Expression of toll-like receptor 4 on human keratinocytes by lipoteichoic acid

Background: We have recently shown that lipopolysaccharide (LPS), a major biologically active component of Gram-negative bacteria, mediate the activation of human keratinocytes by CD14 and Toll-like receptor (TLR 4). However, the mechanism of activation of keratinocytes by Gram-positive bacterial toxins remains unclear. Objective: We investigated the mechanism of activation of human keratinocytes by lipoteichoic acid (LTA), a main stimulatory component of Gram-positive bacteria. Methods: The effects of LTA on CD14, TLR2 and TLR4 mRNA expression were measured by quantitative RT-PCR in cultured human keratinocytes. To determine whether the effects of LTA on CD14, TLR2 and TLR4 expressions of the human keratinocytes were biologically functional, NF-κB nuclear translocation and IL-1α secretion were measured by immune-fluorescence staining and ELISA, respectively. Furthermore, to determine whether these effects by LTA were specific for CD14, TLR2 and TLR4, some cells were pretreated with anti-CD14, anti-TLR2, or anti-TLR2 monoclonal antibodies prior to the addition of LTA. Results: TLR4 mRNA expression on keratinocytes was augmented by exposure to LTA. LTA binding to keratinocytes resulted in NF-κB nuclear translocation and secretion of interleukin-1α. These responses by LTA were effectively abrogated by preincubating cells with anti-TLR4 monoclonal antibody, but not with anti-CD14 or anti-TLR2 monoclonal antibodies. Conclusion: These results indicate that, similar to LPS, LTA induces activation of human keratinocytes mainly through TLR4, however, in contrast to LPS signaling, LTA-induced keratinocyte activation is CD14-independent.