Germinal center CXCR5 + PD-1+ CD4 T cells are susceptible to Fas-mediated cell death. (A) Tonsil CD4 T cell subsets were characterized on the basis of CXCR5 and PD-1 expression. (B-F) Expression levels of ICOS, CD69, CCR5, Fas or FasL on subsets were examined by flow cytometry. (G, H) Purified tonsil CD4 T cells were treated with or without active anti-Fas antibody CH11 (1 μg/ml), or neutralizing anti-Fas antibody ZB4 (1 μg/ml), or both for 8 hours (G) or 24 hours (H). Cell apoptosis was determined by expression of Annexin V (G) and the frequency of different subsets was measured by expression of CXCR5 and PD-1 (H). Data are representative of three independent experiments.