CD4+CD25+FoxP3+ regulatory T cells enhance the allogeneic activity of endothelial-specific CD8+/CD28-CTL

Numerous data indicate that CD4+CD25+FoxP3+ regulatory T cells (Treg cells) can attenuate alloresponses of conventional T lymphocytes against professional antigen-presenting cells and thus qualify for clinical use in various transplant settings. However, it is unknown whether Treg cells also influence T cell-endothelial cell interactions. CD8+ PBMC (CD8+ PBMC, CTL) from healthy human donors were stimulated for 7 days with an allogeneic microvascular endothelial cell line (CDC/EU. HMEC-1, an immortalized human microvascular endothelial cell line, further referred to as HMEC) and additional endothelial cell types and analysed for their lytic activity against these target cells in the presence or absence of Treg cells. Addition of Treg cells (1:1:1) to the CTL/HMEC co-cultures in the efferent immune phase (day -1 prior to the assay) led to an increased cytotoxicity against HMEC. In contrast, Treg cells alone did not lyse HMEC. Treg cell-mediated enhancement of CTL activity was endothelial cell specific since lysis of HLA-matched Epstein-Barr virus-transformed B lymphoblastoid cells (B-LCL) was not influenced by the addition of Treg cells. Further analysis of CD28-positive and CD28-negative CTL sub-populations revealed that only the CD28-negative CTL showed an increased activity against HMEC after Treg cell co-culture. Although there is no doubt about the potential therapeutic efficacy of Treg cells to ameliorate outcome of allogeneic transplants, the endothelium might require additional protective interventions to prevent endothelial cell type-specific alloreactivity.