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 endothe lium is a precondition for subsequent integrin-mediated firm adhesion and extravasation Rolling characteristics of polymorphonuclear leukocy tes (PMNL) and mononuclear leukocytes (MNL) in small venules (15-25 mu m) of the rat mesentery were studied by intravital fluorescence micro scopy under basal conditions and after intravenous treatment with an a nti-rat neutrophil serum (ANS). The baseline rolling fraction of the v enular total leukocyte flux was 36 +/- 15% (mean +/- SD). The PMNL fra ction of the systemic leukocyte count was 27 +/- 9%. Treatment with AN S resulted in total depletion of circulating PMNL and reduced the leuk ocyte rolling fraction to 12 +/- 5%, in this situation represented onl y by MNL. In rats treated intraperitoneally with interleukin (IL)-1 be ta for 4 h, the leukocyte rolling fraction was 53 +/- 13% and was redu ced to 33 +/- 11% after ANS treatment. These data indicated that most, if not all, circulating PMNL rolled along the venular endothelial lin ing in the rat mesentery prepared for intravital microscopy, whereas M NL rolling was minor (similar to 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-I stimulation was likely mediated by alpha(4) integrins, inasmuch as the rolling fra ction of isolated peripheral blood lymphocytes injected into the micro circulation of the cytokine-stimulated mesentery was reduced from 31 /- 14% to 6 +/- 2% by pretreatment of the cells with a monoclonal anti body against the rat integrin alpha(4) chain. In accordance with the i n 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 cytome tric analyses of rat and human leukocytes. Taken together, our finding s in vivo indicate that the adhesive interactions responsible for roll ing of PMNL and MNL, respectively, are distinct in terms of receptor o ccupancy, and may help explain the temporal selectivity in recruitment of different leukocyte subpopulations in inflammatory or immune react ions.