Immunosuppressive therapy with the calcineurin inhibitors (CNI) cyclosporine (CsA) and Tacrolimus (Tac), have radically changed the field of organ transplantation. Ironically, although extensively and effectively used for kidney transplantation and other solid organ transplants, CsA and Tac cause important adverse renal side effects: acute and chronic renal dysfunction, hemolytic-uremic syndrome, hypertension, electrolyte disturbances and tubular acidosis. Chronic nephrotoxicity from CNI has been implicated as a principal cause of post-transplant renal dysfunction and it is characterized by an irreversible and progressive tubular atrophy, interstitial fibrosis, and focal hyalinosis of small renal arteries and arterioles. Attempts to minimize CNIs and their known toxicities have been marginally successful due to unacceptable rates of acute rejection and drug toxicity. Patients are converted to alternative immunosuppressive therapy related to CNI side effects including neurotoxicity, nephrotoxicity, cardiovascular (HTN, hyperlipidemia), metabolic (NODAT), and cosmetic side effects. Furthermore, this class of medications is associated also, by blocking Interleukin 2 (IL2) production, with negative impact on regulatory T cells (Tregs) generation (an important subpopulation of T helper cells that has been associated with positive immunomodulation and donor specific hypo-responsiveness).
Until the approval of Belatacept for adult EBV+ renal transplant recipients, there have been limited alternative immunosuppressive agents available to mitigate drug induced renal impairment. The phase III drug trials of Belatacept in combination with MMF and corticosteroids have resulted in significant and sustained improvement in glomerular filtration rate (GFR) at one year through three years post transplant. The overall safety of belatacept compared to cyclosporine in de novo transplant recipients was similar. However, there was an increased rate and severity of early acute rejection and post-transplant lymphoproliferative disorder (PTLD) of the central nervous system in patients treated with belatacept.
In a phase II switch study conducted by Bristol Myers Squibb (BMS), the incidence of acute rejection at 24 months post conversion was similar in patients remaining on CNI (4%) compared to those converted to belatacept (7%). There were no reported cases of post-transplant lymphoproliferative disorder (PTLD) in this patient population as of two years post randomization. However, one belatacept patient from Mexico developed tuberculosis and there were more non-serious fungal infections in the belatacept treated patients.
Mechanistically, CD28 (Cluster of Differentiation 28) and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) are important for the function of regulatory T cells (Tregs). Belatacept binds to CD80/CD86 (Cluster of Differentiation 80/Cluster of Differentiation 86) ligands on antigen presenting cells (APCs) preventing CD28 to bind with these ligands and deliver the costimulatory signal to activate the T Cell. CTLA-4 is a related receptor expressed on activated T cells that also recognizes CD80/CD86 (Cluster of Differentiation 80/Cluster of Differentiation 86) and is thus termed co-inhibitory. It transmits both cell intrinsic and cell extrinsic negative signals that impair activation.
Investigation of the effect of early conversion to Belatacept at month 3 post-transplant on the subpopulations of T cells and B cells and peripheral blood and allograft biopsy-derived gene expression subpopulation profiles are planned. Optimization of the Belatacept immunosuppressive regimen to achieve good long term renal function and improved graft survival requires understanding the relationships of these cell populations to clinical outcomes.
