A Subpopulation of Germinal Center B Cells Differentiate Directly into Antibody Forming Cells upon Secondary Immunization

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Abstract
Considerable evidence implicates germinal center (GC) B cells in the development of the B memory cell pool (1,2).However, the ability of GC B cells to differentiate directly into antibody forming cells (AFC) has not been well established although the idea that GC B cells may be involved in production of antibody is not new. Plasma cells near follicular dendritic cells (FDC) in the GCs have been observed (3) and electron microscopic (EM) studies have shown specific AFC in GCs after antigen challenge (4,5). The objective of the present study was to determine if GC B cells are capable of differentiating directly into AFCs. The data reported here confirm that: 1- Some GC B cells differentiate into AFCs in the GC. 2- By day five after secondary immunization large numbers of GC B cells appeared to migrate into the medullary cords where they may fully differentiate into plasma cells. 3- GC B cells isolated at various times after immunization produced specific antibody in culture in the absence of any in vitro stimulation by antigen or T cells. This was dramatic in GC B cells isolated 4 days after booster immunization and had declined by day 7. Examination of these cells indicated that many of the cultured cells differentiated into plasma cells in culture. We conclude that at least a subpopulation of GC B cells receive all the stimulation in vivo by antigen and T cells needed to fully differentiate into plasma cells.