A mechanism for antibody-mediated outside-in activation of LFA-1.
MEM83 is an inserted domain (I-domain)-specific antibody that up-regulates the interaction of LFA-1 with ICAM-1 through an outside-in activation mechanism. We demonstrate here that there is no change in the affinity of the MEM83 antibody for the I-domain in either its low (wild-type) or high affinity form and that MEM83 does not enhance the binding of the wild-type I-domain to ICAM-1. Furthermore, we show that the antibody acts as an activating agent to induce LFA-1/ICAM-1-dependent homotypic cell aggregation only as an IgG, but not as a Fab fragment. On the basis of these data, we propose an avidity-based mechanism that requires no direct activation of the LFA-1 I-domain by the binding of the antibody; rather, activation is enhanced when there is an interaction with both arms of the IgG. A molecular model of the antibody interaction with LFA-1 illustrates the symmetry and accessibility of the two MEM83 epitopes across the LFA-1/ICAM-1 heterotetramer. We hypothesize that MEM83 stabilizes adjacent LFA-1 molecules in their active form by the free energy that is gained from the binding of the I-domains to each arm of the IgG. This leads to stabilization of the open state of the integrin and outside-in signaling. Our model supports a mechanism in which both affinity and avidity regulation are required in the activation of LFA-1.
Carreño
Roberto
R
Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204-5001, USA.
Li
Dan
D
Sen
Mehmet
M
Nira
Iris
I
Yamakawa
Tatsuo
T
Ma
Qing
Q
Legge
Glen B
GB
D000911:Antibodies, Monoclonal / Q000737:chemistry; D002460:Cell Line; D006801:Humans; D007074:Immunoglobulin G / Q000737:chemistry; D018799:Intercellular Adhesion Molecule-1 / Q000378:metabolism; D007700:Kinetics; D016169:Lymphocyte Function-Associated Antigen-1 / Q000378:metabolism*; D008954:Models, Biological; D008958:Models, Molecular; D008968:Molecular Conformation; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D020349:Surface Plasmon Resonance; D013816:Thermodynamics
D000911:Antibodies, Monoclonal; D007074:Immunoglobulin G; D016169:Lymphocyte Function-Associated Antigen-1; D018799:Intercellular Adhesion Molecule-1