Transplant rejection. Mechanisms and treatment

In this review, we summarize the cellular and molecular events in the rejection of transplanted allografts, as well as the rationale for the evolving techniques to suppress such rejection. Allogenic major histocompatibility complex antigens expressed on the allograft and/or on the "passenger leukocytes" within the graft are the major antigenic stimuli recognized as being foreign by receptors of CD4+/T helper cells of the host. Host macrophages provide a second signal, interleukin (IL) 1, essential to the activation of T helper cells. Subsequent production of IL-2 by T helper cells leads to activation and proliferation of cytotoxic T cells and lymphokine-activated killer cells and the release of IL-4 and IL-6. In addition, IL-2 promotes release of interferon gamma as well as tumor necrosis factor and other proinflammatory cytokines. Therapeutic options to "downregulate" this cascade have gradually evolved from global nonspecific immunosuppressive techniques (total body irradiation, antilymphocyte serum) to increasingly specific modalities currently being studied, including monoclonal antibodies against the IL-2 receptor (thus targeting only vigorously proliferating T cells), antibodies against specific cytokines (interferon gamma, tumor necrosis factor), and now "designer" antibody-toxin conjugate molecules that deliver toxins to selected receptor targets. Finally, work continues toward inducing preoperative antigen-specific (graft) tolerance, including utilization of gene transfection techniques to transfect donor major histocompatibility complex antigens to recipients before surgery, which has been shown to prolong murine cardiac allografts, perhaps by priming specific suppressor cells. Further understanding of the initiation of, and subsequent events in, transplantation rejection will lead to increasingly effective prolongation of graft survival while minimizing adverse effects on the host.