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Extracellular matrix (ECM) proteins exert comito-genic effects on T cell activation in vitro, and may play a role in selective lymphocyte migration and tissue positioning in vivo. However, putative roles of ECM in host immune responses leading to graft rejection remain largely obscure. The present study evaluates intragraft expression of prominent ECM components, laminin (LN) and fibronectin (FN), and analyzes their interactions with host cellular repertoire in a well-defined cardiac allograft model in sensitized rats.

(LEWxBN)F1 cardiac allografts are rejected within 24 hr in sensitized LEW rats. Immunohistochemical analysis has revealed that this brisk rejection response was associated with an early increase in the intermyocyte and endothelial deposition of LN and FN at the graft site, with their peak at 6 hr after transplantation. The upregulation of ECM preceded intra-graft cellular infiltration, which at 6 hr consisted primarily of monocytes and macrophages. The infiltrating cells localized selectively in FN-rich cardiac interstitial and perivascular areas, as documented by two-color staining and laser scanning confocal microscopy. Marginal and transient increase in ECM expression was noted within control isografts. Next, we tested the role of ECM proteins in systemic lymphocyte recirculation. Specifically sensitized lymph node lymphocytes (LNL) were labelled in vitro with a DNA-binding fluorochrome, H33342, and injected i.v. into secondary engrafted recipients, which were sacrificed 6 hr later. These LNL were detected within host cervical lymph nodes in close association with FN deposits. Moreover, LNL labeled in vitro with a cationic membrane-binding fluorochrome, Di “I”, were traced in hosts the lymph nodes of which were analyzed for simultaneous detection of LN and FN. Again, transferred LNL migrated selectively to FN-containing compartments, as shown by laser scanning confocal microscopy.

Thus, ECM proteins should be regarded as active and important participants in host immune responses leading to graft rejection. FN may act as an ECM component “signal” for graft-infiltrating cells, and may play a key role in selective homing and specific tissue positioning of recirculating specifically sensitized lymphocytes in host peripheral lymphoid tissues.

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