Primary porcine endothelial cells express membrane-bound B7-2 (CD86) and a soluble factor that co-stimulate cyclosporin A-resistant and CD28-dependent human T cell proliferation.
Increasing evidence suggests that endothelial cells can directly activate syngeneic, allogeneic and xenogeneic T cells. In this study we demonstrate that unstimulated, paraformaldehyde-fixed primary porcine aortic endothelial cells (PAEC) and microvascular endothelial cells (PMVEC) can provide co-stimulation for human T cell IL-2 secretion and proliferation. EC-mediated co-stimulation has both cyclosporin A (CsA)-sensitive and CsA-resistant components. The CsA-resistant component is completely suppressed either by blocking with anti-CD28 F(ab) fragments or CTLA-4-Ig. Northern blot analysis of unstimulated PAEC and PMVEC with porcine-specific probes reveals constitutive expression of B7-2 mRNA while B7-1 message was not detected. hCTLA-4-Ig and anti-B7-2 mAb immunoprecipitates a single 79 kDa PMVEC surface protein. Surprisingly, PMVEC conditioned media also has soluble co-stimulatory activity that is blocked by anti-CD28 F(ab) fragments or anti-B7-2 mAb. These findings demonstrate that primary unstimulated porcine EC can co-stimulate CsA-resistant human T cell proliferation through binding of membrane bound, constitutively expressed EC B7-2 (CD86) to human T cell CD28, providing one of the first demonstrations of functional B7-2 on cells outside the immune system. In addition, PMVEC secrete or shed a soluble factor that mediates CD28-dependent human T cell proliferation, demonstrating the existence of soluble mediators of CD28 activation.