Head of Renal Unit
Department of Transplantation
Careggi University Hospital
Ongoing research for new immunosuppressive drugs is driven by two primary goals: the need to improve immunosuppression while reducing side-effects, and the need for effective drugs.
Different drugs for different steps of the immune response
Dendritic and T-cell migration
In the first step of the immune response, dendritic cells and lymphocytes migrate to secondary lymphoid organs, where they initiate the immune response. This process is extremely complex, and some cytokines produced in the paracortex, such as CCL19 and CCL22, play a key role. Cytokines recognise chemokine receptors, such as CCR7, which are present on the membrane of dendritic and T-cells.
FTY720 (Novartis) is currently undergoing phase III clinical trials for the prevention of kidney graft rejection.(1) This drug interferes with cell trafficking between lymphoid organs and blood through the sequestration of naive and activated CD4+ T- and B-cells, as well as CD8 cells, into secondary lymphoid organs.(1) The efficacy of this compound has been demonstrated in a variety of animal models and in clinical trials.(2–6) Immediately after drug administration, the number of lymphocytes in peripheral blood drops dramatically, this drop being reversible after drug withdrawal. FTY720 is phosphorylated by a kinase; after phosphorylation, FTY720 acts as a high-affinity agonist at the G-protein-coupled sphingosine 1-phosphate (S1P) receptor. T-cells are trapped in lymph nodes and Peyer’s patches, probably due to an accelerated chemotactic response, and are unable to recirculate to peripheral inflammatory tissues and graft sites.(7) FTY720 is generally well tolerated in patients. Transient, but asymptomatic, bradycardia is the most common side-effect, as S1P receptors are present in both atrial myocytes and lymph nodes.(4,8)
T-cell receptor activation
The next step of the immune response is the interaction of antigen-presenting cells with T-cells. The primary interaction occurs between a T-cell surface molecule and dendritic cells. This nonantigen-specific binding allows the T-cell receptor to recognise the antigen in the major histocompatibility complex (MHC). This first signal is essential for the activation of the immune response. Therefore, the most potent drugs in immunosuppression are ciclosporin and tacrolimus, as well as some antibodies inhibiting the first signal.
T-cell activation requires a second costimulatory signal, which is essential for full T-cell activation. Three molecules can be targeted: CD154 (CD40 ligand), CD28 and the cytotoxic T-lymphocyte antigen (CTLA)-4. As the CD28 costimulatory signal is blocked, T-cells become inert or apoptotic; this costimulatory signal can be inhibited by fusion protein CTLA-4Ig. The interaction of CD40 and CD154 is another important costimulatory signal. In preclinical trials, combined blockade of CD28 and CD154 has been shown to prolong graft survival almost indefinitely.(9–11) A clinical trial in heart and kidney transplant patients demonstrated that the antibody against CD154 was highly potent for graft rejection prevention; unfortunately, thrombotic events occurred and the study was stopped.(12)
The following step of the immune response is signal transduction to the nucleus, which involves the mammalian target of rapamycin (mTOR).
Sirolimus inhibits mTOR by binding to the FK-binding protein. Sirolimus also inhibits ribosomal-dependent protein synthesis by interfering with p76 kinases. It is well known that sirolimus is a very useful drug in combination with ciclosporin, and also (although data are limited) in combination with mycophenolate mofetil (MMF).(13–16) The mechanism of action of everolimus is similar to that of sirolimus; everolimus, however, has a shorter half-life.(17,18)
A second group of immunosuppressive agents consists of a variety of antibodies with different mechanisms.(12) Antithymocyte globulin (ATG), which has been used for a long time, can deplete subsets of lymphocytes. Alemtuzumab (Campath-1H) has been widely investigated.(19) This drug recognises CD52, which is expressed on a variety of lymphocytes, and induces a “proper” tolerance, allowing long-term immunosuppression with very low doses of ciclosporin. Rituximab is an antibody against CD20 which is mostly used to purge malignant B-cells.(20)
Recently, antibodies against interleukin (IL)-2 receptors have been introduced.(12) Since mouse sequences are often associated with side-effects, Roche has developed a humanised antibody and Novartis a chimeric antibody. In chimeric antibodies variable domains are replaced by mouse sequences, whereas in humanised antibodies only a few amino acids are replaced, these amino acids being necessary for the antibody to recognise the antigen.
The final step of the immune response is T-cell proliferation. Drugs such as MMF selectively inhibit proliferation by interfering with purine synthesis. MMF acts by inhibiting inosine monophosphate dehydrogenase (IMPDH), which results in depletion of guanosine monophosphate.(21,22) A new enteric-coated formulation of MMF (Myfortic), promising improved gastrointestinal tolerance, has recently been licensed.(23)
Leflunomide, another new immunosuppressant, inhibits pyrimidine synthesis. The drug inhibits the flavin enzymes dihydroorotate dehydrogenases (which are involved in the fourth step of pyrimidine synthesis), thereby suppressing T- and B-cell replication. At high concentrations, leflunomide can also block some protein kinases.(24)
In the first step of the immune response, dendritic and T-cells migrate to the paracortex of lymphocytes, and this migration can be disrupted by FTY720.
The next step is the activation of T-cell receptors, which can be inhibited by tacrolimus and ciclosporin. For full T-cell activation, secondary costimulation is necessary, and this can be inhibited using monoclonal antibodies or fusion proteins.
Proliferation can occur only if all these signals are transmitted to the nucleus. Signal transduction can be inhibited by sirolimus.
Finally, lymphocyte proliferation is dependent on DNA synthesis, and a variety of drugs can interfere with DNA synthesis.
Different drugs are currently available that can act at different steps of the immune response. The right combination of these drugs now needs to be identified, in order to get the best immunosuppression while reducing side-effects.
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