IgG2c extrafollicular antibody responses to S. enterica are impaired in the absence of T cell–expressed Bcl-6. (A) Representative flow cytometric plots showing Fas versus GL-7 stains from spleens of 80% Bcl6^−/− CD45.2:20% Cd28^−/− CD45.1 (Tc Bcl6^−/−) chimeras or control 80% Bcl6^+/+ CD45.2:20% Cd28^−/− CD45.1 (Tc Bcl6^+/+) fetal liver chimeric mice injected 7 d previously with S. enterica; oval gates identify GC B cells. (B) Bar graphs show the number of splenic GC B cells in the same mice (left) and a group of Cd28^−/− and control Cd28^+/+ mice infected with S. enterica at the same time (right). (C–F) S. enterica–specific IgM (C and E) and IgG2c (D and F) antibody titers 7 d after S. enterica infection in the same fetal liver chimeric mice as in A (C and D) or Cd28^−/− and Cd28^+/+ mice (E and F). (G and H) S. enterica CFUs in livers 12 d after infection of the chimeras (G) and nonchimeric Cd28^−/− and Cd28^+/+ mice (H). These data are representative of three independent experiments with three mice per PBS-only control groups and five to seven mice per S. enterica–infected groups.