HMGB‐1 induces c‐kit⁺ cell microvascular rolling and adhesion <i>via</i> both toll‐like receptor‐2 and toll‐like receptor‐4 of endothelial cells

Abstract

High-mobility group box 1 (HMGB-1) is a strong chemo-attractive signal for both inflammatory and stem cells. The aim of this study is to evaluate the mechanisms regulating HMGB-1-mediated adhesion and rolling of c-kit(+) cells and assess whether toll-like receptor-2 (TLR-2) and toll-like receptor-4 (TLR-4) of endothelial cells or c-kit(+) cells are implicated in the activation of downstream migration signals to peripheral c-kit(+) cells. Effects of HMGB-1 on the c-kit(+) cells/endothelial interaction were evaluated by a cremaster muscle model in wild-type (WT), TLR-2 (-/-) and Tlr4 (LPS-del) mice. The mRNA and protein expression levels of endothelial nitric oxide synthase were determined by quantitative real-time PCR and immunofluorescence staining. Induction of crucial adhesion molecules for rolling and adhesion of stem cells and leukocytes were monitored in vivo and in vitro. Following local HMGB-1 administration, a significant increase in cell rolling was detected (32.4 ± 7.1% in 'WT' versus 9.9 ± 3.2% in 'control', P < 0.05). The number of firmly adherent c-kit(+) cells was more than 13-fold higher than that of the control group (14.6 ± 5.1 cells/mm(2) in 'WT' versus 1.1 ± 1.0 cells/mm(2) in 'control', P < 0.05). In knockout animals, the fraction of rolling cells did not differ significantly from control levels. Firm endothelial adhesion was significantly reduced in TLR-2 (-/-) and Tlr4 (LPS-del) mice compared to WT mice (1.5 ± 1.4 cells/mm(2) in 'TLR-2 (-/-)' and 2.4 ± 1.4 cells/mm(2) in 'Tlr4 (LPS-del)' versus 14.6 ± 5.1 cells/mm(2) in 'WT', P < 0.05). TLR-2 (-/-) and Tlr4 (LPS-del) stem cells in WT mice did not show significant reduction in rolling and adhesion compared to WT cells. HMGB-1 mediates c-kit(+) cell recruitment via endothelial TLR-2 and TLR-4.