Polyomavirus nephropathy (PVN) is an emerging cause of renal transplant loss. Until now the risk factors of PVN are poorly understood. Tacrolimus (Tacr) and mycophenolate mofetil (MMF) are thought to be associated with a higher risk of developing PVN. However, the way in which Tacr or MMF might enhance the susceptibility for PVN remains largely unknown. In this prospective study we will analyze whether differences in immune-reactivity patterns (Th1, Th2, B cell and monocyte responses, sCD30, immunoregulatory antibodies) of renal transplant patients induced by different immunosuppressive regimens (cyclosporine A [CsA]/MMF, Tacr/MMF, Tacr/MMF with conversion to Tacr/Everolimus [ERL]) or by cytokine promoter gene polymorphisms may account for the different risks of developing PVN.