Extracellular Ca2+ modulates leukocyte function-associated antigen-1 cell surface distribution on T lymphocytes and consequently affects cell adhesion.
Transition of leukocyte function-associated antigen-1 (LFA-1), from an inactive into an activate state depends on the presence of extracellular Mg2+ and/or Ca2+ ions. Although Mg2+ is directly involved in ligand binding, the role of Ca2+ in LFA-1 mediated adhesion remained obscure. We now demonstrate that binding of Ca2+, but not Mg2+, directly correlates with clustering of LFA-1 molecules at the cell surface of T cells, thereby facilitating LFA-1-ligand interaction. Using a reporter antibody (NKI-L16) that recognizes a Ca(2+)-dependent epitope on LFA-1, we found that Ca2+ can be bound by LFA-1 with different strength. We noticed that weak binding of Ca2+ is associated with a dispersed LFA-1 surface distribution on T cells and with non-responsiveness of these cells to stimuli known to activate LFA-1. In contrast, stable binding of Ca2+ by LFA-1 correlates with a patch-like surface distribution and vivid ligand binding after activation of LFA-1. Mg(2+)-dependent ligand binding does not affect binding of Ca2+ by LFA-1 as measured by NKI-L16 expression, suggesting that Mg2+ binds to a distinct site, and that both cations are important to mediate adhesion. Only Sr2+ ions can replace Ca2+ to express the L16 epitope, and to induce clustering of LFA-1 at the cell surface. We conclude that Ca2+ is involved in avidity regulation of LFA-1 by clustering of LFA-1 molecules at the cell surface, whereas Mg2+ is important in regulation of the affinity of LFA-1 for its ligands.
van Kooyk
Y
Y
Department of Tumor Immunology, University Hospital Nÿmegen St. Radboud, The Netherlands.
Weder
P
P
Heije
K
K
Figdor
C G
CG
D002118:Calcium / Q000378:metabolism*; D002448:Cell Adhesion; D002460:Cell Line; D002462:Cell Membrane / Q000276:immunology; D004492:Edetic Acid / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D000939:Epitopes; D006801:Humans; D016169:Lymphocyte Function-Associated Antigen-1 / Q000276:immunology / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology* / Q000378:metabolism
D000939:Epitopes; D016169:Lymphocyte Function-Associated Antigen-1; D004533:Egtazic Acid; D004492:Edetic Acid; D008274:Magnesium; D002118:Calcium