Immunologic 'ignorance' of vascularized organ transplants in the absence of secondary lymphoid tissue

Secondary lymphoid organs (the spleen, lymph nodes and mucosal lymphoid tis sues) provide the proper environment for antigen-presenting cells to intera ct with and activate naive T and B lymphocytes(1). Although it is generally accepted that secondary lymphoid organs are essential for initiating immun e responses to microbial antigens and to skin allografts(2-6), the prevaili ng view has been that the immune response to primarily vascularized organ t ransplants such as hearts and kidneys does not require the presence of seco ndary lymphoid tissue. The assumption has been that the immune response to such organs is initiated in the graft itself when recipient lymphocytes enc ounter foreign histocompatibility antigens presented by the graft's endothe lial cells(7-13). In contrast to this view, we show here that cardiac allog rafts are accepted indefinitely in recipient mice that lack secondary lymph oid tissue, indicating that the alloimmune response to a vascularized organ transplant cannot be initiated in the graft itself. Moreover, we demonstra te that the permanent acceptance of these grafts is not due to tolerance bu t is because of immunologic 'ignorance'.