Castanospermine, an oligosaccharide processing inhibitor, reduces membrane expression of adhesion molecules and prolongs heart allograft survival in rats.

The inhibition of intracellular oligosaccharide processing is a new approach to immunosuppression in allotransplantation. The net effect of such inhibition is reduction in the membrane expression of certain glycoproteins. Hence cell-cell interaction in allorejection may be impaired in the presence of glycoprotein processing inhibitors because the expression of key ligand-receptor pairs of N-linked glycoproteins including adhesion molecules is inhibited. The aims of this study were to measure the immunosuppressive ability of castanospermine (CAST) in a rat heart allograft model, to measure its effect on membrane expression of adhesion molecules (LFA-1 alpha, LFA-1 beta, ICAM-1), class I and class II MHC antigens and on other T cell associated molecules (CD4, CD8, CD39, CD45, W3/13), to test its tolerogenic potential and its toxicity. Membrane expression of these molecules was measured by flow cytometry for single cells and by immunoperoxidase staining for the allograft. In grafted rats CAST significantly reduced the expression of LFA-1 alpha on lymphoid cells in the thymus, lymph node, spleen and heart allografts. ICAM-1 expression on endothelial cells of the allograft vasculature, class I and class II MHC expression on lymphoid cells in the thymus, class II MHC expression on lymphoid cells in the allograft; and CD4, CD8, CD45 and W3/13 expression on lymphoid cells in some organs. By contrast, in non-grafted rats CAST significantly upregulated expression of class I MHC and CD45 in the thymus, lymph node and spleen, ICAM-1 and CD4 on lymphoid cells in the spleen, but reduced expression of LFA-1 alpha on lymphoid cells in the thymus. It also prolonged rat heart allograft survival in a dose-dependent manner and with limited testing was relatively non-toxic. In conclusion, CAST is an immunosuppressive molecule which may work by downregulation of the ligand-receptor adhesion molecule pair, LFA-1 alpha-ICAM-1 although subtle downregulation of class I and II MHC, CD4 and CD8 molecules could also contribute to its immunosuppressive activity. Hence, both lymphocyte-endothelial cell binding and lymphocyte activation may be inhibited by CAST. This work suggests that CAST may hold significant potential as a transplant immunosuppressant probably as an adjuvant agent to inhibitors of interleukin 2 secretion.