Figure 4: Clonal replacement in virus-specific CD4⁺ T cells requires cognate B-cell interaction.

(a) Frequency of germinal center-phenotype (CD38^loGL7^hi) cells in CD19⁺IgD^lo splenic B cells over the course of FV infection or FV/LDV coinfection (n=4–13; P<0.002 between the two infections on days 5–14, Mann–Whitney rank sum test). (b) Germinal center (B220⁺IgD⁻GL7⁺) presence in the spleen of the same mice in a 35 days post infection. The scale bar is 250 μm. (c) MHC II expression in B220⁺CD19⁺ B cells from the spleen of the same mice in a 7 days post infection, compared with uninfected mice. (P=0.0016 between the two infections, Student’s t-test). (d) Absolute numbers (top) and Vα composition (bottom) of WT (left) or Sh2d1a^−/− (right) env-reactive donor EF4.1 CD4⁺ T cells in the spleens of recipient mice after adoptive T-cell transfer and FV infection (n=3–8 mice per time point) or FV/LDV coinfection (n=3–9 mice per time point). (e) Absolute numbers (top) and Vα composition (bottom) of env-reactive donor EF4.1 CD4⁺ T cells in the spleens of WT or Ighm^−/− recipient mice after adoptive T-cell transfer and FV infection (n=4–11 mice per time point) or FV/LDV coinfection (n=4–23 mice per time point). (f) Absolute numbers (top) and Vα composition (bottom) of env-reactive donor EF4.1 CD4⁺ T cells in the spleens of Ighm^−/− mice previously reconstituted with WT (n=18) or H2-Ab1^−/− B cells (n=14) 14 days after adoptive T-cell transfer FV/LDV coinfection, plotted against the level of B-cell reconstitution. In d–f, closed symbols are the means (±s.e.m.); open symbols are individual mice.