Toll-like Receptors and the Control of Immunity

Abstract

The study of innate immunity and its link to inflammation and host defense encompasses diverse areas of biology, ranging from genetics and biophysics to signal transduction and physiology. Central to our understanding of these events are the Toll-like receptors (TLRs), an evolutionarily ancient family of pattern recognition receptors. Herein, we describe the mechanisms and consequences of TLR-mediated signal transduction with a focus on themes identified in the TLR pathways that also explain the operation of other immune signaling pathways. These themes include the detection of conserved microbial structures to identify infectious agents and the use of supramolecular organizing centers (SMOCs) as signaling organelles that ensure digital cellular responses. Further themes include mechanisms of inducible gene expression, the coordination of gene regulation and metabolism, and the influence of these activities on adaptive immunity. Studies in these areas have informed the development of next-generation therapeutics, thus ensuring a bright future for research in this area.

Figure 1.

Multiple TLR family members can detect PAMPs on individual microorganisms. A bacterium (A), virus or parasite (B) are depicted. Insets display PAMPs and the TLRs that are responsible for their detection. The TLR names represent those found in mice, as genetic evidence is available to support the importance of each TLR-PAMP interaction indicated.

Figure 2.

Diverse cellular responses induced by TLRs upon microbial detection. Cell-extrinsic responses induced by TLR signaling are indicated (A). These responses include activities that influence the local (or systemic) environment surrounding the cell that detected a PAMP. Cell-intrinsic responses induced by TLR signaling are indicated (B). These responses occur within the cell that detected a PAMP and contribute to the activities indicates in panel A. Cell type-specific responses induced by TLR signaling are indicated (C). These responses occur uniquely in the cell type indicated, but are mediated by TLR-PAMP interactions. The underlying mechanisms that explain cell type-specific TLR responses are poorly defined.

Figure 3.

TLR signaling is mediated by two SMOCs—the myddosome and triffosome (A) All TLRs, except TLR3, induce the assembly of a supramolecular organizing center (SMOC) called the myddosome upon PAMP detection. Myddosome assembly occurs around the cytosolic tail of dimerized TLRs present at the plasma membrane or endosomes. The enzyme TRAF6 is present in the myddosome. TRAF6 functions to stimulate myddosome-associated TBK1 to drive metabolic changes in the cell and functions to stimulate IKK-and MAPK-dependent transcription factors. The collection of these activities promotes inflammation and host defense. (B) On endosomes TLR4 and TLR3 have the capacity to engage a SMOC called the triffosome. This protein complex is poorly defined, but is believed to be organized and operate as depicted here. TRIF contains a pLxIS motif that promotes TBK1-dependent gene expression and a RHIM domain to promote RIPK3-dependent necroptosis. This latter activity only occurs upon conditions of caspase-inhibition.

Figure 4.

TLR-mediated regulation of inflammasomes TLR signaling can prime cells for an enhanced responsiveness to stimuli that drive the assembly and activity of inflammasomes. Simultaneous exposure of cells to TLR ligands and stimuli that promote inflammasome assembly lead to an immediate pyroptosis. This immediate pyroptosis is dependent on the myddosome component IRAK1, but is independent of any transcriptional response in the cell. In cells that have been pre-exposed to TLR ligands, the transcriptional upregulation of inflammasome regulators NLRP3, Caspase-11 and IL-1β lead to a more efficient assembly and activity of DAMPs such as IL-1β. Other examples of TLR-mediated influence on inflammasome activities are described in the text.

Figure 5.

LncRNAs in TLR signaling. TLR stimulation leads to temporally regulated changes in the expression of a number of lncRNAs which in turn function to regulate the innate immune response. lncRNAs can function in the cytosol or nucleus by binding proteins to either promote or restrain responses. For example, the lincRNA-Cox2 interacts with the SWI/SNF chromatin remodeling complex to regulate expression of immune genes that require chromatin remodeling for their expression. In addition, this same RNA can interact with A2/B1 to downregulate chemokine expression. iNOS-AS functions as a positive regulator of immune gene expression. The iNOS antisense transcript localizes to the cytoplasm where it promotes the stability and subsequent translation of iNOS mRNA through direct base-pair complementation. Lethe and Mirt are induced lncRNAs that inhibit NF B function. Lethe binds RelA to sequester NF B while Mirt2 acts indirectly by reducing Lys63 (K63)-linked ubiquitination of TRAF6 a key regulator of NF B activation. In addition to these inducible RNAs, some lncRNAs are expressed in myeloid cells in the absence of stimulation and can be downregulated upon TLR activation. These lncRNAs can restrain immune gene expression by regulating chromatin accessibility (eg. lnc13 or lincRNA-Eps). It is likely that additional lncRNAs will be identified that modulate IRF signaling and IRF regulated gene expression downstream of TLRs to modulate the entire TLR inducible program.