Direct evidence for two affinity states for lymphocyte function-associated antigen 1 on activated T cells

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Lymphocytes exhibit avid adhesion to ICAM-1 through LFA-1 upon activation.
• The mechanisms driving this enhanced adhesion, whether increased binding affinity or post-receptor events, remain unclear.

Purpose of the Study:

• To quantify the binding affinity of LFA-1 to ICAM-1 on unstimulated and stimulated T cells.
• To elucidate the contribution of LFA-1 affinity modulation to lymphocyte adhesion.

Main Methods:

• Utilized a recombinant soluble ICAM-1 to measure binding affinity to LFA-1 on T cells.
• Employed competition assays with radiolabeled antibody Fab to determine binding equilibrium and kinetics.
• Analyzed binding affinity on unstimulated and phorbol ester-stimulated T cells.

Main Results:

• The binding affinity of LFA-1 to ICAM-1 on unstimulated T cells is very low (approximately 100 microM).
• T cell activation by phorbol esters resulted in a modest increase in average binding affinity.
• Further analysis indicated that a subpopulation of LFA-1 molecules shifted to a state with 200-fold higher affinity.

Conclusions:

• T cell activation significantly enhances LFA-1 binding affinity to ICAM-1.
• This affinity modulation, driven by a fraction of LFA-1 molecules entering a high-affinity state, underlies the avid adhesion of stimulated lymphocytes.