Studying the Effect of Changing Immunosuppression in Case of Polyoma BK Virus Infection of the Renal Transplant

Polyomavirus BK nephropathy is a serious complication after renal transplantation leading to graft loss in 40% of cases. Since no virustatic drug exists, the investigators want to study the best way to manage viral invasion by changing the immunosuppressive treatment comparing two treatment schemes. The investigators hypothesis is that switching to an mTOR-based scheme is superior to a general decrease of a calcineurin inhibitor (CNI)-based scheme. The study will be performed as a prospective, randomized, parallel group comparison.