Expression of obligate covalently linked B7-1 dimers increases CD28-mediated adhesion to T cells. A, CHO-IAd cells expressing the covalently linked dimeric B7-1 show increased adhesion to T cells. CHO-IAd cells untransfected (top row) or expressing WT B7-1 (second row), B7-1–control (third row), or B7-1–cysteine (bottom row) were pulsed with a titrated concentration of OVA peptide, labeled with CMTMR, and mixed with CFSE-labeled DO11.10 TCR Tg T cells. The CMTMR+CFSE+ CHO-IAd–T cell conjugates were scored by flow cytometry. The numbers in the top right corner represent the percentage of T cells as T cell–APC conjugates. B, Quantification of CHO-IAd–T cell conjugates as a percentage of total CD4 T cells. The results and error bars represent the means and SD from four independent experiments. *p < 0.05.