In vivo expansion of OVA-reactive T cells after immunization. (A,C) Absolute number of CD4⁺ T cells per LN and (B,D) relative increase in CD4⁺ T-cell number from preimmunization values were calculated from total cell number and percent CD4⁺ cells by flow cytometric analysis. For C57BL/6 and CCR7-deficient mice, draining popliteal LN cells were pooled from 2 mice and assessed, and the results were expressed as mean values. Baseline CD4⁺ T-cell number at day 0 was 4.38 × 10⁵ ± 1.86 × 10⁵ in BALB/c mice, 0.78 × 10⁵ ± 0.04 × 10⁵ in plt/plt mice, 6.4 × 10⁵ in C57BL/6 mice, and 2.3 × 10⁵ in CCR7-deficient mice. (E) Percentages of cells bearing Vβ6, Vβ8, or Vβ14 in CD4⁺ T cells were determined at the indicated days after immunization. Mean ± SD for 3 to 5 mice was indicated.