In vivo rolling and endothelial selectin binding of mononuclear leukocytes is distinct from that of polymorphonuclear cells

In inflammation, rolling of leukocytes along the microvascular endothelium is a precondition for subsequent integrin-mediated firm adhesion and extravasation. Rolling characteristics of polymorphonuclear leukocytes (PMNL) and mononuclear leukocytes (MNL) in small venules (15–25 μm) of the rat mesentery were studied by intravital fluorescence microscopy under basal conditions and after intravenous treatment with an anti-rat neutrophil serum (ANS). The baseline rolling fraction of the venular total leukocyte flux was 36 ± 15% (mean ± SD). The PMNL fraction of the systemic leukocyte count was 27 ± 9%. Treatment with ANS resulted in total depletion of circulating PMNL and reduced the leukocyte rolling fraction to 12 ± 5%, in this situation represented only by MNL. In rats treated intraperitoneally with interleukin (IL)-1β for 4 h, the leukocyte rolling fraction was 53 ± 13% and was reduced to 33 ± 11% after ANS treatment. These data indicated that most, if not all, circulating PMNL rolled along the venular endothelial lining in the rat mesentery prepared for intravital microscopy, whereas MNL rolling was minor (～ 10%) under the same basal condition. In cytokine-activated tissue, on the other hand, the number of rolling MNL was greatly increased. While PMNL rolling is known to be entirely selectin dependent, the increased MNL rolling after IL-1 stimulation was likely mediated by α₄ integrins, inasmuch as the rolling fraction of isolated peripheral blood lymphocytes injected into the microcirculation of the cytokine-stimulated mesentery was reduced from 31 ± 14% to 6 ± 2% by pre-treatment of the cells with a monoclonal antibody against the rat integrin α₄ chain. In accordance with the in vivo rolling characteristics of the two cell populations, binding of soluble P- or E-selectin (selectin/IgG chimeras) was less intense for blood lymphocytes than for granulocytes, as determined by flow cytometric analyses of rat and human leukocytes. Taken together, our findings in vivo indicate that the adhesive interactions responsible for rolling of PMNL and MNL, respectively, are distinct in terms of receptor occupancy, and may help explain the temporal selectivity in recruitment of different leukocyte subpopulations in inflammatory or immune reactions.