CTLA4 restricts autostimulation of CD4 ⁺ T cells displaying APC-derived CD80. (A) Human CD4⁺ T cells, purified from PBMCs of a healthy donor, were incubated with either CD80^−/−CD86^−/− Raji or CD80^−/−CD86^−/−CD80-GFP⁺ Raji, isolated by FACS, and then cultured with mature DCs derived from human monocytes of another donor, in the presence of SEB, before flow cytometry analysis of IL-2⁺ and IFNγ⁺ T cells, summarized in the bar graphs. (B) Upper: Experimental scheme depicting the assay setup. Human CD4⁺ T cells, purified from PBMCs, were incubated with either CD80^−/−CD86^−/− Raji or CD80^−/−CD86^−/−CD80-GFP⁺ Raji, to allow for trogocytosis, isolated by FACS, and then cultured for 1–4 d with SEB in the presence or absence of ipilimumab before measurement of IL-2 concentration in the medium. Lower: Plot summarizing IL-2 concentrations in the indicated conditions. Error bars: SD. **, P < 0.01; ***, P < 0.001; unpaired two-tailed Student’s t test.