The mosaic of immunosuppressive drugs.

Graft rejections as well as tolerance are true representation of the specificity, sophistication and redundancy of an elegantly and meticulously designed immune system. Tolerance is in a way similar to the process of self-recognition where lymphoid clones, during development, baring self-reactive receptor are eliminated or rendered in active by "clonal deletion" leading to a state of accommodation and acceptance (anergic). On the other hand, both acute and chronic rejections are manifestation of the purpose of existence of the immune system, which is to defend the host against foreign invaders. Thus, in order to treat (control) graft rejection it is necessary to determine and understand the steps leading to recognition, stimulation, activation, and amplification of the immune system. The first step leading to the initiation of the immune system cascade is recognition. Which can either be direct where donor antigens of the major histocompatibility complex (MHC) expressed on the donor cells (passenger leukocytes) or tissues are recognised by the host immune system. The direct recognition pathway initiates acute graft rejection. Alternatively processed donor MHC peptides presented by the recipient antigen presenting cells (APC) initiate the indirect pathway of immune response, which is as important as the direct recognition especially in chronic rejection. Recognition is followed by the ligation of a series of adhesion molecules starting with an antigen to its specific T-cell receptor (TCR)/cluster of differentiation (CD) complex, expressed on the surface of the T cell. In order for the activation to precede additional costimulatory signals, such as ligation of the CD28/B7, CD4/HLA class II and CD/HLA class I antigens are required. The activation process is accompanied by an increase of cytokines production such as interleukin (IL)-2, IL-12, interferon (INF) and tumour necrosis factor (TNF) by the primed T cell. The complexity and the polymorphic nature of the immune system have necessitated designing agents that inhibit the immune system at different levels. Cyclosporine and Tacrolimus, collectively known as calcineurin inhibitors, seems to act on the IL-2 by inhibiting its production thus leading to a decrease in the proliferation of the activated lymphocyte. Rapamycin, which is similar to Tacrolimus, inhibits graft rejection by blocking IL-2 activation and phosphorylation of 70 S6 kinase thus inhibiting the progression of T-cell from G to S phase. While Cellcept (MMF) reduce the proliferation of T cell by inhibiting purine synthesis and by its action on ionosine monophosphate dehydrogenase. Anti-lymphocyte antibodies (ATG) deplete circulating lymphocytes while selective monoclonal antibodies are directed against IL-2 receptor thus reducing the rate of proliferation of activated T cells. Recently, antibodies to the CD40/CD40 ligand have been shown to induce long-term graft survival with the inhibition of the Th1 cytokines (INF), IL-2 and IL-12 and upregulating the Th2 cytokines IL-4 and IL-10. Lastly graft rejection can be reduced by blockade of the B7/CD28 costimulation pathway with the fusion protein CTLA-4Ig. With the availability of such potent and diverse agents it is now possible to develop multi drug regiments that can depress the immune system at the different steps of the activation cascade, with minimal side effects, thus improving graft and patient survival rates.