TLR signalling and the function of dendritic cells.

The recognition of microbes by innate immune cells initiates activation of the whole immune system. Toll-like receptors (TLRs) are known to recognize various components of invading pathogens. At present, the natural ligands for almost all TLR members have been identified. TLRs are expressed on many types of cells including macrophages and dendritic cells (DCs). The recognition of invading microbes by TLRs on DCs induces proinflammatory cytokine production and enhanced antigen presentation to naive T cells, and finally activates antigen-specific adaptive immune responses. The sequential activation of innate and subsequent adaptive immunity are crucial steps to eradicate invading pathogens. Recently, the TLR signalling pathway has been intensively investigated. Accumulating evidence indicates that, at least, four adaptor molecules are involved in TLR signalling and provide their signalling specificities. Distinct TLR ligands provide distinct activation status and cytokine production patterns for antigen presenting cells, resulting in the induction of differential immune responses. Thus, TLRs are critical molecules to induce not only inflammatory responses but also fine-tuned adaptive immune responses depending on invading pathogens.