Pattern Recognition Receptors mini review

Pathogen-associated molecular patterns, or PAMPs, are molecules associated with groups of pathogens that are recognized by cells of the innate immune system. These molecules can be referred to as small molecular motifs conserved within a class of microbes. They are recognized by pattern recognition receptors (PRRs) which divided into Membrane-bound PRRs (which include Toll-like receptors (TLRs), Receptor kinases, and the mannose receptor), Cytoplasmic PRRs (which include NOD-like receptors, RIG-like receptors, and RNA Helicases, Plant PRRs and NonRD kinases) and Secreted PRRs in both plants and animals. They activate innate immune responses, protecting the host from infection, by identifying some conserved non-self-molecules. Toll like receptor mediate immune response through recognition of pathogen-associated molecular patterns (PAMPs), thus play important roles in host defense. The mannose receptor (MR) is a PRR primarily present on the surface of macrophages and dendritic cells. The MR belongs to the multilectin receptor protein group and, like the TLRs, provides a link between innate and adaptive immunity. It recognizes and binds to repeated mannose units on the surfaces of infectious agents and its activation triggers endocytosis and phagocytosis of the microbe via the complement system. The NOD-like receptors (NLRs) are cytoplasmic proteins that regulate inflammatory and apoptotic responses. Approximately 20 of these proteins have been found in the mammalian genome and include two major subfamilies called NODs and NALPs, the MHC Class II transactivator (CIITA), IPAF, BIRC1, and other molecules. NODs are currently known for NOD1 and NOD2. NOD1 recognizes a molecule called meso-DAP, which is a peptidoglycan constituent only of Gram negative bacteria. NOD2 proteins recognize intracellular MDP (muramyl dipeptide), which is a peptidoglycan constituent of both Gram positive and Gram negative bacteria. NODs transduce signals in the pathway of NF-κB and MAP kinases via the serine-threonine kinase called RIP2. NALPs are like NODs, these proteins contain C-terminal LRRs, which appear to act as a regulatory domain and may be involved in the recognition of microbial pathogens. Also like NODs, these proteins also contain a nucleotide binding site (NBS) for nucleoside triphosphates. Interaction with other proteins (e.g. the adaptor molecule ASC) is mediated via N-terminal pyrin (PYD) domain. In this Review, we discuss how distinct PRRs (with particular emphasis on TLRs and RLRs) sense the presence of pathogens and cellular insults and the mechanisms by which these PRR signals elicit TLRs and Their Ligands.