Hypothesis: Early detection, and treatment, of BK virus infection after kidney transplantation will prevent BK virus associated kidney transplant injury.

BK virus associated nephropathy (BKVN) is estimated to cause a progressive kidney transplant injury in 1-10% of renal transplant recipients. Diagnostic and monitoring strategies for BKVN is still being developed. Detectable virus in the blood by polymerase change reaction-test (PCR) is predictive of BKVN. Additionally, PCR provides a objective estimate of the degree of infection.

If early detection and treatment of BK virus infection is effective in preventing subsequent kidney transplant injury has not been studied. However, renal injury and dysfunction develops late in the natural course of BKVN and it seems likely that screening in combination with early treatment would be beneficial for long-term transplant survival.

There is no established treatment for BK virus infection. Nevertheless, in kidney transplanted patients diagnosed with BK virus infection, immunosuppression is reduced to allow the patients own immune system to handle the virus. However, reduction of immunosuppression has not been associated with rejection. This indicate that these patients were over-immunosuppressed, predisposing them to BKVN. Therefore, to compare the degree of immunosuppression in BKVN patients (over-immunosuppressed) to other patients (not over-immunosuppressed) could yield interesting information. One possibility would be to quantify these patients specific cellular immune response to BK virus but also to other viruses (T cell reactivity).

Leflunomide (Arava) is an immunosuppressive drug, approved for the treatment of rheumatoid arthritis, and has been used in more than 300,000 patients worldwide. Furthermore, leflunomide has been used safely in humans after clinical kidney and liver transplantation for more than 300 days. In addition to leflunomide's value in preventing rejection, it has been shown to exert inhibitory effects on different viruses. Recently published pilot studies suggest that leflunomide treatment of patients with BKVN significantly reduces the amount of BK virus in blood and prevents recurrence of kidney transplant injury. At Karolinska University Hospital, leflunomide has been used for treatment of BKVN and, in some of the patients, renal function has stabilized and BK virus load has decreased significantly.

Inclusion Criteria:

• All adult patients undergoing kidney transplantation at Karolinska University Hospital

Exclusion Criteria:

• Absence of informed consent
• Allergy to leflunomide
• Pregnancy