Energetics of Leukocyte Integrin Activation (*)
- From the (1)Laboratory of Cellular Physiology and Immunology, The Rockefeller University, New York, New York 10021
- § To whom correspondence should be addressed: Laboratory of Cellular Physiology and Immunology, Box 303, The Rockefeller University, 1230 York Ave., New York, NY 10021. Tel.: 212-327-8110; Fax: 212-327-7901.
Abstract
Cell adhesion mediated by leukocyte integrin CR3 (CD11b/CD18, αmβ2) may be rapidly modulated without changes in receptor number, and transient changes in adhesivity are thought to be driven by reversible alteration of the affinity of CR3 for ligand. Here we measure the binding affinity of CR3 using purified active and inactive receptor and the ligand, C3bi, coupled to alkaline phosphatase. Immobilized, active CR3 bound saturably and with high affinity (12.5 ± 4.7 nM). In contrast, inactive CR3 exhibited no measurable binding. High affinity binding could be restored by the addition of the activating anti-CR3 monoclonal antibody KIM-127 to inactive CR3. Since the affinity of KIM-127 for active and inactive receptor was identical, it cannot contribute the energy to convert a low affinity receptor into a high affinity receptor. Rather, KIM-127 appears to facilitate binding of C3bi by lowering the activation energy for the shift from an inactive to an active state. These results suggest that CR3-mediated binding and detachment of cells is not driven by a reversible change in affinity but by two mechanistically distinct processes, an energetically neutral activation step for binding and an energy-dependent step that reverses binding of ligand.
Footnotes
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↵* This work was supported by American Cancer Society Grant CB102. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵1 The abbreviations used are:
- CR3
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complement receptor type 3
- PMN
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polymorphonuclear leukocytes
- AP
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alkaline phosphatase
- SPDP
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n-succinimidyl 3-(2-pyridithio)propionate
- PBS
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phosphate-buffered saline
- mAb
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monoclonal antibody.
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↵2K. P. M. van Kessel, M. Diamond, T. A. Springer, and S. D. Wright, unpublished observations.
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↵3S. D. Wright, unpublished observations.
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↵4T. Q. Cai and S. D. Wright, unpublished observations.
