Increased levels of systemic TPO support the accumulation and persistence of plasma cells formed in a specific immune response. (A) Scheme of TPO injection and immunization: Mice received TPO (in PBS) starting from day 4 before immunization as described in “Mice, immunizations, TPO treatment, and BrdU feeding.” Controls received PBS alone. (B) Absolute numbers of total (bottom panels) and Ova-specific (top panels) plasma cells in spleen and bone marrow after secondary immunization are shown at the peak of the anti-Ova response at day 6 (left panels) and day 23 (right panels) when plasma cell counts have reached their plateau. Ova-specific plasma cells were analyzed by flow cytometry. Data are mean ± SD for groups of 6 mice (statistics: unpaired t test).