CD18 activation epitopes induced by leukocyte activation

The cell surface adhesion molecule LFA-1 coordinates leukocyte trafficking and is a costimulatory molecule for T cell activation. We developed a panel of mAbs that recognize activation epitopes on the CD18 subunit, and show t hat stimulation of T lymphocytes appears to be accompanied by a conformatio nal change in a subpopulation of LFA-1 that does not require ligand binding . Activation epitope up-regulation requires divalent cations, is sensitive to cellular signal transduction events, and correlates with cell adhesion. In addition, the stimulated appearance of these activation epitopes is abse nt in cell lines from patients with leukocyte adhesion deficiency-1/variant that has previously been shown to be defective in LFA-1 activation. Thus, these activation epitope Abs can be used to dissect signal transmission to CD18. Evidence suggests that these CD18 activation epitopes are induced ear ly in cellular activation and are independent of actin rearrangement necess ary for avid adhesion. We have also determined that function-blocking CD18 Abs inhibit the induction of activation epitopes. One activation epitope Ab binds to a site on CD18 distinct from that of the blocking Abs, indicating that the blocking Abs suppress a conformational change in LFA-1. We also f ind that these neoepitopes are present on rLFA-1 with high affinity for ICA M-1 and their binding is modulated in parallel with the affinity of LFA-1 f or ICAM-1. Collectively, these neoepitope Abs identify a subpopulation of L FA-1 most likely with high affinity for ICAM-1 and necessary for LFA-1 func tion.