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Sialic acids (Sias) play multifarious roles in immunity, including cell-cell adhesion and activation. Neutrophil sequestration is a hallmark of acute lung injury, and is consistently associated with xenograft lung injury, despite genetic modifications of the donor organ. Here we consider how endothelial and neutrophil Sias modulate the adhesion of human neutrophils (PMN) to human (h) or pig (p) aortic endothelial cells (AECs).
hAECs and pAECs were pretreated with C. perfringens neuraminidase (NA, to remove Sias), human tumor necrosis factor (hTNF, 25 μg/ml), or both. CalceinAM-labelled human neutrophils were treated with rhIL-8, NA. In some experiments PMNs or AECs were pre-treated with N-acetyllactosamine (LacNAc, a pan-galectin inhibitor) or an anti-galectin-3 antibody (to selectively inhibit galectin-3). Neutrophil adhesion was expressed as percent adhesion for static assays, or as number of neutrophils per field for flow assays (Bioflux, 1 dynes/cm2).
NA treatment of AECs increased PMN adhesion to pig (36% vs. 22%) more than to human (24% vs. 21%) AECs statically. PMN adhesion was similarly increased by TNF activation of pig or human AECs in static and flow conditions. IL-8 dramatically increased PMN adhesion to pAECs (77%, p<0.001) and hAECs (45%, p<0.001), with the highest adhesion observed with NA-treated pAECs and IL-8-activated PMNs. Pre-exposure of the PMNs to LacNAc prevented their galectin-mediated adhesion to galactose moieties exposed on NA-treated pAECs (14%, p<0.0001). Under flow conditions (Bioflux, 1 dynes/cm2) when NA-pretreated GalTKO.hCD46 pAECs and human PMNs were both incubated with a blocking anti-human galectin-3 antibody, PMN adhesion to de-sialylated pAECs was decreased in a dose dependent manner (Figure 1).
AEC de-sialylation promotes xenogeneic and, to lesser degree, non-xenogeneic PMN adhesion, an effect completely inhibited by pre-incubation of PMNswith LacNAc and significantly decreased with anti-galectin-3 antibody. These results suggest that inhibition of sialidase activity or of galectin binding (specifically galectin-3) are promising candidate approaches to limit PMN-mediated inflammation in xenogeneic organ injury.