Pathogen recognition is central to the induction of T cell differentiation. Groups of pathogens share similar structures known as pathogen-associated molecular patterns (PAMPs), which are recognised by pattern recognition receptors (PRRs) expressed on dendritic cells (DCs) to induce cytokine expression. PRRs include archetypical Toll-like receptors (TLRs) and non-TLRs such as retinoic acid-inducible gene I (RIG-I)-like receptors, C-type lectin receptors (CLRs) and intracellular nucleotide-binding domain and leucine-rich-repeat-containing family (NLRs). PRR recognition of PAMPs can lead to the activation of intracellular signalling pathways that elicit innate responses against pathogens and direct the development of adaptive immunity.
CLRs comprises a large family of receptors which bind carbohydrates, through one or more carbohydrate recognition domains (CRDs), or which possess structurally similar C-type lectin-like domains (CTLDs) which do not necessarily recognise carbohydrate ligands. Some CLRs can induce signalling pathways that directly activate nuclear factor-kB (NF-kB), whereas other CLRs affect signalling by Toll-like receptors. These signalling pathways trigger cellular responses, including phagocytosis, DC maturation, chemotaxis, the respiratory burst, inflammasome activation, and cytokine production.