PBMCs, peripheral lymph nodes, spleen and bone marrow cells were labeled with a combination of the following mAbs: CD3 PerCP (SP 34–2), CD4 PerCP (L-200), CD8 PerCP (SK1), CD8 APC (SK1), CD95 FITC (DX2), CD95 APC (DX2), and CD28PE (CD28.2) (BD Pharmingen, San Jose, CA). For chimerism analyses, we used an anti-MHC class I HLA mAb (H38, One Lambda, Inc, CA) reacting specifically with certain MHC class I antigens. The recipient and donor pairs were chosen based on their differential reactivity to this mAb. The fluorescence of the stained samples was analyzed using FACS Calibur and FACS Scan flow cytometers and Cell Quest Software (BD), or FlowJo software. For assessing the effect of CTLA4Ig on memory T cell functions, the monoclonal antibodies anti-CD16, anti-CD95, anti-CD4 and anti-CD8 were used to purify CD4 Tmems (CD16−CD4+CD95+) and CD8 Tmems(CD16−CD8+CD95+) using a FACS Vantage cell sorter (BD Immunocytometry System), then those purified populations were tested for their IFNγ production in ELISPOT assay as previously described (11 (link), 12 (link)). The purity of sorted cells was consistently > 95% as previously described.