Activation and differentiation of lymphocytes have profound effects on their trafficking. Whereas naive T cells recirculate through lymphoid organs, activated cells localize predominantly in other compartments. Here, we report that changes in migratory properties of T cells occur immediately upon activation via the TCR. One hour stimulation is enough to target T cells into lung and liver following i.v. injection. The high localization within lung and liver and the lack of recirculation through lymphoid tissues are key features of activated lymphocytes. Regardless of the source, in vitro as well as in vivo activated lymphocytes show this behavior, which is not caused by increased cell size. Accumulation in the lung requires protein synthesis and is partly mediated by LFA‐1, in contrast to the acquisition of liver "homing" properties. Intravital microscopy reveals firm adhesion of activated cells within periportal sinusoids of the liver. Selective homing to other organs, such as skin or mucosa, was not observed, regardless of the cell's origin. These data indicate that activation quickly switches the trafficking program of lymphocytes from recirculation to sequestration; it is tempting to speculate that especially the induced trapping in the liver has a distinct role in limiting systemic T cell responses.

The integrin LFA-1 interacts with a variety of ligands termed ICAMs. ICAM-1 and ICAM-2 are both expressed on endothelium and serve as counterreceptors during lymphocyte trafficking. In this study, we analyzed their relative contribution to lymphocyte recirculation through lymph nodes and to recruitment into lung and inflamed skin by blocking mAbs against ICAM-1 and ICAM-2 and mice deficient for ICAM-1. The entry of lymphocytes into peripheral and mesenteric lymph nodes was found to be unaffected by the functional deletion of either ICAM-1 or ICAM-2. However, when both pathways were blocked, recirculation through lymph nodes was strongly reduced. Trapping of lymphocytes in the lung after i.v. injection is partly mediated by LFA-1/ICAM interactions; the data presented in this study show an exclusive role of ICAM-1 in LFA-1-dependent lung trapping. Similarly, ICAM-1, but not ICAM-2, was required for the migration of T effector cells into the inflamed skin. These results indicate that ICAM-1 and ICAM-2 have redundant functions in lymphocyte recirculation through lymph nodes, but ICAM-1 is unique in supporting migration into inflamed sites and trapping within the lung.

Specific recognition molecules ("homing receptors") on lymphocytes are thought to direct selective entry of cells into different organs. The lectin-related cell adhesion molecule LECAM-1 has previously been supposed to mediate lymphocyte entry into peripheral lymph nodes and, partially, mesenteric nodes but not into Peyer's patches. Here we present evidence that in vivo the molecule is also implicated in homing of mouse lymphocytes to Peyer's patches and may have a more general role as homing receptor for high endothelial venules-bearing lymphoid tissue, but not for most non-lymphoid tissue.

Adhesion molecules play important roles in immune reactions and inflammatory processes and may constitute attractive targets for immunomodulatory approaches. In this study, blocking mAbs against a series of adhesion molecules were tested for their therapeutic effect on developing arthritis in a mouse model. MAbs were given for a period of 4 weeks at the time of exspected incidence of visible disease symptoms, i.e. 4 weeks after priming with collagen type II. A significant reduction of incidence down to values of 13% and 29% of the controls was obtained with mAbs against CD44 and alpha 4-integrin, respectively, during an observation time of 13 weeks. MAbs against CD4 and LFA-1 resulted only in weaker, non-significant effects or a delay in the incidence. MAbs against other molecules including L-selectin, ICAM-1 or VCAM-1 were not effective. The development of antibodies against collagen type II, collagen type I, proteoglycans and the immunogen, bovine collagen type II was affected by mAb treatment to a different extent. In this case, the anti CD4 mAb was the most effective, followed by the anti alpha 4-antibodies in most cases, whereas anti CD44 showed less clear effects on the development of humoral responses. In a skin delayed type hypersensitivity model analyzed for comparison, mAbs against LFA-1/ICAM-1 and alpha 4-integrin showed the largest effects on ear swelling. These data show that mAbs against several adhesion molecules are able to block selectively distinct aspects of immune reactions, and that CD44 and alpha 4-integrins could be promising targets for an immunotherapy of rheumatoid arthritis with receptor-interfering agents.

When lymphocytes are activated in vitro, discrete cell-cell contacts are initiated which result in cluster formation. This contact interaction is found in syngeneic or allogeneic mixed leukocyte reactions as well as in mitogen-stimulated cultures (concanavalin A, periodate, lipopolysaccharide). T cells as well as B cells display the binding phenomenon. This activation-dependent lymphocyte-lymphocyte adhesion involves LFA-1, since monoclonal antibodies (including Fab fragments) against this molecule inhibit adhesion between clustering lymphocytes in a dose-dependent manner, whereas antibodies directed to several other cell surface antigens are inactive. Since a wide variety of functional interactions are inhibited by antibodies to LFA-1, it may be concluded that LFA-1-mediated cell contact is a discrete and essential step between a recognition event and the generation of functional activities by lymphocytes in general.

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Directed migration of lymphocytes from blood into lymph nodes and gut-associated lymphatic tissue, also referred to as homing, is subject to change following activation. Lymphocyte migration into lymphoid organs in vivo and binding to high endothelial venules in vitro is largely suppressed after short-term stimulation with phorbol esters. The observed functional alterations were correlated with changes in the expression of three putative homing receptors, LECAM-1 (MEL-14 antigen), LPAM-1/2 (alpha 4-integrin) and the murine CD44 (Pgp-1, H-CAM, Hermes-antigen equivalent) upon different modes of cellular activation. Expression of LECAM-1 (gp90 MEL-14), a lymphocyte adhesion molecule implicated in targeting extravasation into lymph nodes, was found to be lost almost completely within minutes after protein kinase C activation. LECAM-1 re-expression occurred within less than 24 h. Rapid loss of LECAM-1 was also observed after calcium ionophores whereas anti-CD3 or concanavalin A elicited a gradual and heterogeneous loss of LECAM-1 becoming detectable after several hours only. A number of cytokines tested were not able to induce alterations in LECAM-1 expression. In contrast, expression of LPAM-1/2 (alpha 4-integrin) and CD44 (Pgp-1, H-CAM), two adhesion molecules supposed to direct extravasation into Peyer's patches, remained stable for hours after every stimulus tested; CD44 expression gradually increased 24 h after mitogenic activation, whereas a small reduction only was observed for the expression of the alpha 4-chain under certain conditions. Thus, reduced extravasation of lymphocytes into Peyer's patches after activation is not due to a decline in the surface density of LPAM-1/2 alpha-chain or CD44 whereas alterations in migration into lymph nodes parallel the expression of LECAM-1.

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