Anti-BK Virus Immune Response and Kidney Transplantation

BK virus infections are very frequent during months following a kidney transplantation: a viral reactivation is observed for almost 50% of patients during first year. This reactivation leads to a viremia for 10 to 15% of patient during this same period. The most frequent complication is interstitial nephritis for 2 to 8% of patients (27 patients representing 2.7% during 6 years in Nantes). An intensive et persisting viral replication, assessed by detection of high blood viral load, could evolved to a viral nephropathy which lead to a very pejorative functional issue for the graft. Biological follow-up of these infections lay on the measures of viral load. Their positivity must alert the physician and lead him to modulate immunosuppressive treatment. Actually, there is no real consensus about the modalities of pharmacological immunosuppression decrease (decrease dose or change of molecule). Specific lymphocytic anti-BKv evaluated on several cohorts of patients permit to prove: * weakness of immune cellular response for patient with high viremia * increase of this response when viral load decrease These studies laid on detection of INFg synthesis by Elispot after stimulation with viral antigens and in vitro cellular expansion. New prospective and longitudinal data comparing the immune cellular response (systematic and early) after graft between patients controlling or not BKv infection are necessary to improve the comprehension of illness natural history. The investigators propose to enlarge the investigation of anti-BKv immune cellular response to other functions than IFNg synthesis in the aim of detecting the eventual role of polyfunctional lymphocytes for infection control. Furthermore, the investigators propose to identify better diagnostic and prognostic makers.