The spatial distribution of cell adhesion molecules on the leukocytes has been studied with immuno-SEM (Erlandsen et al., 1993; Berlin et al., 1995; Hasslen et al., 1995, 1996; von Andrian et al., 1995; Fernández-Segura et al., 1996) and immuno-TEM (Picker et al., 1991; Borregaard et al., 1994; Burns and Doerschuk 1994; Moore et al., 1995; Bruehl et al., 1996; Tohya and Kimura 1998) focusing mainly on leukocyte-endothelium interactions, e.g., diapedesis of the lymphocytes into the organ through high endothelial venules, and extravasation of neutrophils through the activated endothelium in inflamed tissue. Sequential adhesion steps, rolling, arrest, strong adhesion, and transendothelial migration, have been postulated in those phenomena, in which interactions of cell adhesion molecules play a central role (Osborn, 1990; Springer, 1994). L-selectin, a cell adhesion molecule mediating the initial rolling contact of leukocytes with vascular endothelial cells, has been localized preferentially on the ruffles/microvilli of neutrophils, lymphocytes, and monocytes in the blood flow or unactivated condition in vitro (Picker et al., 1991; Erlandsen et al., 1993; Borregaard et al., 1994; Hasslen et al., 1995, 1996; von Andrian et al., 1995; Bruehl et al., 1996; Tohya and Kimura, 1998). β2 integrins mediate the subsequent irreversible adhesion with ICAMs expressed by endothelium. Mac-1 (αMβ2) was distributed on the cell body of unactivated human neutrophils, whereas it was localized on both the membrane of the cell body and also of ruffles/microvilli on activate spreading neutrophils (Erlandsen et al., 1993; Hasslen et al., 1996). LFA-1 (αLβ2) was reported to exist on the cell body of lymphocytes in high endothelial venules (Tohya and Kimura, 1998) and of unactivated TK1 lymphoma cells (Berlin et al., 1995), but its distribution in activated leukocytes remains obscure. The binding between VLA-4 on the leukocytes and VCAM-1 on the endothelium also mediates the strong adhesion (Elices et al., 1990), but little is known about the spatial distribution of VLA-4. Although the mechanism of leukocyte transendothelial migration has not yet been fully clarified, PECAM-1 has been suggested to play a central role (Liao et al., 1997; Nakada et al., 2000).
In the present study, we focused on the leukocyte-peritoneum interactions. Morphological changes of the peritoneum presented here were consistent with the previous study on infectious peritonitis (Verger et al., 1983). The αMimmunolabeling showed the Mac-1 distribution on all membrane domains of activated leukocytes in the inflamed peritoneum, consistent with the previous study in vitro by Erlandsen et al. (1993). The αLstaining showed the preferential distribution of LFA-1 on the ruffles/microvilli of activated leukocytes, different from the known distribution of LFA-1 on leukocytes in the blood flow (Tohya and Kimura, 1998). The total expression of β2 subfamily including LFA-1 and Mac-1 on activated leukocytes was reflected by the ubiquitous distribution of β2 integrin molecule. ICAM-1 was localized on the mesothelial cells restricted to their microvilli as well as on the surface of the leukocytes. Thus, the spatial distribution of LFA-1 and Mac-1, and of their ligand ICAM-1 may be involved in leukocyte interaction such as antigen presentation and T cell-B cell collaboration (Dustin and Springer, 1991; Tohma et al., 1992; Sasaki et al., 1998), and also in leukocyte adhesion to mesothelial cell microvilli. The α4 distribution suggested the predominance of VLA-4 on ruffles/microvilli of activated leukocytes, but the possibility could not be excluded that it included α4β7 (LPAM-1) expression by lymphocytes (Berlin et al., 1995). VCAM-1 was associated with the mesothelial cell microvilli, supported by our previous study (Liang and Sasaki, 2000). Fibronectin was localized on the fibrous structures on the exposed smooth muscle cells, presumably collagen fiber or fibrin (Leak, 1983), either are ligands for fibronectin. VLA-4 capable of binding VCAM-1 and fibronectin may mediate leukocyte adhesion to the mesothelial cells (Cannistra et al., 1994) as well as to the exposed surface covered by collagen and/or fibrin.
The present study showed the spatial distribution of integrins on peritoneal leukocytes to be, at least in part, different from that on circulating leukocytes, supporting our state that leukocyte migration in the peritoneal cavity is via a mechanism distinct from leukocyte extravasation (Sasaki and Liang 2000).