Laminin-Mediated Interactions in Heart Graft Rejection

In the context of cardiac allograft rejection, an effective immune response requires the migration of inflammatory cells, driven by the balance between soluble factors and proteins of the extracellular matrix (ECM). In a recent review (1) one ECM protein, fibronectin, was discussed as a potential therapeutic target for graft survival, based in its role “as an adhesive factor,” guiding the influx of infiltrating cells into the allograft. We previously showed that this notion can be applied for laminin, at least when autoreactive cells are involved. This was approached using a murine model of Chagas disease. This parasitic disease is caused by the flagellate protozoan Trypanosoma cruzi and presently affects 18–20 million individuals in Latin America. In the chronic phase of Chagas disease, the presence of cardiac disorders with consequent heart failure relates to death in about 20% of the patients. Although the pathogenesis of this cardiomyopathy remains to be completely determined, an autoimmune reaction is likely to occur. Using a previously established model of heart transplantation into the subcutaneous tissue of the ear, we demonstrated that heart tissues from newborn donors were rejected when grafted within the ears of T. cruzi chronically infected syngeneic recipients, through a CD4+ T cell-dependent mechanism (2). Moreover, when purified spleen-derived CD4+ T cells from chagasic animals were transferred into the ears of naive normal syngeneic recipients, the transplanted hearts were rejected (2). Using this same protocol, we showed that graft rejection was abrogated if CD4+ T cells were treated ex vivo with an anti-laminin receptor monoclonal antibody (mAb) before transfer or if the area adjacent to the graft was treated in vivo with anti-laminin mAb (3). This immune intervention resulted in a down-regulation of the laminin contents within and surrounding the graft, and a decrease in the infiltrating cells, comprising the CD4+ T cells transferred as well as CD8+ T cells and macrophages recruited from the recipients. ECM proteins promote lymphocyte adhesion and positively signal for the secretion of proinflammatory cytokines, such as interferon-γ and tumor necrosis factor (TNF)-α. These cytokines, besides inducing recruitment of immune cells to inflammatory sites, are known to bind laminin (4). We thus evaluated, by confocal microscopy, the colocalization of laminin with interferon-γ or TNF-α, in the situation of syngeneic heart graft rejection elicited by transfer of CD4+ T cells from chagasic animals, and compared this with the findings observed when rejection was blocked in vivo by a single dose of anti-laminin. Such intervention not only reduced the numbers of cytokine-producing cells externally and within the grafts, but also resulted in a decrease in the amounts of the cytokine colocalized with laminin (see Fig. 1). It should be noted that similar findings were obtained when allogeneic mouse hearts were grafted into normal recipients, 1 which is in keeping with previous data showing that systemic anti-laminin polyclonal antibody injection abrogated allogeneic rat heart grafts (5). Figure 1: Confocal microscopy for immunocytochemical localization of TNF-α (red) and laminin (green) in transplanted hearts after local injection of CD4+ T cells derived from spleens of T. cruzi chronically infected mice. In panel a (control treated) and in panel b (anti-laminin mAb treated) the colocalization sites are painted in blue. Note that in the laminin antibody mAb-treated transplant, the degree of colocalization is inferior as compared with the control.Taken together, it seems clear that laminin-mediated interactions are involved in the general mechanism that governs migration of activated T lymphocytes, including their interaction with microenvironmental products, independently of the fact that they recognize self or nonself antigens. This is further emphasized by their role in locally concentrated proinflammatory cytokines. Thus, laminin should also be regarded as a potential target for immune intervention in organ transplantation. Acknowledgments. This work was funded with grants from Fiocruz, Faperj, CNPq, and Pronex/CNPq (Brazil). Suse Dayse Silva-Barbosa Ingo Riederer Wilson Savino1