNA; endolysosome; Golgi; TLR9; trafficking Toll-like receptors (TLRs) are innate immune receptors important for host defense against pathogens. 1 Although many TLRs are present at the cell surface, those dedicated to recognition of various nucleic acid structures are retained in intracellular compartments. Nucleic acid structures recognized by TLRs include double-stranded RNA by TLR3, 2 single-stranded RNA by TLR7 and 8, 3 and CpG DNA by TLR9. 4,5 It is noted that, these structures are also present in selfnucleic acids.Recognition of DNA by TLR9 occurs in endolysosomes, but TLR9 resides predominantly in the endoplasmic reticulum (ER) in resting cells. 5-9 Endolysosomal localization is important for TLR9 response to CpG DNA, as TLR9 binds some DNA preferentially at low pH, endosomal acidification inhibitors block TLR9 signaling and TLR9 undergoes a conformational change upon CpG DNA binding in endolysosomes. [10][11][12] CpG DNAs with different sequence and secondary structure can elicit either production of type I interferon-a or maturation of plasmacytoid dendritic cells depending on the precise location of interaction with TLR9. Those CpG DNAs that induce interferon-a production accumulate in early endosomes, whereas those that induce maturation accumulate in lysosomes. 13 The mechanism regulating TLR9 translocation from the ER to these different compartments remains unclear.Intracellular localization of TLRs 3, 7, 8 and 9 may be a key factor to prevent recognition of self-nucleic acids. Self-nucleic acids are poorly endocytosed and rapidly degraded in the extracellular milieu. However, self-DNA is recognized by TLR9 when it is presented by chromatin-s...

SUMMARY Nucleic acids structures are highly conserved through evolution and when self nucleic acids are aberrantly detected by Toll-Like Receptors (TLRs) they contribute to autoimmune disease. For this reason, multiple regulatory mechanisms exist to prevent response to self nucleic acids. TLR9 is a nucleic acid sensing TLR that is regulated at multiple levels including association with accessory proteins, intracellular localization and proteolytic processing. In the endolysosomal compartment TLR9 is proteolytically processed to an 80 kilodalton form (p80) and this processing is a prerequisite for activation. Here we identified a soluble form of TLR9 generated by a novel proteolytic event that cleaved TLR9 between amino acids 724–735. Similar to p80, sTLR9 was generated in endosomes. However, generation of sTLR9 was independent of the cysteine protease cathepsin B active at acidic pH, but partially dependent on cathepsin S, a protease active at neutral pH. Most importantly, sTLR9 inhibited TLR9-dependent signaling. Together, these data support a model where an intrinsic proteolytic processing mechanism negatively regulates TLR9 signaling. Proper balance between the independent proteolytic events likely contributes to regulation of TLR9 mediated innate immunity and prevention of autoimmune disease.

Macrophages participate in immunity, tissue repair and tissue homeostasis. Activation of Toll-like receptors (TLRs) by conserved exogenous or endogenous structures initiates signaling cascades that result in the release of cytokines such as tumor necrosis factor α (TNFα). Extracellular substrate stiffness is known to regulate functions of non-immune cells through a process called mechanotransduction, yet less is known about how physical cues affect macrophage function or TLR signaling. To investigate this question, we cultured murine primary bone marrow-derived macrophages (BMMs) and RAW264.7 cells on fibronectin-coated polyacrylamide (PA) gels of defined stiffnesses (1, 20 and 150 kPa) that approximate the physical properties of physiologic tissues. BMMs on all gels were smaller and more circular than those on rigid glass. Macrophages on intermediate stiffness 20 kPa PA gels were slightly larger and less circular than those on either 1 or 150 kPa. Secretion of the pro-inflammatory cytokine, TNFα, in response to stimulation of TLR4 and TLR9 was increased in macrophages grown on soft gels versus more rigid gels, particularly for BMMs. Inhibition of the rho-associated coiled-coil kinase 1/2 (ROCK1/2), key mediators in cell contractility and mechanotransduction, enhanced release of TNFα in response to stimulation of TLR4. ROCK1/2 inhibition enhanced phosphorylation of the TLR downstream signaling molecules, p38, ERK1/2 and NFκB. Our data indicate that physical cues from the extracellular environment regulate macrophage morphology and TLR signaling. These findings have important implications in the regulation of macrophage function in diseased tissues and offer a novel pharmacological target for the manipulation of macrophage function in vivo.

Trichinella spiralis is a highly destructive parasitic nematode that invades and destroys intestinal epithelial cells, injures many different tissues during its migratory phase, and occupies and transforms myotubes during the final phase of its life cycle. We set out to investigate the role in immunity of innate receptors for potential pathogen-or danger-associated molecular patterns (PAMPs or DAMPs). Focusing on the MyD88-dependent receptors, which include Toll-like receptors (TLRs) and interleukin-1 (IL-1) family members, we found that MyD88-deficient mice expelled worms normally, while TLR2/4-deficient mice showed accelerated worm expulsion, suggesting that MyD88 was active in signaling pathways for more than one receptor during intestinal immunity. A direct role for PAMPs in TLR activation was not supported in a transactivation assay involving a panel of murine and human TLRs. Mice deficient in the IL-1 family receptor for the DAMP, IL-33 (called ST2), displayed reduced intestinal Th2 responses and impaired mast cell activation. IL-33 was constitutively expressed in intestinal epithelial cells, where it became concentrated in nuclei within 2 days of infection. Nuclear localization was an innate response to infection that occurred in intestinal regions where worms were actively migrating. Th2 responses were also compromised in the lymph nodes draining the skeletal muscles of ST2-deficient mice, and this correlated with increased larval burdens in muscle. Our results support a mechanism in which the immune system recognizes and responds to tissue injury in a way that promotes Th2 responses.