CLL CTLA4 expression is primarily stimulated by CD4 T cells. CLL cells were cocultured with activated total T cells, CD4⁺ T cells, or CD8⁺ T cells. CTLA4 expression on CLL cells was measured 48 hours later by surface and intracellular flow cytometry and compared by ANOVA (n = 7 surface, 5 total CTLA4). *P < .05 by ANOVA; **P < .01 by ANOVA.