The proportion of CMV-specific CD4⁺ T cells increases markedly during treatment with chemotherapy. (A) The temporal kinetics of the CMV-specific CD4⁺ T-cell response was assessed in relation to treatment history. The frequency and absolute number of IFNγ-producing CD4⁺ T cells increases sharply during treatment with chlorambucil or fludarabine. (B) Examples of the kinetics of the CMV-specific CD4⁺ T-cell immune response in 3 patients who were studied serially at 3 monthly intervals. Patient 1 showed a stable immune response prior to commencing treatment with fludarabine, after which the CMV-specific CD4⁺ T-cell response peaked at 37.7% of the CD4⁺ T-cell pool after 2 months and then declined. Patient 2 received treatment with chlorambucil and also exhibited an increase in the CMV-specific CD 4⁺ T-cell immune response, which increased up to 5 months after treatment. Patient 3 was untreated and showed a stable CMV-specific immune response. FACS analysis was gated on CD4⁺ T cells and IFNγ expression is shown on the x-axis and CD69 on the y-axis. The absolute CD4 T-cell counts (cells/μL) were (A) 671, 540, 530, and 642 at 0, 4, 8, and 12 months; (B) 1104, 829, 800, and 983 at 0, 4, 8, and 14 months; and (C) 389, 377, 417, and 97 at 6, 10, 12, and 16 months, respectively.