Role of the microenvironment in immune responses to transplantation.

Through the constant interplay of cellular and extracellular components, the microenvironment of tissues directs immune responses. In solid organ transplantation, one factor that significantly alters the microenvironment of tissues is reperfusion injury, which occurs to a certain extent in essentially all cadaver organs. The damage that results from reperfusion injury initiates a cascade of signals to surveillance cells such as macrophages, mast cells, and dendritic cells, augmenting both innate and allo-immune responses. Chemokines, released from surveillance cells and others, orchestrate an influx of cells into the allograft, and subsequently drive the migration of dendritic cells and lymphocytes to proper areas within lymph nodes for the efficient generation of allo-immune responses. Heparan sulfate, a component of the extracellular matrix, binds chemokines and thus regulates their localization within tissues. This association is one of a multitude of examples of the interplay between cells and their extracellular surroundings. In addition to the association with chemokines, heparan sulfate binds cytokines such as IFN-gamma and IL-2. In the spleen, heparan sulfate localizes IL-2 to the marginal zone, red pulp, and interdigitating dendritic cells of the T cell zone. Our laboratory recently determined that the contribution of heparan sulfate-bound IL-2 to immune responses is substantial, finding that bound, rather than free, IL-2 drives immune responses. This finding reiterates the critical nature of the interaction between cells and the extracellular matrix. Disruptions in these interactions may lead to dysregulation of immune responses and, in turn, pathologies such as tissue fibrosis or autoimmunity. Further studies into the exchange between cells and the extracellular matrix will likely lead to new lines of therapies aimed at correcting these abnormalities before irreversible damage occurs.