One of the mechanisms by which the innate immune system senses the invasion of pathogenic microorganisms is through the Toll-like receptors (TLRs), which recognize specific molecular patterns that are present in microbial components. Stimulation of different TLRs induces distinct patterns of gene expression, which not only leads to the activation of innate immunity but also instructs the development of antigen-specific acquired immunity. Here, we review the rapid progress that has recently improved our understanding of the molecular mechanisms that mediate TLR signalling.

Toll-like receptor signalling

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

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Signaling to NF-kappaB.

Functional characterization of Toll-like receptors (TLRs) has established that innate immunity is a skillful system that detects invasion of microbial pathogens. Recognition of microbial components by TLRs initiates signal transduction pathways, which triggers expression of genes. These gene products control innate immune responses and further instruct development of antigen-specific acquired immunity. TLR signaling pathways are finely regulated by TIR domain-containing adaptors, such as MyD88, TIRAP/Mal, TRIF and TRAM. Differential utilization of these TIR domain-containing adaptors provides specificity of individual TLR-mediated signaling pathways. Several mechanisms have been elucidated that negatively control TLR signaling pathways, and thereby prevent overactivation of innate immunity leading to fatal immune disorders. The involvement of TLR-mediated pathways in autoimmune and inflammatory diseases has been proposed. Thus, TLR-mediated activation of innate immunity controls not only host defense against pathogens but also immune disorders.

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Toll-like receptors in innate immunity.

https://molmed.nl/uploads/abstracts/158/il-33 discovery.pdf

IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines

More than any other cytokine family, the interleukin (IL)-1 family is closely linked to the innate immune response. This linkage became evident upon the discovery that the cytoplasmic domain of the IL-1 receptor type I is highly homologous to the cytoplasmic domains of all Toll-like receptors (TLRs). Thus, fundamental inflammatory responses such as the induction of cyclooxygenase type 2, increased expression of adhesion molecules, or synthesis of nitric oxide are indistinguishable responses of both IL-1 and TLR ligands. Both families nonspecifically affect antigen recognition and lymphocyte function. IL-1β is the most studied member of the IL-1 family because of its role in mediating autoinflammatory diseases. Although the TLR and IL-1 families evolved to assist in host defense against infection, unlike the TLR family, the IL-1 family also includes members that suppress inflammation, both specifically within the IL-1 family but also nonspecifically for TLR ligands and the innate immune response.

Immunological and Inflammatory Functions of the Interleukin-1 Family

The Toll-like receptor–interleukin 1 receptor signaling (TLR–IL-1R) receptor superfamily is important in differentially recognizing pathogen products and eliciting appropriate immune responses. These receptors alter gene expression, mainly through the activation of nuclear factor-κB and activating protein 1. SIGIRR (single immunoglobulin IL-1R-related molecule), a member of this family that does not activate these factors, instead negatively modulates immune responses. Inflammation is enhanced in SIGIRR-deficient mice, as shown by their enhanced chemokine induction after IL-1 injection and reduced threshold for lethal endotoxin challenge. Cells from SIGIRR-deficient mice showed enhanced activation in response to either IL-1 or certain Toll ligands. Finally, biochemical analysis indicated that SIGIRR binds to the TLR–IL-1R signaling components in a ligand-dependent way. Our data show that SIGIRR functions as a biologically important modulator of TLR–IL-1R signaling.

Liping Tian

Papers

SIGIRR, a negative regulator of Toll-like receptor-interleukin 1 receptor signaling.