Phosphatase Inhibitors Promote ICAM-1 and LFA-1 Mediated Homotypic Aggregation of Jurkat and U937 Cells

Here we investigated the role of cellular phosphatases in activation of homotypic aggregation in leukocytes by adding the phosphatase inhibitors okadaic acid (OA) and calyculin A (CA) to cultures of the human monocytic U937 cell line and the human T-lymphocytic Jurkat cell line. We found that OA produces a 120-fold increase of homotypic aggregation with Jurkat cells and a 200-fold increase with U937 cells. Calyculin A increased aggregation of Jurkat cells 10-fold and U937 cells 88-fold. Monoclonal antibodies to LFA-1 completely inhibited the OA and CA induced aggregation in both cell lines, whereas the monoclonal antibody to ICAM-1 only inhibited U937 cell aggregation. These data suggest that phosphatase activity is important in the regulation of ICAM-1 and LFA-1 mediated homotypic aggregation in leukocytes.