Background.

CD40/CD40L (gp39) interactions are known to play a central role in the function of the immune system (1). CD40 is constitutively expressed on professional antigen presenting cells, such as macrophages and dendritic cells, as well as at low levels on other cell lineages, including human umbilical vein endothelial cells (HUVECs). On antigen-presenting cells, ligation of CD40 causes expression of the costimulatory molecule CD80 (B7-1). Similar ligation of CD40 on HUVECs, however, leads to up-regulation of the adhesion molecules VCAM-1, ICAM-1, and E-selectin, but not CD80.

Methods.

In efforts to provide evidence that microvascular endothelial cells (MECs) are distinct from HUVECs and that the distinguishing features play a role in allograft rejection, MEC cultures were prepared from the explanted hearts of human heart transplant recipients and primary cell lines were established. These MECs were induced to express higher levels of CD40 with interferon-gamma pretreatment, co-cultured with CD40L-transfected HeLa cells, and fluorescence-activated cell sorter-assisted phenotypic studies, in addition to functional allogeneic mixed lymphocyte reaction and accessory-cell dependent mitogen induced proliferation assays were performed.

Results.

CD40-CD40L interactions induced the expression of the adhesion molecules VCAM-1 and E-selectin and the costimulatory molecule CD80 but not CD86 (B7-2) on the MECs. The expressed CD80 proved functional in both allo-MLR assays and accessory-cell dependent mitogen proliferation assays.

Conclusions.

MECs are distinct from HUVECs by their potential to express VCAM-1 after interferon-gamma pretreatment and CD80 after CD40 ligation, properties which enable this cell lineage to play a central role in initiating and maintaining allograft rejection in human cardiac transplants.