CRYOPRESERVED AORTIC HOMOGRAFTS CONTAIN VIABLE SMOOTH-MUSCLE CELLS CAPABLE OF EXPRESSING TRANSPLANTATION ANTIGENS

Frozen aortic tissue is increasingly used as homografts in reconstruct ive cardiovascular surgical procedures. The viability of cells within these cryopreserved tissues, their identity, and their potential immun ogenicity have been the subject of controversy. We cultured cells from cryopreserved human aortic homografts that reacted with a monoclonal antibody that recognizes muscle actin isoforms, identifying them as sm ooth muscle cells. Under basal conditions, these smooth muscle cells c ontained messenger ribonucleic acid for class I human leukocyte antige ns detected by northern blotting and expressed class I human leukocyte antigen on their surfaces as measured by enzyme-linked immunoassay an d immunohistochemistry. Unstimulated smooth muscle cells contained no class II human leukocyte antigen messenger ribonucleic acid as determi ned by northern blotting and displayed almost no class II surface anti gen as determined by enzyme-linked immunoassay. Interferon gamma (1000 U/ml, 72 hours), a product of activated T lymphocytes, not only incre ased the expression of class I human leukocyte antigens by smooth musc le cells, but induced class II human leukocyte antigen messenger ribon ucleic acid and elevated surface expression from 22 +/- 7 to 819 +/- 3 5 enzyme-linked immunoassay units (n = 4). Immunohistochemistry reveal ed few class II-positive smooth muscle cells under basal culture condi tions, but all cells showed high levels of DR antigen after exposure t o interferon gamma for 3 days. Similar results were obtained in two in dependent isolates. We conclude that cryopreserved aortic homografts c an contain viable smooth muscle cells capable of expressing major hist ocompatibility antigens that might render them immunogenic and suscept ible to rejection by the recipient's immune system.