Core 2 Project Summary/Abstract The purpose of this core is to provide a centralized laboratory resource for obtaining, cryopreserving and distributing patient samples and to monitor the kinetics of immune reconstitution following allogeneic hematopoietic stem cell transplantation (HCT) and following immune interventions to prevent or treat chronic GVHD. Prior to transplantation, peripheral blood mononuclear cells (PBMC), bone marrow and serum or plasma are obtained from all patients enrolled on clinical trials supported by this Program Project. PBMC, plasma and serum are also obtained from patients undergoing routine allogenic stem cell transplantation and normal stem cell donors for these individuals. Following HCT, peripheral blood, plasma and DNA samples are obtained at regular intervals. All of these samples are processed, cryopreserved, entered into a computerized inventory and made available to all of the investigators in this program. The HCT tissue bank inventory also has a very large number of serial samples from patients who have previously undergone allogeneic HCT at the Dana-Farber Cancer Institute, Brigham and Women’s Hospital and Boston Children’s Hospital. Clinical outcome data on these patients is available in our HCT database. This inventory of annotated samples is also available to Program investigators. Finally, the tissue bank inventory includes serial samples obtained from previous clinical trials evaluating low dose IL-2 and other novel therapeutic agents in patients with chronic graft versus host disease (cGVHD). All banked samples have been collected from patients who have provided informed consent for sample collection for research studies. To monitor immune reconstitution after allogeneic HCT as well as interventions to prevent or treat GVHD, this core utilizes multi-parameter flow cytometry with a panel of fluorochrome- conjugated monoclonal antibodies or mass cytometry with panels of metal-conjugated antibodies. If needed, plasma concentrations of cytokines known to play important roles in immune reconstitution are measured by ELISA or panels of cytokines can be measured by Luminex. Reconstitution of T cell immunity to specific target antigens such as CMV and EBV is determined by ELISPOT and HLA-tetramers. Results of each of these assays are correlated with other parameters of immune function as well as with clinical outcomes.