Characterization of spleen human cell chimerism. (A) Expression of several activation markers among CD3⁺ T cells was analyzed on fresh PBL from donor 1 (□) or engrafted human cells from the spleen of chimeras, killed at days 65, 70, or 77 after the graft (▨). Error bars are indicated. (B) CD28 molecule expression on human CD3⁺ T cells was compared between fresh donor PBL and splenic cells from chimera C5. The filled peaks correspond to the chimera C5 cells, the open peaks correspond to the fresh donor's cells. Isotypically matched negative IgG1 control antibodies were used for each sample. (C) PBL from donor and splenic cells from four chimeras, killed respectively at day 65 (C3, C5, and C6) or day 70 (C19), were cultured for 4 days in medium alone or in the presence of PHA or SEE at 1 μg/mL. Syngeneic T-cell–depleted irradiated feeder cells were added to cultures of chimeras' cells. Results are expressed as cpm and Stimulation Index (SI) = ³H thymidine incorporation put in brackets, in the presence of PHA or SEE/³H thymidine incorporation in medium alone.