Alloproliferative responses of purified CD4+ and CD8+ T cells to endothelial cells in the absence of contaminating accessory cells.

Human capillary endothelial cells (EC), unlike rodent, constitutively express MHC class II antigens, raising the question of whether they can cause direct stimulation of resting T cells. Although previous reports have demonstrated an alloproliferative response between cultured human EC and resting T lymphocytes, they have not stringently proven the absence of contaminating leukocytes in both stimulator and responder populations. Here, we have quantitated the number of contaminating leukocytes in serially passaged human umbilical vein endothelial cells (HUVEC) and have examined the ability of HUVEC and an EC line (EAhy.926) to cause allostimulation of positively selected CD4+ and CD8+ T cells. Contaminating CD45+ leukocytes were found in primary cultures at a mean level of 0.43 +/- 0.49%; by passages 3 and 4 there were less than 1/10,000 cells. CD4+ and CD8+ T cells were purified by positive selection on antibody-coated Dynabeads and residual DR+ cells were removed by complement-dependent lysis. They were shown to be depleted of monocytes by their failure to proliferate in response to OKT3 and PHA. Nevertheless, the same cells gave an unequivocal response to cytokine-treated, serially passaged HUVEC or EAhy.926 cells. The response of CD4+ but not CD8+ T cells was dependent upon pretreatment of HUVEC or EAhy.926 with human rIFN-gamma. Neither CD4+ nor CD8+ T cells responded to MHC class II-bearing fibroblasts or epithelial cells. The CD8+ T cells that recognized EC did not respond to spleen cells in an MLR. The results confirm that EC, unlike other non-bone-marrow-derived cells, have the ability to provide all the signals necessary for direct stimulation of resting allogeneic T cells.