Antibody-mediated lung endothelium targeting: I. In vitro model using a cell line expressing angiotensin-converting enzyme

In order to produce a cell line expressing a level of angiotensin- converting enzyme (ACE) comparable to the one present in lung endothelial cells in vivo, Chinese hamster ovary (CHO) cells were transfected with different human ACE cDNAs. Clones showing ACE expression comparable to that in lung vessels were selected and further characterized. All clones produced both the membrane-bound and the secreted form of ACE. The pattern of ACE secretion to the culture medium was similar to that in cultured endothelial cells, where 80% of the active enzyme is membrane-bound ACE. Both the membrane-bound and secreted ACEs of each cell line bound to a panel of anti- ACE monoclonal antibodies (mAb) in ELISA and ACE precipitation assays. The binding of different preparations of ¹²⁵I-labeled anti-ACE mAb 9B9 and their conjugates to ACE-coated plastic and CHO cells expressing ACE correlates with their specific accumulation in the rat lung after systemic injection. The level of binding of anti-ACE mAbs to different cell lines (CHO cells expressing different forms of ACE, human umbilical vein endothelial cells (HUVEC)), correlates with the level of ACE expression. However the pattern of binding of a set of mAbs to different epitopes of ACE differs in CHO cells in comparison with HUVEC. Up to 30% of the bound ¹²⁵I-labeled anti-ACE mAbs to ACE were internalized by these cells. These data suggest that stable CHO cell lines expressing human ACE can be useful tool to study gene and drug delivery to the lung vasculature using anti ACE mAbs as a carrier.