CEACAM1-mediated interactions inhibit SEB-induced T cell proliferation. Decidual T cell clones were tested for expression of CD4 (a), V_β17 (b), and CEACAM1 (c) by flow cytometry. Bold lines indicate mAb staining and thin lines indicate control staining. (d) Fifty thousand cells of the presented T cell clone were incubated for 2 days with 25,000 irradiated .221 cells or with .221 cells transfected with CEACAM1 (.221/CEACAM1), in the presence of decreasing SEB concentrations as indicated in the figure. Proliferation was measured with ³H-thymidine incorporation. The figure represents the average of ten independent experiments. Similar results were obtained when other T cell clones were used.