Put to good use during the post-transplant follow-up, this type of test can provide the clinician with valuable data to assess the quality of anti- BKV T response compared to the intensity and type of IS treatment. Early identification of patients at risk of reactivation of BKV, or at risk of BKVAN if BKV viremia observed , would adapt the therapeutic response and monitoring arrangements at the fair. The objective of this project is to conduct a feasibility study to assess the relevance of post-transplant monitoring of anti- BKV T response of BKV viremic patients during the first year post- transplant. Immunological data will be analyzed in relation to virological data and bioclinical data.