In secondary antibody (Ab) responses, Ag processing and presentation occur in secondary lymphoid organs but most serum Ab is produced by cells in the bone marrow. Plasma cells in the bone marrow are derived from B cells activated by Ag in secondary lymphoid organs. We hypothesized that germinal center (GC) B cells, which acquire Ag from follicular dendritic cells in draining lymph nodes during the first few days of the secondary response, migrate to the bone marrow to terminally differentiate and produce specific Ab. To test this we looked for GC B cells in the thoracic duct lymph and in peripheral blood after secondary challenge using the peanut agglutininhi phenotype and blast cell morphology as markers for GC B cells. In addition, GC B cells were injected i.v. into naive recipients to determine if they would home to the bone marrow. Finally, to determine if the bone marrow environment supports maturation and Ab production by GC B cells, we cocultured GC B cells with bone marrow cells or bone marrow supernatants. The results indicate that blast cells bearing the GC B cell phenotype were present in both the thoracic duct and the peripheral blood 3 days after antigenic challenge. Day 3 peripheral blood cells secreted specific Ab, whereas cells isolated on day 0, 8, or 11 did not. Furthermore, in adoptive transfer experiments, only the day 3 GC B cells produced specific Ab and migrated to the bone marrow of naive mice. Finally, either bone marrow cells or factor(s) produced by bone marrow cells markedly enhanced Ab production by day 3 GC B cells. These data support the hypothesis that during the first few days after secondary challenge GC B cells seed the bone marrow and differentiate into plasma cells which produce the large quantities of Ab typical of secondary responses.

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