Exposure to CSE during priming reduces CD4 T cell division and increases apoptosis Triplicate wells of equal numbers of naïve HNT cells and irradiated APC were cultured with cognate peptide under Th1-polarizing conditions for 4 days in the presence of stated concentrations of CSE. The number of resulting effector cells was determined in replicate experiments with a broad ( A ) and narrower ( B ) titration of CSE (2 of 5 similar experiments). ( C ) In some experiments naïve CD4 T cells were labeled with CFSE prior to culture. Shown is representative CFSE staining from effectors at 4 days of culture in the presence of 0% (black) or 7.5% (red) CSE versus CFSE staining of undivided naïve cells not exposed to antigen. Day 4 effector cells were analyzed for apoptotic phenotype by staining for Annexin V and 7-AAD. Shown are ( D ) representative staining of effectors cultured with or without CSE and ( E ) summary of 4 wells per condition (one of 2 experiments).