EXPRESSION OF A NOVEL INTEGRIN BETA-1 CHAIN EPITOPE AND ANTI-BETA(1) ANTIBODY-MEDIATED ENHANCEMENT OF FIBRONECTIN-BINDING ARE DEPENDENT ON THE STAGE OF T-CELL DIFFERENTIATION

beta(1) integrins are a family of alpha beta heterodimers that serve a s cell surface receptors for extracellular matrix proteins. We demonst rate that the anti-mouse integrin beta(1) chain mAb KMI6 selectively r ecognizes a beta(1) epitope that is constitutively expressed by certai n immature thymocytes and is induced only slightly on mature thymocyte s and peripheral T cells by activation with Con A. Because virtually a ll cells examined expressed beta(1) integrins on their surface, expres sion of the KMI6 epitope is T cell differentiation stage specific. Mos t CD3(-)4(-)8(-) thymocytes were KMI6(+), with the lowest level of sta ining observed on the earliest CD44(+)IL-2R(-) cells within this subse t. Expression was down-regulated during the CD3(-)4(-)8(-) to CD3(-)4( -)8(+) transition, and lost by the CD4(+)8(+) stage. Mature single pos itive thymocytes and resting peripheral T cells were also KMI6(-). In contrast with the loss of the epitope before TCR expression by other t hymocytes, most CD3(+)4(-)8(-) and certain CD8(+) gamma delta TCR(+) t hymocytes were KMI6(+) Addition of KMI6 to cell adhesion assays enhanc ed CD4(-)8(-) thymocyte, but not activated mature thymocyte or periphe ral T cell, binding to fibronectin (via alpha(4) beta(1) and alpha(5) beta(1)), whereas laminin binding (via alpha(6) beta(1)) was unaffecte d. These properties distinguish the KMI6 epitope from other epitopes i nvolved in beta(1) integrin activation in mice and other species. The unique selectivity of KMI6 recognition of beta(1) integrins, and its s elective enhancement of ligand binding suggest that beta(1) integrin s tructure and factors that regulate beta(1) integrin binding are correl ated with the stage of T cell differentiation.