Toll-like receptors (TLRs) are a family of homologous receptors expressed on many cell types and are involved in innate immune responses

Question

Ten different mammalian TLRs have been identified, and several ligands for many of these receptors are known. Which of the following is a TLR ligand?

A. Single-stranded RNA
B. Transfer RNA
C. Double-stranded DNA
D. Unmethylated CpG DNA
E. Heterochromatin

Answer 1

ANS: D. More than 10 mammalian Toll-like receptors (TLRs) have been identified, and each appears to recognize a different set of structures that are found in pathogenic microbes but not in mammalian cells. Such structures are called pathogen-associated molecular patterns (PAMPs). Unmethylated cytosine guanosine (CpG) motifs are typical of bacterial and protozoan DNA, but not mammalian DNA, and are therefore PAMPs. TLR9 binds CpG DNA. Transfer RNA, single-stranded RNA, double-stranded DNA, and heterochromatin are all normal components of mammalian cells and are not recognized by TLRs. Double-stranded RNA is produced by some viruses but not by mammalian cells and is recognized by TLR3.