Mast cell Toll-like receptors (TLRs)

Toll-like receptors, the most widely studied PRRs, are characterized structurally by the presence of a leucine rich repeat (LRR) domain in their extracellular domain and a Toll/Interleukin-1 receptor (TIR) domain in their intracellular domain. Activation of TLRs leads to the production of cytokines, chemokines, and antimicrobial peptides and to the up regulation of stimulatory and adhesion molecules involved in immune responses (3). MCs, depending on the subtype and tissue distribution pattern, differentially express TLRs (4). Murine connective tissue type MCs express TLR3, TLR7, and TLR9 mRNA. In contrast, BMMCs only weekly express TLR3, TLR7, and TLR9 mRNA (4). P815 cells (Mouse lymphoblast-like mastocytoma cell line) expresses mRNAs as well as protein of TLR3, TLR7 and TLR9 (5). Another study has examine expression of all the murine TLR mRNAs in connective tissue and mucosal mast cells (6). This study indicate that both subtype of mast cells have high expression of TLR2, 4 and 6. The findings of these studies are summarized in Table 1.

The TLRs also appear to be widely expressed by human mast cells and cell lines. Studies on HCMCs indicate that they express TLR1, TLR2, TLR3, TLR4, TLR6, and TLR8 (7). Human mast cell line LAD2 express TLR1, TLR2, TLR4, TLR5 and TLR 9 at RNA as well as at protein level (8). Human cord blood derived mast cells express (CBMC) express mRNA for a number of TLRs including: TLR1, TLR2, and TLR6 but not TLR4 as checked by RT PCR (9), however cord blood-derived MCs expressed mRNA for TLR2, 4, and 6, whereas TLR expression in Peripheral blood derived MCs is restricted to TLR4 and 6 (10). HMC1 cell express expression of TLR2 as well as TLR4 as confirmed by RT PCR and Western blotting (11). Expression of TLR3, TLR5 and TLR 7 were also observed in HMC1 cell line (7). Skin and lung MC express TLR 2, 3 and 4. Skin mast cells also express TLR9 but little TLR7 or TLR10, while lung mast cells express higher levels of TLR7 and TLR10 but no TLR9 (12).