A Toll-like receptor in horseshoe crabs

Non-self-recognition of invading microbes relies on the pattern-recognition of pathogen-associated molecular patterns (PAMPs) derived from microbial cell-wall components. Insects and mammals conserve a signaling pathway of the innate immune system through cell-surface receptors called Tolls and Toll-like receptors (TLRs). Bacterial lipopolysaccharides (LPSs) are an important trigger of the horseshoe crab's innate immunity to infectious microorganisms. Horseshoe crabs' granular hemocytes respond specifically to LPS stimulation, inducing the secretion of various defense molecules from the granular hemocytes. Here, we show a cDNA which we named tToll, coding for a TLR identified from hemocytes of the horseshoe crab Tachypleus tridentatus. tToll is most closely related to Drosophila Toll in both domain architecture and overall length. Human TLRs have been suggested to contain numerous PAMP-binding insertions located in the leucine-rich repeats (LRRs) of their ectodomains. However, the LRRs of tToll contained no obvious PAMP-binding insertions. Furthermore, tToll was non-specifically expressed in horseshoe crab tissues. These observations suggest that tToll does not function as an LPS receptor on granular hemocytes.