We have generated rat monoclonal antibodies (MoAbs) against cell surface an
tigens of the mouse endothelioma cell line bEND.3. Three antibodies (V.1A7,
V.5C7, and V.7C7) were selected, all of which recognize a 75-kD antigen on
bEND.3 cells and bind selectively to endothelial cells in cryostat section
s of mouse tissues. A cDNA for the antigen was isolated from a bEND.3 pCDM8
expression library by using transient expression in COS-7 cells and immuno
selection with the three MoAbs. This cDNA coded for a novel, type I membran
e protein of 248 amino acids with an extracellular domain rich in threonine
and serine residues (35%). The protein is sensitive to O-sialoglycoprotein
endopeptidase, indicating that it belongs to the class of sialomucin-like
proteins. Therefore, we suggest the name endomucin. Treatment of isolated e
ndomucin by sialidase and O-glycosidase reduced the apparent molecular weig
ht to 45 kD and abolished binding of all three antibodies, indicating that
carbohydrates are directly or indirectly involved in the formation of the a
ntibody epitopes. Immunohistological analysis of all examined mouse tissues
showed that endomucin is an endothelial antigen found in venous endotheliu
m as well as in capillaries, but not on arterial endothelium. Interestingly
, high endothelial venules of peripheral and mesenteric lymph nodes as well
as of Peyers's patches were negative for staining with the three MoAbs. (C
) 1999 by The American Society of Hematology.