Thoracic duct and spleen cells of normal (unimmunized) adult rats were fractionated according to size by 1 × G velocity sedimentation. Fractions were tested for their ability to restore the adoptive antibody response of irradiated hosts to horse spleen ferritin. A constant source of T cells (small numbers of unfractionated thoracic duct cells) was added to each fraction in order to monitor the B cell activity of the latter. Although large and small cell fractions of the spleen showed restorative activity, only the small cell fractions of the thoracic duct lymph showed activity.

The turnover rate of the spleen cell fractions was determined by treating donors with high specific-activity 3H-thymidine for 48 hr before splenectomy. Rapidly dividing cells are preferentially killed by this treatment. The results suggest that a considerable proportion of large, intermediate, and small virgin B cells turn over within 48 hr.

The cell surface of the various spleen cell fractions was examined for the presence of immunoglobulin (Ig) and a receptor for complement. The percentage of Ig-bearing cells in the large cell fractions was similar to the percentage of cells bearing IgM and a receptor for complement. However, the majority of Ig-bearing cells in the small cell fractions did not show the latter two surface markers. Experiments with the fluorescence-activated cell sorter showed that the large functionally active B cells bore surface IgM.

The experimental findings suggest that there are subpopulations of virgin B cells in the spleen of the adult rat which differ with respect to size, migration pattern, turnover rate, and cell surface characteristics. The relationship of these cells to one another is discussed in the framework of an antigen-independent model of B cell maturation in the rat.

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