Kidney transplant recipients were previously found to have antibodies that reacted with cells isolated from the endothelium of umbilical cord veins and which were not cytotoxic for lymphocytes from the same donors. Results of the present experiments indicate that endothelial (E) antigens are different from previously known HLA antigens and also from Ia like antigens of bone marrow derived (B) lymphocytes. Attempts to absorb E antibodies with lymphocytes from E positive donors failed in most cases. Antigen redistribution experiments showed that E antigens were located in separate molecules from the products of HLA A, B, and C. Thus, E cells treated with E antibody became resistant to lysis by the antibody used, but remained susceptible to the effects of typing sera for alleles of HLA A, B, and C. Antibodies to E cells were also cytotoxic for blood monocytes. Moreover, monocytes were able to absorb E antibody reactions, indicating that similar antigens were expressed in both cells. E antibodies did not react with B lymphocytes isolated from peripheral blood. In that regard E antibodies were different from antibodies to human Ia like antigens which reacted with E cells, monocytes, and isolated B lymphocytes. Thus it appears that E antigens constitute a system of human alloantigens which has not been previously identified. The possibility that these antigens play a role in kidney allograft rejection should now be investigated since matching can be performed using monocytes isolated from the blood of recipients and donors.
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