Role of Sialic Acids in Xenotransplantation

Abstract

The sialic acid profile on the surface of cells can dynamically change upon cell activation and such change can dramatically affect various cellular functions. One pathway facilitating such change is through the loss of sialic acid by enzymatic cleavage caused by sialidases. In certain cell types, sialidases were found to mobilize from intracellular reservoirs to the cell surface after activation, however, the mechanism of the translocation is still not clear. This surface expressed sialidase activity causes desialylation of the cell surface either in cis or trans on other cell types in close vicinity. One of the mechanisms by which sialic acid cleavage has been shown to affect cell-cell binding is facilitated by the binding of galectins. When sialic acid is cleaved, underlying galactose molecules are exposed. Galactose is in turn the ligand for the lectin receptors galectins. Galectins are defined by sharing a structural homology in their carbohydrate-recognition domains (CRDS). Galectins are expressed in different tissues and cells including endothelial cells (EC) , alveolar macrophages and neutrophils (PMN). It has been found that a change in the level of galectins can affect PMN recruitment. A main limitation of pig lung xenograft injury is the fact that leukocyte (specifically PMN) and platelet sequestration occur in our ex-vivo human blood perfusion system within minutes, and the mechanisms driving these adhesive interactions are largely unknown. We believe that sialidase activity is increased during an ex-vivo lung xeno-perfusion and this causes an increase in desialylation of lung tissue and PMN. Here we characterize sialidase expression during an ex-vivo lung xeno perfusion and evaluate the role of cellular desialylation in PMN and endothelial cell adhesion under xeno settings, most specifically focusing on NEU1 and NEU3. We demonstrate that when sialic acid is cleaved from pig EC/human PMN this increases human PMN adhesion, and that PMN adhesion is further increased when PMN or EC are pre-activated. We show that the galectin binding is a mechanism behind this discovery/phenomenon. These findings provide the first mechanistic explanation of how galectin binding, consequent of sialic acid cleavage, affects human PMN adhesion during xenotransplantation.