Activation of LFA-1 through a Ca2(+)-dependent epitope stimulates lymphocyte adhesion.

Abstract

Abstract. The leukocyte function-associated molecule-1 (LFA-1) plays a key role in cell adhesion processes between cells of the immune system. We investigated the mechanism that may regulate LFA-1-1igand interactions, which result in cell-cell adhesion. To this end we employed an intriguing anti-LFA-1 ot mAb (NKI-L16), capable of inducing rather than inhibiting cell adhesion. Aggregation induced by NKI-L16 or Fab fragments thereof is not the result of signals transmitted through LFA-1. The antibody was found to recognize a unique Ca2+-dependent activation epitope of LFA-1, which is essentially absent on resting lymphocytes, but becomes induced upon in vitro culture. Expression of this epitope correlates well with the capacity of cells to rapidly aggregate upon stimulation by PMA or through the TCR/CD3 complex, indicating that expression of the NKI-L16 epitope is essential for LFA-1 to mediate adhesion. However, expression of the NKI-L16 epitope in itself is not sufficient for cell binding since cloned T lymphocytes express the NKI-L16 epitope constitutively at high levels, but do not aggregate spontaneously. Based on these observations we propose the existence of three distinct forms of LFA-I: (a) an inactive form, which does not, or only partially exposes the NKI-L16 epitope, found on resting cells; (b) an intermediate, NKI-L16 ÷ form, expressed by mature or previously activated cells; and (c) an active (NKI-L16 ÷) form of LFA-1, capable of high affinity ligand binding, obtained after specific triggering of a lymphocyte through the TCR/CD3 complex, by PMA, or by binding of NKI-L16 antibodies.

Adhesion of T and B lymphocytes to extracellular matrix and endothelial cells can be regulated through the beta subunit of VLA

“…Nevertheless, a striking parallel exists between the earlier observations with NKI-L16 and those reported with TS2/16 in this paper ( The induction of VLAmediated cell adhesion by TS2/16 is dependent on metabolic energy, since a combination of deoxyglucose and sodium azide prevents induction of adhesion. We have previously reported that induction of LFA-l-mediated cell adhesion is not affected by sodium azide (Van Kooyk et al, 1991). Similarly deoxyglucose alone does not affect adhesion (our unpublished results).…”

“…The antibody induces homotypic lymphocyte aggregation, as well as binding to purified ICAM-1 (Van Kooyk et al, 1991). In addition, the antibody enhances LFA-l-dependent adhesion to endothelium (our unpublished results).…”

“…These results also demonstrate that high avidity interactions can be obtained in the absence of associated molecules, for instance, cytoskeletal elements. Induction of 131-(this study) and t$2- (Van Kooyk et al, 1991) mediated interactions by the antibodies TS2/16 and NKI-L16, respectively, may also involve conformational changes, but their dependence on metabolic activity of the cell suggests that intraceUular processes play a role. At present it is not known which steps in the adhesion process are affected by these inhibitors of cell metabolism.…”

“…We have previously observed that ~2-mediated adhesion can be induced by an antibody, termed NKI-L16, (Keizer et al, 1988) directed against a Ca2+-dependent epitope located on the c~ subunit of LFAd (Van Kooyk et al, 1991). The antibody induces homotypic lymphocyte aggregation, as well as binding to purified ICAM-1 (Van Kooyk et al, 1991).…”

β1 Integrin-dependent binding of Jurkat cells to fibronectin is regulated by a serine-threonine phosphatase

“…In addition to the amount of expression of cell surface adhesion molecules, the functional state of integrins can be regulated, leading to changes in the affinity for counterligand binding without affecting the level of cell surface expression [11]. Changes in T cell integrin activity can be induced in vitro by a variety of stimulating agents, such as phorbol esters, calcium ionophore, and cross-linking of T cell receptors like CD2, CD3, CD7, and CD28 [3,[12][13][14]. Furthermore, the activation state of integrins can be altered by antibodies against ␤ 1 integrins, like 8A2, that bind to the receptor and induce a more active form [15] or by alterations of the extracellular cation milieu, both of which appear to affect the conformation of the integrin binding site [16,17].…”