Quantitative analysis of lymphocyte CD11a using standardized flow cytometry.

CD11a is the alpha-subunit of the leucocyte adhesion and costimulation molecule LFA-1. We have refined the measurement of lymphocyte CD11a density with a FACScan using commercially available fluorescent beads for standardization and fluorescein-conjugated antibody to CD11a of known fluorescein: protein ratio. The fluorescence intensity of CD11a on peripheral blood CD4+ and CD8+ lymphocytes was measured in 60 healthy subjects. We demonstrated linear correlation between age and mean CD11a density (r = 0.47 for CD4+ cells, r = 0.71 for CD8+ cells). We established that there is a consistent logical cut off point at 4.3 x 10(3) bound antibody molecules between low-expressing and high-expressing subsets of CD8+ cells and we then investigated whether the variation in lymphocyte CD11a expression in healthy subjects was sufficiently small for the application of this method to the detection of abnormal groups or individuals. Analysis of the CD11a high subsets has high statistical power (> 99% in 60 subjects to detect a 25% difference) and good precision (< 4% differences). The advantages of the method for comparative studies of cell surface accessory molecules are discussed. We have also evaluated a frozen cell line for quality control, and demonstrated up-regulation of CD11a density on CD4+ and CD8+ cells measured in three patients with infectious mononucleosis.