Immigration of thoracic duct B lymphocytes into established germinal centers in the rat

Immigration of B lymphocytes into established germinal centers in the rat was studied by transferring genetically marked thoracic duct B cells to non-irradiated congenic hosts at various times between 3 days before and 6 days after host immunization. Seven days after host immunization, the distribution of donor B cells to lymph node germinal centers (relative to their distribution to non-germinal center lymph node areas) was measured by two-color flow cytometry in which (a) donor and host B cells were distinguished by their Ig kappa chain allotypes, and (b) germinal center B cells were distinguished by their lack of labeling with the monoclonal antibody HIS22. Thoracic duct B cells from long-term antigen-primed rats were found to immigrate into host germinal centers much better than B cells from unprimed donors. This effect was antigen specific: primed B cells only immigrated well into host germinal centers induced by the priming antigen. Although B cells localized in germinal centers most efficiently when injected before immunization, specifically primed donor B cells injected after immunization were still found to be at least as evenly distributed to germinal centers as to other lymph node areas, whereas unprimed B cells transferred after immunization localized poorly in host germinal centers. These findings are discussed in light of recent suggestions that memory B cell clones are maintained by continued antigenic stimulation within secondary lymphoid follicles.