Cdc42 is essential to achieve responses against viral infection WT and Cdc42 KO mice were infected with Influenza A virus and immune responses were analyzed at day 9 after infection. (A) Influenza-specific antibody titers were measured by ELISA. Serial dilutions of the sera are shown. (B, D, and F) Mediastinal LNs from WT and Cdc42-KO mice were analyzed by flow cytometry 9 d after infection. Plasmablasts (B; IgD^lo CD138^Int), plasma cells (B; IgD^lo CD138^hi), germinal center cells (D, B220⁺CD95⁺GL7⁺) or Tfh cells (F, PD-1⁺CXCR5⁺) are shown. (C–E) Tile images of spleen sections were acquired and show extrafollicular plasma cells (C; intracellular anti-κ staining) or germinal center cells (E; Bcl6⁺ cells). Quantification charts of spleen sections are shown in the right-hand columns of C and E. Bar, 200 µm. Data are representative of three independent experiments with at least 3 mice in each group. (G) Mixed bone marrow chimeras (either 50:50 WT CD45.1:WT CD45.2 or 50:50 WTCD45.1:Cdc42 KO CD45.2) were infected with Influenza virus, and mLNs were analyzed by flow cytometry. Germinal center (B220⁺CD95⁺GL7⁺) and Tfh (PD-1⁺CXCR5⁺) cells are shown. Quantification charts are shown in the right-hand column and indicate the ratio between CD45.1 and CD45.2 cells in each population.