Porcine vascular cell adhesion molecule (VCAM) mediates endothelial cell adhesion to human T cells : development of blocking antibodies specific for porcine VCAM

Vascular cell adhesion molecule (VCAM) is expressed on activated endothelial cells and binds to the α4β1 integrin receptor, very late antigen-4 (VLA-4), expressed on human lymphoid cells. Anti-VCAM mAbs have been shown to prolong allograft survival. To explore the role of porcine VCAM (pVCAM) in xenotransplantation, a recombinant secreted form of pVCAM (spVCAM) was expressed in 293-EBNA cells and purified by metal affinity chromatography. A human lymphoid cell line bound to spVCAM in a VLA-4-dependent manner. Using spVCAM as an immunogen, we developed three anti-pVCAM mAbs that reacted with cell surface pVCAM on porcine aortic endothelial cells (PAEC) but not to human VCAM on human umbilical vein endothelial cells. Pairwise interaction analysis indicated that these mAbs recognized distinct epitopes on pVCAM. Two anti-pVCAM mAbs, 2A2 and 3F4, inhibited the binding of Ramos cells to spVCAM, while the third, 5D11, did not. Similarly, mAbs 2A2 and 3F4 inhibited binding of Ramos cells or human peripheral blood T cells to activated PAEC. The extent of inhibition with mAbs 2A2 and 3F4 was comparable to the inhibition obtained with a blocking mAb to human VLA-4. These anti-pVCAM mAbs will provide a means to specifically block pVCAM in a xenograft setting and allow the determination of the role of pVCAM in a primary xenogeneic immune response.