A MONOCLONAL-ANTIBODY AGAINST AN ACTIVATION EPITOPE ON MOUSE INTEGRIN CHAIN-BETA(1) BLOCKS ADHESION OF LYMPHOCYTES TO THE ENDOTHELIAL INTEGRIN-ALPHA(6)BETA(1)

We have generated a monoclonal antibody (mAb), 9EG7, against mouse endothelial cells that blocks adhesion of lymphocytes to endothelial cells. Sequencing of four tryptic peptides of the purified antigen revealed its identity with the integrin chain beta1. The only beta1 integrin that is known to mediate cell-cell adhesion is alpha4beta1 (VLA-4). This is not the integrin that is functionally defined by the mAb 9EG7 on endothelial cells. First, alpha4 is not present on the analyzed endothelial cells. Second, mAb 9EG7 does not block the cell-adhesion function of alpha4beta1 on the nonactivated mouse lymphoma L1-2. Thus, the mAb 9EG7 can functionally distinguish between different beta1 integrins and defines a beta1 integrin other than alpha4beta1 as a newly discovered cell-cell adhesion molecule. This integrin is most likely alpha6beta1, Since an antibody against the alpha6 chain blocks lymphocyte adhesion to the same degree as the mAb 9EG7, the effect of both antibodies is not additive, and the alpha6 chain is coprecipitated with beta1 in 9EG7 immunoprecipitations. Surprisingly, activation of alpha4beta1 on L1-2 cells with phorbol ester or Mn2+ allows blocking of alpha4beta1-mediated adhesion of L1-2 cells to endothelial cells with mAb 9EG7. Furthermore, only the activated alpha4beta1 heterodimer, but not the unactivated complex, is detectable with 9EG7 in immunoprecipitations and by flow cytometry. Thus, mAb 9EG7 defines an epitope on integrin chain beta1, which is accessible on the alpha4beta1 heterodimer only after activation of this integrin.