Recipient primary fibroblasts can serve as allo-APCs for CD137-mediated SAD. Purified CD8 T cells of healthy donor 613 were stimulated against CD40L-induced B lymphoblasts of patient MZ506 in haploidentical MLC (Table 1). (A) Day 21 MLC responders were incubated with either MZ506-derived B blasts or fibroblasts, or with K562 cells expressing the patient HLA-B*3501 mismatch allele. After 24 hours, the different cell populations were analyzed for CD137 expression by flow cytometry. Only the fibroblast-stimulated day 22 T-cell culture was depleted using the CD137 approach. Numbers indicate percentages of CD137^pos CD8 T cells. (B) Immediately (day 22) after separation, the CD137^neg fraction (white bars), the CD137^pos fraction (gray bars), and the MLC control population stimulated exclusively with MZ506 B blasts (black bars) were tested in IFN-γ ELISPOT assay. (C) The CD137^neg fraction (white bars) and the MLC control (black bars) were reevaluated on day 33 after stimulation with MZ506 B blasts on day 22 and day 29.