Human vascular ECs in situ basally express MHC class I and class II molecules (2, 3); costimulatory molecules such as LFA-3 (CD58), ICOS ligand (CD275), OX40 ligand (CD252), 41BB ligand (CD137L), CD40, and GITR ligand (4); and cytokines such as IL-1??, IL-6, IL-15, and IL-18 (5???8) that can contribute to T cell activation and differentiation.