Expression of the leukocyte common antigen CD45 by endothelium

The CD45 family of Ag expressed by leukocytes play a key role in lymphocyte activation. In this study, we identify both a restricted pattern of expression of CD45 Ag by human endothelial cells and differences in m.w. of endothelial CD45 compared with lymphocyte CD45. Initially, immunoperoxidase staining of endothelial cells in culture revealed the presence of the CD45RO isoform provided the endothelial cells had been stimulated by IL-1 for several days. No other isoform of CD45 was detected by immunoperoxidase staining of endothelium. Leukocyte common antibodies, reputed to detect all isoforms of CD45, did not detect endothelial CD45RO. A polymerase chain reaction method was then used to demonstrate that the message for the CD45RO isoform was constitutively present, with increased levels after IL-1 stimulation. Western blot confirmed the presence of the RO isoform. No other CD45 isoform was detected, either by PCR amplification for message or by Western blot. There were clear differences between lymphocyte and endothelial CD45. The RO isoform expressed by endothelium has an estimated M(r) of 235,000 in contrast to lymphocyte RO that, on our gels, has an estimated M(r) of 190,000. Given the large surface area of endothelium (approximately 1 km2 in the human), the close apposition of lymphocytes and endothelium in immunologic tissues such as lymph nodes, and the pivotal role CD45 plays in activation, expression of CD45RO by endothelium may have important implications in lymphocyte-endothelial interactions. This is most likely to occur in endothelium after a few days of IL-1 stimulation, e.g., at sites of chronic inflammation, or in the draining lymph node of an inflammatory focus.