*Purpose: BK virus infection represents one of the main viral infections within the first year after kidney transplantation, closely linked to the degree of immunosupresion. We focus on studying risk factors for replication in kidney graft recipients with high immunological risk, treated with strong immunossupression and evaluate the strategies used to achieve BK clearance.

*Methods: Observational retrospective unicentric study, with a cohort of 44 recipients transplanted in 2008- 2016 with high immunological risk (defined as pretransplant PRA ≥ 50%, pretransplant DSAs or acute rejection in the first 3 months after transplantation), with a 3-to-7-years follow-up. The screening method was BK PCR, defining replication as two consecutive positive viral loads.

*Results: 9 of 44 patients (20%) had positive viral load in the 120+/−71 days after transplantation. Risk factors for replication were lower age and number of HLA mismatches. All patients received tacrolimus, mycophenolate, steroids and rATG-Thymoglobulin in the first 3 posttransplant months. Neither tacrolimus plasma levels, nor other treatments including iv Ig, plasmapheresis, rituximab and bortezomib, associated with replication. Among BK-positive patients, the mean period until BK clearance was 200 (40-375) days, with no cases of kidney disfunction or graft failure during replication (3 renal biopsies were performed, only one with BK virus nephropathy). 8 of 9 patients had mycophenolate at viral-positive diagnosis, and in 6 of 8 its reduction/withdrawal got negative viral load. For the 2 patients left, minimization of tacrolimus cleared BK (one received previously cidofovir without success). With respect to the recipient with everolimus instead of mycophenolate at viral-positive diagnosis, the inhibitor of mTOR was replaced with leflunomide, achieving BK clearance 5 months later. In 4 of 9 patients was added iv Ig according to physician judgement to compensate the lower level of immunosuppression, what prevented this group from acute rejection during the follow-up. We didn’t find differences regarding creatinine and proteinuria at 3 and 5 years after transplantation between BK-positive and BK-negative patients

*Conclusions: The replication incidence was similar to that of immunological standard recipients. Risk factors for replication were lower age and number of HLA mismatches. BK-positive patients achieved negative viral load in less than two years, 75% of them with reduction/withdrawal of mycophenolate. A careful screening and a progressive and slow lowering of immunusupresion with the support of iv Ig prevented allograft loss in our cohort.

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