Feasibility objectives: 1. Methodology: Outcome in the control group: what is the BPAR-rate in the control group? 2. Process: Consent rates. How many patients can be included, what is the rate of consent, determining centre willingness and capacity? 3. Scientific (biological plausibility): The primary objective of the pilot study is to estimate the treatment effect using surrogate endpoints with the following immunological parameters: I. Detection of total alloreactive T-cell compartment by CD137+ T staining. II. Leucocyte subsets. III. In vivo assessment of general immune responses: - Vaccination responses: Tetanus, pneumococcus, influenza. IV. The incidence of infection as measured by: a. detection of polyoma BK viral load in serum. b. detection of CMV viral load in serum. c. hospital admission for infection-related problems. d. antibiotic prescriptions (treatment episode defined as 5 or more consecutive days of antibiotic treatment).