The Staphylococcus aureus Lipoprotein SitC Colocalizes with Toll-Like Receptor 2 (TLR2) in Murine Keratinocytes and Elicits Intracellular TLR2 Accumulation

SitC is one of the predominant lipoproteins in Staphylococcus aureus. Recently, SitC was shown to be capable of stimulating Toll-like receptor 2 (TLR2), but the mechanism of TLR2 activation by SitC has not been analyzed in detail so far. In this study, we purified C-terminally His-tagged SitC (SitC-His) from Staphylococcus aureus. SitC-His induced interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-alpha) release in human monocytes and also NF-kappa B activation in TLR2-transfected HEK293 cells, indicating TLR2-specific activation. SitC not only induced a TLR2-dependent release of IL-6 in primary murine keratinocytes (MKs) but also induced intracellular accumulation of TLR2, which was time and concentration dependent. Cy2-labeled SitC-His colocalized specifically with TLR2 in MKs and was also internalized in TLR2 knockout MKs, suggesting a TLR2-independent uptake. Neither activation nor colocalization of SitC-His was observed with TLR4 or Nod2. The results show that the native lipoprotein SitC-His specifically colocalizes with TLR2, is internalized by host cells, induces proinflammatory cytokines, and triggers intracellular accumulation of TLR2.