Antigen-independent processes in antigen-specific immunity: A role for α4 integrin

It has been proposed that the migration of immune T cells out of blood vessels through connective tissue to the site of antigenic challenge is facilitated by the interaction of VLA integrins on lymphocytes with endothelial cell adhesion molecules and matrix proteins. Indeed, we have recently demonstrated that peptides derived from sequences in fibronectin abrogate the T cell-mediated contact hypersensitivity (CHS) reactions in vivo. These peptides blocked the interaction of the integrins VLA-4 and VLA-5 with fibronectin (FN), and our results suggested that by preventing the interaction of T cell integrins with FN, we successfully prevented the migration of T cells to sites of antigenic challenge. To further explore the role of integrins in T cell migration, we have used an antibody (R1-2) specific for the α-chain of α4 integrins. Our data show that this antibody stains more than 90% of CD3⁺ T cells, this percentage does not change after the mice have been immunized with a contact sensitizer. We also show that this antibody blocks the interaction of cells with immobilized FN vascular cell adhesion molecule-1 on activated endothelial cells. Although R1-2 identifies α4 integrin on a large number of T cells, it does not discriminate between activated and resting forms of this integrin, because very few T cells from immune or nonimmune mice actually bind FN or activated endothelium. In vivo, antibody R1-2 effectively blocks Ag-specific CHS, but has little effect on the non-Ag-specific cells that localize to the site of antigenic challenge. The population of cells that adoptively transfer CHS exits with the population of cells that bind to FN or to activated endothelial cells. Our data demonstrate that a small number of Ag-specific T cells use α4 integrin to enter sites of inflammation and mediate effector immune responses. Ag-independent accumulation of T cells near sites of inflammation does not appear to be mediated by α4 integrin.