Pathogen Recognition and Innate Immunity

▪ Abstract The innate immune system is a universal and ancient form of host defense against infection. Innate immune recognition relies on a limited number of germline-encoded receptors. These receptors evolved to recognize conserved products of microbial metabolism produced by microbial pathogens, but not by the host. Recognition of these molecular structures allows the immune system to distinguish infectious nonself from noninfectious self. Toll-like receptors play a major role in pathogen recognition and initiation of inflammatory and immune responses. Stimulation of Toll-like receptors by microbial products leads to the activation of signaling pathways that result in the induction of antimicrobial genes and inflammatory cytokines. In addition, stimulation of Toll-like receptors triggers dendritic cell maturation and results in the induction of costimulatory molecules and increased antigen-presenting capacity. Thus, microbial recognition by Toll-like receptors helps to direct adaptive immune responses ...

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Innate Immune Recognition

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

DNA from bacteria has stimulatory effects on mammalian immune cells, which depend on the presence of unmethylated CpG dinucleotides in the bacterial DNA. In contrast, mammalian DNA has a low frequency of CpG dinucleotides, and these are mostly methylated; therefore, mammalian DNA does not have immuno-stimulatory activity. CpG DNA induces a strong T-helper-1-like inflammatory response. Accumulating evidence has revealed the therapeutic potential of CpG DNA as adjuvants for vaccination strategies for cancer, allergy and infectious diseases. Despite its promising clinical use, the molecular mechanism by which CpG DNA activates immune cells remains unclear. Here we show that cellular response to CpG DNA is mediated by a Toll-like receptor, TLR9. TLR9-deficient (TLR9-/-) mice did not show any response to CpG DNA, including proliferation of splenocytes, inflammatory cytokine production from macrophages and maturation of dendritic cells. TLR9-/- mice showed resistance to the lethal effect of CpG DNA without any elevation of serum pro-inflammatory cytokine levels. The in vivo CpG-DNA-mediated T-helper type-1 response was also abolished in TLR9-/- mice. Thus, vertebrate immune systems appear to have evolved a specific Toll-like receptor that distinguishes bacterial DNA from self-DNA.

A Toll-like receptor recognizes bacterial DNA.

Toll-like receptors (TLRs) are a family of innate immune-recognition receptors that recognize molecular patterns associated with microbial pathogens, and induce antimicrobial immune responses. Double-stranded RNA (dsRNA) is a molecular pattern associated with viral infection, because it is produced by most viruses at some point during their replication. Here we show that mammalian TLR3 recognizes dsRNA, and that activation of the receptor induces the activation of NF-kappaB and the production of type I interferons (IFNs). TLR3-deficient (TLR3-/-) mice showed reduced responses to polyinosine-polycytidylic acid (poly(I:C)), resistance to the lethal effect of poly(I:C) when sensitized with d-galactosamine (d-GalN), and reduced production of inflammatory cytokines. MyD88 is an adaptor protein that is shared by all the known TLRs. When activated by poly(I:C), TLR3 induces cytokine production through a signalling pathway dependent on MyD88. Moreover, poly(I:C) can induce activation of NF-kappaB and mitogen-activated protein (MAP) kinases independently of MyD88, and cause dendritic cells to mature.

Recognition of double-stranded RNA and activation of NF-kappaB by Toll-like receptor 3.

Defective LPS signaling in C3 H/HeJ and C57 BL/10 ScCr mice: Mutations in Tlr4 Gene

Erratum: Genetic and physical mapping of the Lps locus: Identification of the toll-4 receptor as a candidate gene in the critical region (Blood Cells, Molecules, and Diseases (1998) 24:3 (340-355))

The lymphotoxin (LT)-beta heterotrimer was recently identified as a molecule containing LT-alpha subunits, tethered to the cell through non-covalent association with an integral plasma membrane protein, derived from the LT-beta gene. Since knockout mutations of the LT-alpha gene yield animals that lack lymph nodes, whereas animals lacking either or both of the receptors for tumor necrosis factor (TNF) and LT-alpha homotrimers have normal lymph nodes, it has been inferred that the association between the LT-beta heterotrimer and its cognate receptor is required for lymph node ontogeny. Similarly, LT-beta and its receptor are thought to be important for development of the spleen. Since LT-alpha deficient mice lack lymph nodes, it is difficult to assess the extradevelopmental contribution of LT-beta to immune competence. To this end, we employed a strategy for the conditional blockade of LT-beta heteromer activity in normal mice. The interaction between LT-beta and its receptor is essential for the destruction of intracellular Listeria monocytogenes.

C. Van Huffel

Papers

Defective LPS signaling in C3 H/HeJ and C57 BL/10 ScCr mice: Mutations in Tlr4 Gene

Erratum: Genetic and physical mapping of the Lps locus: Identification of the toll-4 receptor as a candidate gene in the critical region (Blood Cells, Molecules, and Diseases (1998) 24:3 (340-355))

Expression of an adenovirally encoded lymphotoxin-β inhibitor prevents clearance of Listeria monocytogenes in mice