The ultrastructure of antigen localization and virus-like particles in mouse spleen germinal centers

The localization of 125I-labeled human gamma globulin in mouse spleen germinal centers was studied by electron-microscopic autoradiography during the first 30 days of the primary immune response. Cells in the germinal centers which were consistently associated with a large number of silver grains exhibited clear voluminous nuclei with deep clefts, and undifferentiated cytoplasm containing isolated groups of polysomes. The outstanding morphologic feature of these cells was the development of highly convoluted infoldings of the plasma membrane which retained the labeled antigen. The term antigen-retaining reticular cell was used to identify this cell type. Villous extensions of immunoblasts were in close association with the antigen-retaining infoldings of these reticular cells. The relationship of the extracellular labeled antigen to the plasma-membrane infoldings of the antigen-retaining reticular cell remained the same until day 20. By 30 days there were no detectable grains present in germinal centers and the amount of plasma-membrane infolding of the reticular cells was decreased. Many virus-like particles were also found in germinal centers. These particles were located primarily in the membrane infoldings of antigen-retaining reticular cells and were also observed to be budding from typical immunoblasts.